What is a small star?

Small Stars: The Unsung Heroes of the Cosmos

We often get wowed by the huge, blazing stars you see in Hubble images, right? But let’s be honest, it’s the little guys, the small stars, that really run the show in our galaxy. They might not be as flashy, but they’re way more common, and they stick around for the long haul. So, what exactly is a “small star”?

Well, “small” is relative, of course. We’re not talking about something the size of Jupiter here. Generally, when astronomers say “small star,” they mean what are called dwarf stars. Think of them as the workhorses of the stellar world. These are stars busily fusing hydrogen into helium in their cores, which is what keeps them shining—and it’s what stars do for most of their lives.

Now, the term “dwarf” itself is a bit of a catch-all. It covers a few different kinds of stellar objects, even some that have already hung up their fusion boots, so to speak. To really get a handle on what a small star is, we need to break things down a bit:

• Main Sequence Dwarfs: These are the real deal—actual, honest-to-goodness stars doing star things. They include:
  • Red Dwarfs: These are everywhere. Seriously, they’re the most common type of star in the Milky Way. They’re smaller and cooler than our Sun, only about 0.08 to 0.6 times its mass. And get this: they can live for trillions of years! Proxima Centauri, the star closest to us, is a red dwarf. Pretty cool, huh?
  • Yellow Dwarfs: Ah, yes, like our very own Sun! These are stars of average size and brightness. Nothing too extreme here.
  • Orange Dwarfs: Think of these as the yellow dwarf’s slightly cooler, slightly smaller cousins.
• Stellar Remnants: These are the leftovers after a star has lived its life and used up all its fuel.
  • White Dwarfs: Imagine squeezing the mass of the Sun into something the size of the Earth. That’s basically a white dwarf! They’re the super-dense cores of stars like our Sun after they’ve run out of gas.
  • Neutron Stars: These are even more extreme. They’re what’s left after a massive star goes supernova. We’re talking about something with more mass than the Sun crammed into a sphere only about 10 kilometers across!
• Substellar Objects: These are the almost-stars, the ones that didn’t quite make the cut.
  • Brown Dwarfs: These are often called “failed stars.” They’re bigger than planets, but not big enough to sustain stable hydrogen fusion. They can fuse a little bit of deuterium (a heavier form of hydrogen), but it’s not enough to really get them going.

What Makes a Small Star “Small”?

So, what are the defining features of these small stars, especially the main-sequence dwarfs like red dwarfs?

• Tiny Compared to the Big Boys: They’re much less massive and smaller in diameter than larger stars. Red dwarfs, for instance, can be anywhere from a tenth to half the mass of our Sun.
• Dim and Cool: They don’t put out a lot of light or heat. Red dwarfs, for example, have surface temperatures of only about 2,500 to 4,000 Kelvin, and most of their light is in the infrared.
• Living the Long Life: Because they burn their fuel so slowly, these stars can last for an incredibly long time. Red dwarfs? Trillions of years, easy.
• Surprisingly Dense: Don’t let their small size fool you—these stars are packed tight!

The End of the Line

What happens when a small star finally kicks the bucket? Well, it depends on its mass. Red dwarfs are predicted to just slowly fade away into white dwarfs after a very long time. Stars like our Sun will eventually swell up into red giants, then shed their outer layers to form a beautiful planetary nebula, leaving behind a white dwarf. Eventually, those white dwarfs will cool down and become black dwarfs, but the universe isn’t old enough for that to have happened yet!

Some Famous Small Stars

• Proxima Centauri: Our nearest stellar neighbor, and a red dwarf.
• Wolf 359: Another well-known red dwarf in the constellation Leo.
• OGLE-TR-122b: One of the smallest red dwarfs we’ve accurately measured.
• Sirius B: The white dwarf companion to Sirius A, the brightest star in the night sky.

The Takeaway

Small stars might not grab headlines like supernovas or black holes, but they’re a vital part of the universe. From the incredibly long-lived red dwarfs to the super-dense white dwarfs, these stars teach us a lot about how stars are born, how they evolve, and what their ultimate fate will be. So next time you look up at the night sky, remember the little guys—they’re more important than you might think!