What is the Smallest Star?

Article written: 4 Dec , 2014
Updated: 1 Mar , 2017

We’ve talked about the biggest stars, but what about the smallest stars? What’s the smallest star you can see with your own eyes, and how small can they get?

Space and astronomy is always flaunting its size issues. Biggest star, hugest nebula, prettiest most talented massive galaxy, most infinite universe, and which comet came out on top in the bikini category. Blah blah blah.

In an effort to balance the scales a little we’re going look at the other end of the spectrum. Today we’re talking small stars. First, I’m going to get the Gary Coleman and Emmanuel Lewis joke out of the way, so we can start talking about adorable little teeny tiny fusion factories.

We get big stars when we’ve got many times the mass of the Sun’s worth of hydrogen in one spot. Unsurprisingly, to get smaller stars we’ll need less hydrogen, but there’s a line we can’t cross where there’s so little, that it won’t generate the temperature and pressure at its core to ignite solar fusion. Then it’s a blob, it’s a mess. It’s clean-up in aisle Andromeda. It’s who didn’t put the lid back on the jar marked H.

So how small can stars get? And what’s the smallest star we know about? In the traditional sense, a star is an object that has enough mass and pressure in its core that it can ignite fusion, crushing atoms of hydrogen into helium.

Fusion is exothermic, releasing energy. It’s this energy that counteracts the force of gravity pulling everything inward. That gives you the size of the star and keeps it from collapsing in on itself.

By some random coincidence and fluke of nature our Sun is exactly 1 solar mass. Actually, that’s not true at all, our shame is that we use our Sun as the measuring stick for other stars. This might be the root of this size business. We’re in an endless star measuring contest, with whose is the most massive and whose has the largest circumference?

So, as it turns out, you can still have fusion reactions within a star if you get all the way down to 7.5% of a solar mass. This is the version you know as a red dwarf. We haven’t had a chance to measure many red dwarf stars, but the nearest star, Proxima Centauri, has about 12.3% the mass of the Sun and measures only 200,000 kilometers across. In other words, the smallest possible red dwarf would only be about 50% larger than Jupiter.

There is an important distinction, this red dwarf star would have about EIGHTY times the mass of Jupiter. I know that sounds crazy, but when you pile on more hydrogen, it doesn’t make the star that much bigger. It only makes it denser as the gravity pulls the star together more and more.

At the time I’m recording this video, this is smallest known star at 9% the mass of the Sun, just a smidge over the smallest theoretical size.

X-Ray image of Proxima Centauri. Image credit: Chandra

X-Ray image of Proxima Centauri. Image credit: Chandra

Proxima Centauri is about 12% of a solar mass, and the closest star to Earth, after the Sun. But it’s much too dim to be seen without a telescope. In fact, no red dwarfs are visible with the unaided eye. The smallest star you can see is 61 Cygni, a binary pair with one star getting only 66% the size of the Sun. It’s only 11.4 light years away, and you can just barely see it in dark skies. After that it’s Spock’s home, Epsilon Eridani, with 74% the size of the Sun, then Alpha Centauri B with 87%, and then the Sun. So, here’s your new nerd party fact. The Sun is the 4th smallest star you can see with your own eyes. All the other stars you can see are much bigger than the Sun. They’re all gigantic terrifying monsters.

And in the end, our Sun is absolutely huge compared to the smallest stars out there. We here like to think of our Sun as perfectly adequate for our needs, it’s ours and all life on Earth is there because of it. It’s exactly the right size for us. So don’t you worry for one second about all those other big stars out there.

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4 Responses

  1. UFOsMOTHER says

    Our life giving Sun may not be the biggest but for us tiny creatures it is by far the Best…..

  2. ari1413 says

    That’s a really cool way of looking at all the starts. This in no way diminishes the neatness of that but if I’m not mistaken there might be a handful of other small stars visible.
    Looking at the wikipedia list of nearest stars http://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs
    it appears as though epsilon indi and tau ceti are also smaller than the sun and visible. Alpha mensae might also be smaller (wiki lists it as .99+/- 0.03).
    Looking at this list:
    I see another possible few including 70 Ophiuchi and Gliese 117 (there might be others but I’m too lazy to comb through that list, which isn’t formatted in a way that lends itself to quickly looking up radii).
    Still, it’s cool to think that almost any star you see is going to be beefier than the sun.

  3. Member
    Tihomir says

    It seems the slide in your video shows the Sun smaller than Alpha Centauri B – your text however got it right.

  4. crash68 says

    This “nerd party fact” is utterly wrong.
    Defining stars visible with the naked eye as V<6 mag, and taking spectral type of main sequence stars as a proxy for "size" (either mass or radius), there are 76 stars "smaller" than the sun (search done with the SIMBAD database). While the exact number depends on many details and would require some effort toget right, this is definitely more than 3 as suggested in this article.
    So if somebody tells you that the sun is the 4th smallest star that you can see with your own eyes tell them that they are completely wrong.

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