Citizen Science by Trisha Muro

Sunshine and Starshine: Exploring How Stars, Planets Differ

Hey, NASA girl!”

It was a warm summer night, and the man’s voice rang out clearly across the field where we’d all gathered. Even though I’ve never worked at NASA, I knew he was calling out for me.

I’d been leading a tour of sorts, pointing out how to hopscotch across the celestial dome studded with constellations. A midwestern summer abundance of mosquitoes joined the 100 or so humans who stood with heads craned upward to gaze into the star-filled night sky.

The man went on to say, “This might be a stupid question, but…” He paused. “What’s the difference between a star and a planet? They both shine pretty bright.”

Definitely not a stupid question.

And as we talked through a few basic criteria, more people wanted to know: What’s the difference between an asteroid and a comet? A galaxy and a nebula? All these things are visible in the night sky, sometimes to the naked eye. They all shine. So how do astronomers classify these objects?

So let’s start with this man’s question.

You might hear responses like, “A star is a gas, but a planet is solid.” That’s a reasonable place to start; however, the outer planets (Jupiter, Saturn, Uranus and Neptune) are all gaseous with no solid surface. How about, “A star gives off energy, and a planet doesn’t,” then? Now we’re heading in a good direction, but several planets—including Earth—generate a substantial amount of energy. There’s a kernel of truth in there, though, because stars shine due to the energy they produce, whereas planets only reflect light from their sun.

Another common suggestion is, “A star doesn’t move around, but planets orbit around their star.” In my mind, one of the most mind-boggling concepts in astronomy is the fact that everything in space is moving, in all directions, all the time. There’s no such thing as “standing still” in space. Even stars move.

The simplest difference between a star and a planet, then, is that a star shines by generating its own energy through fusing hydrogen into helium in its core. A planet may generate lots of energy—like Earth does—but nuclear fusion is a whole different ballgame in terms of energy production. A planet just won’t be able to do that.

Combustion, meaning the chemical burning of fuel in the presence of oxygen, is a type of reaction that humankind has put to tremendous use. Any time you light a match, click the gas starter of a stove or grill, or turn the key in your (gas-powered) car, you’re starting up a combustion process. Rockets launch with combustion. But because it’s chemical in nature, combustion relies only on reactions that involve the electrons of atoms and molecules. Those electrons exist on the far outskirts of an atom, and even though they can be held quite tightly they’re still relatively easy to manipulate under the right conditions.

Nuclear reactions, in contrast, involve the protons and/or neutrons of an atom. These subatomic particles are deep, deep inside an atom and are held together with a force that is literally the strongest force in the universe. (It’s actually called the strong nuclear force.) By their nature, nuclear reactions involve overcoming these ridiculously powerful forces, thus the energies released are similarly enormous. We’re talking E = mc2 territory, here.

A star becomes a star when it’s able to sustain nuclear reactions in its core. Fusion, which is what stars do, refers to combining two or more lighter elements into a slightly heavier element. Most stars fuse hydrogen into helium for the vast majority of their lifetimes. As a star begins to run out of hydrogen, it can begin to move up the Periodic Table to fuse helium, lithium, carbon and so on. Fission, which is what nuclear power plants do, is what happens when a very heavy element decays into two or more lighter elements. (Nuclear processes truly transform one element into another. Chemical processes do not.)

The man who shouted out this seemingly simple question—What’s the difference between a star and a planet?—lit the spark for a cascade of profoundly curious questions about the cosmos. What we see in the sky reveals the tiniest glimpse of the science churning within.

Look deeper. And keep asking questions.

Trisha Muro is a freelance science writer specializing in physics and astronomy and, although she’s never worked for NASA, she’s been a space nerd for as long as she can remember. Her first book, “It’s (Just) Rocket Science,” will be published this spring by Johns Hopkins University Press.