Astronomy

How much of a molecular cloud can end up as “starstuff”?

How much of a molecular cloud can end up as “starstuff”?


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Stars form within molecular clouds. These clouds can be up to 6 millions solar masses. When the cloud collapses into stars, is it possible to know a rough figure for how much of this material actually turns into stars? 1%? 99%?

Do larger clouds tend to be more mass-efficient in star formation than smaller ones?


This quantity is referred to as star formation efficiency (SFE) by astronomers who study star formation and galaxy evolution. Estimates can vary but typically are around a few percent. In Sec. 4.1 of this paper Inoue et al. review some estimates from the literature. Those numbers are for the central regions of spiral galaxies, and the rate may be lower in the outer regions where the density is lower.

Note that even if the rate is a few percent in a given cloud for a given episode of star formation, the rest of that gas mass will be dispersed into the interstellar medium and eventually find its way back into another cloud to form stars later - star formation is ongoing in spiral galaxies.

Note that you will often see SFE expressed as star formation rate (solar masses per year converted to stars) divided by total gas mass. That has units of (mass/yr) / mass, which reduces to units of 1/yr. You can think of that as the fraction of gas that forms into stars per year, or alternatively, the inverse of that number gives you a timescale in years to deplete the gas entirely.


Large Galaxies Strip Molecular Gas from Their Satellites

A research team led by International Centre for Radio Astronomy Research (ICRAR) astronomers has studied the molecular-gas content of nearby massive galaxies in a cosmological simulation, focusing on how it depends on galaxy environment.

An artist’s impression showing the increasing effect of ram-pressure stripping in removing gas from the spiral galaxy NGC 4921 and its satellite galaxies. Image credit: ICRAR / NASA / ESA / Hubble Heritage Team / STScI / AURA.

“Our study provides new systematic evidence that small galaxies everywhere lose some of their molecular gas when they get close to a larger galaxy and its surrounding hot gas halo,” said Dr. Adam Stevens, an astrophysicist at ICRAR and the ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D).

“Gas is the lifeblood of a galaxy. Continuing to acquire gas is how galaxies grow and form stars. Without it, galaxies stagnate.”

“We’ve known for a long time that big galaxies strip atomic gas from the outskirts of small galaxies. But, until now, it hadn’t been tested with molecular gas in the same detail.”

“Galaxies don’t typically live in isolation. When a galaxy moves through the hot intergalactic medium or galaxy halo, some of the cold gas in the galaxy is stripped away. This fast-acting process is known as ram pressure stripping,” said Dr. Barbara Catinella, also of ICRAR.

Using the TNG100 cosmological, hydrodynamic simulation, the astronomers made direct predictions for the amount of atomic and molecular gas that should be observed by specific surveys on the Arecibo telescope in Puerto Rico and the IRAM 30-m telescope in Spain.

They then took the actual observations from the telescopes and compared them to their original predictions. The two were remarkably close.

“The IRAM 30-m telescope observed the molecular gas in more than 500 galaxies,” Dr. Catinella said.

“These are the deepest observations and largest sample of atomic and molecular gas in the local Universe. That’s why it was the best sample to do this analysis.”

The finding fits with previous evidence that suggests satellite galaxies have lower star formation rates.

“Stripped gas initially goes into the space around the larger galaxy,” Dr. Stevens said.

“That may end up eventually raining down onto the bigger galaxy, or it might end up just staying out in its surroundings.”

“But in most cases, the little galaxy is doomed to merge with the larger one anyway.”

“Often they only survive for one to two billion years and then they’ll end up merging with the central one. So it affects how much gas they’ve got by the time they merge, which then will affect the evolution of the big system as well.”

“Once galaxies get big enough, they start to rely on getting more matter from the cannibalism of smaller galaxies.”

The study was published in the Monthly Notices of the Royal Astronomical Society.


Alien Molecules of the Cosmos –“Never Before Seen in Space”

“The amazing thing about these observations, about this discovery, and about these molecules, is that no one had looked, or looked hard enough,” said Michael McCarthy , an astrochemist and Acting Deputy Director of Harvard Center for Astrophysics (CfA) about the discovery of a vast, previously unknown reservoir of new molecules in a cold, dark molecular cloud in the interstellar medium for the first time, “It makes you wonder what else is out there that we just haven’t looked for.”

