Table of absolute magnitudes of stars by spectral type and photometric band

Table of absolute magnitudes of stars by spectral type and photometric band

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I would like to find a table where I can, say, look for K5 dwarfs and read off that stars of that type have on average an absolute magnitude of X.XX in the infrared L band and Y.YY in the G band. I've looked everywhere and I can't find a compilation of this kind of information. Does any such thing exist?

There are a couple of standard papers containing the table you want.

Kenyon & Hartmann (1995)… 101… 117K

Table A5 contains many colours for stars as a function of spectral type. You need to combine this with something that gives absolute V magnitude along the main sequence, like that of Schmidt-Kaler (1982).

An alternative, that has colours for both main sequence and pre main sequence stars (though the absolute magnitude of a PMS star is age-dependent) is found in Pecaut & Mamajek (2013).

Online table at

There are a lot of stars [citation needed] and they are organised into catalogs by various organisations, well known catalogs include the UNSO catalog (produced by the united states Naval office) and the Guide Star Catalog (used by Hubble to accurately posistion itself)

I think the best for you is the Hipparcos catalog of stars, as it can be browsed quite easily by spectral type.

HEASARC Browse (Catalog search)

The Spectral Types of Stars

By: Alan MacRobert August 1, 2006 0

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What's the most important thing to know about stars? Their brightness (apparent magnitude) might top the list, but right behind would be the spectral types. Without its spectral type a star is a meaningless dot. Add a few letters and numbers like "G2V" or "B5IV-Vshnne" and the star suddenly gains personality and character. To those who can read its meaning, the spectral code tells at a glance just what kind of object the star really is: its color, size, and luminosity compared to the Sun and stars of all other types its peculiarities, its history, and its future.

Mass-producing stellar spectra. A thin prism was placed in front of the telescope to spread out starlight from the Hyades cluster into little rainbows, or spectra. Spectra contain the 'fingerprints' astronomers use to deduce myriad stellar properties. Click on this image to see some of these stellar spectra up close.

University of Michigan Department of Astronomy

absolute magnitude) and often its evolutionary status (young, middle-aged, or nearing death). The luminosity (when compared to the star's apparent brightness in our sky) also gives a good idea of the star's distance. Appended to the basic spectral type may be letters for chemical peculiarities, an extended atmosphere, unusual surface activity, fast rotation, or other special characteristics.

Every starwatcher needs to have a feel for spectral types.

GCVSNSVARS - General Catalog of Variable Stars, March 2012 Version: Suspected Variable Stars

The data contained in the present catalog include positions, magnitudes, variability types, references to the literature, and spectra.

Catalog Bibcode





The GCVS Suspected Variable Star prefix NSV (for New Suspected Variable) together with a running number (from 1 to 14811 for stars in the original 1982 NSV Catalog, and from 15001 to 26206 for stars in the 1998 Supplement to the NSV). Leading zeroes have been removed from the running number. One entry (NSV 10360) has a letter suffix of A, and thus, to be completely accurate, should be referred to as NSV 10360A, according to the CDS documentation.

The NSV Catalog running number (from 1 to 14811 for stars in the original 1982 NSV Catalog, and from 15001 to 26206 for stars in the 1998 Supplement to the NSV). Leading zeroes have been removed from the running number, notice.

The NSV Catalog letter suffix, used only for one entry (NSV 10360) which has a letter suffix of A, and thus should be referred to as NSV 10360A, according to the CDS documentation.

This is a flag that is set to the value of '-' if a star's variability seems doubtful or erroneous according to the compilers of the NSV Catalog.

This is a flag that is set to 'Y' if there are remarks on the star in the file

The remarks contain information covering situations such as, e.g.:

The Right Ascension of the star in the specified equinox. This was given to a precision of 0.1 seconds of time in the originating table, and in both B1950 and J2000 equatorial coordinates.

The Declination of the star in the specified equinox. This was given to a precision of 1 arcsecond in the originating table, and in both J2000 and B1950 equatorial coordinates.

The Galactic Longitude of the star.

The Galactic Latitude of the star.

This is a code flag describing the positional accuracy, as follows:

The type of variability according to the GCVS (4th Edition) variability classification scheme, with the addition of six new types (ZZO, AM, R, BE, LBV, BLBOO) that were introduced in the Name-Lists 67-72 and in the GCVS vol. V. Descriptions of the 6 new types only are given below, while the entire classification scheme is described in the file The new types are:

This is a limit flag for the maximum magnitude: '<' indicates that the max_mag value given is a numerical upper limit, i.e., the actual maximum magnitude may be brighter than this value, while '>' means that the max_mag value given is a numerical lower limit, i.e., the actual maximum magnitude may be fainter than this value.

The apparent magnitude at maximum brightness. Magnitudes are reported to hundredths if the observations are photoelectric or CCD, to tenths or whole magnitudes if they are not. If only an amplitude has been measured photoelectrically, then the maximum magnitude is generally given to tenths only and the minimum is reported to hundredths.

This flag is set to ':' if the maximum magnitude is considered uncertain.

