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URL: http://sciastro.astronomy.net/
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part7
Subject: Introduction
sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.
However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.
This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from <URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/>,
and it is on the World Wide Web at
<URL:http://sciastro.astronomy.net/> and
<URL:http://www.faqs.org/faqs/astronomy/faq/>. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
<URL:http://sciastro.astronomy.net/mirrors.html>. (As a general note,
many other FAQs are also available from
<URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/>.)
Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio (jlazio@patriot.net).
Subject: G.00 Stars
[Dates in brackets are last edit.]
G.01 What are all those different kinds of stars?
01.1 General overview and main sequence stars [1996-01-02]
01.2 White dwarfs [2003-04-27]
01.3 Neutron stars [2003-04-27]
01.4 Black holes [2003-04-27]
G.02 Are there any green stars? [1995-12-28]
G.03 What are the biggest and smallest stars? [1998-06-03]
G.04 What fraction of stars are in multiple systems? [1995-06-27]
G.05 Where can I get stellar data (especially distances)?
[2003-05-08]
G.06 Which nearby stars might become supernovae? [1995-12-29]
G.07 What will happen on Earth if a nearby star
explodes? [2000-02-20]
G.08 How are stars named? Can I name/buy one? [1995-12-28]
G.09 Do other stars have planets?
G.10 What happens to the planets when a planetary nebula is
formed? Do they get flung out of the solar system?
[2002-05-04]
G.11 How far away is the farthest star? [1999-01-01]
G.12 Do star maps (or galaxy maps) correct for the motions of the
stars? [2003-10-18]
For an overall sense of scale when talking about stars, see the Atlas
of the Universe, <URL:http://anzwers.org/free/universe/>.
Subject: G.01.1 What are all those different kinds of stars?
General overview and main sequence stars
Author: Steve Willner <swillner@cfa.harvard.edu>,
Ken Croswell
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.
Subject: G.01.2 What are all those different kinds of stars?
White Dwarfs How are white dwarfs classified? What
do the spectral types DA, DC, etc. mean?
Author: Mike Dworetsky <mmd@star.ucl.ac.uk>
The MK classification system for the vast majority of stars works
remarkably well for one simple reason: most stars in the Galactic disk
have surface chemical compositions that are broadly similar to each
other and the Sun's composition. They are 71 percent hydrogen, 27
percent helium, and 2 percent "metals" (Li--U). Thus, the differences
in spectral line strengths that give rise to the familiar OBAFGKM
sequence are due to their vast range in surface temperature. The MK
system can also classify by absolute stellar brightness: the more
subtle differences in the strengths of certain lines at various
classes, caused by the different surface gravities of main sequence
and supergiant stars, for example, are spoken of as luminosity
criteria, because they depend on the size of the star (big stars
radiate much more energy than small stars, but their atmospheres are
much less dense).
The name "white dwarf" for these stars comes from the observed colors
of the first examples discovered. They caught the attention of
astronomers because they had large masses comparable to the Sun but
were hot and very faint, hence extremely small and dense. We now know
that there are a few "white dwarfs" that are actually cool enough to
look red.
The first spectroscopic investigators of white dwarfs tried to fit
them into a descriptive system parallel to the MK classes, using the
letter D plus a suffix OBAFGK or M, with the letter C added for the
cases when the spectra showed no lines (continuous spectra). The
types were sometimes supplemented by cryptic abbreviations like "wk"
for weak; "s" for sharp-lined, and so on.
When the spectra of white dwarfs were investigated in more detail, it
proved impossible to categorize them neatly for one increasingly
apparent reason: the surface compositions of white dwarfs varied
enormously from star to star. Astronomers needed a new scheme to
reflect this. In the revised classification scheme, white dwarf
designations still start with the letter D to indicate dwarf or
"degenerate" stellar structure. A second letter indicates the main
spectral features visible: C for a continuous spectrum with no lines,
A for Balmer lines of hydrogen with nothing else, B for He I (neutral
helium) lines, O for He II with or without He I or H, Z for metal
lines (often, strong Ca II lines are seen), and Q for atomic or
molecular lines of carbon (C is used for continuous spectra; K for
Karbon could be confused with the K stars; so try to think of
Qarbon!).