“Biologically Relevant”

“These molecules represent a vast reservoir of elemental carbon. Many of the molecules we consider to be possibly ‘biologically relevant’ – things such as glycolaldehyde and formamide – have carbon as the foundation of their structures, wrote Brett McGuire, Assistant Professor of Chemistry at the Massachusetts Institute of Technology, and the Project Principal Investigator for GOTHAM, using Green Bank Telescope (GBT) data, in an email to The Daily Galaxy. “

“Thus, it is important for us to be able to ask: are the PAHs ( polycyclic aromatic hydrocarbon) a carbon sink or a potential source of reactive carbon?” added McGuire. “That is to say: are we building PAHs and steadily removing carbon from the chemical reaction networks that build smaller prebiotic molecules, or are these PAHs reacting and being broken down, serving as a feedstock for reactions generating prebiotic molecules? This is something we can now start to explore in more detail now that we have the ability to detect and study individual PAH molecules and their reactions.”

“It is all about being able to track this reservoir of carbon. How many of these PAHs survive the formation of a star? Are they incorporated into small rocky/icy bodies like comets and meteorites to rain down reactive organic carbon on the surfaces of planets?” continued McGuire in his email ” Do they get broken down by the radiation of the new star, driving further carbon chemistry in this phase of star/planet formation? These are harder questions to answer – as it will be substantially harder to observe these molecules in sources where stars are being born. But, if we can build models based on our observations of these species early in the process, we can make predictions about how their chemistry will evolve alongside the changing physical conditions of the forming planetary system, even if we cannot observe that evolution directly.”

200 Types of Molecules Known Floating in Space –Until Now

The compounds that are found naturally on Earth such as water (H2O) and carbon dioxide (CO2)—that make up the huge diversity of materials on this planet—are just a fraction of those discovered so far in the cosmos. Scientists have found some 200 types of molecules floating in space. In 2015, astronomers studying spectroscopic data –light broken down into their constituent wavelengths–displayed on screen from Hubble’s iconic Horsehead Nebula revealed the chemical makeup of the nebula that looked like blips on a heart monitor, with each wiggle indicating that some molecule had emitted light of a particular wavelength. What the researchers saw displayed was a mystery—several small unidentified lines of a molecule completely unknown to science.

Every molecule has its own unique wiggles based on the orientation of its protons, neutrons and electrons. Most of the wiggles in the Horsehead data, reported Clara Moskowitz for Scientific American in The Hunt for Alien Molecules , were easily attributable to common chemicals such as carbon monoxide, formaldehyde and neutral carbon. But one spot in the Horsehead had several small unidentified lines equally separated from one another in frequency– an enigma.

The Horsehead Nebula,” writes Moskowitz , “is no aberration. Almost everywhere in the universe astronomers look—if they peer closely enough—they see unidentified spectral lines. The compounds we humans are familiar with, the species responsible for the huge diversity of materials on this planet, are just a fraction of those nature has created.

Answers to a Three-Decades-old Scientific Mystery

The new discoveries announced by the Harvard CfA, made by detecting individual polycyclic aromatic hydrocarbon molecules, are beginning to answer a three-decades-old scientific mystery: how and where are these molecules formed in space?

“We had always thought polycyclic aromatic hydrocarbons were primarily formed in the atmospheres of dying stars,” said McGuire. “In this study, we found them in cold, dark clouds where stars haven’t even started forming yet.”

Aromatic molecules, and PAHs–shorthand for polycyclic aromatic hydrocarbons–are well known to scientists. Aromatic molecules exist in the chemical makeup of human beings and other animals, and are found in food and medicines. As well, PAHs are pollutants formed from the burning of many fossil fuels and are even amongst the carcinogens formed when vegetables and meat are charred at high temperatures.

PAHs are considered as precursors to the formation of molecular clouds—the so-called “molecular factories” of more complex organic molecules that can include the precursors to life as we know it—This could open up new models of how carbon-containing material in deep space and in the rich atmospheres of planets and their moons in our solar system evolve and originate.

“Polycyclic aromatic hydrocarbons are thought to contain as much as 25-percent of the carbon in the universe,” said McGuire, who is also a research associate at the Center for Astrophysics. “Now, for the first time, we have a direct window into their chemistry that will let us study in detail how this massive reservoir of carbon reacts and evolves through the process of forming stars and planets.”