This field may contain flag(s) indicating limit and/or amplitude for the min_mag value.

If the field contains '(', that indicates that the quoted min_mag value is an amplitude relative to the value of the max_mag parameter, rather than the actual minimum magnitude, and the limit, if present, applies to this amplitude.

If the value is '>', then the min_mag value given is a bright limit, i.e., the actual minimum magnitude may be fainter than this value, while if the value is '<', then the min_mag value given is a faint limit, i.e., the actual minimum magnitude may be brighter than this value.

The minimum apparent magnitude, or the amplitude of variability. If the value of limit_min_mag contains '(', this indicates that the min_mag value given is an amplitude rather than an observed minimum magnitude. Magnitudes are reported to hundredths if the observations are photoelectric or CCD, to tenths or whole magnitudes if they are not. If only an amplitude has been measured photoelectrically, then the maximum magnitude is generally given to tenths only and the minimum is reported to hundredths. If the value of min_mag_flag is 'st', then the quoted value of min_mag is an amplitude in steps rather than magnitudes, and the original references should be consulted for further information.

This flag is set to ':' if the min_mag value is considered uncertain, and is set to 'st' if the quoted value of min_mag is an amplitude in steps rather than magnitudes, and the original references should be consulted for further information. In 3 cases (NSV 25926, NSV 25927 and NSV 25981) the value is set to 'A' (the HEASARC changed the original '*' since an asterisk is used as a wild card character in its system): the significance of this flag value is not specified in the original CDS documentation.

A code for the photometric system in which the min_mag value is reported (given only if this is different from the photometric system in which the max_mag value is reported). The main codes are V (visual, photovisual, or Johnson V), B (Johnson B) and p (photographic magnitudes). The designations u, v, b, y refer to the Stroemgren system. The symbols Ic and Rc mean magnitudes in the Cousins I, R system, g designates magnitudes in the system of Thuan and Gunn, T stands for broad-band Tycho magnitudes formed from B and V measurements, while r are red magnitudes not tied to a particular system. A value of 'w' for this parameter means that the quoted magnitude is for a white-light measurement. Several stars from the 68th Name List have values of Ƈ.' for this parameter to designate the 1.04 micron band of the system introduced by G.W. Lockwood.

A code for the photometric system in which the max_mag and min_mag value are reported (if the value of min_mag_system is blank), or for the photometric system in which the max_mag value only is reported (if the value of min_mag_system is non-blank). The main codes are V (visual, photovisual, or Johnson V), B (Johnson B) and p (photographic magnitudes). The designations u, v, b, y refer to the Stroemgren system. The symbols Ic and Rc mean magnitudes in the Cousins I, R system, g designates magnitudes in the system of Thuan and Gunn, T stands for broad-band Tycho magnitudes formed from B and V measurements, while r are red magnitudes not tied to a particular system. A value of 'w' for this parameter means that the quoted magnitude is for a white-light measurement. Several stars from the 68th Name List have values of Ƈ.' for this parameter to designate the 1.04 micron band of the system introduced by G.W. Lockwood.

This is a reference code referring to a study of the star, the key to which can be found in the file

This is a reference code referring to a study that contains a chart or photograph of the star field, the key to which can be found in the file

The identification of the suspected variable in the paper whose code reference is given in the ref_star parameter. DM numbers are given without a prefix, the standard naming convention of "The Henry Draper Catalogue" being used (BD for declination zones +89 to -22, CD for zones -23 to -51, and CP for zones -52 to -89). Some identifications are given by coordinate designations, a 6-digit number consisting of hours, minutes, and seconds (or tenths of a minute) of time and degrees of declination with sign included. A value of 'Y' signifies that a designation is given in the remarks to the published catalog. This parameter is not populated for the stars in the NSV Supplement (entries with values of the nsv parameter of 15001 or greater.

The spectral type and luminosity class of the suspected variable star, if known. Spectral types from the HD Catalog are generally given in parentheses. Also, the following (non-standard) symbols may be used:

The alternative name of the suspected variable star either as given in the main Catalog of Galactic Variable Stars (the HEASARC GCVS table, created from the CDS table B/gcvs/gcvs_cat.dat), a Name-List number, if it starts with the 2-digit number 67-77 and is followed by the 4-digit number of the star in the corresponding Name List, or as given in the GCVS Catalog of Extragalactic Variable Stars (the HEASARC GCVSEGVARS table, created from the CDS table B/gcvs/evs_cat.dat). In the last case, the name may be followed by an asterisk: this indicates that there are remarks about this star in GCVS Volume V. Notice that the HEASARC removed a number of entries (which are listed in the file that were present in the CDS table B/gcvs/nsv_cat.dat but had no coordinates given, indicating that the status of these suspected variable stars had changed. The HEASARC retained other entries that have non-blank values of the alt_var_name parameter but that have valid positional coordinates and other data.

The HEASARC Browse object classification, based on the spectral type parameter (spect_type), if there is information in this field, else based on the variability type parameter (variability_type).

