Density of stars in Milky Way

Amtmann · 16-04-2011 12:27AM #1

So the galaxy is 100,000 light years across, and there are 200 billion stars within it.

Where do they all fit within the 100,000 light years? They must be very densely packed; yet the closest star to us is 4.2 light years away.

Can someone explain how the stars of the Milky Way are positioned across the galaxy?

Plug · 16-04-2011 12:56AM

Tremelo wrote: »

So the galaxy is 100,000 light years across, and there are 200 billion stars within it.

Where do they all fit within the 100,000 light years? They must be very densely packed; yet the closest star to us is 4.2 light years away.

Can someone explain how the stars of the Milky Way are positioned across the galaxy?

Lots of stars are in Globular clusters, a big packed space full of stars.

Rubecula · 16-04-2011 01:56AM

You have to think in 3 dimensions. OK the galaxy is pretty thin in comparison to the diameter, but it is still thick enough for stars to be dotted around in 3 dimensions. Now think about the overall area the milky way covers.

Pi is 22/7 area of galaxy is Pi (r squared) I don't know how to type the equation... sorry

Anyway I think it works out at nearly 500,000 square light years Then if you take into account the thickness of the galaxy at the centre (This is sometimes described as two fried eggs back to back) So there is a large ball of stars there, very closely packed.

Apparently multiple stars are very very common too.

silverharp · 16-04-2011 08:53AM

side question, what would the view at night be like from a star system near the centre? and not near the black hole:) , would the night sky almost have a glow?

Cú Giobach · 16-04-2011 08:56AM

Maths is not one of my strong points but I'm gonna give this a go, for the craic.

Using VERY rough figures.
Radius = 50,000 ly
Thickness = 4,000 ly (my own arbitary figure because of the shape of the galaxy, not a perfect cylinder, thought to be around 10,000 thick at centre and around 3,000 in the disk, not including the gas which increases the dimensions)

Pi*(r*r)*thickness = Area Volume (;))

3.14*2,500,000,000*4,000 = 31,400,000,000,000 Square Ly's

With 200,000,000,000 stars that gives 157 square ly's per star = (very) rough separation of 5.5 ly's.

Please don't take this as my attempt at an accurate figure, it is just to show how using the rough dimensions and numbers there is nothing odd about the 4.2 ly distance to our nearest companion.

Rubecula · 17-04-2011 07:05AM

Thank you Cu Giobach. However I think what you just posted is the equation for volume not area? (Please forgive me if I am wrong as my maths is not that great either.):D

However the idea is correct as far as I am concerned. There is more than enough space in the galaxy for all the stars... (Obviously or they wouldn't be here):pac:

Cú Giobach · 17-04-2011 07:30AM

Rubecula wrote: »

Thank you Cu Giobach. However I think what you just posted is the equation for volume not area? (Please forgive me if I am wrong as my maths is not that great either.):D

Yea it was volume (which was what was needed),

It has been 30 years since I had to work out something like that

, I must admit I was well chuffed when I pressed the final "=" on the calculator and the nice realistic figure of 5.5 (actually 5.395) popped up.

Rubecula · 17-04-2011 08:02AM

I am suitably impressed. I didn't know the 'height' of the galaxy and I was not sure how to work out the volume and density average either. (Well I did know.... once upon a time)

What you did was:

area * height = volume

volume / No. stars = density

I think thats right isn't it? (I am getting too old for this hard thinking stuff):pac:

I know I keep getting different figures when I try it LOL. But I got the same as you when I followed what you did. Apart from the average space between the stars. I can not get 5.5 light years.

Edit:
Yes I did. I did the thing wrong... sorry

The Hill Billy · 17-04-2011 12:04PM

From what I've read in other articles, the stars actually take up very little 'space' in our galaxy. When galaxies collide, for example, they pretty much 'pass through' each other & impacts between objects are thought to rarely, if ever, occur.

silverharp · 17-04-2011 12:51PM

I posted the video below here recently but worth a watch if you havnt seen it. One analogy is that is you put a basketball representing the Sun in London, the nearest star would have to be placed in Kiev :cool:

https://www.youtube.com/watch?v=rcLnMe1ELPA

Cú Giobach · 17-04-2011 02:11PM

My favourite analogy for stellar distances is.....a pea in each of the cities and larger towns in Ireland (rough average 100kms apart), Cork, Limerick, Galway, Athlone etc......

Imaging if you were blindfolded and walked across the country, the chances of stepping on one of those peas would be pretty slim.

Space really is space.

Rubecula · 17-04-2011 02:37PM

Hill Billy wrote: »

From what I've read in other articles, the stars actually take up very little 'space' in our galaxy. When galaxies collide, for example, they pretty much 'pass through' each other & impacts between objects are thought to rarely, if ever, occur.

Yes this is true. You can somewhere on the internet find computer simulations of galaxies colliding. They pass through each other with mutual gravity disturbing the star formations as they pass through. Then they come back and 'bounce' through, then back again and so on until the super massive black holes at the centres coalesce and become one. Then slowly the milling stars settle down to become a more huge galaxy. A few stars get flung out and sometimes a few clusters are left to orbit the galaxy.

It is what will happen to the Milky Way and the Andromeda galaxies sometime in the distant future.

djhaxman · 17-04-2011 03:35PM

Khan academy doing astronomy videos now, good stuff. Excellent site that for everything science and maths related really.

Density of stars in Milky Way

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