fundamentals: the stars

Stellar Evolution Demystified

/ Emma / 7 Comments

Whaddya know…after what seems like a geological age, we’re finally done with stellar evolution! And we’ve covered a truly ridiculous amount of information.

We’ve covered a star’s relatively gentle, humble beginnings within the collapsing cores of giant molecular clouds (or GMCs). We’ve explored how stars begin fusing hydrogen nuclei for fuel and how their interiors work.

We’ve covered how they evolve across the main sequence, and how they eventually exhaust their fuel, lose stability, and expand into giants.

We’ve delved into the way low- and medium-mass stars quietly expel their atmospheres and shrink into inert balls of carbon called white dwarfs. And we’ve watched as massive stars burst apart in brilliant supernova explosions and then collapse into some of the most extreme objects in the universe, neutron stars and black holes.

Those three end states–white dwarfs, neutron stars, and black holes–are known as compact objects, and we’ve explored them too.

If it all seems super complicated…I understand. But now, just as I did once with types of stars, I’m going to give you an overview to put it all together.

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The Average Star

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solar_system_poster.jpg

What the heck is the average star like?

We’ve talked about a lot of stars over the past few weeks. We’ve discovered the vast distances between the stars, looked more closely at what really makes a star bright, and covered all kinds of ways to classify stars—from their spectral type to their luminosity class.

Most importantly, we’ve looked at the H-R diagram, the diagram that classifies stars by their color, temperature, composition, and luminosity…and relates those properties with many other features stars have.

We know what kinds of stars are out there. We know they range from thousands of times smaller than the sun to thousands of times larger. We know they range from desperately faint to incredibly luminous. We know they come in all the colors of the rainbow.

But how many blue stars are there? How many small stars are there? Are most of them small, or are there about the same number of small stars as large ones?

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Binary Stars

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We know how big stars are; they range from the size of the Earth to over a thousand times the size of the sun (which is in itself over one hundred times the size of the Earth). We know they’re huge.

But how massive are they?

Yes, that’s a different thing.

A pingpong ball and a golf ball are close to the same size, but a golf ball is much more massive—in that it has more stuff in it. A pingpong ball is hollow and easily tossed; a golf ball has more matter in it and will hit the ground with a harder thunk.

Stars are similar. They have a wide range of sizes, but nothing I’ve described thus far has told us about their masses. That is, how much stuff is in them? Are they like puffy gaseous balls, or are they more dense, like planets?

The best way to learn about stars’ masses is by studying binary stars. But what exactly are binary stars?

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Star Types Demystified

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image.png

By now, I’ve introduced you to a lot of different ways to classify stars.

Months ago, I talked about the different spectral classes—O, B, A, F, G, K, and M. Even before that, I told you about apparent visual magnitude, our ranking system for how bright stars appear to the naked eye.

More recently, we explored absolute visual magnitude and the related absolute bolometric magnitude and luminosity. All these are related to a star’s actual brightness, not just how bright they seem to be from Earth.

And last but not least, we talked about the H-R diagram and how to rank stars by their luminosity classification.

In short, it may seem like sorting stars is a complicated business. But it’s not really. And here, I intend to give you an overview to put all this together.

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Just How Big Are Stars?

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Tell me about the stars you see in this image.

They look like billions of little pinpricks of light, right? It’s hard to imagine that each one of these is probably the size of the sun…or much larger. And the sun, by the way, is about 109 times Earth’s diameter.

So if you thought those stars were small…not so.

It makes sense that they would be very large. Their light reaches us from many light years away, with the nearest star 4.3 light years away and the most distant one likely trillions.

In order to radiate that far out and stay bright enough to speckle the night, they would have to be very luminous, and that means having a large surface area, even if they’re not particularly hot.

So how do we know how big the stars are?

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The H-R Diagram

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Planet-Bearing-Stars

There are 250 billion stars in our galaxy alone. Many are much like the sun, labeled with the Latin sol for “sun” in this diagram. But many more are not quite what we might expect stars to be like, after living under the light of a white G2 star our whole lives.

Wait a second. White G2? Since when is the sun white? And what the heck does G2 mean?

I’m talking about its spectral type—a classification system that organizes stars by their temperatures, determined by what they’re made of. The sequence is O, B, A, F, G, K, and M, in order from hottest to coolest. The sun is a fairly cool star.

But the thing is, the spectral types don’t actually tell you anything about how bright the star is, how big it is, how luminous it is…I could go on.

So how can we make things easy for ourselves and classify stars according to spectral type, size, and luminosity all at the same time?

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Distances Between Stars

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milkyway

When you look up into the sky on a clear night away from the glare of the city, you see trillions upon trillions of stars.

Thousands of years ago, the classical astronomers saw the same thing you do today—except perhaps a little different, due to the ever-changing cosmos. And, like you, they weren’t satisfied with just looking. They wanted to know what was out there.

For hundreds of years, they developed model after model to explain why the stars seemed to orbit the Earth and why certain objects in the sky—which they named planets—seemed to wander backwards from time to time.

Tycho Brahe, an astronomer known mainly for what he got wrong, dismissed the idea of the Earth orbiting the sun because he could detect no parallax between the stars.

If he had been able to measure parallax, he might have realized that the universe was much larger than any of his fellow classical astronomers imagined.

So what is parallax…and how can it help us measure the distances between stars?

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