Color and Magnitude Definitions

The characteristics of the Johnson-Cousins color bands are given in the following table. The band width is defined as the full-width half-maximum of each curve (half the distance between the two points that have a value that is half of the peak value). The center frequency is half-way between the half-maximum points. The 0 magnitude flux is the power per unit area per unit wavelength received in the wave band from a 0 magnitude star. In other words, this flux is almost precisely the flux one measures at each wave band for the star Vega. The data in this table is from Lamla (1982) via the ADPS. [5,6]

Table of absolute magnitudes of stars by spectral type and photometric band - Astronomy

This page contains a sample queries designed to serve as templates for writing your own SQL queries. The first section, Basic SQL, serves as an introduction to the syntax of the SQL database access language. The sections that follow feature queries written to solve real scientific problems submitted by astronomers. Those queries are divided by scientific topic.

Click on the name of the query in the list below to go directly to that sample query. You can load the query into SkyServer's SQL Query tool by clicking on the button above each query. You can then modify the query to suit your needs. Alternatively, you can send each query to the database and see the results by clicking on the button.

NOTE: Please read the query hints below before you try any queries, especially if you are new to SQL or the SkyServer.

Some hints on searching SkyServer:

  1. For an introduction to SQL, see the interactive SQL tutorial. For more information on how to use SQL with the SkyServer database, see SQL in SkyServer.
  2. Please read the Optimzing Queries section in the SQL in SkyServer help page as well as the Query Limits page (to see the timeouts and row limits on queries) before you attempt any complex queries of your own.
  3. If you're not sure how many objects a query is going to return, it's always a good idea to first do a "count" query, e.g. "SELECT count(*) FROM Galaxy WHERE . " so as to get an idea of how many objects will be returned, so you don't find yourself waiting a lot longer than you expected to.
  4. If even a count takes a long time, this is a good indication that the actual query will take a much longer time to run, so you should check if you have formulated the query correctly and in the most efficient way. Some advice on how to write efficient queries can be found in the Optimzing Queries section of the SQL in SkyServer.
  5. If you have optimized your query and it still takes much longer to run than you think it should, try again at a different time. Sometimes when many queries are being run simultaneously, the servers can take a longer time to return. This in turn can sometimes result in queries timing out, even though they would run perfectly well on the server when the server load is less.
  6. A good way to find if there are any objects that meet the search criteria in a given query is to use the "TOP <n>" SQL construct. For example, "SELECT TOP 10 FROM . " will only return the first 10 objects that meet your criteria. Note that this is NOT a deterministic ordering the "TOP 10" objects may be different if you run the same query again.
  7. If your query returns a lot of output (more than a few thousand objects), it is generally not a good idea to select the HTML output format (which is selected by default in the SQL Search tool). Instead, try selecting CSV (comma-separated value) output. However, for all queries that return many objects, you are much better off using the CasJobs batch query service or one of the command-line query interfaces (sqlcl or the emacs query interface).
  8. Be sure to exclude invalid values (unset or uncalculated quantities), as described on the SQL in SkyServer page under Excluding Invalid Data Values.
  9. SkyServer returns all values with a default precision. For some applications (such as doing high-precision astrometry with RA and Dec values), the default precision is not enough. For these applications, use the STR(column,n,d) SQL construct (where n is the total number of digits and d is the number of decimal places) to set the precision of the column that your query requests. The SkyServer returns values with a default precision that is set for each data type, and this may not be enough for columns like ra, dec etc. See the Selected neighbors in run sample query below for examples of how to use STR.


-- This query shows the basic structure of a SQL query:
-- SELECT [variables] FROM [table] WHERE [constraints]
-- Although many of your SQL queries will be more complex,
-- they will all follow this same basic structure.

-- This sample query finds unique objects in an RA/Dec box.
-- For a more efficient way to find objects by position, see the next query,
-- Searching around a sky position.

Basic position search

-- To see how to limit the search only to objects with clean photometry, see the
-- Clean imaging query.

Using the PhotoTag table

-- This sample query finds data for all objects in fields with desired PSF width.

Search for a Range of Values: BETWEEN

-- The BETWEEN statement can be used to set constraints on a range of values. -- This sample query finds galaxies with g magnitudes between 18 and 19.
SELECT TOP 10 objID, cModelMag_g
FROM Galaxy
cModelMag_g between 18 and 19 -- 18 < cModelMag_g < 19

Rectangular position search

-- a) Rectangular search using straight coordinate constraints:
SELECT objID, ra, dec
FROM PhotoObj
WHERE (ra between 179.5 and 182.3) and (dec between -1.0 and 1.8)

-- b) This query can be rewritten as follows to use the HTM function that returns a
-- rectangular search area (The "Run this Query" button above will run this query):

SELECT p.objID, p.ra, p.dec
FROM PhotoObj p JOIN dbo.fGetObjFromRectEq(179.5, -1.0, 182.3, 1.8) r ON p.objID = r.objID

Searching more than one table: JOIN. ON

-- The syntax of the statement is:
-- JOIN [the second table] ON [a variable the tables have in common].
-- The variable the tables have in common is called the "key" (think of it
-- as the key that unlocks your ability to search two tables).
-- The key variable(s) in each table are shown on the About the Database page.
-- Find the two tables you want to join and look for a key variable they have in common.

