Depending on their mass, stars can remain stable for millions and even billions of years. The most massive stars live for “only” about 10 million years, but models predict that the least massive live for much longer—longer than cosmologists believe the universe has existed.
As stars exhaust their fuel, their internal structures change drastically. Their cores contract, but their outer layers are forced to expand, and they become giants. You’d think the next thing we’d cover would be what happens to these giant stars, right?
Well…not quite! At this point, something downright weird is going on in their cores, and it’s well worth a closer look…
Paradoxically, stars begin in the galaxy’s coolest places: the dense giant molecular clouds (or GMCs).
This is not quite the paradox it seems, as in the beginning, stars require little else but gravity to form. And that’s really quite lucky, because one thing they do need is regions of high density, and high density is unlikely to occur where temperatures are high.
And so stars begin in perhaps the most surprising of ways: as a high-density bundle of very cool gases within an equally cool interstellar cloud.
Astronomers have discovered that the clouds of gas and dust—the interstellar medium (ISM)—found between the stars are made of the same materials as the stars themselves. In fact, hydrogen is the most common element in both stars and the ISM, followed closely by helium.
But it would be more accurate to say that stars are made of the same material as the ISM, not the other way around.
This is because all of the stars formed out of material in the ISM at some point millions to hundreds of billions of years ago. And when they die, they return that material—what’s left of it—to the ISM.
Really hot. Hot enough to have energy to spare for their planets. If our star wasn’t hot, we couldn’t live on Earth. And our star isn’t even particularly hot for a star. It’s a middle-aged star of low mass, so it’s relatively cool compared to other stars.
You might also notice that stars aren’t all the same color. There are redder stars and bluer stars and more whitish stars.
We know stars are hot. They’re also bright. And they’re different colors. But how does that all translate to radiation—and how can we see it?