Frequently Asked Questions Regarding Galactic Halo Dark Matter
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
105-24
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
France
+33 04 72 72 87 06
[email protected]
© Copyright 2001 by Ben R. Oppenheimer and Penelope E. Kneebone