Frequently Asked Questions Regarding Galactic Halo Dark Matter

What is dark matter?
I refer the reader to an excellent review of this problem by Prof. Joseph Silk:

What is a white dwarf?
94% of stars end their lives as white dwarfs.  In the last stage of stellar evolution, before turning into a white dwarf, stars begin to shed a large fraction of their mass into the surrounding interstellar space.  During this time the star is consuming the last of the fuel used in nuclear fusion, the process which  makes stars shine.  Once this stage ends only the dense core of the star remains.  This core is approximately the size of the Earth but contains about half the mass of the Sun.  White dwarfs, because they have no more nuclear fuel and no capacity to fuse elements and therefore generate energy, cool as they age.  They may start out at temperatures near 100,000 Kelvin and after 10 to 12 billion years reach temperatures near 4,000 Kelvin.  For reference the Sun is approximately 6,000 Kelvin, although it is not cooling off the way white dwarfs do.

Why are they called white dwarfs?
They actually are white, although as they cool they become redder and redder.  However, recently astrophysicists such as Brad Hansen and Didier Saumon have shown that once they cool below 4,000 Kelvin they begin to turn bluer.  This effect has been confirmed with observations.  The term "dwarf" refers to what astronomers call the "luminosity class" of an object and is meant to indicate that in the range of luminosities that stars posess dwarfs rank relatively low.  In this sense, the Sun is also a dwarf star.

What is the ultimate fate of white dwarfs?
Our current understanding of white dwarfs is that they simply continue to cool for eternity, with no additional changes in their structure.

What is the chemical composition of white dwarfs?  What are they made of?
Because white dwarfs are the remnant cores of normal stars, they are primarily made of the "waste" products of the nuclear fusion reactions that made them shine before they turned into white dwarfs.  These "waste" products are primarily carbon and oxygen, with traces of other elements.  The outer part of a white dwarf contains helium and hydrogen.  It is the tremendous gravitational force associated with these dense stars that stratifies the material within them, with the heaviest elements residing at the deepest depths in the star.  The atmospheres of white dwarfs are only about ten meters thick.

Why can you not see the white dwarfs responsible for the microlensing effect?
It may be possible to observe directly one of the objects responsible for the microlensing events, but they are likely to be extremely distant from the Earth and therefore far too faint to be detected.

Why should we be able to see any dark matter at all?
Some dark matter, such as dim stars, is extremely faint but does emit a minute amount of light.  Other forms of dark matter may emit no light at all.

What is the overall structure of the Galaxy?
The Milky Way is a spiral galaxy which consists of a relatively thin disk of stars arranged in a spiral pattern.  At the center of the pattern is a roughly spherical but elongated bulge.  Completely enveloping this disk is a vast spherical component called the halo.  It is in this halo that most of the dark matter in the Galaxy resides.  The disk rotates and at the distance of the Sun from the center of the Galaxy (roughly halfway along a radius of the disk) it is spinning at approximately 220 kilometers per second.  The halo does not have an overall rotation.  Instead, the orbits of stars in the halo about the center of the Galaxy are randomly distributed and usually highly elliptical.  These orbits dictate that the stars in the halo move very rapidly, some as fast as hundreds of kilometers per second.  For a reference of scale, the Sun lies approximately 26,000 light years from the center of the Galaxy and the halo may be as large as 150,000 light years in radius.

What is the halo of the Galaxy?
Please see the previous question.

What is the disk?
Please see the previous question.

How far out into the halo is 450 light years?
450 light years from the Sun is a distance that is still within the disk of the Galaxy.  However, the material that makes up the halo pervades the disk because it completely envelopes it.  What this means is that some stars or a portion of the matter that is part of the halo can be found close to the Sun relative to the size of the Galaxy.

How are Halo stars distinguished from stars of the disk?
The halo is believed to be the oldest component of the Galaxy.  As such, halo stars are generally older than stars in the disk.  In the disk stars are being formed continuously, whereas there is no evidence for any star formation in the Galaxy's halo.  When one observes a particular star, it is not always possible to determine its age.  Therefore, the way astronomers usually distinguish members of the halo from members of the disk is by measuring their velocities through space.  Disk stars have a tightly constrained distribution of velocities, while halo stars generally have very high velocities.  In addition, if one is studying a large number of stars, halo stars can be distinguished because they do not exhibit the overall rotation about the center of the Galaxy that disk stars possess.  (Individual halo stars do orbit the center of the Galaxy, but taken together, the population of the halo does not rotate en masse about the Galaxy's center.)

