Astronomy

Why have brown dwarf classes been dubbed L, T and Y?

Why have brown dwarf classes been dubbed L, T and Y?


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The classes used to categorize stars (O, B, A, F, G, K, M) are in a bizarre order for historical reasons. Stars were labeled based on the spectral lines that were visible, then the categories were put in a different order to account for their temperature. When discovering the temperature of stars, the categories were not renamed, so that the old catalogs could still be used, and everyone just learned "Oh be a fine guy/gal kiss me".

The first seven classes were updated as knowledge improved, but brown dwarfs were discovered more recently, so why have the letters L, T and Y been chosen, rather than the easier to remember P, Q and R (or some other set of letters), for instance?


The choice of L and T is explained in Kirkpatrick et al. (1999) 'Dwarfs Cooler than "M": The Definition of Spectral Type "L" Using Discoveries from the 2 Micron All-Sky Survey (2MASS)'. The principles behind the choice are given at the start of section 5.1:

In choosing a letter designation for the new spectral class, three important points must be considered : (1) The letter must be unambiguous, having not been used for any currently recognized spectral type. For example, though "N" follows "M" in the alphabet, it would be a poor choice of letter since it is used for a class of carbon stars. (2) The letter must create a taxonomy that is clearly distinguished from other types of astronomical objects. In this case, the letter must be accepted by the entire community, both by researchers involved in low-mass star and brown dwarf science and by astronomers in general. Although stellar spectroscopists might find "E0," "E1," "E2," etc., perfectly acceptable as new spectral subclasses, extragalactic morphologists already recognize these designations as elliptical galaxy types. (3) The letter must stand the test of time. For example, choosing "D" to mean that these objects are "degenerate" brown dwarfs would be flawed reasoning. Some of these dwarfs certainly are substellar (see § 7), but such a designation cannot be tied uniquely to any particular spectroscopic trait. The designation should apply to spectral features alone and be free of physical interpretation. Our understanding of the underlying physics may change with time ; our choice of letter should be impervious to such changes.

Applying these, they find the letters H, L, T and Y to be ok: the rationale for excluding the other letters is given in their Table 5:

LetterStatusNotes
AIn useStandard spectral class
BIn useStandard spectral class
CIn useStandard carbon-star class
DAmbiguousConfusion with white dwarf classes DA, DB, DC, etc.
EAmbiguousConfusion with elliptical galaxy morphological types E0-E7
FIn useStandard spectral class
GIn useStandard spectral class
HOK
IProblematicTranscription problems with I0 (10, Io) and I1 (11, II, Il)
JIn useStandard carbon-star class
KIn useStandard spectral class
LOK
MIn useStandard spectral class
NIn useStandard carbon-star class
OIn useStandard spectral class
PProblematic?Incorrect association with planets?
QProblematic?Incorrect association with QSOs?
RIn useStandard carbon-star class
SIn useStandard spectral class for ZrO-rich stars
TOK
UProblematic?Incorrect association with ultraviolet sources?
VProblematicConfusion with vanadium oxide (V0 vs VO)
WAmbiguousConfusion with Wolf-Rayet WN and WR classes
XProblematicIncorrect association with X-ray sources
YOK
ZProblematic?Incorrect implication that we have reached "the end"?

They go with "L" as it is the closest letter to "M" that is still available. They prefer it to "H" because of the occasional use of the term "hydride dwarf" to refer to M subdwarfs which are dominated by CaH and MgH bands.

After that, the sequence is going alphabetically through the remaining available letters, so they put Gl 229B, which has a notably different spectrum to the other (L-class) brown dwarfs known at the time, into spectral class T. Spectral type Y took a few more years to show up in observations, and was chosen by the same principle.

If observations get to the point where it is worth erecting a new spectral class beyond Y then things will get interesting.


Review Questions

1: What two factors determine how bright a star appears to be in the sky?

2: Explain why color is a measure of a star’s temperature.

3: What is the main reason that the spectra of all stars are not identical? Explain.

4: What elements are stars mostly made of? How do we know this?

5: What did Annie Cannon contribute to the understanding of stellar spectra?

6: Name five characteristics of a star that can be determined by measuring its spectrum. Explain how you would use a spectrum to determine these characteristics.

7: How do objects of spectral types L, T, and Y differ from those of the other spectral types?

8: Do stars that look brighter in the sky have larger or smaller magnitudes than fainter stars?

9: The star Antares has an apparent magnitude of 1.0, whereas the star Procyon has an apparent magnitude of 0.4. Which star appears brighter in the sky?

10: Based on their colors, which of the following stars is hottest? Which is coolest? Archenar (blue), Betelgeuse (red), Capella (yellow).

11: Order the seven basic spectral types from hottest to coldest.

12: What is the defining difference between a brown dwarf and a true star?


Exploring the Different Types of Stars

Stars are divided into spectral classes, which in turn help to identify their color, size, and luminosity. The seven main types of stars are assigned one of the letter O, B, A, F, G, K, and M, remembered by the classic mnemonic “Oh Be A Fine Girl (Guy), Kiss Me,” with their individual colors, effective temperatures, and size and masses compared to the Sun (sol) as follows:

Harvard Spectral Classification

O: Blue, 28,000-50,000K, radius 20, mass 40,
B: Blue-white, 10,000-28,000K, radius 5, mass 0.1,
A: White, 7,500-10,000K, radius 2, mass 10,
F: White-yellow, 6,000-7,500K, radius 1.2, mass 1.5,
G: Yellow, 4,900-6,000K, radius 1, mass 1,
K: Orange, 3,500-4,900K, radius 0.3, mass 0.5,
M: Red, 2,000-3,500K, radius 0.1, mass 0.1,

The stellar classification sequence has since been extended to include the spectral types L, T, and Y, with L type stars ranging in temperature from 1,300K to 2,000K, and usually red-brown in color T type stars between 700K and 1,300K K and of a purplish-red hue and Y type stars showing temperatures of less than 600K.

Life Cycle of Stars

Nebula are large expanses of interstellar gas which mostly contain vast amounts of hydrogen and helium, and when a dense region of a nebula starts to gravitational collapse and heat up, stars begin to form. This may in turn result in the birth of just a few dozen stars to many thousands, and to put the process into perspective, our Sun, which has a diameter of 864,938 miles (1.392 million km), would require an amount of gas hundred of times the size of our solar system to form.

Protostars/T Tauri phase or Brown Dwarfs

The process of star formation begins by hot clumps of molecules forming inside a gas cloud to create a protostar, with the object remaining in this contraction stage as long as material continues to fall inward. For our Sun, this protostar phase would have lasted around 100,000 years, after which it would have entered the T Tauri phase for 100 million years, in which it shines using only energy produced by its ongoing gravitational collapse. Eventually, it would have acquired enough size and mass, as well as temperatures and pressures at its core to sustain nuclear reactions (hydrogen fused into helium), after which the outward force of its emitted radiation is balanced by its own inward gravity resulting in a hydrostatic equilibrium state referred to as the main sequence.

Those balls of gas whose mass is less than 8% that of the Sun, however, are unable to ignite nuclear fusion, and end up as Brown Dwarfs, or a failed star. These dim and cool objects fall into the M, L and T spectral class, and have between 13 and 90 times the mass of Jupiter. They also emit so little light and energy that they are difficult to detect.

Main Sequence

The main sequence is where a star will spend 90% of its life fusing hydrogen into helium in its core. These type of stars account for around 90% of all stars in the universe, and range in mass from 1/10th to 200 times that of the Sun, with their life spans mostly depending upon their mass and chemical compositions the least massive stars last for tens of billions of years, while for the heaviest stars their estimated lifetimes may only be a few million years.

Going from coolest to hottest, the different types of Main Sequence stars include Red Dwarfs (K to M), Orange Dwarfs (K), Yellow Dwarfs (G), white stars (F to A ), and blue stars (B to O).

Leaving the Main Sequence

Mass also determines how a star leaves the main sequence phase of its life, and what type of star it then becomes.

1) Those stars with solar masses less than 0.5 do not have enough size or pressure in their core to fuse helium, and so collapse directly into a ‘dead’ star known as a White Dwarf. These type of stars can be a million times denser than that the Sun, but have only 1% the Sun’s diameter and luminosity. Over several billion years, the leftover heat it still emits will subsequently radiate away to leave a Black Dwarf, which is a hypothetical stellar remnant that has no heat or light.

2) Those stars with solar masses between a 0.5 and 8 continue to fuse hydrogen into helium in their core until the hydrogen available runs out and hydrogen fusion takes place in a shell surrounding the core, which then expands to the star’s outer layers, resulting in it growing in size and luminosity to form a Subgiant, and then a Red Giant. In the meantime, the star’s helium rich core starts to fuse helium into carbon and oxygen, and after its helium supply is exhausted the star’s outer layers will be ejected to form a planetary nebula, while its core becomes a white dwarf.

3) More massive stars will either evolve into Red Giants, or even Red Supergiants as they fuse heavier and heavier elements in their cores. Over time, they may oscillate between existence as a red and Blue Supergiant before being unable to fuse the iron which has formed in its core, leading to it becoming unstable and collapsing. A massive explosion then causes the star to go supernova, in the process creating many elements heavier than iron, such as uranium and plutonium, with those stars with 8 or more solar masses leaving behind a Neutron Star, and those with 30 or more Sun’s masses transforming into a Black Hole.


Why have brown dwarf classes been dubbed L, T and Y? - Astronomy

By the end of this section, you will be able to:

  • Describe how astronomers use spectral classes to characterize stars
  • Explain the difference between a star and a brown dwarf

Measuring colors is only one way of analyzing starlight. Another way is to use a spectrograph to spread out the light into a spectrum (see the Radiation and Spectra and the Astronomical Instruments chapters). In 1814, the German physicist Joseph Fraunhofer observed that the spectrum of the Sun shows dark lines crossing a continuous band of colors. In the 1860s, English astronomers Sir William Huggins and Lady Margaret Huggins ([link]) succeeded in identifying some of the lines in stellar spectra as those of known elements on Earth, showing that the same chemical elements found in the Sun and planets exist in the stars. Since then, astronomers have worked hard to perfect experimental techniques for obtaining and measuring spectra, and they have developed a theoretical understanding of what can be learned from spectra. Today, spectroscopic analysis is one of the cornerstones of astronomical research.