-- The sample query looks for spectra of quasars and shows the date and time at which
-- each spectrum was taken.

Photometry and Spectroscopy: SpecPhoto

-- You should use specPhoto whenever your variables of interest
-- can be found there using specPhoto means that your queries will
-- return results much faster than using JOIN. ON.
-- that your queries will return results much faster than using JOIN. ON.

-- This sample query retrieves both magnitudes (from photometry) and
-- redshifts (from spectroscopy) of 100 quasars. Note that this query
-- also introduces the use of mathematical operators (in this case subtraction)
-- in queries.

Counting objects by type or category

-- When included in the SELECT block, the count(*) statement returns the number of objects that
-- meet your search criteria.
-- The GROUP BY statement sorts results into groups (categories) based on the value of a data column.

-- In this query, the grouping occurs on the 'class' column of the SpecObj view, which contains
-- the spectral classification of the object.
-- The query can be easily modified to find object counts sorted by other columns.

-- For another example, see the Counts by Type and Program sample query.

Using flags

-- Important caveat: The 'flags' column is the union of the flag values in
-- each band,and using it may produce unexpected results, for example if a
-- bad flag bit is set in only one band (e.g. in the 'flags_r' value), that bit
-- will be bad in the overall 'flags' value as well. For more information,
-- please see the 'Photometric flags detail' entry in Algorithms.

-- NOTE: This query takes a long time to run without the "TOP 1000".

Photometric data only for stars or galaxies

-- This sample query shows how you can search for data
-- for galaxies using the Galaxy view. Searching for stars
-- using the Star view would work the same, but with
-- "Star" in the FROM clause.

-- This sample query finds 1000 galaxies
-- brighter thanr-magnitude 22, extinction-corrected.
-- You could remove the "TOP 1000" to find all galaxies brighter
-- than r=22, but it would take a long time to run and might
-- time out.

Clean photometry: the CLEAN flag

-- In some contexts, it is important to know what conditions
-- are required for theimaging pipeline to set the CLEAN flag,
-- so that you can adjust these requirements to suit your needs.
-- The CLEAN flag works differently for stars and galaxies. For stars,
-- see the Clean photometry - stars sample query. For galaxies,
-- see the Clean photometry - galaxies sample query.

SELECT top 10 objID, ra, dec, g, clean
FROM PhotoObj

Using Field MJD (Modified Julian Date)

-- This query uses the Field table, which contains data on SDSS fields.
-- It is also possible to search for the observation date of a specific
-- object by joining the Field table with the PhotoObj table or its
-- associated views.

SELECT top 100 run, rerun, camcol, field, mjd_r as primary_mjd
FROM Field
WHERE mjd_r > 53140

Finding objects by their spectral lines

-- Spectral lines for galaxies are calculated using the MPA-JHU
-- spectroscopic reanalysis (Tremonti et al. 2004 Brinchmann et al. 2004)
-- and are stored in the galSpecLine table. For more on how spectral lines
-- of galaxies are found, see the Galspec page of the website.
-- Spectral lines for stars are calculated using the SEGUE Stellar Parameter
-- Pipeline (SSPP Lee et al. 2008) and are stored in the sppLines
-- table. For more on how spectral lines of stars are found, see
-- the SSPP page of the website.

-- The "Run this Query" button will run the second query, which finds stars
-- by searching CaII lines.

-- a) Finding galaxies by their emission lines:
-- This query selects galaxy spectra with high internal reddening,
-- as measured by the standard Balmer decrement technique. It
-- makes use of the galSpec tables for the measurements of
-- galaxy lines. In this case we use galSpecLine, which has
-- emission line measurements.

s.plate, s.fiberid, s.mjd, s.z, s.zwarning,
g.h_beta_flux, g.h_beta_flux_err,
g.h_alpha_flux, g.h_alpha_flux_err
FROM GalSpecLine AS g
JOIN SpecObj AS s
ON s.specobjid = g.specobjid
h_alpha_flux > h_alpha_flux_err*5
AND h_beta_flux > h_beta_flux_err*5
AND h_beta_flux_err > 0
AND h_alpha_flux > 10.*h_beta_flux
AND s.class = 'GALAXY'
AND s.zwarning = 0

-- b) This query selects red stars (spectral type K), with
-- large CaII triplet eq widths, with low errors on the CaII triplet
-- equivalent widths.

FROM sppLines AS sl
JOIN sppParams AS sp
ON sl.specobjid = sp.specobjid
fehadop < -3.5
AND fehadopunc between 0.01
and 0.5 and fehadopn > 3

Finding spectra by classification (object type)

-- Other possible values with of class are 'QSO', 'GALAXY' and 'UNKNOWN'.

Moving Asteroids

-- This query is an example of a situation where you must
-- search the full PhotoObj view, since the
-- columns rowv and colv are not in PhotoTag.