“What They Found was Astonishing”–Zooming in on The Taurus Molecular Cloud

Scientists have suspected the presence of PAHs in space since the 1980s but the new research, detailed in nine papers published over the past seven months, provides the first definitive proof of their existence in molecular clouds. To search out the elusive molecules, the team focused the 100m behemoth radio astronomy GBT on the Taurus Molecular Cloud , or TMC-1–a large, pre-stellar cloud of dust and gas located roughly 450 light-years from Earth that will someday collapse in on itself to form stars–and what they found was astonishing: not only were the accepted scientific models incorrect, but there was a lot more going on in TMC-1 than the team could have imagined.

These dark clouds are the initial birthplaces of stars and planets. So, these previously invisible aromatic molecules will also need to be thought about at each later step along the way to the creation of stars, planets, and solar systems like our own.

“From decades of previous modeling, we believed that we had a fairly good understanding of the chemistry of molecular clouds,” said McCarthy , whose research group at the CfA made the precise laboratory measurements that enabled many of these astronomical detections to be established with confidence. “What these new astronomical observations show is these molecules are not only present in molecular clouds, but at quantities which are orders of magnitude higher than standard models predict.”

Radio Astronomy Reveals Individual Molecules

“For the last 30 years or so,” said McGuire, about previous studies revealed only that there were PAH molecules out there, but not which specific ones, “scientists have been observing the bulk signature of these molecules in our galaxy and other galaxies in the infrared, but we couldn’t see which individual molecules made up that mass. With the addition of radio astronomy, instead of seeing this large mass that we can’t distinguish, we’re seeing individual molecules.”

Much to their surprise, the team didn’t discover just one new molecule hiding out in TMC-1. Detailed in multiple papers, the team observed 1-cyanonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanoacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans-(E)-cyanovinylacetylene, HC4NC, and propargylcyanide, among others.

Discovered a Giant Warehouse of Molecules and Chemistry

It’s like going into a boutique shop and just browsing the inventory on the front-end without ever knowing there was a back room. We’ve been collecting little molecules for 50 years or so and now we have discovered there’s a back door. When we opened that door and looked in, we found this giant warehouse of molecules and chemistry that we did not expect,” said McGuire. “There it was, all the time, lurking just beyond where we had looked before.

McGuire and other scientists at the GOTHAM project have been “hunting” for molecules in TMC-1 for more than two years, following McGuire’s initial detection of benzonitrile in 2018. The results of the project’s latest observations may have ramifications in astrophysics for years to come.

Unlike Anything We’ve Previously Been Able to Detect

“We’ve stumbled onto a whole new set of molecules unlike anything we’ve previously been able to detect, and that is going to completely change our understanding of how these molecules interact with each other. It has downstream ramifications,” said McGuire, adding that eventually these molecules grow large enough that they begin to aggregate into the seeds of interstellar dust. “When these molecules get big enough that they’re the seeds of interstellar dust, these have the possibility then to affect the composition of asteroids, comets, and planets, the surfaces on which ices form, and perhaps in turn even the locations where planets form within star systems.”

The discovery of new molecules in TMC-1 also has implications for astrochemistry, and while the team doesn’t yet have all of the answers, the ramifications here, too, will last for decades.

“We’ve gone from one-dimensional carbon chemistry, which is very easy to detect, to real organic chemistry in space in the sense that the newly discovered molecules are ones that a chemist knows and recognizes, and can produce on Earth,” said McCarthy. “And this is just the tip of the iceberg. Whether these organic molecules were synthesized there or transported there, they exist, and that knowledge alone is a fundamental advance in the field.”

Before the launch of GOTHAM in 2018, scientists had cataloged roughly 200 individual molecules in the Milky Way’s interstellar medium. These new discoveries have prompted the team to wonder, and rightly so, what’s out there.

This new aromatic chemistry that scientists are finding isn’t isolated to TMC-1. A companion survey to GOTHAM, known as ARKHAM –A Rigorous K/Ka-Band Survey Hunting for Aromatic Molecules–recently found benzonitrile in multiple additional objects.

Pushing the Limit of Chemistry in Space

“Incredibly, we found benzonitrile in every single one of the first four objects observed by ARKHAM,” said Andrew Burkhardt , a Submillimeter Array Postdoctoral Fellow at the CfA and a co-principal investigator for GOTHAM. “This is important because while GOTHAM is pushing the limit of what chemistry we thought is possible in space, these discoveries imply that the things we learn in TMC-1 about aromatic molecules could be applied broadly to dark clouds anywhere.

