# Frequency of multiple star systems in our galaxy? **URL:** \ **Category:** Factual Questions\ **Created:** [December 27, 2008, 9:27pm UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878 "2008-12-27T21:27:04Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Cerowyn](https://avatars.discourse-cdn.com/v4/letter/c/82dd89/32.png) [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)\ **Post date:** [December 27, 2008, 9:27pm UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/1 "2008-12-27T21:27:04Z")
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? ---
**Author:** ![Julius\_Henry](https://avatars.discourse-cdn.com/v4/letter/j/8797f3/32.png) [@Julius\_Henry](https://boards.straightdope.com/u/Julius_Henry)\ **Post date:** [December 27, 2008, 11:24pm UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/2 "2008-12-27T23:24:24Z")
This is from [Ask an Astrophysicist](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980402c.html) on NASA’s Goddard Space Flight website. > [@](#): > > Somebody once said that “2 out of every 3 stars are in a binary”. Seriously, the fraction is very high, but it’s difficult to be precise, because it’s difficult to prove that a certain star is definitely single. Of the stars nearest to the Sun, about half are known to be in multiple systems. > Koji Mukai > for Ask an Astrophysicist ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [December 27, 2008, 11:38pm UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/3 "2008-12-27T23:38:05Z")
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. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 28, 2008, 3:03am UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/4 "2008-12-28T03:03:47Z")
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](http://www.chara.gsu.edu/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](http://www.chara.gsu.edu/RECONS/census.posted.htm) > [@](#): > > As of January 1, 2008, the complete RECONS Census of objects known > within 10 parsecs included 354 objects in 249 systems. Note that the 354 objects counts 10 planets and 12 brown dwarfs. > [@](#): > > Note that the multiplicity rate, or the probability that a given > system has more than one component, is only 31%. The low rate is > because M-type dwarfs (also called red dwarfs) dominate the solar > neighborhood (a full 72% of the stellar sample, i.e. not L/T/P > objects, are M-type dwarfs), and do not have companions as often as > their more massive stellar cousins. 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%. ---
**Author:** ![Cerowyn](https://avatars.discourse-cdn.com/v4/letter/c/82dd89/32.png) [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)\ **Post date:** [December 28, 2008, 7:22pm UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/5 "2008-12-28T19:22:32Z")
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. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [December 29, 2008, 2:54am UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/6 "2008-12-29T02:54:17Z")
> [@Julius\_Henry](#): > > This is from [Ask an Astrophysicist](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980402c.html) on NASA’s Goddard Space Flight website. 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” ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 29, 2008, 8:08am UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/7 "2008-12-29T08:08:30Z")
> [@Cerowyn](#): > > 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. 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. > [@Sapo](#): > > 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” 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. ---
**Author:** ![Cerowyn](https://avatars.discourse-cdn.com/v4/letter/c/82dd89/32.png) [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)\ **Post date:** [December 29, 2008, 4:11pm UTC](https://boards.straightdope.com/t/frequency-of-multiple-star-systems-in-our-galaxy/478878/8 "2008-12-29T16:11:27Z")
> [@dtilque](#): > > 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. 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.