These basic types can sometimes mix; DAQ stars are known, for example.
A further suffix can be added: P for magnetic stars with polarized
light, H for magnetic stars that do not have polarized light, and V
for variable. (There is a class of short-period pulsating white
dwarfs, called ZZ Ceti stars.) There may be emission lines (E). And if
an unusual star still defies classification, it goes into type X.
Finally, a number is appended that classifies the star according to
its effective temperature based on formulae which use the observed
colors: the number is 50400/T rounded to the nearest 0.5, i.e., the
value of 50400/temperature, rounded. If white dwarfs with T much
higher than 50,000 K are ever found, they could have the number 0 or
0.5 appended. The coolest designation is open-ended; there is a star
classified as DC13, for example, which is actually rather red, not
white.
Thus a hot white dwarf with neutral helium lines might be described as
DB2.5; a cooler white dwarf with hydrogen lines, a magnetic field,
polarized light, and a trace of carbon might be DAQP6.
This system can provide good summary descriptions of the vast majority
of white dwarf stars. However, it is a definite move away from the
original concept of spectral classification, because it requires
photometry and polarimetry as well as visual inspection of a spectrum,
in order to make an assignment. But most leading experts on the
subject have agreed it was necessary to move in this direction.
Some references:
Sion, E.M., et al. 1983. Astrophys. J., 269, 253--257
Greenstein, J. 1986. Astrophys. J., 304, 334--355
Wesemael, F. et al. 1993. Publ. Astr. Soc. Pacif., 105, 761--778
(Electronic versions of journal articles can be found on the WWW in
postscript and pdf formats via the Astronomical Data Center and its
mirrors in Europe, South America and Asia. Start from
http://adswww.harvard.edu/ and locate the best mirror for your location.)
Subject: G.01.3 What are all those different kinds of stars?
Neutron Stars
Author: Joseph Lazio <jlazio@patriot.net>
Neutron stars are the remnants of massive stars. Sufficiently massive
stars form iron in their cores during the process of nuclear fusion.
Iron proves problematic for the star, though, as iron is among the
most tightly bound nuclei. Nuclear fusion involving iron actually
requires energy to occur, as opposed to nuclear fusion involving
lighter nuclei in which the fusion produces energy. At some point so
much iron accumulates in the core of the star that its nuclear
reactions do not produce enough heat (i.e., pressure) to
counter-balance the force of gravity due to the star's mass. The star
implodes in a supernova, blowing off much of its outer layers and
leaving an NS as a remnant. A star has to be (roughly) at least 8
times as massive as the Sun and not more than 25--50 times as massive
as the Sun to form an NS. (The upper limit is quite uncertain.)
(There has been a second mechanism postulated as a way to form neutron
stars. There is an upper limit to the mass of a white dwarf, 1.4
times the mass of the Sun, called the Chandrasekhar limit after
Subrahmanyan Chandrasekhar who first described it. Above this mass
the force of gravity overwhelms the internal pressure provided by the
electrons in the WD. If one had a WD that was quite close to the
Chandrasekhar limit and a small amount of mass was added to it, it
might collapse to form an NS. This process is called
"accretion-induced collapse." It is not clear if this mechanism
actually occurs, however.)
NSs can be divided into three broad classes, rotation-powered pulsars,
accretion-powered pulsars, and magnetars.