The Daily Galaxy with Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research, via CfA. Avi was formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL)


How Cloud Chemistry Differs from Earth Chemistry

A major difference is that hydrogen exists as free atoms, while on Earth only hydrogen molecules exists. With the cloud’s low density and rate of collision between atoms, the hydrogen atoms react at a much slower rate. Also, ultraviolet photons break up the hydrogen molecules.

Single hydrogen atoms aren’t the only thing which exists in space but not on Earth. Another large group are the radicals, like CH + , CN + , OH ─ , C2H + , and HCO + . Radicals are charged molecules. They form by a photon knocking an electron free or by the photon knocking an atom free. Since their charge comes from a dangling chemical bond, they are very reactive and grab the first available atom which comes along. Just like free hydrogen atoms, radicals exist in the cloud because of the low collision rates.


The Birth of a Star

Although regions such as Orion give us clues about how star formation begins, the subsequent stages are still shrouded in mystery (and a lot of dust). There is an enormous difference between the density of a molecular cloud core and the density of the youngest stars that can be detected. Direct observations of this collapse to higher density are nearly impossible for two reasons. First, the dust-shrouded interiors of molecular clouds where stellar births take place cannot be observed with visible light. Second, the timescale for the initial collapse—thousands of years—is very short, astronomically speaking. Since each star spends such a tiny fraction of its life in this stage, relatively few stars are going through the collapse process at any given time. Nevertheless, through a combination of theoretical calculations and the limited observations available, astronomers have pieced together a picture of what the earliest stages of stellar evolution are likely to be.

The first step in the process of creating stars is the formation of dense cores within a clump of gas and dust, shown in Figure 7 (a). It is generally thought that all the material for the star comes from the core, the larger structure surrounding the forming star. Eventually, the gravitational force of the infalling gas becomes strong enough to overwhelm the pressure exerted by the cold material that forms the dense cores. The material then undergoes a rapid collapse, and the density of the core increases greatly as a result. During the time a dense core is contracting to become a true star, but before the fusion of protons to produce helium begins, we call the object a protostar.

Figure 7. (a) Dense cores form within a molecular cloud. (b) A protostar with a surrounding disk of material forms at the center of a dense core, accumulating additional material from the molecular cloud through gravitational attraction. (c) A stellar wind breaks out but is confined by the disk to flow out along the two poles of the star. (d) Eventually, this wind sweeps away the cloud material and halts the accumulation of additional material, and a newly formed star, surrounded by a disk, becomes observable. These sketches are not drawn to the same scale. The diameter of a typical envelope that is supplying gas to the newly forming star is about 5000 AU. The typical diameter of the disk is about 100 AU or slightly larger than the diameter of the orbit of Pluto.

The natural turbulence inside a clump tends to give any portion of it some initial spinning motion (even if it is very slow). As a result, each collapsing core is expected to spin. According to the law of conservation of angular momentum (discussed in the chapter on Orbits and Gravity), a rotating body spins more rapidly as it decreases in size. In other words, if the object can turn its material around a smaller circle, it can move that material more quickly—like a figure skater spinning more rapidly as she brings her arms in tight to her body. This is exactly what happens when a core contracts to form a protostar: as it shrinks, its rate of spin increases.

But all directions on a spinning sphere are not created equal. As the protostar rotates, it is much easier for material to fall right onto the poles (which spin most slowly) than onto the equator (where material moves around most rapidly). Therefore, gas and dust falling in toward the protostar’s equator are “held back” by the rotation and form a whirling extended disk around the equator as shown in Figure 7 (b). You may have observed this same “equator effect” on the amusement park ride in which you stand with your back to a cylinder that is spun faster and faster. As you spin really fast, you are pushed against the wall so strongly that you cannot possibly fall toward the centre of the cylinder. Gas can, however, fall onto the protostar easily from directions away from the star’s equator.

The protostar and disk at this stage are embedded in an envelope of dust and gas from which material is still falling onto the protostar. This dusty envelope blocks visible light, but infrared radiation can get through. As a result, in this phase of its evolution, the protostar itself is emitting infrared radiation and so is observable only in the infrared region of the spectrum. Once almost all of the available material has been accreted and the central protostar has reached nearly its final mass, it is given a special name: it is called a T Tauri star, named after one of the best studied and brightest members of this class of stars, which was discovered in the constellation of Taurus. (Astronomers have a tendency to name types of stars after the first example they discover or come to understand. It’s not an elegant system, but it works.) Only stars with masses less than or similar to the mass of the Sun become T Tauri star s. Massive stars do not go through this stage, although they do appear to follow the formation scenario illustrated in Figure 7.


