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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Stars: The Limits of “Normal”

How big—or small—can a star get?

As with most questions in astronomy, the answer to that is not definitive. But stellar models can give us a pretty good idea.

Mathematical models of stars tell us that their life—or, to use a less personifying term, function—depends on the balance between two opposing forces: internal pressure and gravity.

Stars produce energy to function. They don’t just do this to light up our skies and provide for life on their orbiting worlds. They need to produce energy to constantly support the weight of their own mass.

The more massive stars are, the more energy they need to produce—and the reverse is true too. There has to be a balance.

But is there a limit? Is there a point where balance is impossible?

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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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What Goes On Inside a Star?

Our sun is undoubtedly the star we know the best. It’s only 93 million miles away—which might seem far, but isn’t that large a distance when you realize that the nearest neighboring star is a whole 4.3 light-years away.

As in, it takes light—yeah, that same stuff that hits the ground from your flashlight in a split second—a whole 4.3 years to get here.

We’re pretty familiar with our star’s interior. We know it produces most of its energy in its core, a relatively small but very hot region at its center. We also know that energy then radiates outward until it hits the convective layer.

There, the energy gets stuck in circulation for a bit until it finally manages to leave the sun’s surface.

But…how normal is that? Is it the same for all stars, or just the sun?

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What Keeps a Star Stable?

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All life as we know it has to maintain homeostasis—that is, keep internal goings-on regulated. Body temperature is just one example. Mammals can maintain a stable body temperature with no trouble. Reptiles have to bask in the sun to keep warm.

You’re probably familiar with this idea. When you sweat, your body is trying to cool down. When you shiver, it’s trying to warm up. These are all examples of your own body maintaining its own homeostasis.

And then there’s blood pressure, heart rate, hormones, and pH—not that I have any real idea how all that works, but I know they’re all things that your body regulates on its own. Homeostasis is an important thing. Basically, when it fails, things go wrong.

And stars do the same thing. Continue reading

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

What is Contagious Star Formation?

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Recognize this nebula?

Yeah…we’re talking about the Orion Nebula again. I know, we already took a tour through the Orion constellation in my last post…but there’s still more to cover about how stars come to life, and Orion is still the best case study I know.

So…hold up a second. Contagious star formation? What’s that supposed to mean? I mean, usually, when you think about “contagion,” you think of catching diseases from others around you. So…can stars get sick?

Well, no. Stars are pretty good at maintaining their own homeostasis, something I’ll explain in a later post. By “contagious” star formation, I mean that star formation can trigger more star formation.

Basically…forming stars is contagious.

But how the heck does that happen? Continue reading