How many white dwarf stars are there in the Milkyway Galaxy? How old are the white dwarf stars in our galaxy?

Answer 1

That depends on how many stars were originally in the galaxy and on how old the galaxy is. Small stars have much longer lives than more massive stars do.

Here are some (local) reciprocal densities for the various spectral types of main sequence stars.

M stars occur about 1 per 400 cubic lightyears.
K stars occur about 1 per 2300 cubic LY.
G stars occur about 1 per 8300 cubic LY.
F stars occur about 1 per 13000 cubic LY.
A stars occur about 1 per 55000 cubic LY.
B stars occur about 1 per million cubic LY.

Stars of spectral type M are...
6 times more common than type K
21 times more common than type G
33 times more common than type F
138 times more common than type A
2500 times more common than type B

A fair estimate of the lifespan of a star prior to its becoming a white dwarf (or whatever) is

Tms = 1E10 years (M/Ms)^(-2.5)

where Tms = time on main sequence
where M/Ms = star mass in solar masses

The galaxy is estimated to be about 12.2 billion years old. Of the stars originally present in the galaxy, all the G's, F's, A's, B's and O's have finished their lives on the main sequence and are now in their respective retirement homes. The former G's, F's, and A's (and maybe some of the B's) are white dwarfs now.

I don't know the formation distribution of stars by spectral type. A lot of the proportional abundance of type M stars might be due to their longevity.

Answer 2

The exact number is surely not known, but they must be fairly common since most stars with less than about 8 solar masses will end up as one, and stars of that description are very common.

So, how come we don't see them scattered all over the sky? Two reasons. First, they are small, so for all their heat (white heat to be sure!) they are difficult to see. Second, they cool over time so many are now too dim to see.

HTH

Charles

Answer 3

http://www.space.com/scienceastronomy/mystery_monday_040705.html