How big—or small—can a star get?

As with most questions in astronomy, the answer to that is not definitive. But stellar models can give us a pretty good idea.

Mathematical models of stars tell us that their life—or, to use a less personifying term, function—depends on the balance between two opposing forces: internal pressure and gravity.

Stars produce energy to function. They don’t just do this to light up our skies and provide for life on their orbiting worlds. They need to produce energy to constantly support the weight of their own mass.

The more massive stars are, the more energy they need to produce—and the reverse is true too. There has to be a balance.

But is there a limit? Is there a point where balance is impossible?

The answer—to our knowledge—is yes.

Meet the star R136a1, the most massive star ever discovered. Its mass is estimated at 315 M_☉—315 times the mass of our sun. But it used to be bigger. It’s lost a lot of mass due to its own strong solar winds.

The problem that massive stars face is that supporting that much mass requires a lot of energy. That means the most massive stars need to be very hot and powerful. Stars that powerful emit floods of radiation with enough force to, over time, blow their mass away.

And it’s rare for very massive stars to form in the first place.

Stars form from the dense cores of giant molecular clouds. A shock wave, often the result of a nearby supernova, compresses the cloud enough that gravity can begin to do the rest of the work and form a protostar—essentially a baby star.

Astronomers have observed that, for some reason, giant molecular clouds—or GMCs—tend to fragment as they collapse. That’s why multiple stars are formed. Each separate cloud core forms a different star. In future posts, we’ll explore what that means for the “sibling” stars.

The point is, though, that if GMCs naturally fragment as they collapse, it’s kinda hard to form a giant star—instead of lots of little and medium ones.

Some do form, as in the case of R136a1. But stars like that one are rare. And that makes them hard to study. It’s just that much harder to actually find massive stars to investigate.

So what about the lower end of normal stars, then?

Meet a brown dwarf.

I suppose technically, this mysterious object isn’t a star. Not if you define a star as an object that fuses atomic nuclei for fuel, that is.

You could think of a brown dwarf as a “failed” star. It’s a star that didn’t form from enough material to ignite hydrogen fusion in its core. Its internal pressures were never high enough. It’s actually a lot like a gas giant planet.

Whoa…is it just me, or does that brown dwarf look about the size of Jupiter?

It’s true. They’re tiny. And like I said, they’re not truly stars. Extremely massive stars are unstable because their powerful cores blow away their mass so quickly, and extremely small stars can’t form properly because their cores can’t get powerful enough to begin with.

The smallest true stars discovered are actually even smaller than Jupiter—about the size of Saturn. But…how can that be possible?

Well, stars don’t need size to function—they need internal pressure, which depends on mass. So a star smaller than a brown dwarf could theoretically be more massive, massive enough to ignite hydrogen fusion.

Actually, it’s not even theoretical. It’s proven by observations. We’ve observed brown dwarfs the size of Jupiter, and true stars the size of Saturn. That’s solid evidence that there’s no true size correlation.

So…how common are these brown dwarfs, anyway?

Surprisingly common, actually.

This image shows just one star-forming region in our galaxy. We’ve discovered at least ten brown dwarfs in the area. Since they’re so faint, barely brighter than planets, and there are brighter stars near them, they’re hard to find—so it’s quite possible there are more.

It makes sense that they’d be common. They’re failed stars, and “functional” stars are essentially an accident. They are the result of nature stumbling upon just the right conditions for star formation. If those conditions aren’t met…well, we get a brown dwarf instead.

But I haven’t even told you the coolest thing about brown dwarfs.

(Get it? Cool?)

Yeah…I know, that was weak.

Anyway, the coolest thing about these mysterious objects is that they actually have weather. And not solar weather. Stars have solar winds, sun quakes, sunspots, prominences, flares, and other weather associated with their magnetic fields, but brown dwarfs have actual clouds. Just like a gas giant.

Weird, huh?

One thing you’ll find as you study astronomy—and science in general—is that nature rarely conforms to our expectations. We give objects names and distinctions, like planets and stars, to make things easier on ourselves.

But nature doesn’t care. It does what it likes. It forms objects that are not quite planets and not quite stars. Most of what we observe exists on a sort of spectrum of related objects, and we make definitions and distinctions for our own convenience, but nature just ignores us.

Coming up, we’ll take a closer look at “normal” stars.