Why do objects in the halo have high velocities?
Objects that are members of the halo possess orbits which can take them to very large distances from the center of the Galaxy.  These orbits are often highly elliptical.  Due to the scale of the orbits, and the laws of gravity, the stars must move at rather high velocities.

What about dark matter outside the Galaxy, or non-baryonic dark matter?
Studies concerning white dwarfs in the Galaxy's halo do not have much bearing on the question of non-baryonic dark matter.  Although it is possible that white dwarfs may comprise a substantial fraction of the baryonic dark matter, non-baryonic dark matter must still exist and by most estimations it dominates the mass of the universe.  It is not possible in the current astrophysical conception of the universe that white dwarfs can solve the whole dark matter problem.  See the essay by Prof. Silk, mentioned in the answer to the first question.

Would the presence of many white dwarfs in the halo impact current notions of star formation?
Let's assume for the moment that there is a very large population of white dwarfs in the halo.  As mentioned above, most stars end up as white dwarfs, but the lowest mass stars (so-called M and K dwarfs, or red dwarfs) take an extremely long time to evolve from their stellar states to the white dwarf stage.  In fact, according to our current understanding of stellar evolution, these stars take so long to evolve into white dwarfs that even one born close to the beginning of time (some 10 to 13 billion years ago), would still have not reached the white dwarf stage.  People have actually measured the numbers of these M and K dwarfs in the Galaxy's halo.  There are very few, and it is possible that there are many times as many white dwarfs in the halo as one would expect based on the numbers of M and K dwarfs in the halo.  That statement assumes that the stars in the halo formed exactly the same way as stars are forming now in the Galaxy's disk.  In other words, we find that stars form at a range of masses according to certain rules that establish the relative numbers of massive and low mass stars.  Those rules cannot apply if there are so many more white dwarfs.  The possible solution to this problem is that perhaps the rules were different when the Galaxy was young.  Perhaps star formation in the early epochs of the universe strongly favored higher mass stars, the stars that would eventually make the population of white dwarfs in the halo.  At the same time it would have formed relatively few low-mass stars.

Do stars go out into the halo when they become white dwarfs?  Or are there lots of white dwarfs in the disk also?
There are thousands of white dwarfs known in the disk and the study of those has been a major sub-field in astronomy since white dwarfs were concieved and the first example found orbiting the star Sirius.  Stars in the disk that evolve into white dwarfs will remain in the disk.

What information do the spectra provide?
Spectroscopy is the astronomer's most powerful tool.  It is a technique which allows one to determine the brighness of an object as a fuction of wavelength of light, or equivalently color.  It turns out that the properties of atoms and molecules leave tell-tale signs in the spectra of virtually all things.  By measuring the spectrum of an object one can determine what sorts of elements and molecules exist in the object.  In the case of stars the spectrum reveals the contents of only the atmosphere.  The interior of the star is shrouded by the atmosphere and cannot be observed directly using spectroscopy.

Whom can I talk to for additional information or impartial comments?

Here are a few suggestions.

Dr. Brad Hansen
Theory of white dwarfs, their structure and cooling mechanisms
Dept. of Astrophysical Sciences
Princeton University
Peyton Hall
Princeton, NJ 08544-1001
(609) 258-3588
Fax: (609) 258-1020
[email protected]

Prof. Gilles Fontaine
Theory of white dwarfs and their ages
Dept. de Physique
University of Montreal
CP 6128 Succursale A
Montreal, PQ H3C 3J7 Canada
(514) 343-6111 x3212
[email protected]

Prof. Roger Blandford
Theorist with a very broad range of interests
Dept. Physics
California Institute of Technology
Pasadena, CA 91125
(626) 395-4200
[email protected]

Prof. Charles Alcock
Head of the MACHO microlensing project, which found the first evidence that white dwarfs might exist in abundance in the halo
Dept. of Physics and Astronomy
209 South 33rd Street, 4N8
Philadelphia, PA 19104-6396
(215) 898-1975
fax: (215) 898-2010
[email protected]

Prof. Harvey Richer
Observations of white dwarfs and globular clusters
University of British Columbia
Dept. of Physics and Astronomy
2219 Main Mall
Vancouver, BC V6T 1Z4 Canada
(604) 822-4134
fax: (604) 822-6047
[email protected]

Prof. Gilles Chabrier
Theory of white dwarf interiors and atmospheres
C.R.A.L., Ecole Normale Superieure
69364 Lyon Cedex 07
+33 04 72 72 87 06
[email protected]

© Copyright 2001 by Ben R. Oppenheimer and Penelope E. Kneebone