Re: How can you see black Dwarfs?

Black dwarf stars are theoretical objects,
because they haven't been seen yet.  Since they are
black and therefore don't radiate much, and are at
great distance from the Earth, we couldn't see them.
They are the predicted outcomes of white dwarf stars,
after many billions of years.

White dwarf stars are remnants of stars like the Sun.
For most of its lifetime the Sun will burn its store
of hydrogen into helium in its core.  When it runs out
of hydrogen, it can burn the helium into carbon, nitrogen,
and oxygen, but this reaction is much less stable.  The
Sun's outer layers will be heated up and will expand so
much that the Sun will be called a "red giant" star.
Eventually, the nuclear reactions become so fast and
furious that the energy produced ejects the outer layers
of the giant star, revealing a hot white core of helium,
carbon, nitrogen, and oxygen.  It appears white because
of its temperature (roughly 100,000 degrees Kelvin) and
because of its composition.  This is a white dwarf.

White dwarfs continue to burn small amounts of helium
and carbon, but these soon run out, and only the star's
own gravity and nuclear pressures produce energy afterwards.
It takes a LONG time to cool, however.  The theoretical
estimate of the cooling time lies in the TRILLIONS of
years.  For that reason, even if black dwarf stars do
exist, there wouldn't be many of them in a universe only
10 to 20 billion years old.

"Brown dwarfs", on the other hand, have recently been found
to exist.  They are a completely different kind of object
from white and black dwarfs, because they are "failed stars",
or objects with masses lying in between stars and planets.
Brown dwarfs are believed to be roughly 10 to 80 times the
mass of Jupiter, which is the largest planet in our Solar
System.  In comparison, the Sun is roughly 1000 times the
mass of Jupiter.  Brown dwarfs are not hot enough in their
centers to ignite thermonuclear reactions like the Sun has,
but glow in the infrared due primarily to their gravitational
energies.  If astronomers look carefully in the infrared part
of the electromagnetic spectrum, for certain molecules like
methane - which couldn't exist on the surface of a star -
they can find brown dwarfs.
--