William and Margaret Huggins were the first to identify the lines in the spectrum of a star other than the Sun they also took the first spectrogram, or photograph of a stellar spectrum.

Formation of Stellar Spectra

When the spectra of different stars were first observed, astronomers found that they were not all identical. Since the dark lines are produced by the chemical elements present in the stars, astronomers first thought that the spectra differ from one another because stars are not all made of the same chemical elements. This hypothesis turned out to be wrong. The primary reason that stellar spectra look different is because the stars have different temperatures. Most stars have nearly the same composition as the Sun, with only a few exceptions.

Hydrogen, for example, is by far the most abundant element in most stars. However, lines of hydrogen are not seen in the spectra of the hottest and the coolest stars. In the atmospheres of the hottest stars, hydrogen atoms are completely ionized. Because the electron and the proton are separated, ionized hydrogen cannot produce absorption lines. (Recall from the Formation of Spectral Lines section, the lines are the result of electrons in orbit around a nucleus changing energy levels.)

In the atmospheres of the coolest stars, hydrogen atoms have their electrons attached and can switch energy levels to produce lines. However, practically all of the hydrogen atoms are in the lowest energy state (unexcited) in these stars and thus can absorb only those photons able to lift an electron from that first energy level to a higher level. Photons with enough energy to do this lie in the ultraviolet part of the electromagnetic spectrum, and there are very few ultraviolet photons in the radiation from a cool star. What this means is that if you observe the spectrum of a very hot or very cool star with a typical telescope on the surface of Earth, the most common element in that star, hydrogen, will show very weak spectral lines or none at all.

The hydrogen lines in the visible part of the spectrum (called Balmer lines) are strongest in stars with intermediate temperatures—not too hot and not too cold. Calculations show that the optimum temperature for producing visible hydrogen lines is about 10,000 K. At this temperature, an appreciable number of hydrogen atoms are excited to the second energy level. They can then absorb additional photons, rise to still-higher levels of excitation, and produce a dark absorption line. Similarly, every other chemical element, in each of its possible stages of ionization, has a characteristic temperature at which it is most effective in producing absorption lines in any particular part of the spectrum.

Classification of Stellar Spectra

Astronomers use the patterns of lines observed in stellar spectra to sort stars into a spectral class . Because a star’s temperature determines which absorption lines are present in its spectrum, these spectral classes are a measure of its surface temperature. There are seven standard spectral classes. From hottest to coldest, these seven spectral classes are designated O, B, A, F, G, K, and M. Recently, astronomers have added three additional classes for even cooler objects—L, T, and Y.

At this point, you may be looking at these letters with wonder and asking yourself why astronomers didn’t call the spectral types A, B, C, and so on. You will see, as we tell you the history, that it’s an instance where tradition won out over common sense.

In the 1880s, Williamina Fleming devised a system to classify stars based on the strength of hydrogen absorption lines. Spectra with the strongest lines were classified as “A” stars, the next strongest “B,” and so on down the alphabet to “O” stars, in which the hydrogen lines were very weak. But we saw above that hydrogen lines alone are not a good indicator for classifying stars, since their lines disappear from the visible light spectrum when the stars get too hot or too cold.

In the 1890s, Annie Jump Cannon revised this classification system, focusing on just a few letters from the original system: A, B, F, G, K, M, and O. Instead of starting over, Cannon also rearranged the existing classes—in order of decreasing temperature—into the sequence we have learned: O, B, A, F, G, K, M. As you can read in the feature on Annie Cannon: Classifier of the Stars in this chapter, she classified around 500,000 stars over her lifetime, classifying up to three stars per minute by looking at the stellar spectra.

For a deep dive into spectral types, explore the interactive project at the Sloan Digital Sky Survey in which you can practice classifying stars yourself.

To help astronomers remember this crazy order of letters, Cannon created a mnemonic, “Oh Be A Fine Girl, Kiss Me.” (If you prefer, you can easily substitute “Guy” for “Girl.”) Other mnemonics, which we hope will not be relevant for you, include “Oh Brother, Astronomers Frequently Give Killer Midterms” and “Oh Boy, An F Grade Kills Me!” With the new L, T, and Y spectral classes, the mnemonic might be expanded to “Oh Be A Fine Girl (Guy), Kiss Me Like That, Yo!”

Each of these spectral classes, except possibly for the Y class which is still being defined, is further subdivided into 10 subclasses designated by the numbers 0 through 9. A B0 star is the hottest type of B star a B9 star is the coolest type of B star and is only slightly hotter than an A0 star.

And just one more item of vocabulary: for historical reasons, astronomers call all the elements heavier than helium metals, even though most of them do not show metallic properties. (If you are getting annoyed at the peculiar jargon that astronomers use, just bear in mind that every field of human activity tends to develop its own specialized vocabulary. Just try reading a credit card or social media agreement form these days without training in law!)

Let’s take a look at some of the details of how the spectra of the stars change with temperature. (It is these details that allowed Annie Cannon to identify the spectral types of stars as quickly as three per minute!) As [link] shows, in the hottest O stars (those with temperatures over 28,000 K), only lines of ionized helium and highly ionized atoms of other elements are conspicuous. Hydrogen lines are strongest in A stars with atmospheric temperatures of about 10,000 K. Ionized metals provide the most conspicuous lines in stars with temperatures from 6000 to 7500 K (spectral type F). In the coolest M stars (below 3500 K), absorption bands of titanium oxide and other molecules are very strong. By the way, the spectral class assigned to the Sun is G2. The sequence of spectral class es is summarized in [link].

This graph shows the strengths of absorption lines of different chemical species (atoms, ions, molecules) as we move from hot (left) to cool (right) stars. The sequence of spectral types is also shown.
Spectral Classes for Stars
Spectral Class Color Approximate Temperature (K) Principal Features Examples
O Blue > 30,000 Neutral and ionized helium lines, weak hydrogen lines 10 Lacertae
B Blue-white 10,000󈞊,000 Neutral helium lines, strong hydrogen lines Rigel, Spica
A White 7500󈝶,000 Strongest hydrogen lines, weak ionized calcium lines, weak ionized metal (e.g., iron, magnesium) lines Sirius , Vega
F Yellow-white 6000� Strong hydrogen lines, strong ionized calcium lines, weak sodium lines, many ionized metal lines Canopus, Procyon
G Yellow 5200� Weaker hydrogen lines, strong ionized calcium lines, strong sodium lines, many lines of ionized and neutral metals Sun , Capella
K Orange 3700� Very weak hydrogen lines, strong ionized calcium lines, strong sodium lines, many lines of neutral metals Arcturus, Aldebaran
M Red 2400� Strong lines of neutral metals and molecular bands of titanium oxide dominate Betelgeuse , Antares
L Red 1300� Metal hydride lines, alkali metal lines (e.g., sodium, potassium, rubidium) Teide 1
T Magenta 700� Methane lines Gliese 229B
Y Infrared 1 < 700 Ammonia lines WISE 1828+2650

To see how spectral classification works, let’s use [link]. Suppose you have a spectrum in which the hydrogen lines are about half as strong as those seen in an A star. Looking at the lines in our figure, you see that the star could be either a B star or a G star. But if the spectrum also contains helium lines, then it is a B star, whereas if it contains lines of ionized iron and other metals, it must be a G star.

If you look at [link], you can see that you, too, could assign a spectral class to a star whose type was not already known. All you have to do is match the pattern of spectral lines to a standard star (like the ones shown in the figure) whose type has already been determined.

This image compares the spectra of the different spectral classes. The spectral class assigned to each of these stellar spectra is listed at the left of the picture. The strongest four lines seen at spectral type A1 (one in the red, one in the blue-green, and two in the blue) are Balmer lines of hydrogen. Note how these lines weaken at both higher and lower temperatures, as [link] also indicates. The strong pair of closely spaced lines in the yellow in the cool stars is due to neutral sodium (one of the neutral metals in [link]). (Credit: modification of work by NOAO/AURA/NSF)

Both colors and spectral classes can be used to estimate the temperature of a star. Spectra are harder to measure because the light has to be bright enough to be spread out into all colors of the rainbow, and detectors must be sensitive enough to respond to individual wavelengths. In order to measure colors, the detectors need only respond to the many wavelengths that pass simultaneously through the colored filters that have been chosen—that is, to all the blue light or all the yellow-green light.

Annie Jump Cannon was born in Delaware in 1863 ([link]). In 1880, she went to Wellesley College, one of the new breed of US colleges opening up to educate young women. Wellesley, only 5 years old at the time, had the second student physics lab in the country and provided excellent training in basic science. After college, Cannon spent a decade with her parents but was very dissatisfied, longing to do scientific work. After her mother’s death in 1893, she returned to Wellesley as a teaching assistant and also to take courses at Radcliffe, the women’s college associated with Harvard.

Cannon is well-known for her classifications of stellar spectra. (credit: modification of work by Smithsonian Institution)

In the late 1800s, the director of the Harvard Observatory, Edward C. Pickering, needed lots of help with his ambitious program of classifying stellar spectra. The basis for these studies was a monumental collection of nearly a million photographic spectra of stars, obtained from many years of observations made at Harvard College Observatory in Massachusetts as well as at its remote observing stations in South America and South Africa. Pickering quickly discovered that educated young women could be hired as assistants for one-third or one-fourth the salary paid to men, and they would often put up with working conditions and repetitive tasks that men with the same education would not tolerate. These women became known as the Harvard Computers. (We should emphasize that astronomers were not alone in reaching such conclusions about the relatively new idea of upper-class, educated women working outside the home: women were exploited and undervalued in many fields. This is a legacy from which our society is just beginning to emerge.)