GOTHAM Investigators Uncover Warehouse-Full of Complex Molecules Never Before Seen in Space

In a series of nine papers, scientists from the GOTHAM — Green Bank Telescope Observations of TMC-1: Hunting Aromatic Molecules — project described the detection of more than a dozen polycyclic aromatic hydrocarbons in the Taurus Molecular Cloud, or TMC-1. These complex molecules, never before detected in the interstellar medium, are allowing scientists to better understand the formation of stars, planets, and other bodies in space. In this artist’s conception, some of the detected molecules include, from left to right: 1-cyanonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanoacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans-(E)-cyanovinylacetylene, HC4NC, and propargylcyanide, among others. Credit: M. Weiss / Center for Astrophysics | Harvard & Smithsonian

Radio observations of a cold, dense cloud of molecular gas reveal more than a dozen unexpected molecules.

Scientists have discovered a vast, previously unknown reservoir of new aromatic material in a cold, dark molecular cloud by detecting individual polycyclic aromatic hydrocarbon molecules in the interstellar medium for the first time, and in doing so are beginning to answer a three-decades-old scientific mystery: how and where are these molecules formed in space?

“We had always thought polycyclic aromatic hydrocarbons were primarily formed in the atmospheres of dying stars,” said Brett McGuire, Assistant Professor of Chemistry at the Massachusetts Institute of Technology, and the Project Principal Investigator for GOTHAM, or Green Bank Telescope (GBT) Observations of TMC-1: Hunting Aromatic Molecules. “In this study, we found them in cold, dark clouds where stars haven’t even started forming yet.”

Aromatic molecules, and PAHs — shorthand for polycyclic aromatic hydrocarbons — are well known to scientists. Aromatic molecules exist in the chemical makeup of human beings and other animals, and are found in food and medicines. As well, PAHs are pollutants formed from the burning of many fossil fuels and are even amongst the carcinogens formed when vegetables and meat are charred at high temperatures. “Polycyclic aromatic hydrocarbons are thought to contain as much as 25-percent of the carbon in the universe,” said McGuire, who is also a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA). “Now, for the first time, we have a direct window into their chemistry that will let us study in detail how this massive reservoir of carbon reacts and evolves through the process of forming stars and planets.”

Green Bank Telescope in West Virginia, USA. Credit: GBO / AUI / NSF

Scientists have suspected the presence of PAHs in space since the 1980s but the new research, detailed in nine papers published over the past seven months, provides the first definitive proof of their existence in molecular clouds. To search out the elusive molecules, the team focused the 100m behemoth radio astronomy GBT on the Taurus Molecular Cloud, or TMC-1 — a large, pre-stellar cloud of dust and gas located roughly 450 light-years from Earth that will someday collapse in on itself to form stars — and what they found was astonishing: not only were the accepted scientific models incorrect, but there was a lot more going on in TMC-1 than the team could have imagined.

“From decades of previous modeling, we believed that we had a fairly good understanding of the chemistry of molecular clouds,” said Michael McCarthy, an astrochemist and Acting Deputy Director of CfA, whose research group made the precise laboratory measurements that enabled many of these astronomical detections to be established with confidence. “What these new astronomical observations show is these molecules are not only present in molecular clouds, but at quantities which are orders of magnitude higher than standard models predict.”

McGuire added that previous studies revealed only that there were PAH molecules out there, but not which specific ones. “For the last 30 years or so, scientists have been observing the bulk signature of these molecules in our galaxy and other galaxies in the infrared, but we couldn’t see which individual molecules made up that mass. With the addition of radio astronomy, instead of seeing this large mass that we can’t distinguish, we’re seeing individual molecules.”

Much to their surprise, the team didn’t discover just one new molecule hiding out in TMC-1. Detailed in multiple papers, the team observed 1-cyanonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanoacetylene, 2-cyano-cyclopentadiene, benzonitrile, trans-(E)-cyanovinylacetylene, HC4NC, and propargylcyanide, among others. “It’s like going into a boutique shop and just browsing the inventory on the front-end without ever knowing there was a back room. We’ve been collecting little molecules for 50 years or so and now we have discovered there’s a back door. When we opened that door and looked in, we found this giant warehouse of molecules and chemistry that we did not expect,” said McGuire. “There it was, all the time, lurking just beyond where we had looked before.”

McGuire and other scientists at the GOTHAM project have been “hunting” for molecules in TMC-1 for more than two years, following McGuire’s initial detection of benzonitrile in 2018. The results of the project’s latest observations may have ramifications in astrophysics for years to come. “We’ve stumbled onto a whole new set of molecules unlike anything we’ve previously been able to detect, and that is going to completely change our understanding of how these molecules interact with each other. It has downstream ramifications,” said McGuire, adding that eventually these molecules grow large enough that they begin to aggregate into the seeds of interstellar dust. “When these molecules get big enough that they’re the seeds of interstellar dust, these have the possibility then to affect the composition of asteroids, comets, and planets, the surfaces on which ices form, and perhaps in turn even the locations where planets form within star systems.”

The discovery of new molecules in TMC-1 also has implications for astrochemistry, and while the team doesn’t yet have all of the answers, the ramifications here, too, will last for decades. “We’ve gone from one-dimensional carbon chemistry, which is very easy to detect, to real organic chemistry in space in the sense that the newly discovered molecules are ones that a chemist knows and recognizes, and can produce on Earth,” said McCarthy. “And this is just the tip of the iceberg. Whether these organic molecules were synthesized there or transported there, they exist, and that knowledge alone is a fundamental advance in the field.”

Before the launch of GOTHAM in 2018, scientists had cataloged roughly 200 individual molecules in the Milky Way’s interstellar medium. These new discoveries have prompted the team to wonder, and rightly so, what’s out there. “The amazing thing about these observations, about this discovery, and about these molecules, is that no one had looked, or looked hard enough,” said McCarthy. “It makes you wonder what else is out there that we just haven’t looked for.”

This new aromatic chemistry that scientists are finding isn’t isolated to TMC-1. A companion survey to GOTHAM, known as ARKHAM — A Rigorous K/Ka-Band Survey Hunting for Aromatic Molecules — recently found benzonitrile in multiple additional objects. “Incredibly, we found benzonitrile in every single one of the first four objects observed by ARKHAM,” said Andrew Burkhardt, a Submillimeter Array Postdoctoral Fellow at the CfA and a co-principal investigator for GOTHAM. “This is important because while GOTHAM is pushing the limit of what chemistry we thought is possible in space, these discoveries imply that the things we learn in TMC-1 about aromatic molecules could be applied broadly to dark clouds anywhere. These dark clouds are the initial birthplaces of stars and planets. So, these previously invisible aromatic molecules will also need to be thought about at each later step along the way to the creation of stars, planets, and solar systems like our own.”

Reference: “Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering” by Brett A. McGuire, Ryan A. Loomis, Andrew M. Burkhardt, Kin Long Kelvin Lee, Christopher N. Shingledecker, Steven B. Charnley, Ilsa R. Cooke, Martin A. Cordiner, Eric Herbst, Sergei Kalenskii, Mark A. Siebert, Eric R. Willis, Ci Xue, Anthony J. Remijan and Michael C. McCarthy, 19 March 2021, Science.
DOI: 10.1126/science.abb7535

In addition to McGuire, McCarthy, and Burkhardt, the following researchers contributed to and led research for this project: Kin Long Kelvin Lee of MIT Ryan Loomis, Anthony Remijan, and Emmanuel Momjian of the National Radio Astronomy Observatory Christopher N. Shingledecker of Benedictine College Steven B. Charnley and Martin A. Cordiner of NASA Goddard Eric Herbst, Eric R. Willis, Ci Xue, and Mark Siebert of the University of Virginia and, Sergei Kalenskii of the Lebedev Physical Institute. The project also received research support from the University of Stuttgart, Max Planck Institute, and The Catholic University of America.

Funding: Center for Astrophysics | Harvard & Smithsonian, Massachusetts Institute of Technology, National Radio Astronomy Observatory, Benedictine College, National Aeronautics and Space Administration Goddard Flight Center, University of Virginia, Lebedev Physica


Flows of Interstellar Gas

Figure 1. Large-Scale Distribution of Interstellar Matter: This image is from a computer simulation of the Milky Way Galaxy’s interstellar medium as a whole. The majority of gas, visible in greenish colors, is neutral hydrogen. In the densest regions in the spiral arms, shown in yellow, the gas is collected into giant molecular clouds. Low-density holes in the spiral arms, shown in blue, are the result of supernova explosions. (credit: modification of work by Mark Krumholz)

The most important thing to understand about the interstellar medium is that it is not static. Interstellar gas orbits through the Galaxy, and as it does so, it can become more or less dense, hotter and colder, and change its state of ionization. A particular parcel of gas may be neutral hydrogen at some point, then find itself near a young, hot star and become part of an H II region. The star may then explode as a supernova, heating the nearby gas up to temperatures of millions of degrees. Over millions of years, the gas may cool back down and become neutral again, before it collects into a dense region that gravity gathers into a giant molecular cloud (Figure 1).

At any given time in the Milky Way, the majority of the interstellar gas by mass and volume is in the form of atomic hydrogen. The much-denser molecular clouds occupy a tiny fraction of the volume of interstellar space but add roughly 30% to the total mass of gas between the stars. Conversely, the hot gas produced by supernova explosions contributes a negligible mass but occupies a significant fraction of the volume of interstellar space. H II regions, though they are visually spectacular, constitute only a very small fraction of either the mass or volume of interstellar material.

However, the interstellar medium is not a closed system. Gas from intergalactic space constantly falls onto the Milky Way due to its gravity, adding new gas to the interstellar medium. Conversely, in giant molecular clouds where gas collects together due to gravity, the gas can collapse to form new stars, as discussed in The Birth of Stars and the Discovery of Planets outside the Solar System. This process locks interstellar matter into stars. As the stars age, evolve, and eventually die, massive stars lose a large fraction of their mass, and low-mass stars lose very little. On average, roughly one-third of the matter incorporated into stars goes back into interstellar space. Supernova explosions have so much energy that they can drive interstellar mass out of the Galaxy and back into intergalactic space. Thus, the total amount of mass of the interstellar medium is set by a competition between the gain of mass from intergalactic space, the conversion of interstellar mass into stars, and the loss of interstellar mass back into intergalactic space due to supernovae. This entire process is known as the baryon cycle—baryon is from the Latin word for “heavy,” and the cycle has this name because it is the repeating process that the heavier components of the universe—the atoms—undergo.


Science at Your Doorstep

Paradoxically, stars begin in the galaxy’s coolest places: the dense giant molecular clouds (or GMCs).

This is not quite the paradox it seems, as in the beginning, stars require little else but gravity to form. And that’s really quite lucky, because one thing they do need is regions of high density, and high density is unlikely to occur where temperatures are high.

And so stars begin in perhaps the most surprising of ways: as a high-density bundle of very cool gases within an equally cool interstellar cloud.

But they do heat up eventually. How?

Remember how I said that stars need little else but gravity to form?

Well, that’s the short answer.

For this post, we’re going to need to consider two different types of energy: gravitational energy and thermal energy. I discussed thermal energy in my previous post. It’s the total energy of all the moving particles within an object—in this case, a giant molecular cloud.

Gravitational energy, on the other hand, is much like kinetic energy.

Kinetic energy is the energy a particle possesses due to its motion. Think about walking across your bedroom, versus jogging around your neighborhood, versus running a marathon. The faster you’re moving, the more energy you need to have, and the more you need to eat.

Humans actually need energy from food to move for the same reason, but I’ll elaborate on that when I finally move on to writing about the life sciences. Right now, remember that moving particles have energy—and the faster they’re moving, the more energy they have.

Gravitational energy is a similar concept.

When the gravity of a molecular cloud’s dense cores begins to pull material inward, that material is now falling. Think about it: on Earth, “falling” means motion toward the Earth, or to be more precise, toward the Earth’s center of gravity.

In a giant molecular cloud, for any one particle, “falling” means moving in the direction of the gravitational force of one of the cloud’s dense cores. These falling particles have “gravitational energy.” And, like kinetic energy, this is energy due to motion.

Let’s focus on what’s happening to just one of these dense cores of material. As the cores grow denser, the GMC fragments into smaller but denser clouds. Material falls inward and picks up speed, just like an object falling on Earth.

The cosmological principle, which tells you that the laws of physics here on Earth are the same anywhere else in the universe, is a well-tested and accepted theory. It means that if objects accelerate—pick up speed—as they fall on Earth, then so do the particles that fall toward the center of a dense cloud.

This state of picking up speed as they fall inward is known as free-fall collapse. Towards the outer reaches of a core’s gravity, particles may be moving slowly, but by the time they reach the center, they are moving very rapidly.

The particles trapped in the gravity of this core have gravitational energy. But do they have thermal energy?

You can say that they have kinetic energy—the energy of movement. But that doesn’t mean they have thermal energy. While both concern the movement of the particles, thermal energy requires the particles to be moving in random directions, and right now, they’re all falling in toward the center.

