# Frequency of multiple star systems in our galaxy?

Is there a consensus (or even just respected) best-guess estimate for the frequency of multiple star systems in the Milky Way? I’ve done a bit of online research, and I haven’t come up with anything as straight forward as “one in three star systems in the galaxy may be binary systems” (for instance).

Is it possible to venture a guess at the distribution of multiple stars? I know that there’s a lot of work going into identifying physically-multiple systems (as opposed to stars that are optically close, but physically far apart), but have any models or projections been done that would indicate how common or rare such systems should be in our galaxy?

This is from Ask an Astrophysicist on NASA’s Goddard Space Flight website.

While getting a degree in astronomy and physics 30 years ago, I was taught that a majority of the things that look like single stars from a distance were binary stars.

One must be careful whether to count on the basis of the stars themselves or on the basis of the points of light you see in the sky. If half the points of light are binary stars, then 2/3 of the stars themselves are in binary systems.

Star systems with more than two stars are less common, in part because they aren’t very stable so they don’t last so long. A common configuration is to have a close binary and a third star at a much bigger distance, all orbiting. For the purposes of the third star, the thing it is dancing with might as well be a single star. In the limit where the distance between the closest two is very small relative to either’s distance from the further, it’s stable, and may be thought of as a binary within a binary. The same is true of pairs of binaries.

Bright stars tend to be multiple much more often than your common run-of-the-mill red dwarf. So to avoid selection effects, you have to count all stars within a given region and see how many are double. RECONS is a project attempting to find all stars within 10 parsecs of the Earth, so it’s the closest thing we have to a count of all stars within a region. They’re still discovering more, but all additions are going to be red dwarfs, brown dwarfs or planets.They say

Note that the 354 objects counts 10 planets and 12 brown dwarfs.

L and T objects are brown dwarfs; P objects are planets.

As far as I can tell, that multiplicity rate counts planetary systems as multiple objects. The 10 planets in their count are distributed among 6 systems, which are probably all single stars. So that would lower the multiplicity rate to 29%.

Thanks, folks, this seems to be pretty close to what I wanted to find out. I knew that the local neighborhood has a disproportionate number of M class stars (76%, according to Wikipedia), so it doesn’t necessarily represent the mix of the galaxy as a whole.

When we say “2 out of 3 stars are in a binary”, do we mean 3 stars, 1 lone, 2 in the binary, or do we mean 5 starts, 1 lone, 2 sets of binaries, 3 “star systems”

Perhaps not the Galaxy as a whole, but it’s expected to be representative of the galactic disk. Which is a large percentage of the Galaxy.

I’m not sure why you think the fraction is ‘disproportionate’. M dwarfs represent the smallest stars, so you’d expect many more of them than larger stars. Well, except for brown dwarfs, but we’re a very long way from cataloging a significant fraction of those.

I expect they mean the former. That translates to a multiplicity of 50% (half of all systems are multiple) which is considerably higher rate than the ~30% from the RECONS data.

A poor choice of words on my part. I meant that every source I’ve come across mentions that the local neighborhood has a large percentage of M class dwarfs; I’ve always taken that distinction to mean that on a larger scale (i.e. not just in our neighborhood), either there is a lower frequency of such stars, or we don’t know what the frequency is.