Star Distribution In The Milky Way

[Kelzan]

I've heard wildly different numbers for how many stars there are in our galaxy. On the low end I head about 100 billion, this was several years ago. Today I read numbers like 200 billion to 400 billion. Whatever the numbers are this doesn't tell the whole story.

 

I know stars in the bulge are packed a lot tighter, than in the spiral arms and the halo. 

 

But are the percentages known?

 

What percent of the galaxy's stars are in the bulge? the spiral arms? the halo?

 

Are the majority of the galaxy's stars in the bulge? Does anyone know how many stars are in the arms?

[CraigD]

Hi Kelzan - welcome to Hypography!

 

There's much uncertainty about the number of stars in the Milky Way galaxy, for mainly 2 reasons: uncertainty about its mass, and uncertainty about the abundance of low mass, low luminosity (and thus practically invisible beyond a few hundred lightyears) stars.

 

All have led to an increasing consensus that the Milky Way has about the same mass as our similar neighbor Andromeda galaxy, and that about 80% of its stars are red dwarfs (about 10% the Sun’s mass. A reasonable estimate of the actual number of stars in either the Milky Way or Andromeda is 1,000,000,000,000 (1 trillion).

 

Uncertainty about the percentage of stars in the Milky way’s central bulge (about 5,000 lightyears radius) comes from not knowing its thickness precisely (newer models suggest it’s much thinner than most otherwise similar distant spiral galaxies)), not knowing if the precise abundance of low mass/luminosity stars there, and uncertainty about the mass of the central supermassive black hole (though recent space telescopes have helped reduce that uncertainty). My guess would be that about 33%, based on observation and models suggesting the bulge’s is 1% of total disk’s, but has about 50 times the number of stars per unit volume – but this is only rough, amateur guess.

[Kelzan]

So your guess is the bulge contains about 33% of the stars.

So would the halo have about 1-2% of the stars?

That being the case (and realizing it was an educated guess) is this a reasonable distribution?:

 

Bulge- 33%

Disk- 65-66%

Halo- 1-2%

Edited May 27, 2014 by Kelzan

[CraigD]

So would the halo have about 1-2% of the stars?

Estimating the number of stars in the halo is tricky.

 

If we just count the ones that can be directly detected, there are only on the order of 100,000,000, about half in gravitationally bound globular clusters – about 0.1%.

 

This gets complicated by solutions to the missing mass/galactic rotation problem that strongly suggest that the mass of the halo is about equal to the mass of the rest of the galaxy. So, if the halo has lots of low luminosity stars – red dwarfs and similar – the number may be 5%, or even approaching 50%.

 

There’s evidence, though, such as gravitational lensing of images of distant galaxies - that much of the mass may be much higher mass but still very low luminosity bodies, such as neutron stars and stellar mass black holes. If this is the case, the number may be close to the 0.1% that are visible.

 

A key feature of the halo is that it it’s almost certainly so much less mass dense that the disk of the galaxy that, except for perhaps the globular cluster stars, stars and other stellar mass objects likely don’t form there, but are mostly either ejected from the galactic disk, or form far outside of the galaxy – that is, are essentially mini-galaxies captured by the Milky way. I don’t believe there’s a clear scientific consensus of how the visible stars in the halo got there, so it’s hard to use modeling to confidently estimate the number and kind of objects in it.

 

I find it a bit surprising that recent supercomputer modeling of the entire universe (see this thread, which actually is based mostly on directly un-observed dark matter and energy, is more conclusive that models of galaxy formation. It’s less surprising when you consider that models like these involve fewer than 100,000 objects – each a galaxy - a tiny fraction of the estimate 1,000,000,000,000 stars in our galaxy.