When particles begin to accumulate at the center of the core, they can’t fall any further. They begin colliding with one another in the central region of the cloud. Now their motion becomes randomized and jumbled.

At this point, the cloud begins to grow hotter, and we can say that gravitational energy has been converted to thermal energy.

I would liken it to converting potential energy to kinetic energy.

Here’s that diagram of potential and kinetic energy again. Potential energy isn’t so much energy as an object’s potential to have energy. If it’s going to be dropped, then the higher up it is, the more time it’ll have to accelerate and the more kinetic energy it will have.

So when dense clouds are contracting, they have gravitational energy. This is just the potential to have thermal energy, as once the material gets to the center and begins to collide, thermal energy will be generated.

There are many cases in astronomy where gravitational energy gets converted to thermal energy. Interestingly enough, we see one such case when stars nearing the end of their life cycle begin to expand and contract. We see this conversion from gravitational to thermal energy both at the beginning and at the end of a star’s life.

Now here’s the million-dollar question. Before star formation begins, giant molecular clouds resist the forces pushing them to contract simply with the energy of motion of their particles colliding and repelling one another.

So in a dense cloud, when the material begins to heat up, will this stop the contraction?

It won’t—not if it has a way to rid itself of the thermal energy.

I’ll bet I know what you’re going to ask next. What’s the point of all this, if the newly forming star has to get rid of its energy? How can a star ever form, if it can’t contract without losing what little energy it’s got?

Because it still has gravitational energy, being constantly converted to more thermal energy. The cloud has not finished contracting, and it is essential that it continue to contract. It needs to gain enough mass so that pressures in its core will be high enough to ignite hydrogen fusion.

And in order to continue to contract, the cloud must radiate away the thermal energy as it is converted. What I find incredible is the sheer perfection of this process.

The core that will form a new star is still ensconced deep within a thick cloud of gas and dust. Only the longer wavelengths of radiation can penetrate the cloud. And by chance, the cloud’s low thermal energy means it can only radiate those longer wavelengths.

If that didn’t work, stars couldn’t form. Heat would get trapped inside the contracting cloud, and it would cease to contract. A star could never form there.

Star formation is possible simply because of a quirk of the electromagnetic spectrum—that cool objects emit long wavelengths, and the longer the wavelength, the better the radiation can penetrate thick clouds.

How do astronomers know all this? Because longer-wavelength radiation must escape the cloud in order for it to contract, we can look for that radiation with telescopes—or, specifically, with a spectroscope.

As I’ve described in many posts, a spectroscope separates out the many wavelengths of radiation an object produces and shows us which wavelengths are being emitted the most intensely—and which wavelengths are being completely blocked by certain atoms.

An emission spectrum, specifically, is produced by the excited atoms of a hot gas, like that of a contracting cloud.

If we aim a spectroscope at a suspected region of star formation, we observe emission lines in the far infrared, dubbed cooling lines. I imagine they’re called “cooling lines” because they are evidence of a cloud regulating its temperature by cooling off.

But this can’t last forever. Remember, the dust in the cloud is opaque to the shorter wavelengths of radiation—which carry more energy. And as the core continues to contract and get hotter, it will emit exactly that. These wavelengths won’t be able to escape and let the cloud cool off.

At this point, the cloud’s contraction slows. We can track its path on the H-R diagram…

The H-R diagram—named Hertzsprung-Russel for its creators—is a plot of all stars according to their temperature and luminosity. Temperature, as you can see, corresponds directly to color, and luminosity is a measure of the total energy emitted by the star—which corresponds directly to its surface area.

Meaning, a star could be very cool and still very luminous, as long as it is very large. Conversely, a star could be very hot and very faint, as long as it is very small. However, this graph only shows the main-sequence, the part of a star’s life cycle where temperature corresponds almost directly to size and luminosity.

You can see on the H-R diagram that a giant molecular cloud starts out very cool and very faint, fainter than most stars. When it breaks up into dense contracting clouds, the clouds are hotter and still very large. As the clouds contract further, they are also accumulating mass, so they grow hotter and more luminous.

But once a protostar is born, it stops accumulating mass. It continues to contract within its cocoon of dust and gases, causing it to shrink, and its luminosity drops as a result. And roughly at this point, a star is born—which we’ll explore in posts coming up.


Watch the video: Άσπρα Καράβια-Καίτη Χωματά u0026 Μιχάλης Βιολάρης (February 2023).