red dwarfs

The Composition of the Milky Way

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What is our home galaxy made up of?

In the broadest sense, it’s made up of stars, clouds of dust and gas, and the mysterious dark matter.

We could also get a little more detailed. We could say that it is a great wheel of stars, made up of a thin disk component, a central bulge, and a broader spherical halo that surrounds the disk.

We could even build on that, and say that the thin disk is where all the youngest stars are found. We could say that within the thin disk are spiral arms, where the star formation actually happens. We could say that the oldest stars are found in the central bulge and the halo, where there is very little dust and gas to make new stars.

But…what about its chemical composition? If we could explore our galaxy and bring home test tubes of “star stuff,” what would we find? And what can that tell us about our galaxy’s history?

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How Low-Mass Stars Die

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When we talk about star death, we’re not really talking about death. We’re talking about the end of a functioning star. Astronomers tend to personify cosmic objects like stars, saying that they are born and die, when it’s more like they transition into something new.

With stars in particular, there’s two main courses their “life cycles,” such that they are, can take: one for massive stars and one for low-mass stars.

We can further subdivide low-mass star “deaths” into those of red dwarfs—like our nearest stellar neighbor, Proxima Centauri—and those of medium-mass stars, like the sun.

But before we dive into the final stages of these stellar life cycles, let’s review what kinds of stars we’re talking about here…

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The Adult Life of a Star

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Stars are like cars. They need fuel to go. And also like cars, they don’t have an infinite supply.

But here’s where the metaphor breaks down. They can never refuel.

Yup. That’s right. For their entire lives, stars are stuck with only the amount of fuel they formed with. They can’t get more.

What happens when you’re driving, and you run out of gas?

Well, if you can’t refuel, you’re gonna have to call a tow truck. But stars don’t have tow trucks, and for them, it’s not a matter of moving or not—it’s a matter of life and death, such as it is.

But how does that work?

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The Average Star

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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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Eclipsing Binary Stars

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Imagine a frisbee.

At the center of this frisbee lies the sun—our sun, for simplicity’s sake. And sprinkled around the surface of its disk are all nine…excuse me, eight…planets of the solar system, plus the dwarf planets, asteroids, moons, Kuiper belt objects, Oort Cloud objects, comets, cosmic dust…

Okay, I could go on, but I’ll stop there. You get the picture. The whole solar system is on this frisbee. It’s a flat plane, disk-like. There aren’t orbits that put the planets up in the air  above or below the frisbee. They all lie, more or less, in the same basic plane.

Wait a second though…isn’t this post supposed to be about eclipsing binary stars? What the heck does our frisbee-like solar system have to do with that?

A lot, actually.

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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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