At the lower end, and to the bitter end, defining a star is tougher than you might expect
I love simple questions that wind up having complicated—or at least not straightforward—answers. Astronomers twist themselves into knots, for example, trying to define what a planet is, even though it seems like you'd know one when you see it. The same is true for moons; in fact, the International Astronomical Union, the official keeper of names and definitions for celestial objects, doesn't even try to declare what a moon is. That's probably for the best because that, too, is not so easy. What about stars, though? Do they also confound any sort of palatable definition? In a very broad sense, a star is simply one of those twinkling points of light you can see in the night sky. But that's not terribly satisfying in either lexicological or physical terms. After all, we also know the sun is a star—but, by definition, we never see it in Earth's night sky, and it's certainly not a dot (unless you're viewing it from well past Pluto, that is). On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. If such a basic definition leaves us a bit dry, then perhaps we can do better. From centuries of scientific observations and theoretical physics, we can say more. Stars are massive, hot and roughly spherical. They're held together by their own gravity, and they consist of plasma (gas heated so much that electrons are stripped from its constituent atoms). And, of course, they're luminous. They shine, which is probably their most basic characteristic. That's descriptive, certainly, but still doesn't really tell us what a star is. What makes one different from, say, a planet? Can there be a smallest star or a biggest one? To sensibly answer such questions, we need to understand the core mechanism that makes a star luminous in the first place. Then we can use…