What are White Dwarfs?

Now that we’re finally talking about white dwarfs, we’re getting into the really cool stuff.

In my last post, we explored planetary nebulae, and we left off with a question: where does the fast wind that forms planetary nebulae come from? Well, remember that planetary nebulae are formed from the atmospheres of medium-mass stars, but there’s still the stellar interior to worry about.

White dwarfs are objects comparable in size to our own Earth. They are the remains of medium-mass stars like our own sun. Often, you can see a white dwarf at the center of a planetary nebula with a large telescope. Together, they form what’s left of a star after it loses stability completely.

But there’s way more to a white dwarf than that…

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How Low-Mass Stars Die

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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What are Variable Stars?

What if I told you that the “two” stars you see here are actually one and the same?

This star, known as L Carinae after its location in the southern constellation Carina, is actually what we call a variable star. It is fairly bright, and its brightness varies significantly. And it’s not alone.

You might be familiar with a few variable stars. Betelgeuse, the bright giant in Orion’s shoulder, was all the rage among astronomers not too long ago. Polaris, the North Star, is also a variable. So is Algol in Perseus.

We’ve actually talked about one type of variable stars before. A variable star is any star whose brightness varies significantly and repeatedly. That means that eclipsing binaries fall within the definition. Algol is this type of variable star.

Now, though, we’re interested specifically in intrinsic variables, stars whose brightness changes because of something going on internally—not because another object passes in front of them and dims their light similarly to casting a shadow, as is the case with eclipsing binaries.

But…why would a star change in brightness like that?

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Story of a Star Cluster

Meet M13, one of my favorite globular star clusters.

M13, also known as Messier 13 or the Hercules Cluster, is found—surprise surprise—in the constellation Hercules in the northern hemisphere.

The really cool thing about star clusters is that they look just as spectacular through a telescope as they do in a good image—that is, on a clear, dark night with good seeing conditions.

So…why am I showing you a picture of a star cluster? (Besides the fact that they’re gorgeous?)

Well…after all the talk I’ve done of stellar evolution, I know what you’re going to ask me next…how the heck do we know all this?

That’s a very good question—and one that star clusters can answer.

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What Happens After Helium Fusion?

Back in August—sorry I took so long!—we talked about the helium flash, an explosion that occurs within stars when helium nuclei begin to fuse within a degenerate core.

So…this is not what the helium flash would look like.

Even though it’s a powerful explosion, it happens in such a small region in the center of the star that we wouldn’t see it at all, and the star’s outer layers absorb most of the energy from the explosion. I just thought it was a cool picture 🙂

In any case…what happens after the helium flash?

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How a Star Expands

Well, everyone, look who’s back!

For those of you who are not signed up for my newsletter, I’m sorry I’ve been away forever—life happened. It’s been a very rough three months. I hope you’re all doing well in light of the COVID-19 pandemic. I know it’s pretty tough right now, but we’ll pull through. Hang in there! 🙂

And now, for some long-awaited astronomy…

Meet Betelgeuse, a bright star in the winter constellation Orion.

Betelgeuse is a cool red supergiant that we’ll talk about a lot more in just a couple weeks, when we cover variable stars. Not too long ago, it was the height of excitement among astronomers. No one was sure why it…well…appeared to be dimming.

Yeah. Like a lightbulb. It was literally getting fainter—considerably fainter.

It’s pretty normal for Betelgeuse, like any other variable star, to fluctuate in brightness over time, but it was doing something downright weird. We’ll explore what was going on with it soon enough.

For now, let’s take a look at why Betelgeuse, as a supergiant, is so darn big.

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The Adult Life of a Star

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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What is a “Normal” Star?

If we were talking about people, I’d say there’s no such thing as a “normal” person. We’re all weird in our own way—that’s what makes us unique and ourselves.

However, there’s such a thing as a functional human—a human with a combination of functional organ systems and/or prosthetics that makes daily life navigable. And just as no star is exactly alike, there are functional stars.

Nature makes mistakes all the time. It is not intelligent—it doesn’t know the best way to do anything. It doesn’t know the path of least resistance or least effort. It just tries everything at random, and we get to observe what happens.

A “normal” star is what happens when nature stumbles upon the right conditions. But…what does that mean?

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Observations of Star Birth

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Astronomers have a pretty solid idea of how stars are born. They begin within the dense, cold dust of an interstellar cloud such as this one. They heat up and get more luminous as they contract, and then drop in luminosity as they continue to contract steadily toward the main sequence.

I’m going to spend at least the next ten or so posts talking about the main-sequence portion of a star’s life cycle. Basically, we’re talking about a star’s adulthood.

You know what, while we’re at it, why don’t I draw up an analogy between a star’s life cycle and that of a human:

  1. When a human is a mere fetus developing within its mother, a star is a protostar.
  2. We say a star has been “born” when it crosses the birth line—basically, satisfies certain expectations for its temperature and luminosity for its specific mass—and becomes visible.
  3. After that, a star steadily approaches adulthood. A “child” star is referred to as a Young Stellar Object (YSO) or a pre-main-sequence star.
  4. An “adult” star is one that has begun to fuse atomic nuclei in its core for fuel. At this stage, the star has reached the main sequence.
  5. When a star runs out of fuel, it leaves the main sequence. We’ll cover this evolution in depth very soon.

I explained this process in depth in my last post. But I also posed the question: how do astronomers know all this? Where’s the evidence? Continue reading

The Story of a Newborn Star

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What happens when a star is born?

A couple of posts ago, I explained how a protostar forms out of a dense cloud core within the interstellar medium. But…wait. What exactly is a protostar again?

A protostar forms when one dense core of an interstellar cloud condenses enough so that gravity can overcome the repulsive forces between the particles, and collapse the cloud. A very cool object then forms in the cloud’s depths, visible only at infrared wavelengths—known as a protostar.

A protostar is compressed enough to be opaque no matter the wavelength—that is, no radiation can pass through it due to its density. However, what separates it from a “true” star is that it’s not compressed enough to generate energy by nuclear fusion.

Astronomers also define a protostar specifically as a young star that’s not yet detectable at visible wavelengths. In other words, protostars emit only longer-wavelength light—that is, infrared and radio waves.

You’d think that becoming a true star would be the next step for a protostar. But that’s not quite how it happens… Continue reading