Rotation-powered pulsars are the kind of pulsars most commonly
described and were the first kind of NSs observed. These NSs have
powerful magnetic fields and rotate. If the axes of the star's
rotation and magnetic field are not aligned, this rotating magnetic
field produces an electric field; in the case of NSs, the electric
fields are strong enough to rip particles from the crust of the NS and
accelerate them. The accelerated particles radiate. The magnetic
field collimates the accelerated particles, so the radiation from the
NS is emitted in two narrow beams. If one of the beams sweeps across
the Earth, we observe a pulsating source---a pulsar. Most of the
known rotation-powered pulsars are observed in the radio (though the
radio emission itself is a usually just a tiny fraction of the
rotation energy of the NS).
Rotation-powered pulsars are often further sub-divided into
strong-field and recycled pulsars. Strong-field pulsars have magnetic
fields of about 10^8 Tesla and observed pulse periods about 1 second.
As the pulsars lose energy, their rates of spin slow down. At some
point, the rotating magnetic field is no longer produces electric
fields strong enough to power the pulsar mechanism, and the pulsar
"shuts off." However, if the NS is a member of a binary system, its
companion star, during the course of its own evolution, increase in
size and start spilling matter onto the NS. As the matter spills onto
the NS, if it hits the NS in the same direction that the NS is
rotating, it can increase the rate at which the NS is spinning or
"spin-up" the NS. If this spin-up process goes on for a long enough
period of time, the NS may "turn on" as a pulsar again. The process
of matter spilling onto the pulsar tends to suppress the magnetic
field, though. With a weaker magnetic field, the spun-up pulsar
doesn't spin down as fast as before. So, these recycled pulsars are
distinguished by having very slow spin-down rates. As it turns out,
they also tend to have very short pulse periods, typically less than
0.1 seconds, with the shortest being 0.00156 seconds.
Accretion-powered pulsars are NSs onto which matter is spilling. The
gravity well around an NS is so deep, it is actually fairly difficult
for matter to fall onto the NS. Only matter that starts at rest with
respect to the NS can fall directly onto its surface. If the matter
has any velocity relative to the NS, as it falls toward the NS, it
will begin to orbit the NS. (This is the same principle that causes a
skater to spin faster as she pulls in her arms.) If a lot of matter
is falling toward the NS, a disk is formed around the NS. Due to
"frictional" forces within the disk, matter slowly works its way
closer to the NS until finally falling a short distance onto its
surface. The process of the matter falling onto the NS' surface is
known as accretion, so the disk is called an accretion disk. The
gravitational potential of a NS is so deep that a lot of energy can be
released as the matter forms an accretion disk and spills onto the NS'
surface. Consequently, accretion-powered NSs are typically seen as
X-ray sources.
Magnetars are a recently recognized class of NSs. It is thought that
rotation-powered pulsars only work if the magnetic field is not too
strong. If the magnetic field is too strong, it can effectively shut
down the process by which the particles are produced. The critical
field seems to be about 10^10 Tesla. Only a few examples of magnetars
are known. These generally appear as fairly constant X-ray sources,
though magnetars have also been suggested to be responsible for
sources known as soft-gamma ray repeaters.
Subject: G.01.4 What are all those different kinds of stars?
Black Holes
Author: Joseph Lazio <jlazio@patriot.net>
A black hole is any object for which its entire mass M is contained
within a radius
2GM
R = ---
c^2
where G is the universal gravitation constant (G = 6.67 x 10^-11
m^3/kg/s^2) and c is the speed of light. An object this compact will
have an escape velocity larger than light so nothing can escape from
it. (For an object with the mass of the Sun, this radius is 3 km.)
BHs can be divided into (at least) three classes: primordial,
stellar-mass, and supermassive. Primordial BHs, if they exist, were
formed during the initial instants of the Big Bang. The initial
Universe was not perfectly smooth, there were slight fluctuations in
its density. Some of these density fluctuations could have satisfied
the above criterion. In that case, BHs would have formed. These
primordial BHs could have a range of masses, anywhere from milligrams
to 10^17 times the mass of the Sun. Currently, however, there is
little evidence to suggest that any primordial BHs did form. (In
fact, the available evidence suggests that no primordial BHs formed.)