Finding plates with repeat spectra

SELECT first.plate, other.plate, COUNT(DISTINCT other.mjd) + COUNT(DISTINCT first.mjd) AS nightsObserved, otherPlate.programname, count(DISTINCT other.bestObjID) AS objects
FROM SpecObjAll first JOIN SpecObjAll other ON first.bestObjID = other.bestObjID JOIN PlateX AS firstPlate ON firstPlate.plate = first.plate JOIN PlateX AS otherPlate ON otherPlate.plate = other.plate
WHERE first.scienceprimary = 1 AND other.scienceprimary = 0 AND other.bestObjID > 0
GROUP BY first.plate, other.plate, otherPlate.programname
ORDER BY nightsObserved DESC, otherPlate.programname, first.plate, other.plate

Galaxies blended with stars

-- This query introduces the use of multiple tables or views with a table JOIN clause.
-- You can assign nicknames to tables as in the FROM clause below. Since you are using
-- multiple tables, you must specify which table each quantity in the SELECT clause
-- comes from. The "ON " part of the JOIN clause specifies the joining
-- condition between the two tables, which is achieved by requiring that a quantity
-- present in both tables be equal.

Object counts by type and program

SELECT plate.programname, class, COUNT(specObjId) AS numObjs
FROM SpecObjAll JOIN PlateX AS plate ON plate.plate = specObjAll.plate
GROUP BY plate.programname, class
ORDER BY plate.programname, class

Clean photometry with flags - Stars

-- For queries on star objects, when you use PSF mags, use only PRIMARY objects
-- and the flag combinations indicated below. If you use the Star view as this sample
-- query does, you will get only primary objects otherwise you will need to add
-- a "mode=1" constraint. For example, if you are interested in r-band magnitudes
-- of objects, perform the following checks (add analogous checks with AND for
-- other bands if you are interested in multiple magnitudes or colors).

-- In this query, the human-readable flag names (through calls to the flag
-- functions have been replaced with explicit values. Using explicit values for flags
-- makes the queries run much faster.

Cataclysmic variables (CVs) using colors

Binary stars colors

Using the sppLines table

Using the sppParams table

SELECT top 10 so.bestobjid, so.specobjid, so.plate, so.mjd, so.fiberid, sp.teffadop, sp.fehadop, sp.loggadop, sp.snr, sp.flag
FROM specobjall so -- get the stellar params for each spectrum. -- the sciencePrimary=1 guarantees no repeats in this query, but not all -- these spectra will be from SEGUE JOIN sppparams sp ON so.specobjid = sp.specobjid and so.sciencePrimary=1 -- get the photometry info. note the "dr7" context prefix that will go -- away when we get the dr8 imaging
WHERE sp.teffadop > 4500 and sp.teffadop and fehadop > -2 and fehadop and loggadop > 2 and loggadop -- demand that the first two letters of the sspp flags be n (see web page docs) -- the like and % are for sub-string comparisons and sp.flag like '_n%' and sp.snr > 30

Stars multiply measured

Proper motions in an open cluster

Clean photometry with flags - Galaxies

-- For galaxies (i.e. not using PSF mags): Again use only PRIMARY objects. Other
-- cuts are nearly the same, but remove the cut on EDGE. Possibly also remove
-- the cut on INTERP flags.

-- In this query, the human-readable flag names (through calls to the flag
-- functions have been replaced with explicit values. Using explicit values for flags
-- makes the queries run much faster.

Galaxies with blue centers

Diameter limited sample

SDSS luminous red galaxy selection

Classifications from Galaxy Zoo

-- The first query below finds the weighted probability that a given galaxy
-- has each of the six morphological classifications.
-- The second query finds 100 galaxies that have clean photometry,
-- at least 10 Galaxy Zoo volunteer votes and at least an 80% probability
-- of being clockwise spirals.

BOSS Target Selection Sample

FROM photoprimary
WHERE (dered_r-dered_i) < 2 AND cmodelmag_i-extinction_i BETWEEN 17.5 AND 19.9 AND (dered_r-dered_i) - (dered_g-dered_r)/8. > 0.55 AND fiber2mag_i < 21.7 AND devrad_i < 20. AND dered_i < 19.86 + 1.60*((dered_r-dered_i) - (dered_g-dered_r)/8. - 0.80)

The absolute magnitudes of stars.

SIRIUS is the brightest star in the night sky. Or is it? From our vantage point here on Earth, it certainly looks brighter than all the other stars. But actually Sirius emits just a fraction of the light produced by our galaxy's most luminous stars. It appears bright simply because it is relatively close by, 8.6 light-years away. Since the apparent brightness of a star diminishes as the square of the distance -- the well-known inverse square law -- a nearby, modest star like Sirius can outshine a remote supergiant like Rigel.

How would the night sky look if all the naked-eye stars were the same distance from Earth? Imagine watching Sirius grow dimmer as we move it farther away. Now bring more-distant Rigel alongside its glow overwhelms the former "brightest star in the heavens." Even relatively faint 4 Lacertae, a far-flung supergiant, would stand among the most prominent luminaries in this imaginary sky. And our Sun? An average star in terms of its light output, the Sun would hardly be noticed among its more luminous stellar cohorts.

