White Dwarfs Could be Dark Matter

Porfiry writes "An international team of astronomers has detected what could be a significant portion of the galactic dark matter that has eluded astronomers for nearly 70 years. Scanning digitized images of the southern sky, the team found 38 previously unseen cool white dwarfs within about 450 light years of Earth. If the density of these newly discovered white dwarfs is indicative of the rest of the galaxy's halo, these dead stars would comprise at least three percent of the dark matter of the halo, and perhaps as much as 35 percent."

Comments

Re:Dark matter

[skwang]

Being as dense as it supposedly is with such great gravitational attraction, how does it keep from folding in on itself like a black hole?

There are two answers:

1.

What determines the final stage of a collapsed star is its main sequence mass. Your sun's main sequence mass is 2 x 10^30 kg or one solar mass (Mo). Stars that are (less than)1-3 Mo become white dwarves, 3-10 Mo become neutron stars, >10Mo become black holes. Note, these numbers are approximate, look in a astronomy text for details.

2.

White dwarves are "held up" by what is known as degenerate electron pressure. To understand this phenomenon, I have to explain the Pauli-Exclusion Principle. The Pauli-Exclusion Principle states that no two electrons can occupy the same energy state. (In general the Pauli-Exclusion Principle says that no two identical fermions can occupy the same state. Fermions are particles with a spin of one-half.) A good analogy (which was given to me) is as follows: think of a classroom with 50 seats. Since only one student can occupy one seat, there are exactly fifty students in the room. If the room suddenly shrinks to 30 seats, 20 students must leave, and only 30 students remain.

As a main sequence star with mass (less than)1-3 Mo collapses, it reaches a point where every electron fills every energy state. At this point, there is an outward pressure because all the electron energy states are filled. This outward pressure balances against the inward pressure of gravitational collapse. At this point, we have a white dwarf.

If a star is sufficiently large, the gravitational pressure is enough to force electrons to bind with protons making neutrons. Neutrons are also fermions. So they also obey the Pauli-Exclusion Principle. Neutron stars are "held up" by degenerate neutron pressure.

I don't understand how black holes form...