Stellar-mass BHs are those with masses of roughly 10 times the mass of
the Sun. These are formed from processes involving one or a few
stars. For instance, a star more massive than 50 solar masses will
also start to form a iron core. Unlike a less massive star that forms
an NS during the supernova, though, the iron core becomes so massive
that it collapses to form a BH. Another possibility for the formation
of a stellar-mass BH is the collision of two stars, such as might
happen in the center of dense globular cluster of stars or two
orbiting NSs. A Stellar-mass BH is identified typically when it is
orbited by a lower mass star. Some of the material from the companion
star may be stripped away from it and fall into the BH, producing
copious amounts of radio and X-ray emission in the process.
Supermassive BHs are those with masses exceeding roughly 1 million
times that of the Sun. These are found at the center of galaxies. It
is not clear how these form, but it probably involves the accumulation
of many smaller mass BHs, NSs, and perhaps interstellar gas during the
formation of galaxies. Recent work shows a correlation between the
mass of the central parts of galaxies and the mass of the central BH.
This has led to some speculation at to whether the central BHs form
first and "seed" the formation of galaxies or if there is a symbotic
process in which the central BH and the galaxy are created
simultaneously.
There have also been suggestions of "intermediate mass" BHs. These
would be objects whose mass is roughly 100--1000 times that of the
Sun. The suggestions that such intermediate mass BHs might exist
arise from X-ray observations of other galaxies showing strong X-ray
sources not associated with the centers of the galaxies. Certain
assumptions must be used in relating the X-ray brightness of the
objects to their mass, though, so whether such intermediate mass BHs
actually exist is still somewhat controversial.
Subject: G.02 Are there any green stars?
Author: Paul Schlyter <pausch@electra.saaf.se>,
Steve Willner <swillner@cfa.harvard.edu>
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.
Subject: G.03 What are the biggest and smallest stars?
Author: Ken Croswell,
John E. Gizis <jeg@pistol.caltech.edu>
[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
Subject: G.04 What fraction of stars are in multiple systems?
Author: John E. Gizis <jeg@pistol.caltech.edu>
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
Subject: G.05 Where can I get stellar data (especially distances)?
Author: Steve Willner <swillner@cfa.harvard.edu>,
John Ladasky Jr. <ladasky@my-deja.com>
Two key sites for stellar data are the Astronomical Data Center,
<URL:http://adc.gsfc.nasa.gov/adc.html>, and the CDS Service for
Astronomical Catalogues,
<URL:http://cdsweb.u-strasbg.fr/cats/Cats.htx>, both of which maintain
large inventories of astronomical catalogs, including star catalogs.
Another important site is SIMBAD,
<URL:http://simbad.u-strasbg.fr/sim-fid.pl>, as one can use it to find
alternate names for a star. (For instance, what is another name for
the variable star V* V645 Cen?)
Distances in astronomy are always problematic, and it is important to
keep in mind that all astronomical data have uncertainties. It is
vital to understand what the uncertainties are. Moreover, if one is
interested in constructing 3-D star maps, one should recognize that
astronomical data are not stored in XYZ coordinates. Science-fiction
writers and people who want to make 3-D maps of local space like them,
but astronomers don't use them. Astronomers need polar coordinates
(right ascension and declination) centered on Earth, so that they know
where to point their telescopes.
Three useful sites for distance data are
* One large (3803 stars) compilation of nearby stars is the
"Preliminary Version of the Third Catalogue of Nearby Stars," which
aims to catalog all known stars within 25 pc (~ 75 light years) of
the Sun. The "ReadMe" file for the catalog is at
<URL:ftp://adc.gsfc.nasa.gov/pub/adc/archives/catalogs/5/5070A/ReadMe>.
* The Internet Stellar Database
<URL:http://www.stellar-database.com/> attempts to synthesize
information about the nearest stars from various catalogs.
* 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>.