In an attempt to improve on mere imagination, I have designed a new type of constellation chart that represents such a hypothetical sky. The map's stars have circular symbols indicating their absolute rather than apparent brightnesses, measured in magnitudes. The absolute value tells how bright a star would be if viewed from a standard distance of 32.6 light-years. The first chart based on this idea, which I created in 1992, is called SC006 and is available from Sky Publishing Corp. as part of its ESSCO series of educational materials. On the next two pages the same approach is used in a chart that can be compared more readily with this magazine's center sky maps.

Astronomers have long known that stars displaying identical spectral characteristics have approximately the same intrinsic brightness. Therefore, I generally adopted the stars' absolute magnitudes, derived from spectral type, that are listed in the second edition of Sky Catalogue 2000.0, Vol. 1, the Observer's Handbook of the Royal Astronomical Society of Canada, or the Yale Bright Star Catalogue. The absolute magnitudes of the various spectral types are based on calculations by Robert F. Garrison published in 1984. For double stars, I took the spectral types of both members into account.

Representing the entire range of absolute magnitudes of the stars on these charts, from +6 to -8, proved difficult. I wanted to base the diameters of the star symbols on an exponential law involving the stellar magnitude, as recommended by Andrew T. Young (S&T: March 1990, page 311). But in a preliminary trial I found that adopting a single dot-size formula for the entire range led to some bizarre results.

Even if the 6th-magnitude stars were represented by the smallest available symbol, those of magnitude -8 became too large. Orion's familiar form became almost unrecognizable due to overlapping circular symbols. On the other hand, if the most luminous stars were shown in a reasonable fashion, the faintest ones virtually disappeared from the page. In the end I settled on two formulas, one for magnitudes +6 through +1, and another that is more gradual for 0 through -8.

The Milky Way is easy to spot as a diagonal band by virtue of the massive, luminous stars that hug the galaxy's plane. The constellation Orion dominates this portion of the "absolute-magnitude sky," its outline demarcated by supergiants such as Rigel, Betelgeuse, and Saiph. A pair of nearby stars become much fainter: Sirius has faded from magnitude -1.5 to +1.4 and Procyon from +0.3 to +2.6. Other examples are Altair, which dims from magnitude +0.8 to +2.2 Fomalhaut, +1.2 to +2.0 and Vega, 0.0 to + 0.5.

The collective absolute magnitude of the members of a cluster or a galaxy can also be computed. The total output from the Pleiades, for example, is the equivalent of a single star of absolute magnitude -4. Other clusters are even more luminous: h Persei, -8 Chi ([Chi]) Persei, -7 Messier 13, -8 and Omega ([Omega]) Centauri, -10. The galaxies M31 and M51 have absolute magnitudes of -21 each. (By comparison, the Sun's apparent magnitude is -26. Thus the Sun at its present distance would still outshine a galaxy's worth of stars located 32.6 light-years away!)

Clearly, no chart of this type can really show how the absolute-magnitude sky would look to the naked eye. For the SC006 I felt it best to include only those stars actually plotted on a preexisting companion chart, the SC001. Many of these delineate familiar constellation patterns thus the SC001 itself has no precise brightness cutoff. In fact, very dim and remote stars exist that are even more luminous than Epsilon ([Epsilon]) Aurigae and Deneb, which at magnitude -8 are the brightest stars portrayed on the SC006.

According to a 1978 study by Roberta Humphreys (University of Minnesota), the variable star Rho ([Rho]) Cassiopeiae has an average absolute magnitude of about -9.5. And the star known merely as No. 12 in the stellar association Cygnus OB2 has an absolute magnitude of -9.9, the equivalent of 760,000 Suns, making it a likely candidate for the brightest individual star in the absolute-magnitude sky of our own galaxy. (Actually, these figures represent only a star's emission at visible wavelengths. If the energy produced outside the visible range is also considered, the winner would likely be HD 93129A in the constellation Carina, with a total absolute magnitude of -12.)

Most of the stars on this type of chart have absolute magnitudes around zero, and of these most are either main-sequence stars of spectral type A or giants of spectral type G5 through K5. The reason is straightforward. Luminous O and B stars, with absolute magnitudes between -6 and -8, are relatively rare in our galaxy they appear on the chart only because they can be seen to such great distances. On the other hand, the ubiquitous main-sequence stars of types K and M put out so little light that few are visible to the naked eye hence, they are not included. An absolute-magnitude chart based on a sample of the visually brightest stars favors those with intermediate absolute magnitudes.

A random-sample chart of this type -- one that portrays the actual distribution of luminosities found among the Milky Way galaxy's stars -- would contain only a sprinkling of red giants and stars of spectral types O and B. Most of the plotted stars would be type-M main-sequence stars, with absolute magnitudes of +10 or fainter. On our adopted magnitude scale, such stars would be represented on the full-size chart by dots no larger than 0.08 millimeter across, five times smaller than the dot in this exclamation mark! The least luminous star known, RG 0050-2722, has an absolute magnitude of +19 or fainter (S&T: July 1982, page 21).

