White dwarfs are the burned-out cores of collapsed stars that, like
dying embers, slowly cool and fade away. They are the remnants of low mass
stars, among the dimmest objects observable in the Universe. They are low to
medium (less than ten solar mass) Main Sequence stars which have burned through
their reservoirs of both hydrogen and helium, passed through the giant phase,
were not hot enough to ignite their carbon, puffed off their outer layers to
form colorful planetary nebula, and then collapsed and cooled into small
glowing coals. This beautiful Hubble Space Telescope image shows a nearby
white dwarf, and the outer layers of the former star's atmosphere which have
been blown away. The resultant planetary nebula will shine for the next
20,000 to 50,000 years, expanding outwards and fading slowly with time.
Nearby planetary nebula NGC 2440, surrounding a piping hot white dwarf
with a surface temperature of 200,000 degrees Celsius.
[NASA/HST, H. Bond, R. Ciardullo]
A typical white dwarf has a carbon and oxygen mass similar to the Sun,
but is much smaller in size (similar to the Earth). It is much hotter (25,000
K), but because of its small size its luminosity is low. How then can we
find them?
The nearby globular cluster M4, observed from the ground (left) and with the Hubble
Space Telescope (right).
[NOAO/KPNO, M. Bolte & NASA/HST, H. Richer]
Some very nearby white dwarf stars can be observed directly through
telescopes, though they are extremely faint. M4, shown above, is the nearest
globular cluster to the Earth. It contains hundreds of thousands of stars
visible with ground-based telescopes, and is expected to contain about 40,000
white dwarfs. This globular cluster formed early in the history of the Milky
Way, and today is a veritable stellar retirement community. It is so ancient
(14 billion years old) that all of its stars that began with 80% or more of
the Sun's mass have already evolved off of the Main Sequence to become red
giants, and many have turned into white dwarfs. In the Hubble Space Telescope
picture shown above, the brightest of the detected white dwarfs (the
faint pin-pricks circled on the right hand side of the figure) is no more
luminous than a 100-watt light bulb seen at the Moon's distance (239,000
miles).
A thin layer of hydrogen is transferred from the red giant, and
slowly accumulates about the surface of the white dwarf. When
the hydrogen builds up to a critical level, it triggers a supernova
1a explosion. [NMSU, N. Vogt & NASA/STScI]
We can also observe the presence of a white dwarf in a binary
system through its effect on its stellar companion. Consider a solar system
composed of a white dwarf and a Main Sequence star. For a long, long time,
the two stars will orbit each other in harmony. But eventually, the Main
Sequence star will run out of hydrogen in its core and begin to swell up into
a red giant. At some point, the star will balloon out to such a large radius
that the outer atmosphere becomes more gravitationally attracted to the white
dwarf than the star, and it will begin to drift towards it. The white dwarf
star slowly accumulates a layer of hydrogen on its surface (see a
beautiful simulation of this process by John
Blondin). When about 10-8 solar masses of hydrogen has been
accumulated, the temperature and pressure at the base of this layer will be
great enough so that thermonuclear reactions begin (just like in a stellar
core). The hydrogen quickly begins to fuse into helium. This reaction is
explosive, and ejects the burning layer of gas into space. Under certain
conditions the result is a type of supernova (a type 1a supernova).
With such a large mass concentrated into such a small size, we can deduce
that white dwarfs are extremely high density objects. In a normal Main
Sequence star, the pressure of gravity is withstood by the forces of nuclear
fusion. In a white dwarf, however, all nuclear fuel has been exhausted and
gravity compresses the core inwards, forcing the matter into a degenerate
state. We find ionized carbon and oxygen, and a sea of electrons which have
been forced out of the atoms.
Degeneracy pressure (by degenerate we mean that the electrons
and the protons have separated) may sound complicated, but it is actually a
familiar process. It is no more than the pressure that prevents liquids and
solids from being compressed. Gas pressure is squishy, and
gases are compressible, as you can verify by pumping your bicycle tire with a
hand pump. At a density of a few grams per cubic centimeter, however,
ordinary matter becomes liquid or solid and is nearly incompressible. Squeeze
on any solid object, and it will push back. No apparatus on Earth can
compress water (or steel) to half its volume. The same degeneracy pressure
prevents atoms from collapsing within white dwarfs. Moreover, the tendency of
liquids and solids to resist compression is almost independent of their
temperature. Liquids and solids do not shrink much when cooled, even to
almost absolute zero temperature (-273 C). Contrary to our usual experience,
if we could heat a balloon of degenerate gas it would not expand. This has
very important consequences for thermonuclear reactions, such as those that
occur in stellar cores. Because of this, a white dwarf is able to radiate
heat and slowly cool down, without the internal pressure dropping. (Contrast
this with the pressure due to the heat and excitation of nuclear reaction when
the star is in its Main Sequence hydrogen-burning phase: without the heat, the
pressure drops and gravity wins the battle to compress the star.)