Cannon was hired by Pickering as one of the “computers” to help with the classification of spectra. She became so good at it that she could visually examine and determine the spectral types of several hundred stars per hour (dictating her conclusions to an assistant). She made many discoveries while investigating the Harvard photographic plates, including 300 variable stars (stars whose luminosity changes periodically). But her main legacy is a marvelous catalog of spectral types for hundreds of thousands of stars, which served as a foundation for much of twentieth-century astronomy.

In 1911, a visiting committee of astronomers reported that “she is the one person in the world who can do this work quickly and accurately” and urged Harvard to give Cannon an official appointment in keeping with her skill and renown. Not until 1938, however, did Harvard appoint her an astronomer at the university she was then 75 years old.

Cannon received the first honorary degree Oxford awarded to a woman, and she became the first woman to be elected an officer of the American Astronomical Society, the main professional organization of astronomers in the US. She generously donated the money from one of the major prizes she had won to found a special award for women in astronomy, now known as the Annie Jump Cannon Prize. True to form, she continued classifying stellar spectra almost to the very end of her life in 1941.

Spectral Classes L, T, and Y

The scheme devised by Cannon worked well until 1988, when astronomers began to discover objects even cooler than M9-type stars. We use the word object because many of the new discoveries are not true stars. A star is defined as an object that during some part of its lifetime derives 100% of its energy from the same process that makes the Sun shine—the fusion of hydrogen nuclei (protons) into helium. Objects with masses less than about 7.5% of the mass of our Sun (about 0.075 MSun) do not become hot enough for hydrogen fusion to take place. Even before the first such “failed star” was found, this class of objects, with masses intermediate between stars and planets, was given the name brown dwarfs .

Brown dwarfs are very difficult to observe because they are extremely faint and cool, and they put out most of their light in the infrared part of the spectrum. It was only after the construction of very large telescopes, like the Keck telescopes in Hawaii, and the development of very sensitive infrared detectors, that the search for brown dwarfs succeeded. The first brown dwarf was discovered in 1988, and, as of the summer of 2015, there are more than 2200 known brown dwarfs.

Initially, brown dwarfs were given spectral classes like M10 + or “much cooler than M9,” but so many are now known that it is possible to begin assigning spectral types. The hottest brown dwarfs are given types L0–L9 (temperatures in the range 2400� K), whereas still cooler (1300� K) objects are given types T0–T9 (see [link]). In class L brown dwarfs, the lines of titanium oxide, which are strong in M stars, have disappeared. This is because the L dwarfs are so cool that atoms and molecules can gather together into dust particles in their atmospheres the titanium is locked up in the dust grains rather than being available to form molecules of titanium oxide. Lines of steam (hot water vapor) are present, along with lines of carbon monoxide and neutral sodium, potassium, cesium, and rubidium. Methane (CH4) lines are strong in class-T brown dwarfs, as methane exists in the atmosphere of the giant planets in our own solar system.

In 2009, astronomers discovered ultra-cool brown dwarfs with temperatures of 500� K. These objects exhibited absorption lines due to ammonia (NH3), which are not seen in T dwarfs. A new spectral class, Y, was created for these objects. As of 2015, over two dozen brown dwarfs belonging to spectral class Y have been discovered, some with temperatures comparable to that of the human body (about 300 K).

This illustration shows the sizes and surface temperatures of brown dwarfs Teide 1, Gliese 229B, and WISE1828 in relation to the Sun, a red dwarf star (Gliese 229A), and Jupiter. (credit: modification of work by MPIA/V. Joergens)

Most brown dwarfs start out with atmospheric temperatures and spectra like those of true stars with spectral classes of M6.5 and later, even though the brown dwarfs are not hot and dense enough in their interiors to fuse hydrogen. In fact, the spectra of brown dwarfs and true stars are so similar from spectral types late M through L that it is not possible to distinguish the two types of objects based on spectra alone. An independent measure of mass is required to determine whether a specific object is a brown dwarf or a very low mass star. Since brown dwarfs cool steadily throughout their lifetimes, the spectral type of a given brown dwarf changes with time over a billion years or more from late M through L, T, and Y spectral types.

Low-Mass Brown Dwarfs vs. High-Mass Planets

An interesting property of brown dwarfs is that they are all about the same radius as Jupiter , regardless of their masses. Amazingly, this covers a range of masses from about 13 to 80 times the mass of Jupiter (MJ). This can make distinguishing a low-mass brown dwarf from a high-mass planet very difficult.

So, what is the difference between a low-mass brown dwarf and a high-mass planet? The International Astronomical Union considers the distinctive feature to be deuterium fusion. Although brown dwarfs do not sustain regular (proton-proton) hydrogen fusion, they are capable of fusing deuterium (a rare form of hydrogen with one proton and one neutron in its nucleus). The fusion of deuterium can happen at a lower temperature than the fusion of hydrogen. If an object has enough mass to fuse deuterium (about 13 MJ or 0.012 MSun), it is a brown dwarf. Objects with less than 13 MJ do not fuse deuterium and are usually considered planets.

Key Concepts and Summary

The differences in the spectra of stars are principally due to differences in temperature, not composition. The spectra of stars are described in terms of spectral classes. In order of decreasing temperature, these spectral classes are O, B, A, F, G, K, M, L, T, and Y. These are further divided into subclasses numbered from 0 to 9. The classes L, T, and Y have been added recently to describe newly discovered star-like objects—mainly brown dwarfs—that are cooler than M9. Our Sun has spectral type G2.


Thought Questions

13: If the star Sirius emits 23 times more energy than the Sun, why does the Sun appear brighter in the sky?

14: How would two stars of equal luminosity—one blue and the other red—appear in an image taken through a filter that passes mainly blue light? How would their appearance change in an image taken through a filter that transmits mainly red light?

15: Table 17.2 lists the temperature ranges that correspond to the different spectral types. What part of the star do these temperatures refer to? Why?

16: Suppose you are given the task of measuring the colors of the brightest stars, listed in Appendix J, through three filters: the first transmits blue light, the second transmits yellow light, and the third transmits red light. If you observe the star Vega, it will appear equally bright through each of the three filters. Which stars will appear brighter through the blue filter than through the red filter? Which stars will appear brighter through the red filter? Which star is likely to have colors most nearly like those of Vega?

17: Star X has lines of ionized helium in its spectrum, and star Y has bands of titanium oxide. Which is hotter? Why? The spectrum of star Z shows lines of ionized helium and also molecular bands of titanium oxide. What is strange about this spectrum? Can you suggest an explanation?

18: The spectrum of the Sun has hundreds of strong lines of nonionized iron but only a few, very weak lines of helium. A star of spectral type B has very strong lines of helium but very weak iron lines. Do these differences mean that the Sun contains more iron and less helium than the B star? Explain.

19: What are the approximate spectral classes of stars with the following characteristics?

  1. Balmer lines of hydrogen are very strong some lines of ionized metals are present.
  2. The strongest lines are those of ionized helium.
  3. Lines of ionized calcium are the strongest in the spectrum hydrogen lines show only moderate strength lines of neutral and metals are present.
  4. The strongest lines are those of neutral metals and bands of titanium oxide.

20: Look at the chemical elements in Appendix K. Can you identify any relationship between the abundance of an element and its atomic weight? Are there any obvious exceptions to this relationship?

21: Appendix I lists some of the nearest stars. Are most of these stars hotter or cooler than the Sun? Do any of them emit more energy than the Sun? If so, which ones?

22: Appendix J lists the stars that appear brightest in our sky. Are most of these hotter or cooler than the Sun? Can you suggest a reason for the difference between this answer and the answer to the previous question? (Hint: Look at the luminosities.) Is there any tendency for a correlation between temperature and luminosity? Are there exceptions to the correlation?

23: What star appears the brightest in the sky (other than the Sun)? The second brightest? What color is Betelgeuse? Use Appendix J to find the answers.

24: Suppose hominids one million years ago had left behind maps of the night sky. Would these maps represent accurately the sky that we see today? Why or why not?

25: Why can only a lower limit to the rate of stellar rotation be determined from line broadening rather than the actual rotation rate? (Refer to Figure 17.14.)

26: Why do you think astronomers have suggested three different spectral types (L, T, and Y) for the brown dwarfs instead of M? Why was one not enough?

27: Sam, a college student, just bought a new car. Sam’s friend Adam, a graduate student in astronomy, asks Sam for a ride. In the car, Adam remarks that the colors on the temperature control are wrong. Why did he say that?

Figure 9. (credit: modification of work by Michael Sheehan)

28: Would a red star have a smaller or larger magnitude in a red filter than in a blue filter?

29: Two stars have proper motions of one arcsecond per year. Star A is 20 light-years from Earth, and Star B is 10 light-years away from Earth. Which one has the faster velocity in space?

30: Suppose there are three stars in space, each moving at 100 km/s. Star A is moving across (i.e., perpendicular to) our line of sight, Star B is moving directly away from Earth, and Star C is moving away from Earth, but at a 30° angle to the line of sight. From which star will you observe the greatest Doppler shift? From which star will you observe the smallest Doppler shift?

31: What would you say to a friend who made this statement, “The visible-light spectrum of the Sun shows weak hydrogen lines and strong calcium lines. The Sun must therefore contain more calcium than hydrogen.”?


By Jesse Emspak

Brown dwarfs were once called failed stars — more massive than planets but without enough heft to ignite hydrogen fusion and shine under their own power. In recent years, astronomers have learned that they are among the most complex objects in the sky: Pressure has crushed their interiors into super-dense states scientists call “degenerate” while their cool atmospheres may harbor clouds of iron and silicon. They could hold the keys to understanding why solar systems form the way they do and serve as clocks for determining ages throughout the galaxy — if astronomers can pin down how they change with time.

“They show us that our [stellar] evolutionary models are wrong,” says Emily Rice, an astrophysicist at the American Museum of Natural History and the College of Staten Island in New York City. Brown dwarfs have had a habit of defying expectations, and their sheer variety keeps them interesting, she says. “There are a lot of big ideas and open questions [surrounding them]. ”

From Theory to Reality
Astronomer Shiv Kumar, then at NASA’s Goddard Space Flight Center Institute for Space Studies in New York, first proposed the existence of brown dwarfs in the 1960s. Kumar constructed models of low-mass stars and found the mass limit for objects capable of fusing hydrogen — about 0.07 solar mass for a gas cloud with a similar composition to the Sun and about 0.09 solar mass for one made of pure hydrogen. Such an object would contract until it reached a certain size, where the pressure exerted by degenerate electrons — they occupy all of the lowest possible energy states in the gaseous interior — would halt the collapse. At the time, Kumar called them “black dwarfs,” but that name already was taken by white dwarf stars that had cooled to the point where they no longer shine. In 1975, Jill Tarter, then a newly minted Ph.D. and now at the SETI Institute in Mountain View, California, proposed the name “brown dwarf,” and the moniker stuck.

Yet it took until 1995 to finally see one, when astronomers discovered Teide 1 in the Pleiades star cluster. After that, the sightings came thick and fast — astronomers now have identified more than 1,000

brown dwarfs thanks to better detectors, particularly in the infrared part of the spectrum where brown dwarfs radiate most of their energy. The big players include the Two-Micron All-Sky Survey (2MASS), the Spitzer Space Telescope, and the Wide- Field Infrared Survey Explorer (WISE).

With greater numbers, however, has come greater complexity.

Acting your age
Stars fuse hydrogen into helium during most of their lives, a stage scientists refer to as the “main sequence.” A star’s size depends on the balance between the inward pull of gravity and the outward push of gas pressure caused by heat. Heavier stars go through their stores of hydrogen faster, and thus are more luminous, and a star’s color and size tend to stay the same until it’s almost out of fuel. Once you know a star’s mass, intrinsic luminosity, and color, it’s not difficult to put constraints on how old it is and how long it will live.

But brown dwarfs behave differently. Lacking the mass of stars, they don’t generate the necessary heat and pressure at their cores to turn hydrogen into helium. The core may get hot enough to fuse deuterium, a heavy isotope of hydrogen with one neutron, or even lithium. But neither process lasts long because such elements form only a tiny percentage of a brown dwarf’s mass. Electron degeneracy puts a lower limit on the size of the dwarf, which cools slowly as it radiates away its internal heat.

Astronomers classify brown dwarfs as L, T, and Y, running from hottest to coolest. Theoretically, this sequence also should run from youngest to oldest, reflecting the dwarfs’ slow cooling.

“Stars stay on [the main sequence] and at an absolute brightness and color for a long time,” says Adam Burgasser, an astrophysicist at the University of California, San Diego and head of its Cool Star Lab. While it’s possible to put a lower limit on a star’s age, the evidence is indirect until they start moving off the main sequence. “But the luminosity of a brown warf is the main thing we measure — it’s more directly accessible — so if that is time variable, it’s a much better clock.”

The problem is getting a good handle on a brown dwarf’s mass and, from that, the rate at which it cools. A massive brown dwarf will lose heat much more slowly than a less massive one.

The difficulty of determining a brown dwarf’s mass stems from their location — they often exist in isolation. A companion star or planet makes the task easy because scientists can measure the dwarf ’s gravitational pull and thus its mass. So the key, says Burgasser, is to find lots of brown dwarfs in binary systems. “A lot of work is being done to make that a reality,” he adds.

Another way to learn a brown dwarf’s age is to measure its surface gravity. By breaking down an object’s light into individual colors, a spectrum can show not only what compounds are in the brown dwarf’s atmosphere but also the gravitational force there. In stronger gravity fields, spectral lines broaden because the atmospheric gases are more compressed and therefore the molecules move more rapidly. So, by looking at the width of spectral lines, scientists can estimate a brown dwarf’s surface gravity, which in turn tells them how much it has contracted and thus approximately how old it is.

True colors and stormy weather
Meanwhile, some astronomers strive to see into the atmospheres and come up with models that describe the clouds there. Brown dwarfs are cool enough to have weather, but it isn’t like anything on Earth.

For a brown dwarf, cloud composition depends on temperature. Younger objects are relatively hot, sometimes up to about 3,000 kelvins. As the dwarf cools, different compounds will condense. At higher temperatures, the clouds might be made of silicon or iron, while lower temperatures mean clouds of methane or water. In both cases, a lot of complex molecular chemistry takes place.

In fact, the only place that water clouds have been definitely observed beyond the solar system is on cool Y-class brown dwarfs. Jackie Faherty, an astronomer at the Carnegie Institution of Washington and the American Museum of Natural History, recently published a study of a particularly cool dwarf with a temperature of only about 250 K (–10° Fahrenheit) and a mass of six to 10 Jupiters. “What I think that I have is the first object that there’s verifiable evidence of water clouds outside our solar system,” she says. The object, cataloged as WISE 0855–0714, lies only about 7 light-years from Earth.

Another way of using a brown dwarf’s atmosphere to get at deeper truths involves looking at how much light it lets through. Kay Hiranaka, a graduate student at Hunter College in New York City, is working on how to identify a brown dwarf’s age by how deep into the dwarf an observer can see. A younger, warmer brown dwarf will tend to have a thicker atmosphere. As the dwarf cools, heavier elements will con- dense into larger droplets and dust grains that eventually rain out of the atmosphere. So, as a brown dwarf ages, it should become less cloudy, making it easier to see light from deeper in the interior.

Adding complications
But the story of brown dwarf atmospheres isn’t so simple. Hunter College astronomer Kelle Cruz (Hiranaka’s advisor) has been studying the spectra of low-mass brown dwarfs using 2MASS data for more than a decade. In a 2009 study published in The Astronomical Journal, she found that while many of these objects had spectra that looked normal, some showed absorption lines that didn’t match expected strengths, and the overall light coming from the dwarf was either bluer or redder than it should be.

For example, Cruz found that the spectral lines for sodium, cesium, rubidium, potassium, iron hydrides, and titanium oxide were weak while those for vanadium oxide were relatively strong. These results differ from most brown dwarfs of the same class but with higher surface gravities.

Another odd aspect of the spectra was lithium, the third-lightest element. In ordinary stars, lithium atoms fuse with hydrogen to create two helium nuclei, so the lithium gets depleted quickly. No object below 65 Jupiter masses (0.06 solar mass) can build up enough heat to fuse lithium, which means that it should show up in absorption spectra. Many of the low-mass objects Cruz and her colleagues studied failed the so-called lithium test, however, because they showed none of this element.

Cruz’s team considered various explanations for the lack of lithium and concluded that the dwarfs’ low gravity is the likely culprit. Cruz says clouds also may help block lithium’s signature. For example, a brown dwarf with lots of dust particles in its atmosphere might preferentially scatter shorter wavelengths of light where the lithium lines occur.

Clouds also have been a focus of Stanimir Metchev, an astronomer at the University of Western Ontario in London, who studied brown dwarf rotations to learn more about these atmospheric phenomena. By tracking the brightnesses of the dwarfs, he could use the variability to map visible features. “It’s the oldest technique in astronomy,” he says, “just measuring the total brightness over time.”

“The bottom line from our study of weather on brown dwarfs is that virtually all of them have spots on their surfaces, perhaps not much unlike the weather systems that we observe on Jupiter and other giant planets in the solar system,” he says. “The state-of-the-art understanding before our survey was that spotted brown dwarfs may be confined to a narrow temperature range, between 1,300 and 1,500 K, where their atmospheres were expected to undergo the greatest changes because of the disruption of silicate [dusty] clouds. Our survey has shown that these clouds are visible in all brown dwarfs, not on just those special ones.”

In addition, Metchev found that younger, hotter brown dwarfs show a greater temperature contrast between regions than older ones. Temperature contrasts across a dwarf’s surface provide the driving force for storms that can be every bit as violent as those on Jupiter or Saturn, and possibly many times that size.

Clouds on brown dwarfs also can add complexity to the models for how the luminosities of these objects change over time. Astronomer Trent Dupuy of the University of Texas at Austin recently found evidence that the models are off, perhaps by as much as a factor of two. He looked at a binary system for which he could get an accurate mass for the dwarf and checked its luminosity against available models. He found that the dwarf was too bright given the system’s estimated age.

Dupuy thinks a big reason is that the clouds are irregular — no planet or dwarf is uniformly cloudy everywhere. At the same time, clouds act like a blanket and help the dwarf hang on to more energy. Models, he says, tend to assume that temperatures are uniform across the surface. Dupuy doesn’t think the discrepancy is too bad. Saturn, for example, is also hotter than it should be according to models that work well for Jupiter. “On the one hand, they are a factor of two off,” he says. “On the other, it’s only a factor of two.”

Spin doctors
Metchev and his colleagues found that the rotational periods of brown dwarfs don’t match theory either. As a body gravitationally con- tracts, the law of conservation of angular momentum dictates that it will rotate faster, like a spinning figure skater who pulls in her arms. Although the researchers found that a significant fraction of brown dwarfs spin in about 10 hours or more, Metchev says the expected average should be even faster. Without tidal forces — from a planet orbiting the brown dwarf or the dwarf cir- cling a star — there are not many ways to slow down a rapidly rotating object.

One possibility would be for the dwarf’s magnetic field to couple with the interstellar medium. The problem with this idea is that there might not be enough matter to generate a coupling strong enough. “Within about 300 light-years of the Sun, we’re in a local bubble,” says Metchev. “A long-ago supernova cleared this region.”

How low can you go?
These problems connect with how brown dwarfs are born in the first place. Before they were discovered, it wasn’t clear how they could form at all.

University of Western Ontario astronomer Shantanu Basu, who studies star formation, says that most scientists around 1990 said that forming a star would require a gas cloud of at least one solar mass. But most stars are smaller than the Sun, so clearly it’s possible to generate objects with lower masses, perhaps through fragmentation. But can you get down as low as a brown dwarf?

“It’s actually rather hard to get some- thing that low mass to collapse directly,” says Basu. He adds that the debate now is whether brown dwarfs form “top down,” from collapsing gas clouds as stars do, or “bottom up,” by accreting matter like planets. The evidence is not conclusive, and it’s possible that both processes occur.

Astronomer Kevin Luhman at Pennsylvania State University isn’t so sure. “I think that observations indicate that most brown dwarfs probably form in the same manner as stars, through the gravitational collapse of a cloud core,” he says. “They are just born from smaller molecular cloud cores than stars.”

It’s possible, he adds, that turbulence within the gas cloud causes some parts of it to turn into stars and others into brown dwarfs. Through surveys of star-forming regions, he has found objects as small as 0.005 solar mass (about five Jupiters).

Basu notes that a protostar’s accretion disk can contain a lot of mass, so it’s pos- sible that brown dwarfs form the same way as gas giant planets. If so, some of these bodies should get ejected into deep space as they get jostled. This could happen even before they have fully formed — creating clumps of half-contracted matter that eventually will form free-floating brown dwarfs.

If true, a large number of free-floaters should exist in star-forming regions and at the periphery of local star systems. The problem with confirming such objects is that their ejection speeds would tend to be slow, on the order of a mile per second, which is equivalent to moving a light-year in a few hundred thousand years. So, it would be difficult to tell if a brown dwarf formed in place or elsewhere.

Basu hopes new observations with the Atacama Large Millimeter/submillimeter Array in Chile will reveal brown dwarfs in the dust disks surrounding stars. Isolated brown dwarfs have been observed, though it’s not clear yet if they were ejected from a parent system. “We don’t have any observations of the early stages, the first 10,000 years,” he says.

Planet stand-ins
The fuzzy boundary between brown dwarfs and giant planets is part of what makes these objects worthy of study, says Faherty. “Some of these would be without question a planet [if they orbited a star].”

And their masses can get close to some of the Jupiter-class worlds found by the Kepler space telescope. “It’s a gateway to under- standing giant exoplanets,” she says.

“One reason brown dwarfs are interesting is that they allow us to study the process of star formation over a very wide range of masses, from 100 solar masses to 0.005 solar mass [and perhaps lower],” says Luhman. “At the same time, we can exam- ine how planet formation varies over that same range of masses for the central ‘sun.’ By doing so, we can test theories for star formation and planet formation since those theories often make predictions that depend on the stellar mass.”

That’s part of what makes the study of brown dwarfs so exciting, says Faherty. When we study the origins of these objects, “We’re playing detective for something [that happened anywhere] from 10 million to 3 billion years ago.”


Freelance Traveller

This article originally appeared in the July 2015 issue.

As many of us who have studied stellar masses know, each star can have its own special characteristics that make it as unique as a fingerprint. For example, our own sun can be considered a stellar variant due to cycles of sunspots and stellar ejecta that emanate from the sun’s surface.

With that in mind, I went about looking at other types of variables and found a wide variety listed. Some of the better known types are as follows:

Alpha Cygni (α Cyg) variables are non-radial pulsating variables of spectral type B or A and luminosity class Ia. Due to their immense size and high luminosity, one can expect to see periodic rapid increases of EMP (Electromagnetic Pulses) and intense radiation, ranging from 5-30 days in length. The ‘non-radial pulsating variable’ description means that one area of this star can be expanding while another side may be shrinking. Deneb (α Cyg, type A2Ia) is the prototypical example of this star type. Inhabited planets orbiting α Cyg variables would likely have “hardened” subsurface habitats with heavily shielded local communications, and transiting starships would use microwave beacons and masers for communications with the world(s). Beta Cephei (β Cep) variables are rapidly pulsating variables of spectral type B0-B2 and luminosity class III-V. Their variability is a function of changes in radius driven by the opacity of the stellar atmosphere to the star’s own radiation (the “κ-mechanism” or “kappa mechanism”). Most κ-mechanism variables display higher than normal levels of ionized hydrogen and helium in their spectra β Cep variables are driven by high levels of iron in the depths of the star, causing spherical contraction and pressure build-up until expansion back to the original shape. One would expect occasional disruption of non-protected communications when the star contracts. Cepheid variables are radial pulsating variables of spectral type F, G or occasionally “hot” K, and luminosity class Ia-III. Their most notable feature is a nearly lockstep relationship between the period and absolute magnitude, allowing them to be used for accurately determining distances on interstellar and intergalactic scales. For 2-40 days this star ejects positively ionized Helium particles. The star increases in luminosity during this time. Expect an increase in the solar winds while these ions emanate from the star. Because this ejecta is helium-based, most planets with even a trace atmosphere will be unaffected. The first such Cepheid variable discovered was Eta Aquilae (η Aql) in 1784, but the class was named after Delta Cephei (δ Cep). Flare stars are eruptive variables, mostly of spectral types K (cooler end) and M, luminosity class V (the “red dwarf” stars on the Main Sequence). As with solar flares, the flares on this type of star are caused by magnetic buildup and subsequent “reconnection” in the stellar atmosphere, causing an almost epileptic shaking of its outer surface and erratic bursts of thermal and radioactive energy in irregular patterns. Examples of flare stars include Barnard’s Star, Proxima Centauri and Wolf 359. Expect a radically changing solar wind in these system and most settlements will need to be underground or shielded in some fashion. Mira variables, or Omicron Ceti (ο Ceti) variables are red giants (spectral class M, luminosity class II-IV) that have evolved off the Main Sequence and onto the Asymptotic Giant Branch of the of the H-R diagram. As the star heads towards the final stages if its existence, it ejects elements ranging from Helium to oxygen for 50 – 550 days. These free-ranging elements may form small planetary nebulas as they get blown away from the star and begin to interact with other stellar and planetary bodies. This type of variable is named after the star Mira (ο Ceti) in the constellation Cetus. Any planets attendant on a variable of this type will have formerly been outer planets of a Main Sequence star, and as such will probably not have more than outposts or resource acquisition (e.g. mining) stations. RR Lyrae variables are low-mass Population II stars of spectral type A (or “hot” F) and luminosity class III. Their variability mechanism is similar to that of the δ Cep variables, and they are similarly used for establishing interstellar distances, for relatively near objects. From 1 to 6 days this star ejects ionized Helium and greatly increases in luminosity. These stars are generally in multiple-star systems and in globular clusters. This type of star is named after the first such variable found (RR Lyrae). Settlements in these systems will need protection from a searing increase of light and radiation of some sort. RV Tauri variables differ from the otherwise similar Cepheid variables in that the spectral type of RV Tauri change from F or G at the brightest to K or M at their dimmest. They also exhibit alternating primary and secondary minima at their fundamental period, with the interval between the two primary minima (or two secondary minima) being twice the fundamental period. Over its period, the star greatly varies in luminosity due to a rapid expansion and deflation of its diameter. Severe stellar flares, with their attendant disruption of communications, will be common. Planets with weak or nonexistent magnetic fields will experience high radiation. Settlements in any habitable zones will need to have a safe haven underground. Semiregular variables exhibit significant variation in their cycles, often resolving on analysis to multiple overlapping periods. The majority of these stars are of spectral type M, S, or C, and luminosity class Ia to III, but one of the four subclasses (SRD, exemplified by SV Ursae Majoris) is of spectral type F, G, or K. All subclasses may have mean periods ranging from approximately a month to several thousand days. Subclass SRA stars (exemplified by Z Aquarii) are essentially the same as ο Ceti variables, except that where ο Ceti stars pulsate in the fundamental period, SRA stars pulsate in a harmonic or “overtone” mode. Subclass SRB stars (exemplified by RR Coronae Borealis) may not show any significant periodicity, and some are known to have stopped varying for a length of time, and others have been shown to have multiple overlapping variation periods. Subclass SRC stars are supergiants (luminosity class Ia or Ib) with variability over only about 1 magnitude. An example of this type is Betelgeuse (Alpha Orionis [α Ori], M2Iab). T Tauri variables are aperiodic variable protostars in the process of contracting to the Main Sequence. They are low-mass (less than 3Msol) protostars of spectral type F, G, K, or M. These are stars that gain and lose luminosity rapidly in a stellar nursery, with gravitational contraction being the driving mechanism (as they are yet too cool to sustain fusion). They have up to a thousand times more sunspots than a normal star and they will sometimes eject energy and stellar ejecta in jets coming out of both poles. These stars also eject Lithium at a much higher rate than most stars. Wildly chaotic in the extreme, any sustained life in these systems will be difficult at best. Wolf-Rayet stars are eruptive variables of spectral type O and luminosity class Ia or Ib, but because of their unique characteristics (including strong emission lines), they have been given their own classification as type W (with several subtypes). This type of variable is an aging type O star that has blown off much of its outer surface. The hydrogen is gone and it is now using helium as fuel (or something heavier). This is a classic example of a live fast, die young star – any planets in this system will be blasted or well out in the outer zones. The inner zones will likely be covered in the ejected hydrogen as a thin nebula has formed like a globe surrounding the star. An adventurous starship captain may be able to skim this hydrogen, but that ship will need to be mindful of the radiation and solar winds coming from a dying beast of a star. The characteristics of type W stars are such that most are expected to finally die as supernovae. The first types of these stars were discovered by Charles Wolf and Georges Rayet in the constellation Cygnus in 1867.

Generating Variable Stars

Variable stars should only be placed by referee fiat, but an appropriate type of variable can explain why a world with a UWP characteristic of a highly-desirable world might have a low population. Use the existing stellar type (but see the Revised Stellar Classifications and Dwarf Classification sections below) to determine where on the table below to roll to determine the type of variable.

Chance of Variables (for Traveller star system generation)
Stellar Type Chance of variance Type of variable star
O/Ia and O/Ib 28% (9+:2d6) Wolf-Rayet stars (reclassify as a type W)
B0-B2/III to V 28% (9+:2d6) β Cep
B/Ia and A/Ia 28% (9+:2d6) α Cyg
A (any luminosity) 17% (10+:2d6) RR Lyrae
F/I to III 8% (11+: 2d6) Roll 1d6: 1-2 Cepheid,
3-4 RV Tauri,
5-6 SRD
F (any) 3% (12:2d6) Roll 1d6: 1-3 RV Tauri,
4-6 SRD
G/I to III 8% (11+:2D6) Roll 1d6: 1-2 Cepheid,
3-4 RV Tauri,
5-6 SRD
G (any) 3% (12:2d6) Roll 1d6: 1-3 RV Tauri,
4-6 SRD
K 3% (12:2d6) Roll 1d6: 1-3 Flare,
4-6 SRD
M/Ia, M/Ib, M/II-III 8% (11+:2d6) Roll 1d6: 1-3 SRB
4-6 SRC
M/II to IV 28% (9+:2d6) Roll 2d6: 2-5 ο Ceti,
6-8 SRA,
9-10 Type S,
11-12 Type C
M/V 28% (9+: 2d6) Roll 1d6: 1-3 Type L,
4-5 Type T,
6 Type Y

Extended Classifications for Dying Stars

Since the writing of Classic Traveller Book 6: Scouts , the stellar classification system has been updated and extended. Thanks to the Hubble and Kepler telescopes, along with a better understanding of how stars work, the catalog has expanded to include star types L, T, Y, S and C. This is by no means the end of the classification, but I will stay with these types as they are the most common.

The stars under the L, T and Y spectral classes are commonly referred to as Brown Dwarfs. These are M/V and M/VI stars that have cooled off over time (stars of spectral class M7V or M7VI or cooler are also considered Brown Dwarfs). They still are hot enough to be gaseous and emissive, so approaching them is impossible. Such Brown Dwarfs will progress from type M through L, T, and Y in that order as they cool the lowest-mass type Y stars have little to distinguish them from the highest-mass jovian planets. All the surviving planets that may still be surrounding these burned-out stars will be considered in the outer zone the only heat would come from tectonics affected by a larger planet (such as Jupiter’s gravity affects the moon Io). These essentially are massive obstacles in space for any traveler, but their gravity is strong enough to have some gravitational effects on hyperdrives.

L Brown Dwarf 1300K-2400K
T Methane Dwarf 700K-1300K
Y Sub-Brown Dwarf Less than 700K

The H-R types C and S (together called, informally, ‘carbon stars’) are used to identify those Red stars (Type M) nearing the end of their main life-cycle. Not to be grouped with the regular variables, this is a special type of star to be used very sparingly by any GM.

Type S Giants (luminosity Ia, Ib, or II) emit a combination of carbon and oxygen (or carbon monoxide) from a star that is acting like a ο Ceti variable. Solar winds will be intense and any settlements that may have been here for millennia will likely need to be relocated – and soon.

Type C stars are either Giants or main sequence stars (luminosity III-V) that emit carbon. The hydrogen and the helium are gone. They should again be treated as a ο Ceti variables, but with half to a quarter of the time listed. As above, any settlements/civilizations that are in the vicinity of this star should be exiting the system as absolutely soon as possible.

Dwarf Classification

Earlier in this article, stars of spectral type M and the cooler end of K and luminosity class V or VI were referred to as “red dwarfs”. These stars are part of the Main Sequence, and represent stars at the end of their life that are insufficiently massive to expand into giants. There is a separate area on the H-R diagram for “white dwarfs” (whose spectral “color” extends from spectral type B to K), which are typed with the Dx nomenclature – but the x does not represent the spectral type ( Book 6: Scouts errs in this) rather, it represents the composition of the outer layers of the star’s atmosphere. The ‘D’ classification stands for ‘degenerate’, rather than ‘dwarf’, and stars in this class are no longer undergoing fusion their temperature is sustained by gravitational collapse. Change stars of type DG, DK, or DM to the appropriate spectral type and luminosity class V (Main Sequence dwarf) or VI (subdwarf can also be prefixed ‘sd’, i.e., G5VI and sdG5 are equivalent) stars of type DB, DA, and DF should be changed to DB2, DB5, or DB8 respectively (DB is a type for helium-rich white dwarfs the number indicates the effective temperature, with higher numbers indicating cooler stars).

Summary

For those of us who like a little science with our storytelling I hope this can add a twist or two to your colonies or to those intrepid scouts who are conducting system surveys. I hope you can find this information useful as I have.


Shared Flashcard Set

: Not enough thermal energy to create fusion in the core, quantum pressure stops the star from contracting anymore.

electron degeneracy pressure

cooling/fading track in the HR diagram

- direct vs indirect detection methods

1. direct is seeing the actual spectra and/or image of the planet ,

2. indirect is measuring the parent star in order to receive information on the orbiting planet includes

1.astrometric - looking at the wobble of the star

2. transit - when planet passes infront of star making a dip "eclipse" so its edge on

3. dopper/spectroscopic- Doppler shift of the star, we get the minimum mass of the planet, velocity, period à which gives us the radius

position wobble (astrometry)

the star does a little orbital tilt.

it is the accurate measurement of stellar objects&rsquo positions.

doppler wobble or radial velocity method (spectroscopy)

: Gives minimum mass and orbital period combined astrometric and spectroscopic methods to get orbital tilt and mass.

--> Doppler shift of the star, we get the minimum mass of the planet, and the velocity,

(period gives us the radius)

jupiter-size planets orbiting another star very closely therefore making it very hot in temperature. they have considerably short periods

The habitable zone is region in which a planet orbiting a particular star would be capable of having liquid water (which could possibly support life) depending on the planet, i.e. its luminosity, distance from the star, and temperature factors, the HZ is either further or closer away from the parent star.


&rdquoGoldilocks planets&rdquo-planets where water can exist in liquid state

1. pre main sequence or protostar phase

contraction under gravity

1. pre main sequence or protostar phase

- conversion to thermal energy

1. pre main sequence or protostar phase

beginning of core H fusion

1. pre main sequence or protostar phase

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)


double shell burning stage (Asymptotic giant branch &ldquoAGB&rdquo)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

: it is the tendency for AGB stars to pulsate every 1-2 years, which means they swell up and contract. mira the wonderful&rdquo is the most famous example of this.

"dredge up" which release "thermal pulses"

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

neutron capture reactions :

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

2. post M.S evolution of low & intermediate mass stars (up to 8 SM)


2. post M.S evolution of low & intermediate mass stars (up to 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)


Triple alpha and higher fusion reactions

3. post M.S evolution of High mass stars ( over 8 SM)


- &ldquoonion&rdquo structure of evolved star

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

: The outer layers of the star lose pressure support, so fall inward. The core collapse is suddenly halted by neutron degeneracy pressure, so the outer layers crash into it and bounce off, creating the supernova explosion. Neutrinos are involved somehow.

3. post M.S evolution of High mass stars ( over 8 SM)


more neutron capture reactions ( rapid &ldquoR&rdquo process)

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

3. post M.S evolution of High mass stars ( over 8 SM)

: The shock from the supernova that travels outward at
about 10% the speed of light.

The ejecta carries the newly synthesized elements with it into the interstellar medium, ready to become the next generation of stars.

3. post M.S evolution of High mass stars ( over 8 SM)

1. white dwarf, former cores of AGB stars

end states of low & intermediate mass stars

1. white dwarf, former cores of AGB stars


- electron degeneracy pressure

1. white dwarf, former cores of AGB stars

1. white dwarf, former cores of AGB stars


- cooling tracks and cooling ages

2. neutron stars, collapsed cores of high mass star supernova

- neutron degeneracy pressure

2. neutron stars, collapsed cores of high mass star supernova

2. neutron stars, collapsed cores of high mass star supernova

: The magnetic radiation in the pulsar beams is not thermal emission. It has a different spectral shape from a blackbody, being very strong at radio wavelengths.

2. neutron stars, collapsed cores of high mass star supernova

2. neutron stars, collapsed cores of high mass star supernova


- pulsar slowdowns and glitches

2. neutron stars, collapsed cores of high mass star supernova


- association of pulsars with supernova remnants

2. neutron stars, collapsed cores of high mass star supernova

2. neutron stars, collapsed cores of high mass star supernova

2. neutron stars, collapsed cores of high mass star supernova

2. neutron stars, collapsed cores of high mass star supernova

the CNO cycle occurs in higher mass stars and it means that carbon is a catalyst in nuclear fusion, or the fusion of hydrogen into helium. requires a high core temperature.


brown dwarfs are very cool and lie beyond the red dwarfs, so it is logical their spectrum would appear in the infrared. just a guess: you could measure the radius to see how small the object is if it is smaller than a red dwarf needs to be then it is probably a red dwarf. you could use the reflex motion method.

Wouldn&rsquot you just take its spectral lines to confirm if it was a brown dwarf, i.e. L, T, Y?

-
Yes. while it is true that brown dwarfs peak in the infrared, they are defined by the pattern of their absorption lines L,T,Y, which reveal larger molecules and therefore cooler end on the EM spectrum.

White dwarf minimum mass is 0.08, max mass is 1.4

Neutron star minimum mass is 1.4 , max mass is 2-3

Blackhole minimum mass 2-3, no maximum

the minimum mass for a main sequence is 0.08 solar masses / the maximum for a main sequence is 250+ sm

the maximum mass for a brown dwarf is anything less than 0.08 solar masses / the minimum is probably not known, or negligible


1. transit method - measure the transit of a planet in front of a star. (Finds radius of planet and is edge on)
2. positional shift - measure the shift in the star&rsquos position as a planet orbits it
3. doppler shift - measure the doppler shift of the star&rsquos position as a planet orbits it (Minimum mass) First successful method of finding exoplanets.

transit and doppler are successful working together!

9. What are the observable properties of a protostar, and where does it lie in the HR diagram?

The core begins helium-burningunder pressure from the star it fuses three helium atoms to create a carbon atom. By adding another helium atom oxygen is formed. The core continues this process until it is entirely carbon and oxygen.

- 1. Main Sequence: H core burning: H &rarr He in core ! (small)


2. Red Giant: H shell-burning: H &rarr He outside the He core! (bigger) Goes towards the upper right corner of the H-R diagram while the core contracts.


3. Horizontal Branch: He &rarr C in the core, H &rarr He in shell! (gets smaller again) Moves left on the H-R diagram towards the blue side.


4. AGB or Double Shell Burning: H and He both fuse in shells,CO core becomes degenerate! (gets even bigger again) Moves back right towards the Red Giants.


5. Planetary Nebula lifts off, leaves white dwarf behind!


6. White dwarf: Excretes the energy produced, does not grow or shrink due to becoming electron degenerate and slides down along a line of constant radius in the H-R diagram.
Crystallizes and becomes a diamond.

12. Cite the major stages in the life cycle of the higher-mass stars (more than 8 M¤). Summarize the physical properties of the star, the energy production mechanisms, and the path in the HR diagram. Which stages are in common with the lower-mass stars, and which are different?

- life cycle - protstar, massive star, red supergiant, blue supergiant, supernova, black hole or neutron star
physical properties - Massive star that his hot and large, gets larger and cools off, starts to &ldquoonionize&rdquo(outside ---> inside) hydrogen, helium, carbon, oxygen, neon, magnesium, silicon, and finally iron, collapses into a neutron star with a supernova remnant.
energy production - massive star H -> He in core. Red supergiant H shell burns, Blue Supergiant He -> C and O in core. Once core has turned into Iron it collapses.

Similar stages as lower-mass stars


&ndash Hydrogen core fusion (Main Sequence)
&ndash Hydrogen shell burning (red supergiant)
&ndash Helium core fusion (blue supergiant)

position in HR diagram : Starts in the upper left of the main sequence, begins moving right on diagram by cooling off and expanding, as the shells of fusion around the core increase in number, the star swings &ldquoback and forth&rdquo between the red and blue sides of the HR diagram.

The differences is that low mass stars expel all their outer material layers leaving behind the core, while the high mass stars there core gets so dense with iron that all the material falls in at once and they just &ldquobounce off&rdquo which creates a supernova explosion.

18. Compare white dwarfs and neutron stars in terms of: mass, radius, density, composition.

- White dwarfs tend to have a smaller mass than neutron stars they are formed from lower mass stars. However, their radii are much bigger than that of neutron stars, which tend to be the size of a small city (such as Austin). Therefore the density of neutron stars is far greater than that of white dwarfs.


As far as composition goes, white dwarfs are composed of mostly carbon and oxygen, with a hydrogen outer layer neutron stars are composed entirely of neutrons (who would have guessed).

19. What can be learned from a pulsar that is found to be in a binary system?

It can be learned that new planets may form from the material after a supernova explosion in the accretion disk around the pulsar.

We also see the period slowing down over time, 2 pulsars getting closer, and periods are increasing, they have to give up energy (gravitational waves) carrying energy away from the binary system.

- this prove einsteins theory of general relativity

- Well it was the 1st supernova observed in modern astronomy. And its important because according to our theories about supernovae, they should leave behind neutron stars. But this 1987A has not shown evidence of a neutron star. Does this mean that there isn&rsquot one there? Not exactly.


The key point about SN 1987A was that a neutrino burst was seen , emitted when the core is &ldquoneutronized.&rdquo However, no pulsar has been seen in the remnant of SN 1987A to date. !


Freelance Traveller

This article originally appeared in the July 2015 issue.

As many of us who have studied stellar masses know, each star can have its own special characteristics that make it as unique as a fingerprint. For example, our own sun can be considered a stellar variant due to cycles of sunspots and stellar ejecta that emanate from the sun’s surface.

With that in mind, I went about looking at other types of variables and found a wide variety listed. Some of the better known types are as follows:

Alpha Cygni (α Cyg) variables are non-radial pulsating variables of spectral type B or A and luminosity class Ia. Due to their immense size and high luminosity, one can expect to see periodic rapid increases of EMP (Electromagnetic Pulses) and intense radiation, ranging from 5-30 days in length. The ‘non-radial pulsating variable’ description means that one area of this star can be expanding while another side may be shrinking. Deneb (α Cyg, type A2Ia) is the prototypical example of this star type. Inhabited planets orbiting α Cyg variables would likely have “hardened” subsurface habitats with heavily shielded local communications, and transiting starships would use microwave beacons and masers for communications with the world(s). Beta Cephei (β Cep) variables are rapidly pulsating variables of spectral type B0-B2 and luminosity class III-V. Their variability is a function of changes in radius driven by the opacity of the stellar atmosphere to the star’s own radiation (the “κ-mechanism” or “kappa mechanism”). Most κ-mechanism variables display higher than normal levels of ionized hydrogen and helium in their spectra β Cep variables are driven by high levels of iron in the depths of the star, causing spherical contraction and pressure build-up until expansion back to the original shape. One would expect occasional disruption of non-protected communications when the star contracts. Cepheid variables are radial pulsating variables of spectral type F, G or occasionally “hot” K, and luminosity class Ia-III. Their most notable feature is a nearly lockstep relationship between the period and absolute magnitude, allowing them to be used for accurately determining distances on interstellar and intergalactic scales. For 2-40 days this star ejects positively ionized Helium particles. The star increases in luminosity during this time. Expect an increase in the solar winds while these ions emanate from the star. Because this ejecta is helium-based, most planets with even a trace atmosphere will be unaffected. The first such Cepheid variable discovered was Eta Aquilae (η Aql) in 1784, but the class was named after Delta Cephei (δ Cep). Flare stars are eruptive variables, mostly of spectral types K (cooler end) and M, luminosity class V (the “red dwarf” stars on the Main Sequence). As with solar flares, the flares on this type of star are caused by magnetic buildup and subsequent “reconnection” in the stellar atmosphere, causing an almost epileptic shaking of its outer surface and erratic bursts of thermal and radioactive energy in irregular patterns. Examples of flare stars include Barnard’s Star, Proxima Centauri and Wolf 359. Expect a radically changing solar wind in these system and most settlements will need to be underground or shielded in some fashion. Mira variables, or Omicron Ceti (ο Ceti) variables are red giants (spectral class M, luminosity class II-IV) that have evolved off the Main Sequence and onto the Asymptotic Giant Branch of the of the H-R diagram. As the star heads towards the final stages if its existence, it ejects elements ranging from Helium to oxygen for 50 – 550 days. These free-ranging elements may form small planetary nebulas as they get blown away from the star and begin to interact with other stellar and planetary bodies. This type of variable is named after the star Mira (ο Ceti) in the constellation Cetus. Any planets attendant on a variable of this type will have formerly been outer planets of a Main Sequence star, and as such will probably not have more than outposts or resource acquisition (e.g. mining) stations. RR Lyrae variables are low-mass Population II stars of spectral type A (or “hot” F) and luminosity class III. Their variability mechanism is similar to that of the δ Cep variables, and they are similarly used for establishing interstellar distances, for relatively near objects. From 1 to 6 days this star ejects ionized Helium and greatly increases in luminosity. These stars are generally in multiple-star systems and in globular clusters. This type of star is named after the first such variable found (RR Lyrae). Settlements in these systems will need protection from a searing increase of light and radiation of some sort. RV Tauri variables differ from the otherwise similar Cepheid variables in that the spectral type of RV Tauri change from F or G at the brightest to K or M at their dimmest. They also exhibit alternating primary and secondary minima at their fundamental period, with the interval between the two primary minima (or two secondary minima) being twice the fundamental period. Over its period, the star greatly varies in luminosity due to a rapid expansion and deflation of its diameter. Severe stellar flares, with their attendant disruption of communications, will be common. Planets with weak or nonexistent magnetic fields will experience high radiation. Settlements in any habitable zones will need to have a safe haven underground. Semiregular variables exhibit significant variation in their cycles, often resolving on analysis to multiple overlapping periods. The majority of these stars are of spectral type M, S, or C, and luminosity class Ia to III, but one of the four subclasses (SRD, exemplified by SV Ursae Majoris) is of spectral type F, G, or K. All subclasses may have mean periods ranging from approximately a month to several thousand days. Subclass SRA stars (exemplified by Z Aquarii) are essentially the same as ο Ceti variables, except that where ο Ceti stars pulsate in the fundamental period, SRA stars pulsate in a harmonic or “overtone” mode. Subclass SRB stars (exemplified by RR Coronae Borealis) may not show any significant periodicity, and some are known to have stopped varying for a length of time, and others have been shown to have multiple overlapping variation periods. Subclass SRC stars are supergiants (luminosity class Ia or Ib) with variability over only about 1 magnitude. An example of this type is Betelgeuse (Alpha Orionis [α Ori], M2Iab). T Tauri variables are aperiodic variable protostars in the process of contracting to the Main Sequence. They are low-mass (less than 3Msol) protostars of spectral type F, G, K, or M. These are stars that gain and lose luminosity rapidly in a stellar nursery, with gravitational contraction being the driving mechanism (as they are yet too cool to sustain fusion). They have up to a thousand times more sunspots than a normal star and they will sometimes eject energy and stellar ejecta in jets coming out of both poles. These stars also eject Lithium at a much higher rate than most stars. Wildly chaotic in the extreme, any sustained life in these systems will be difficult at best. Wolf-Rayet stars are eruptive variables of spectral type O and luminosity class Ia or Ib, but because of their unique characteristics (including strong emission lines), they have been given their own classification as type W (with several subtypes). This type of variable is an aging type O star that has blown off much of its outer surface. The hydrogen is gone and it is now using helium as fuel (or something heavier). This is a classic example of a live fast, die young star – any planets in this system will be blasted or well out in the outer zones. The inner zones will likely be covered in the ejected hydrogen as a thin nebula has formed like a globe surrounding the star. An adventurous starship captain may be able to skim this hydrogen, but that ship will need to be mindful of the radiation and solar winds coming from a dying beast of a star. The characteristics of type W stars are such that most are expected to finally die as supernovae. The first types of these stars were discovered by Charles Wolf and Georges Rayet in the constellation Cygnus in 1867.

Generating Variable Stars

Variable stars should only be placed by referee fiat, but an appropriate type of variable can explain why a world with a UWP characteristic of a highly-desirable world might have a low population. Use the existing stellar type (but see the Revised Stellar Classifications and Dwarf Classification sections below) to determine where on the table below to roll to determine the type of variable.

