What is the Smallest Star?

What is the Smallest Star?
The Sun by Paul Stewart

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?

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?

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 . 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.

What is the Smallest Star?
X-Ray image of Proxima Centauri. Credit: Chandra

Proxima Centauri is about 12% of a , 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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Dec 05, 2014
What about white dwarfs and neutron stars? Or are you limiting this to main sequence stars? But even then you should include brown dwarf stars.

Dec 05, 2014
Like said in article, by definition stars are active fusion reactors, and author specifically pointing on the fact of fusion of hydrogen into helium in star internals. So yes, its only about major sequence stars. Brown dwarfs are just dissipating heat of their contraction and stellar mass objects like white dwarfs and neutron stars do not generate their energy by fusion either.

Dec 05, 2014
But where is the line between brown dwarf and planet? Jupiter still radiates heat via the mechanism you describe, just like a brown dwarf, only less. Point being, why should brown dwarfs be considered stars?


its all about hydrogen fusion. if an object achieves this then it Is considered a star. some brown dwarfs have achieved this and others have not. so a brown dwarf doesn't by definition have to be a star but it can be. also

Currently, the International Astronomical Union considers an object with a mass above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas an object under that mass (and orbiting a star or stellar remnant) is considered a planet.

found that on wiki too. hope it helps

Dec 06, 2014
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.

Of course there's a lot more we can't see. But that's not how I read that statement.

Dec 16, 2014
What would be the life span of a star 12% the size of our star? Are there advantages to having an earth size planet with our moon orbiting a small star like that? I assume we would be orbiting closer it order to have the correct temps, but could our magnetic shield be big enough to ward off the effects of solar flares, etc.

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