Why are luminous stars such as Rigel so rare? The lifetime of a star is governed by its overall fuel supply -- primarily the amount of hydrogen available to stoke its thermonuclear furnace -- and the rate at which the star consumes the fuel. The first factor depends on the star's mass, the second on its luminosity.

A star with the Sun's mass and luminosity can last about 10 billion years. Lower-mass stars, which have less fuel but use it very sparingly, will survive even longer, perhaps hundreds of billions of years. Conversely, higher-mass stars have several times more fuel but consume it far faster. Thus they have relatively short lifetimes -- tens or hundreds of millions of years.

Consequently, virtually all of the galaxy's stars created with absolute magnitudes fainter than the Sun's are still shining today. But the stars' frequency begins to decline markedly at absolute magnitudes brighter than about +3.5, close to the Sun's value of +4.8. Such stars blaze forth only briefly, then burn out and become inconspicuous white dwarfs, neutron stars, or black holes.

Magnitudes, Spectral Types, and Distances

THE GREEK ASTRONOMER Hipparchus invented the stellar-magnitude system over 2,000 years ago. He called the brightest stars in the sky 1st magnitude and the faintest ones visible to the naked eye 6th magnitude. Intermediate stars were labeled 2nd through 5th magnitude.

By modern definition a star of magnitude 1.0 is precisely 100 times brighter than one of 6.0, so that each magnitude step represents a ratio in brightness of 2.512 (the fifth root of 100). For example, a 2nd-magnitude star outshines one of 5th magnitude by a factor of 2.512 x 2.512 x 2.512, or about 16. The brightest celestial objects have negative magnitudes, such as the Sun at -26 or Sirius at -1.5.

Astronomers also categorize stars according to the prominence of various absorption-line patterns in their spectra. The categories, called spectral types, are used to infer important characteristics of stars, such as surface temperature and luminosity. The primary spectral types are represented by the letters O, B, A, F, G, K, and M, arranged in order of decreasing surface temperature (and with some exceptions in order of decreasing mass).

Another characteristic of any star is its position along its own evolutionary track. The Sun, for example, is a main-sequence star of spectral type G and should remain so for a few more billion years. It and other venerable main-sequence stars then segue to briefer-lived giants and supergiants, which ultimately end their lives as dense white dwarfs, neutron stars, or black holes.

Cosmic distances are commonly expressed in either of two units: light-years or parsecs. A light-year is the distance light travels through empty space in one year -- approximately 9 trillion kilometers. The term parsec is a contraction of "parallax of one second." At a distance of one parsec, 3.26 light-years, the radius of the Earth's orbit would subtend an angle of 1 arc second, or 1/3600, of a degree. A star's absolute magnitude is defined as the magnitude it would have if it were located 10 parsecs (32.6 light-years) away.

Alan Hirshfeld (University of Massachusetts at Dartmouth) is coeditor of Sky Catalogue 2000.0, Vols. 1 and 2, and wrote the computer programs that were used to generate the ESSCO series of star charts.

Metallicity and absolute magnitude calibrations for F-G type main-sequence stars in the Gaia era

In this study, photometric metallicity and absolute magnitude calibrations were derived using F-G spectral type main-sequence stars in the Solar neighbourhood with precise spectroscopic, photometric and Gaia astrometric data for UBV photometry. The sample consists of 504 main-sequence stars covering the temperature, surface gravity and colour index intervals (5300< T_ < 7300) K, (log g > 4) (cgs) and (0.3<(B-V)_<0><0.8) mag, respectively. Stars with relative trigonometric parallax errors (sigma _/pi leq 0.01) were preferred from Gaia DR2 data for the estimation of their (M_) absolute magnitudes. In order to obtain calibrations, ((U-B)_<0>) and ((B-V)_<0>) colour indices of stars were preferred and a multi-variable second order equation was used. Calibrations are valid for main-sequence stars in the metallicity and absolute magnitude ranges (-2<<[mbox/mbox]><0.5) dex and (2.5< M_<6) mag, respectively. The mean value and standard deviation of the differences between original and estimated values for the metal abundance and absolute magnitude are (langle Delta <[mbox/mbox]> angle =0.00pm 0.11) dex and (langle Delta M_ angle =0.00pm 0.22) mag, respectively. In this work, it has been shown that more precise iron abundance and absolute magnitude values were obtained with the new calibrations, compared to previous calibrations in the literature.

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Table of absolute magnitudes of stars by spectral type and photometric band - Astronomy

We present results of our ongoing photometric survey of Local Group galaxies, using a four filter technique based on the method of Wing (cite) to identify and characterise the late-type stellar content. Two narrow band filters centred on spectral features of TiO and CN allow us to distinguish between AGB stars of different chemistries [M-type (O-rich) and C-type (C-rich)]. The major parts of two dwarf galaxies of the M 31 subgroup - NGC 185 and NGC 147 - were observed. From photometry in V and i we estimate the tip of the RGB, and derive distance moduli respectively. With additional photometric data in the narrow band filters TiO and CN we identify 154 new AGB carbon stars in NGC 185 and 146 in NGC 147. C/M ratios are derived, as well as mean absolute magnitudes <M_i>, bolometric magnitudes M_bol, luminosity functions, and the spatial/radial distributions of the C stars in both galaxies.

Based on observations made with the Nordic Optical Telescope operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.


We have selected from the Hipparcos catalogue 6262 unreddened and reddened OB stars as well as 430 OeBe stars, for which the spectral classifications and luminosity classes are known. The Hipparcos catalogue ( ESA 1997) itself gives the Sp/L estimates. The latter have been applied if not different by more than 0.5 spectral type from those found as mean values in the SIMBAD data base. This allowed us to segregate the selected targets into homogeneous samples of the same Sp/L classes. All stars inside one such sample should be of the same MV. One can believe that a possible scatter results from errors of Sp/L estimates, uncertain RV values and erroneous Hipparcos parallaxes.

The visual magnitudes V, colour indices (BV) and observed parallaxes π with their errors σπ, were taken from the Hipparcos catalogue ( ESA 1997). The intrinsic colour indices (BV)o were taken from Wegner (1993, 1995). The values of AV=RV× E(BV) for stars with E(BV) ≥ 0.05 were calculated using individual RV values calculated from the infrared (IR) photometric data when the latter were not available, the ‘canonical’ value of 3.1 was used. Some of the individual RV estimates were taken from Wegner (2002) others were calculated inside the present project by means of extrapolating the IR segments of the extinction curves. The latter have been derived from the published photometric IR measurements using the artificial standards of Wegner (1994) and Wegner (1995) in the form of the ratios of colour excesses kλ=E(λ−V)/E(BV). For the details of the procedure, see Wegner (2003).

The RIJHKLM magnitudes of OB- and Be-type stars of luminosity classes I–V having the Hipparcos parallaxes have been selected from objects catalogued by Gezari, Schmitz & Mead (1984), Gezari et al. (1993), Whittet & van Breda (1980) and The, Wesselius & Jansen (1986). We applied also the list of 199 standard stars for the European Southern Observatory (ESO) standard photometric systems at J, H, K, L and M ( Bouchet, Manfroid & Schider 1991 Blieck, Manfroid & Bouchet 1996). The Johnson VRI data are taken from the published catalogues by Johnson et al. (1966) and by Fernie (1983). The accuracy of these data is of the order of ±0.01. The red VRI measurements from Cousins (see The et al. 1986) were transformed to Johnson's revised photometric system – see Wegner (1993). The effective wavelengths of R and I bands are 0.71 and 0.97 μm, respectively. All JHKL(M) measurements were reduced to the Glass (1974),JHKL photometric system, which is tied to the Johnson system, by comparing with standard stars of Glass (1974) transmission curves for the filters are presented by Glass (1973) and the effective wavelengths are 1.25, 1.65, 2.2 and 3.5 μm. The system M is defined by the effective wavelength of 4.8 μm.

Currently, the largest IR photometric catalogue giving stellar magnitudes in the three IR bands J, H and Ks (the latter centred at 2.16 μm) is the Two-Micron All-Sky Survey (2MASS) – Skrutskie et al. (1997). Despite the difference between the central wavelength of Ks (2.16 μm) and the Johnson K band (2.22 μm), the Ks magnitudes do not differ greatly from those measured in the classical K band.

Fig. 1 demonstrates the comparison between the 2MASS and Johnson K-band magnitudes of 680 stars, while Fig. 2. demonstrates the comparison between the 2MASS (KsJ) and Johnson (KJ) colour indices of 603 stars. It is evident that data from both sources are very closely related, except for several stars which are very likely variables observed in different phases. Thus, the values of the total-to-selective extinction ratios, calculated using both systems, should be practically identical.

A comparison of Johnson K-band (2.22 μm) magnitudes with the 2MASS Ks-band (2.16 μm) ones, based on a sample of 680 stars.

17. Does a big telescope suffer more from light pollution than a small telescope?

Long answer: The simple-minded claim is that under conditions of bad light pollution, beyond some certain aperture, you gain very little or nothing over a smaller telescope. The claim also is that this crossover aperture becomes smaller under progressively worse conditions of light pollution.

The physics is pretty simple: "aperture always wins". It is a simple matter of signal-to-noise: in any scene a telescope does not magically brighten the sky background while not brightening the celestial target at the same time. Yet the claim being made by most folks is that the background is indeed enhanced.

Take this notion to an extreme of really bad light pollution: daytime! If the expections of this urban legend hold, then "theoretically" the naked eye should show things in the sky better than any telescope. But you can see no stars with the naked eye in daylight (let's talk noontime here, not 10 minutes before sunset), whereas you can see stars in even small a telescope without much difficulty. (Try it! First a bright star like Vega. Figure out how to get to it with your telescope, and go look at it some clear day at 11 in the morning. Is Vega easier to see in a 6x30mm finder or a 16-inch? Now go for Altair, then Deneb. and how much fainter?)