Chance of Variables (for Traveller star system generation)
Stellar Type Chance of variance Type of variable star
O/Ia and O/Ib 28% (9+:2d6) Wolf-Rayet stars (reclassify as a type W)
B0-B2/III to V 28% (9+:2d6) β Cep
B/Ia and A/Ia 28% (9+:2d6) α Cyg
A (any luminosity) 17% (10+:2d6) RR Lyrae
F/I to III 8% (11+: 2d6) Roll 1d6: 1-2 Cepheid,
3-4 RV Tauri,
5-6 SRD
F (any) 3% (12:2d6) Roll 1d6: 1-3 RV Tauri,
4-6 SRD
G/I to III 8% (11+:2D6) Roll 1d6: 1-2 Cepheid,
3-4 RV Tauri,
5-6 SRD
G (any) 3% (12:2d6) Roll 1d6: 1-3 RV Tauri,
4-6 SRD
K 3% (12:2d6) Roll 1d6: 1-3 Flare,
4-6 SRD
M/Ia, M/Ib, M/II-III 8% (11+:2d6) Roll 1d6: 1-3 SRB
4-6 SRC
M/II to IV 28% (9+:2d6) Roll 2d6: 2-5 ο Ceti,
6-8 SRA,
9-10 Type S,
11-12 Type C
M/V 28% (9+: 2d6) Roll 1d6: 1-3 Type L,
4-5 Type T,
6 Type Y

Extended Classifications for Dying Stars

Since the writing of Classic Traveller Book 6: Scouts , the stellar classification system has been updated and extended. Thanks to the Hubble and Kepler telescopes, along with a better understanding of how stars work, the catalog has expanded to include star types L, T, Y, S and C. This is by no means the end of the classification, but I will stay with these types as they are the most common.

The stars under the L, T and Y spectral classes are commonly referred to as Brown Dwarfs. These are M/V and M/VI stars that have cooled off over time (stars of spectral class M7V or M7VI or cooler are also considered Brown Dwarfs). They still are hot enough to be gaseous and emissive, so approaching them is impossible. Such Brown Dwarfs will progress from type M through L, T, and Y in that order as they cool the lowest-mass type Y stars have little to distinguish them from the highest-mass jovian planets. All the surviving planets that may still be surrounding these burned-out stars will be considered in the outer zone the only heat would come from tectonics affected by a larger planet (such as Jupiter’s gravity affects the moon Io). These essentially are massive obstacles in space for any traveler, but their gravity is strong enough to have some gravitational effects on hyperdrives.

L Brown Dwarf 1300K-2400K
T Methane Dwarf 700K-1300K
Y Sub-Brown Dwarf Less than 700K

The H-R types C and S (together called, informally, ‘carbon stars’) are used to identify those Red stars (Type M) nearing the end of their main life-cycle. Not to be grouped with the regular variables, this is a special type of star to be used very sparingly by any GM.

Type S Giants (luminosity Ia, Ib, or II) emit a combination of carbon and oxygen (or carbon monoxide) from a star that is acting like a ο Ceti variable. Solar winds will be intense and any settlements that may have been here for millennia will likely need to be relocated – and soon.

Type C stars are either Giants or main sequence stars (luminosity III-V) that emit carbon. The hydrogen and the helium are gone. They should again be treated as a ο Ceti variables, but with half to a quarter of the time listed. As above, any settlements/civilizations that are in the vicinity of this star should be exiting the system as absolutely soon as possible.

Dwarf Classification

Earlier in this article, stars of spectral type M and the cooler end of K and luminosity class V or VI were referred to as “red dwarfs”. These stars are part of the Main Sequence, and represent stars at the end of their life that are insufficiently massive to expand into giants. There is a separate area on the H-R diagram for “white dwarfs” (whose spectral “color” extends from spectral type B to K), which are typed with the Dx nomenclature – but the x does not represent the spectral type ( Book 6: Scouts errs in this) rather, it represents the composition of the outer layers of the star’s atmosphere. The ‘D’ classification stands for ‘degenerate’, rather than ‘dwarf’, and stars in this class are no longer undergoing fusion their temperature is sustained by gravitational collapse. Change stars of type DG, DK, or DM to the appropriate spectral type and luminosity class V (Main Sequence dwarf) or VI (subdwarf can also be prefixed ‘sd’, i.e., G5VI and sdG5 are equivalent) stars of type DB, DA, and DF should be changed to DB2, DB5, or DB8 respectively (DB is a type for helium-rich white dwarfs the number indicates the effective temperature, with higher numbers indicating cooler stars).

Summary

For those of us who like a little science with our storytelling I hope this can add a twist or two to your colonies or to those intrepid scouts who are conducting system surveys. I hope you can find this information useful as I have.


How realistic is it to have spacecraft brightly illuminated when journeying the solar system?

This image from NASA's Cassini spacecraft shows three moons -- Titan, Mimas, and Rhea. Titan, the largest moon shown here, appear fuzzy because we only see its cloud layers. Image credit: NASA/JPL-Caltech/Space Science Institute

This is a great question, but before we get to the meat of your query, I want to clear up two misconceptions that are present in the question itself.

The first is that the Moon has something to do with the day/night cycle. Days and nights occur because the Earth is spinning rapidly on its own axis. The Sun, which is relatively stationary with respect to the Earth on the timeframes of a few days, continues to shine from the same point. As the part of the Earth that you or I live on rotates to face towards or away from the Sun, we get day and night respectively. The Moon orbits much, much slower around the Earth - approximately once every month. The Moon can occasionally cast a shadow onto the Earth, but that’s a rare event we know as a solar eclipse.

The second is why you sunburn at altitude. You absolutely are more prone to sunburns at higher altitudes, but it’s not because you’re significantly closer to the Sun. The Sun is 93 million miles away - getting a single mile or two closer isn’t going to make a significant change to the amount of sunlight that your skin’s getting. What happens instead is that you’re rising above some of the protective layer of our atmosphere, which allows more ultraviolet radiation to reach you. This UV radiation is what triggers a sunburn, and the more atmosphere above you, the more protected you are. If you’re on a snowy mountain, you have the additional complication of being able to get sunburned in really strange places, like the underside of your earlobes and the bottom of your chin, because of the reflected light off of the snow.

This image of a crescent Jupiter and the iconic Great Red Spot was created by a citizen scientist (Roman Tkachenko) using data from Juno's JunoCam instrument. Image credit: NASA/JPL-Caltech/SwRI/MSSS/Roman Tkachenko

With those two points addressed, your question about lighting in space is an excellent one. There’s a couple things to think about with lighting, so let’s begin with a spacecraft which is near the Earth. If you are in a position where nothing is blocking the sunlight coming your way, you would be constantly bombarded by the Sun’s rays, exactly as you suspect. However, this is an extremely harsh lighting system - with no atmosphere in space to diffuse the light a little, spacecraft are in pure sunlight or deepest shade. If a spacecraft is moving around the Sun, that means that the sunward facing side of the spacecraft would be illuminated, and the other half of your spacecraft would be in shadow - triggering a pretty intensive temperature gradient between the two sides. As a point of reference, the temperature on the surface of the Moon swings between 224F (106C) and negative 298F (-183C) when the surface is illuminated versus when it is in shadow.

This temperature cycling causes stress on most materials you could build a spacecraft out of, and is a challenge we face already as a moderately spacefaring species. The International Space Station, which orbits around the Earth, alternates between spending 45 minutes in the shadow of the Earth and 45 minutes in direct sunlight. Without intensive, intensive insulation, our astronauts would alternate between freezing to death and boiling to death. We have to manage this same situation on a smaller scale for space suits in the sunlight, your suit has to keep you cool and protect your eyes from glare. In the shadows, it must keep you warm.

These considerations will only get worse as you get closer to the Sun, or really around any star. As we proceed inwards, closer to the sun, the sunlight gets more intense, and the amount of work you’d need to do to stay cool would increase. The cool side of your craft wouldn’t get any colder, but the temperature stress would get more severe between the sun and shaded sides of your craft, so your insulation would have to get much better. This intensity doesn’t change linearly though - if you got twice as close to the star, the sunlight won’t be twice as intense. It will be four times as intense.

This works just as well in the other direction - go twice as far out in the solar system, and your sunlight will drop off by a factor of four. Go four times as far, and you’re dealing with light intensity 16 times fainter than you have at the distance of the Earth. However, the Sun is very bright. Jupiter is 5.2 au - and Neptune at 30 au. At 5.2 au, you’re dealing with sunlight 27 times fainter than what we receive on Earth. It’s still going to be the brightest thing in the sky. Neptune is much further, but even at 900 times fainter than the Sun appears from an Earth distance away, it still hasn’t faded to anywhere near the relative faintness of the full moon in the sky, and you can do a lot in the light of a full moon, visibility wise.

The way that astronomers measure brightness is with a counterintuitive system called a magnitude, where 1 magnitude is about a factor of 2.5 in brightness. Every magnitude is multiplicative, so five magnitudes is a difference in brightness of a factor of 100. A difference of ten magnitudes is a factor of 10,000 in brightness. At Jupiter’s distance, then, the Sun will appear about 3.6 magnitudes fainter than it does from the Earth. At Neptune’s distance, it’s something like 7.5 magnitudes fainter. The brightest star in the night sky, Sirius, is 25 magnitudes fainter than the Sun, so even at the distance of Neptune, the Sun will appear more than 10 million times brighter than Sirius appears on Earth. The full Moon, which I mentioned earlier, is fourteen magnitudes fainter than the Sun, so the Sun would be shining on Neptune about 390 times more intensely than the full moon.

If your fictional craft is within the bounds of a solar system then, I’d say having the craft be brightly illuminated on one side is pretty reasonable. If you’re going beyond that, though, you’d start to descend into full darkness. You’d have to be very far away from our star before the Sun sank to the brightness of Sirius. In fact, you’d need to be almost 1.5 light years away from our star. The spaces within the stars, which is the majority of the Milky Way Galaxy, are going to be very dark. In those places, the only bright lights will be the ones you bring with you. You probably would want to have a few spotlights around, if any of the crew ever has to go outside for any kind of repair operations, but it wouldn’t have the same aesthetics as the harshly lit side of a spacecraft that many shows like to go for.

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