(Note that these sites tend to focus on *nearby* stars---that's a
result of the difficulty of obtaining accurate distances for distant
stars.)
If an object is close enough to Earth to have a significant parallax
(an apparent yearly wobble in the sky that results from the change in
observing position of the Earth), then its distance can be determined
by triangulation. With two angles and a distance, you can compute
Cartesian coordinates if you want them. If you'd like to use the
astronomical data, say, to calculate distances between stars, a useful
reference is <URL:http://www.projectrho.com/starmap.html>. (Note that
many astronomical catalogs do not include parallax measurements.)
The best parallax data collected thus far comes from the European
astrometry satellite, Hipparcos,
<URL:http://astro.estec.esa.nl/Hipparcos/>, and it 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).
Both the CDS and the Hipparcos Web site offer online tools for
searching the Hipparcos catalog as well as the full catalog itself.
Two important aspects of the Hipparcos catalog are how distances are
described and the names given to stars. First, distances are
described by the parallax in milliarcseconds. The distance d in
parsecs is given by d = 1000/p for a parallax p in milliarcseconds.
To obtain a distance in light years, multiply by 3.26. Thus, a star
with a parallax of 100 milliarcseconds is at a distance of 10 pc (~ 30
light years).
Second, all of the Hipparcos catalog "names" will be unfamiliar to
you, as they are just numbers. One can use SIMBAD to convert from
Hipparcos catalog names to more familiar names.
Subject: G.06 Which nearby stars might become supernovae?
Author: Steve Willner <swillner@cfa.harvard.edu>
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?
Subject: G.07 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."
Subject: G.08 How are stars named? Can I name/buy one?
Author: Kevin D. Conod <kdconod@delphi.com>
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.
OFFICIAL STAR-NAMING PROCEDURES
Bright stars from first to third magnitude have proper names that have
been in use for hundreds of years. Most of these names are Arabic.
Examples are Betelgeuse, the bright orange star in the constellation
Orion, and Dubhe, the second-magnitude star at the edge of the Big
Dipper's cup (Ursa Major). A few proper star names are not Arabic. One
is Polaris, the second-magnitude star at the end of the handle of the
Little Dipper (Ursa Minor). Polaris also carries the popular name, the
North Star.
A second system for naming bright stars was introduced in 1603 by
J. Bayer of Bavaria. In his constellation atlas, Bayer assigned
successive letters of the Greek alphabet to the brighter stars of each
constellation. Each Bayer designation is the Greek letter with the
genitive form of the constellation name. Thus Polaris is Alpha Ursae
Minoris. Occasionally Bayer switched brightness order for serial order
in assigning Greek letters. An example of this is Dubhe as Alpha Ursae
Majoris, with each star along the Big Dipper from the cup to handle
having the next Greek letter.
Faint stars are designated in different ways in catalogs prepared and
used by astronomers. One is the _Bonner Durchmusterung_, compiled at
Bonn Observatory starting in 1837. A third of a million stars to a
faintness of ninth magnitude are listed by "BD numbers." The
_Smithsonian Astrophysical Observatory (SAO) Catalog_, _The Yale Star
Catalog_, and _The Henry Draper Catalog_ published by Harvard College
Observatory all are widely used by astronomers. The Supernova of 1987
(Supernova 1987A), one of the major astronomical events of this century,
was identified with the star named SK -69 202 in the very specialized
catalog, the _Deep Objective Prism Survey of the Large Magellanic
Cloud_, published by the Warner and Swasey Observatory.
These procedures and catalogs accepted by the International Astronomical
Union are the only means by which stars receive long-lasting names. Be
aware that no one can buy immortality for anyone in the form of a star
name.
Subject: Do other stars have planets?
Author: needed
Yes!
This is an active area of research, and since 1992 astronomers have
found planets around two pulsars (PSR 1257+12 and 0329+54) and about a
half-dozen main-sequence stars.
