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 Stars (7/9)

G.01 What are all those different kinds of stars?
G.02 Are there any green stars?
G.03 What are the biggest and smallest stars?
G.04 What fraction of stars are in multiple systems?
G.05 Where can I get stellar data (especially distances)?
G.06 Which nearby stars might become supernovae?
G.07 What will happen on Earth if a nearby star explodes?
G.08 How are stars named?  Can I name/buy one?
    

G. What are all those different kinds of stars?

There are lots of different ways to classify stars.  The most important
single property of a star is its mass, but alas, stellar masses for most
stars are very hard to measure directly.  Instead stars are classified
by things that are easier to measure, even though they are less
fundamental.

There are three separate classification criteria commonly used: surface
temperature, surface gravity, and heavy element abundance.  The familiar
"spectral sequence" OBAFGKM is a _temperature_ sequence from the hottest
to the coolest stars.  Strictly speaking, the letters describe the
appearance of a star's spectrum, but because most stars are made out of
the same stuff, temperature has the biggest effect on the spectrum.  O
stars are hotter than 30000 K and show ionized helium in their spectra.
M stars are cooler than 4000 K and show molecular bands of TiO.  Others
are in between.

The ordinary spectral classes are divided into subclasses denoted by
numbers; thus G5 is a medium temperature star a little cooler than G2.
The Sun is generally considered a G2 star.  Not all the subclasses are
used, or at least generally accepted; G3 and G4 are absent, for example.

For historical reasons, hotter stars are said to have "earlier"
spectral types, and cool stars to have "later" spectral types.  An
"early A" star might mean somewhere between A0 and A3, while "late A"
might denote roughly A5--A8.  Or "early type stars" might mean
everything from O through A or F.  There's nothing terribly wrong with
this bit of jargon, but it can be confusing if you haven't seen it
before.

There are several spectral types that don't fit the scheme above.  One
reason is abnormal composition.  For example, some stars are cool enough
for molecules to form in their atmospheres.  The most stable molecule at
high temperatures is carbon monoxide.  In most stars, oxygen is more
abundant than carbon, and if the star is cool enough to form molecules,
virtually all the carbon combines with oxygen.  Leftover oxygen can form
molecules like titanium oxide and vanadium oxide (neither of which is
particularly abundant but both of which have prominent spectral bands at
visible wavelengths), but no carbon-containing molecules other than CO
can form.  (This is only approximately true.  Weak CN lines can often be
seen, for example, and all kinds of stuff will show up if you look hard
enough.  This article just gives a summary of the big picture.)  In a
minority of stars, however, the situation is reversed, and there is no
(or rather very little) oxygen to form molecules other than CO.  These
stars show lines of CH, CC, and CN, and they are called (not
surprisingly!) "carbon stars."  They are nowadays given spectral
classifications of C(x,y) where x is a temperature index and y is
related to heavy element abundance and surface gravity.  These stars
were formerly given "R" and "N" spectral types, and you occasionally
still see those used.  Roughly speaking, R stars have temperatures in
the same range as K stars and N stars in the same range as M, though the
correspondence is far from exact.

Another interesting group is the S stars.  In these, the atmospheric
carbon and oxygen abundances are nearly equal, and neither C nor O (or
at least not much of either) is available to form other molecules.
These stars show zirconium oxide and unusual metal lines such as barium.

There are other stars with unusual abundances: CH, CN, SC, and probably
more.  They are rare.  There are also stars that are peculiar in one way
or another and have spectral types followed by "p."  The "Ap" stars are
one popular class.  And finally, some stars have extended atmospheres
and show emission lines instead of the normal absorption lines.  These
get an "e" or "f."

The second major classification is by surface gravity, which is
proportional to the stellar mass divided by radius squared.  This is
useful because spectra can measure the gas pressure in the part of the
atmosphere where the spectral lines are formed; this pressure depends
closely on surface gravity.  But because surface gravity is related to
stellar radius, it is also related to the stellar luminosity.  Every
unit of stellar surface area emits an amount of radiation that mostly
depends on the temperature, and for a given temperature the total
luminosity thus depends on surface area which is proportional to radius
squared hence inversely proportional to surface gravity.  The upshot of
all this is that we have "dwarf" stars of relatively high surface
gravity, small radius, and low luminosity, and "giant" stars of low
surface gravity, large radius, and high luminosity _and their spectra
look different_.  In fact, many "luminosity classes" are identified in
spectra.  For normal stars, these are designated by Roman numerals and
lower case letters following the spectral class in the order: Ia+, Ia,
Iab, Ib, II, III, IV, V.  Class I stars are also called "supergiants,"
class II "bright giants," class III "giants," class IV "subgiants," and
class V either "dwarfs" or more commonly "main sequence stars."  By the
way, not all luminosity classes exist for every spectral type.

The importance of all this is that the luminosity classes are closely
related to the evolution of the stars.  Stars spend most of their
lives burning hydrogen in their cores.  For stars in this evolutionary
stage, the surface temperature and radius, hence spectral type and
luminosity class, are determined by stellar mass.  If we draw a
diagram of temperature or spectral type on one axis and luminosity
class on the other and plot each star as a point in the correct
position, we find nearly all stars fall very close to a single line;
this line is called the "main sequence."  (This kind of diagram is
called a "Hertzsprung-Russell" or "H-R" diagram after two astronomers
who were among the first to use it.)  Stars at the low mass end of the
main sequence are very cool (spectral type M) and are called "red
dwarfs."  This term is not very precise and may include K-type stars
as well.

As stars age, they expand and cool off; stars in this stage of evolution
account for the brighter luminosity classes mentioned above.  If they
happen to be cool, they are called "red giants" or perhaps "red
supergiants."  One interesting special case is for the hottest stars,
spectral classes O and early B.  Normally main sequence stars are hotter
if they have more mass, but not once they reach such high temperatures.
Instead more massive stars have larger radii but about the same surface
temperature, so an O I star is likely more massive but no more evolved
than an O V star.  These stars are called "blue giants" or "blue
supergiants."

After stars finally burn out their nuclear fuel, any of several thing
can happen, depending mainly on their initial mass and perhaps on
whether they had a nearby companion.  Some stars explode and are
entirely destroyed, but most leave remnants: white dwarfs, neutron
stars, or black holes.  

White dwarfs have high density because they are supported by "electron
degeneracy pressure."  This is a kind of pressure that arises from the
Fermi exclusion principle in nuclear physics.  A white dwarf has roughly
the radius of the Earth but a mass close to that of the Sun.  No white
dwarf can have a mass greater than the "Chandrasekhar limit," about 1.4
solar masses.  White dwarfs are given spectral type designations DA, DB,
and DC according to the spectral lines seen.  These lines represent the
composition of just a thin layer on the star's surface, so the spectral
classifications aren't terribly fundamental.

White dwarfs radiate solely by virtue of their stored heat.  As they
radiate, they cool off, eventually turning into "black dwarfs."  Because
their radii are so small, though, white dwarfs take billions of years to
cool.  There may be few or no black dwarfs in our galaxy simply there
has not been time for many white dwarfs to cool off.  Of course it's not
obvious how one would detect black dwarfs if they exist.

Neutron stars are even more compact; the mass of the Sun in a radius of
order only 10 km.  These stars are supported by "neutron degeneracy
pressure," in which Fermi exclusion acts on neutrons.  Neutron stars
have a maximum mass of around 2 solar masses, although the exact
theoretical value depends on properties of the neutron that are not
known terribly accurately.  Because the radius is so small, these stars
don't emit significant visible light from their surfaces.  They may emit
radio energy as pulsars.

Some properties of black holes are discussed elsewhere in the FAQ.

All types of "compact remnants," white dwarfs, neutron stars, and black
holes, may emit energy from an accretion disk around them if a nearby
companion is transferring mass to the compact remnant.  The emission
often comes out at X-ray and ultraviolet wavelengths.

The third classification is by composition and specifically by "heavy
element abundance."  In astronomy, "heavy elements" or "metals" refers
to all elements heavier than helium.  Since heavy elements are created
in stars, stars formed later in the life of the galaxy have more heavy
elements than found in older stars.

The term "subdwarf" or occasionally "luminosity class VI" refers to
stars of low metallicity.  Because they have so few metals, they look a
little hotter than they "ought" to be for their masses or equivalently
have lower luminosity than main sequence stars of the same color.
Physically, these stars are burning hydrogen in their cores and are
similar to main sequence stars except for the lower metallicities.
Since all these stars are old, they are of low luminosity.  Their higher
luminosity counterparts no doubt existed but have long since evolved
away, most of them presumably into some form of compact remnant.


The following material is adapted from Ken Croswell's book The Alchemy
of the Heavens (Doubleday/Anchor, 1995) and is reprinted here with
permission of the author.

The terms "Population I" and "Population II" originated with Baade,
who in 1943 divided stars into these two broad groups.  Today, we
know the Galaxy is considerably more complicated, and we recognize
four different stellar populations.  To make a long story short, the
modern populations are:

      THIN DISK      metal-rich, various ages
      THICK DISK     old and somewhat metal-poor
      STELLAR HALO   old and very metal-poor; home of the subdwarfs
      BULGE          old and metal-rich

To make a long story longer: as astronomers presently understand the
Milky Way, every star falls into one of these four different stellar
populations.  The brightest is the thin-disk population, to which the
Sun and 96 percent of its neighbors belong.  Sirius, Vega, Rigel,
Betelgeuse, and Alpha Centauri are all members.  Stars in the thin
disk come in a wide variety of ages, from newborn objects to stars
that are 10 billion years old.  As its name implies, the thin-disk
population clings to the Galactic plane, with a typical member lying
within a thousand light-years of it.  Kinematically, the stars revolve
around the Galaxy fast, having fairly circular orbits and small U, V,
W velocities.  (These are the intrinsic space velocities with respect
to the average of nearby stars.  Zero in all components means rotating
around the center of the Galaxy at something like 220 km/s but no
other motion.)  Thin-disk stars are also metal-rich, like the Sun.

The second stellar population in the Galaxy is called the thick disk.
It accounts for about 4 percent of all stars near the Sun.  Arcturus is
a likely member.  The thick disk is old and forms a more distended
system around the Galactic plane, with a typical star lying several
thousand light-years above or below it.  The stars have more elliptical
orbits, higher U, V, W velocities, and metallicities around 25 percent
of the Sun's.

The third stellar population is known as the halo.  Halo stars are old
and rare, accounting for only 0.1 to 0.2 percent of the stars near the
Sun.  Kapteyn's Star is the closest halo star to Earth.  These stars
make up a somewhat spherical system, so most members of the halo lie far
above or far below the Galactic plane.  Kinematically, halo stars as a
group show little if any net rotation around the Galaxy, and a typical
member therefore has a very negative V velocity.  (This is a reflection
of the Sun's motion around the Galactic center in the +V direction.)
The halo stars often have extremely elliptical orbits; some of them may
lie 100,000 light-years from the Galactic center at apogalacticon but
venture within a few thousand at perigalacticon.  Metallicities are even
lower than in the thick disk, usually between 1 and 10 percent of the
Sun's.  Subdwarfs are members of this population.

The fourth and final stellar population is the bulge, which lies at the
center of the Galaxy.  Other galaxies have bulges too; some can be seen
in edge-on spiral galaxies as the bump that extends above and below the
galaxy's plane at the center.  The Galactic bulge is old and metal-rich.
Most of its stars lie within a few thousand light-years of the Galactic
center, so few if any exist near the Sun.  Consequently, the bulge is
the least explored stellar population in the Milky Way.

References:

Ken Croswell, _The Alchemy of the Heavens_ (Doubleday/Anchor, 1995)
(See http://www.ccnet.com/~galaxy)

James B. Kaler, _Stars and their Spectra: an Introduction to the
    Spectral Sequence (Cambridge U. Press, 1989)

Most any introductory astronomy book.

G. Are there any green stars?

The color vision of our eyes is a pretty complicated matter.  The
colors we perceive depend not only of the wavelength mix the eye
receives at a perticular spot, but also on a number of other factors.
For instance the brightness of the light received, the brightness and
wavelength mix received simultaneously in other parts of the field
of view (sometimes visible as "contrast effects"), and also the
brightness/wavelength mix that the eye previously received (sometimes
visible as afterimages).

One isolated star, viewed by an eye not subjected to other strong
lights just before, and with very little other light sources in the
field of view, will virtually never look green.  But put the same
star (which we can assume to appear white when viewed in isolation)
close to another, reddish, star, and that same star may immediately
look greenish, due to contrast effects (the eye tries to make the
"average" color of the two stars appear white).

Also, stars generally have very weak colors.  The only exception is
perhaps those cool "carbon" stars with a very low temperature---they
often look quite red, but still not as red as a stoplight.  Very hot
stars have a faint bluish tinge, but it's always faint---"blue" stars
never get as intense in their colors as the reddest stars.  Once the
temperature of a star exceeds about 20,000 K, its temperature doesn't
really matter to the perceived color (assuming blackbody
radiation)---the star will appear to have the same blue-white color no
matter whether the temperature is 20,000, 100,000 or a million degrees K.

Old novae in the "nebular" phase often look green.  This is because
they are surrounded by a shell of gas that emits spectral lines of
doubly ionized oxygen (among other things).  Although these object
certainly look like green stars in a telescope---the gas shell cannot
usually be resolved---the color isn't coming from a stellar
photosphere.

G. What are the biggest and smallest stars?

[Table reflects most recent distances from Hipparcos.]
The most luminous star within 10 light-years is Sirius.
The most luminous star within 20 light-years is Sirius.
The most luminous star within 30 light-years is Vega.
The most luminous star within 40 light-years is Arcturus.
The most luminous star within 50 light-years is Arcturus.
The most luminous star within 60 light-years is Arcturus.
The most luminous star within 70 light-years is Aldebaran.
The most luminous star within 80 light-years is still Aldebaran.
The most luminous star within 100 light-years is still...Aldebaran.
The most luminous star within 1000 light-years is Rigel.  
  (Honorable mentions: Canopus, Hadar, gamma Velae, Antares, and
   Betelgeuse.) 
The most luminous star within 2000 light-years is Rigel.
The most luminous star in the whole Galaxy is *drum roll, please*
  .... Cygnus OB2 number 12, with an absolute magnitude around -10.
  (also known as VI Cygni No 12).

A table listing the nearest stars (within 12 light years) may be found
at http://www.ccnet.com/~galaxy/tab181.html.  The faintest star
within that distance is Giclas 51-15 with absolute visual magnitude
16.99 and spectral type M6.5.

Wielen et al. published the following as the local luminosity function
(total number of stars within 20 parsecs = 65 lightyears).  At the faint
end (abs. magnitude >12) this table is bit out of date and the numbers
are probably too high.  Everything from abs. magnitude 9 to 18 is
considered an M dwarf (shows TiO and other molecules) or a white dwarf.

abs. mag	Number
-1		1
0		4
1		14
2		24
3		43
4		78
5		108	Sun is here!
6		121
7		102
8		132
9		159
10		245
11		341
12		512
13		597
14		427
15		427
16		299
17		299
18		>16

G. What fraction of stars are in multiple systems?

According to the work of A. Duquennoy and M. Mayor, 57% of systems
have two or more stars.  They were working with a sample of F and G
stars, i.e., stars like the Sun.  It appears that for the coolest,
low-luminosity stars (the M-dwarfs) there are fewer binaries.  Fischer
and Marcy found that only 42% of M-dwarfs are binaries.  Neill Reid
and I have used HST images to find that for the coolest stars in the
Hyades cluster (absolute magnitude > 12, or mass < 0.3 solar masses)
only 30% are binaries.

[There's also the tongue-in-cheek answer that three out of every two
stars is in a binary.  TJWL]

References:
Gizis, J. & Reid, I. Neill  1995, "Low-Mass Binaries in the Hyades,"
     Astronomical Journal, v. 110, p. 1248

G. Where can I get stellar data (especially distances)?

The Astronomical Data Center maintains a large inventory of
astronomical catalogs, including star catalogs.  Access at
<URL:http://adc.gsfc.nasa.gov/adc.html>.  The HIPPARCOS catalog,
<URL:http://astro.estec.esa.nl/Hipparcos/>, represents a gigantic
improvement both in systematic accuracy and in precision over previous
catalogs, but it is limited to fairly bright stars (magnitude limit
around 11).  Keep in mind that all astronomical data have
uncertainties.  Distances can be especially problematic, and it is
vital to know what the uncertainties are.  Recent research on refining
astronomical data for the nearby stars can be found at the Research
Consortium on Nearby Stars (RECONS),
<URL:http://tarkus.pha.jhu.edu/%7Ethenry/RECONS.html>.

One large (3803 stars) compilation of nearby stars can be found at
<URL:ftp://adc.gsfc.nasa.gov/pub/adc/archives/catalogs/5/5070A/catalog.gz>.
An excerpt from the "ReadMe" file,
<URL:ftp://adc.gsfc.nasa.gov/pub/adc/archives/catalogs/5/5070A/ReadMe>
follows:

   Preliminary Version of the Third Catalogue of Nearby Stars
   GLIESE W., JAHREISS H.
       <Astron. Rechen-Institut, Heidelberg (1991)>
  
   Description:
        The present version of the CNS3 contains all known stars within
    25 parsecs of the Sun. It depends mainly on a preliminary version
    (Spring 1989) of the new General Catalogue of Trigonometric
    Parallaxes (YPC) prepared by Dr. William F. van Altena (Yale
    University).
        The catalogue contains every star with trigonometric parallax
    greater than or equal to 0.0390 arcsec, even though it may be
    evident from photometry or for other reasons that the star has a
    larger distance. For red dwarf stars, new color-magnitude
    calibrations for broad-band colors were carried out and applied.
    For white dwarfs, the recipes of McCook and Sion in ApJS, 65, 603
    (1987) were applied. Stroemgren photometry was used (not yet
    systematically) for early-type stars and for late dwarfs, the
    latter supplied by E. H. Olsen from Copenhagen Observatory
    (private communication).
        Contrary to the CNS2 (Gliese 1969) trigonometric parallaxes
    and photometric or spectroscopic parallaxes were not combined.
    The resulting parallax in the present version is always the
    trigonometric parallax---if the relative error of the
    trigonometric parallax is smaller than 14 percent. The resulting
    parallax is the photometric or spectroscopic parallax only if no
    trigonometric parallax is available or if the standard error of
    the trigonometric parallax is considerably larger.

The Internet Stellar Database <URL:http://www.stellar-database.com/>
attempts to synthesize information about the nearest stars from
various catalogs.

If you'd like to use the astronomical data, say, to calculate
distances between stars, a useful reference is
<URL:http://www.clark.net/pub/nyrath/starmap.html>.

G. Which nearby stars might become supernovae?

Obvious candidates are alpha Orionis (Betelgeuse, M1-2 Ia-Iab), alpha
Scorpii (Antares, M1.5 Iab-Ib), and alpha Herculis (Rasalgethi, M5
Ib-II).  Spectral types come from the Bright Star Catalog.  Although
trigonometric parallaxes are listed in the catalog, they will not be
very accurate for stars this far away.  I derive photometric distances
of around 400 light years for the first two and 600 light years for
alpha Her.  (Anybody have better sources, or do we have to wait for
Hipparcos?)  Anybody want to suggest more?

G. What will happen on Earth if a nearby star explodes?

A nice article by Michael Richmond <mwrsps@rit.edu> may be found at
<URL:http://a188-L009.rit.edu/richmond/answers/snrisks.txt>.  His
conclusion is:

"I suspect that a type II explosion must be within a few parsecs of
the Earth, certainly less than 10 pc, to pose a danger to life on
Earth.  I suspect that a type Ia explosion, due to the larger amount
of high-energy radiation, could be several times farther away.  My
guess is that the X-ray and gamma-ray radiation are the most important
at large distances."

G. How are stars named?  Can I name/buy one?

Official names for celestial objects are assigned by the International
Astronomical Union.  Procedures vary depending on the type of object.
Often there is a system for assigning temporary designations as soon as
possible after an object is discovered and later on a permanent name.
See E.05 of this FAQ.

Some commercial companies purport to allow you to name a star.
Typically they send you a nice certificate and a piece of a star atlas
showing "your" star.  The following statement on star naming was
approved by the IPS Council June 30, 1988.

    The International Planetarium Society's Guidelines on Star Naming

    SELLING STAR NAMES

    The star names recognized and used by scientists are those that have
    been published by astronomers at credible scientific institutions.  The
    International Astronomical Union, the worldwide federation of
    astronomical societies, accepts and uses _only_ those names.  Such names
    are never sold.

    Private groups in business to make money may claim to "name a star for
    you or a loved one, providing the perfect gift for many occasions."  One
    organization offers to register that name in a Geneva, Switzerland,
    vault and to place that name in their beautiful copyrighted catalog.
    However official-sounding this procedure may seem, the name and the
    catalog are not recognized or used by any scientific institution.
    Further, the official-looking star charts that commonly accompany a
    "purchased star name" are the Becvar charts excerpted from the _Atlas
    Coeli 1950.0_.  [Other star atlases such as _Atlas Borealis_ may be used
    instead.]  While these are legitimate charts, published by Sky
    Publishing Corporation, they have been modified by the private "star
    name" business unofficially.  Unfortunately, there are instances of news
    media describing the purchase of a star name, apparently not realizing
    that they are promoting a money-making business only and not science.
    Advertisements and media promotion both seem to increase during holiday
    periods.

    Planetariums and museums occasionally "sell" stars as a way to raise
    funds for their non-profit institutions.  Normally these institutions
    are extremely careful to explain that they are not officially naming
    stars and that the "naming" done for a donation is for amusement only.