See
<URL:http://cannon.sfsu.edu/~gmarcy/planetsearch/planetsearch.html>,
<URL:http://www.obspm.fr/planets>,
<URL:http://techinfo.jpl.nasa.gov/WWW/ExNPS/HomePage.html>, and
<URL:http://ast.star.rl.ac.uk/darwin/> for more information.
Subject: G.10 What happens to the planets when a planetary nebula is
formed? Do they get flung out of the solar system?
Author: Joseph Lazio <jlazio@patriot.net>
A couple of possibilities exist. Prior to forming a planetary nebula,
a low-mass star (i.e., one with a mass similar to that of the Sun)
forms a red giant. Planets close to the star are engulfed in the
expanding star, spiral inside it, and are destroyed. In our own solar
system, Mercury and Venus are doomed.
As the star expands to form a red giant, it also starts losing mass.
All stars lose mass. For instance, the Sun is losing mass. However,
at the rate at which the Sun is currently losing mass, it would take
over 1 trillion years (i.e., 100 times longer than the age of the
Universe) for the Sun to disappear. When a star enters the red giant
phase, the rate at which it loses mass can accelerate. The mass of a
star determines how far a planet orbits from it. Thus, as the Sun
loses mass, the orbits of the other planets will expand. The orbit of
Mars will almost certainly expand faster than the Sun does, thus Mars
will probably not suffer the same fate as Mercury and Venus. It is
currently an open question as to whether the Earth will survive or be
engulfed.
The orbits of planets farther out (Jupiter, Saturn, Uranus, Neptune,
and Pluto) will also expand. However, they will not expand by much
(less than double in size), so they will remain in orbit about the Sun
forever, even after it has collapsed to form a white dwarf.
(Any planets around a high-mass star would be less lucky. A high-mass
star loses a large fraction of its mass quickly in a massive explosion
known as a supernova. So much mass is lost that the planets are no
longer bound to the star, and they go flying off into space.)
As for the material in the planetary nebula, it will have little
impact on the planets themselves. The outer layers of a red giant are
extremely tenuous; by terrestrial standards they are a fairly decent
vacuum!
Subject: G.11 How far away is the farthest star?
Author: Joseph Lazio <jlazio@patriot.net>
This question can have a few answers.
1. The Milky Way galaxy is about 120,000 light years in diameter.
We're about 25,000 light years from the center. Thus, the most
distant stars that are still in Milky Way galaxy are about 95,000
light years away, on the opposite side of the center from us. Because
of absorption by interstellar gas and dust, though, we cannot see any
of these stars.
2. The most distant object known has a redshift of just over 5. That
means that the light from this object started its journey toward us
when the Universe was only 30% of its current age. The exact age of
the Universe is not known, but is probably roughly 12 billion years.
Thus, the light from this object left it when the Universe was a few
billion years old. Its distance is roughly 25 billion light years.
3. Existing observations suggest that the Universe may be infinite
in spatial extent. If so, then the farthest star would actually
be infinitely far away!
Subject: G.12 Do star maps (or galaxy maps) correct for the motions of the
stars?
Author: Joseph Lazio <jlazio@patriot.net>
In general, no.
The reason is that stellar distances are so large. Over human time
spans, the typical velocity of a star is so low that its distance does
not change appreciably.
Let's consider a star with a velocity of 10 km/s, typical of most
stars. In 1000 yrs, this star moves about 300 billion kilometers (or
3E11 km). Suppose the star is 100 light years (about 1E15 km or 1
quadrillion kilometers) distant. Thus, in 1000 yrs, the star moves
about 0.03% of its distance from the Sun. This is such a small
change, it's not worth worrying about it.
The situation is even more extreme in the case of galaxies. Typical
galaxy velocities might be hundreds to thousands of kilometers per
second. However, their distances are measured in the millions to
billions of light years.
Subject: Copyright
This document, as a collection, is Copyright 1995--2003 by T. Joseph
W. Lazio (jlazio@patriot.net). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.
This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
|
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP