# Eclipsing Binary Stars

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.

# How Far Are the Stars?

Stars don’t look small because they’re really the size of pinholes in a blanket. The smallest are the size of Earth. The largest have 128,865,170 times Earth’s diameter.

They look small in the sky because they’re distant. It’s for the same reason you can tell how far away your surroundings are by how small they appear; you know the mountains on the horizon are far away because they look shorter than your house.

The nearest star to our solar system is 4.3 light-years away. But what exactly is a light-year?

Light seems to travel instantaneously from your flashlight to the nearest surface, but it actually has a finite speed. In one second, it travels 299,792 km—fast enough to wrap itself around Earth’s equator 7.5 times.

In one year, light covers 9,460,730,472,580.8 kilometers, enough to wrap around the sun’s equator 2160.5 times. Four times that is the distance to the nearest star.

But how do we know this?

# The H-R Diagram

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?

# The Starlight We Can’t See

Find yourself a dark, unpolluted night sky on a clear night free of clouds, and you are very likely to look up into the heavens and see a sight quite like this. It’s what we see of the Milky Way, our galaxy.

When I’m at an astronomy event with a sky like the one above, I find it absolutely incredible. Do you notice how the stars don’t all look the same?

A couple are startlingly bright, there are numerous stars that are somewhat dimmer, and if you look really hard, you notice that even the dark night background is sprinkled with stars so faint they can barely be seen.

But what if I told you that you’re not even seeing the half of it?

# The True Brightness of Stars

Have you ever looked up at the night sky and noticed that while relatively bright stars outline the constellations, there are numerous other stars that are almost too faint to see with the naked eye?

If you ever noticed this, you probably guessed that the brighter stars are literally brighter, and the fainter stars truly are fainter. Or maybe you guessed that they don’t vary in brightness that much, but fainter stars are much farther away.

But that’s not really true…or, at least, it’s not the whole answer.

So what’s the real reason why some stars appear to be brighter than others—and how can we tell how bright they really are?

Stars are hot.

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?

# Info in a Rainbow

What do you see in this image?

If you’re from a larger city and haven’t had the opportunity to venture into a place like the desert, you might not know what you’re looking at. That’s the Milky Way, our name for our galaxy.

Inside this galaxy are billions of stars, including our own. Galileo Galilei was the first to discover that it was really many tiny points of light, not just a cloud-like haze across the dark night sky.

We can’t see our galaxy from outside, but we can learn a lot about it by looking out at it from within. It’s difficult. It’s like trying to learn about a building if you can never step outside one of its rooms.

But we can do it, with the help of the spectrograph.

# Einstein: Space-Time Curvature

When you hear about “space-time,” it’s just a way to say that space is related to time. And the curvature of space-time, as Albert Einstein predicted, is the way space and time alike literally bend around a mass such as the Earth or the sun.

That’s what’s diagramed above. This is a three-dimensional concept diagram of the way space sort of “clings” to an object. Notice the way it sort of tightens up when you get close to Earth? And because time is part of this whole equation…time sort of tightens up, too.

I assume that explains the “twin paradox,” as it’s called. That’s where the space-traveling twin returns home to Earth younger than their Earth bound twin.

Why? Seems to me it’s because time was tighter and passed faster on Earth, while it spread out and passed a bit slower for the traveler. (Don’t quote me on that, I just guessed that from this diagram.)

Einstein figured all this out. But scientists need evidence. Trusting Einstein’s genius wasn’t enough for them. How did they accept relativity as fact?

# Stars: Naming and Brightness

Meet Pegasus, and the constellations surrounding it. As I said in my last post, constellations are just regions of space.

Yes, they are named after mythical beasts and ancient queens, but for scientific purposes, all that matters are the regions they denote.This way, astronomers can easily find obscure, faint objects in the sky.

And telescopes can be easily programmed to find the same objects for those with less experience.

Keep in mind, though, that constellations only appear to fall in the same horizontal plane over Earth’s surface. Some of these stars, even in the same constellation, are light-years apart from one another.

So, in that case, the brighter stars must be closer to us and the dimmer stars farther away, right?

Wrong.

I know—not what you were expecting, was it? Well, like I mentioned in my last post, the brightest star in the sky is Sirius. It’s very far away from us. Proxima Centauri, on the other hand, is the nearest star to the sun…but it’s pretty dim, comparatively.

Fun Fact: Proxima Centauri is suspected to be part of a triple-star system also consisting of Alpha Centauri A and Alpha Centauri B. All three stars are gravitationally interlocked.

Proxima Centauri is the nearest of the three, making it the nearest star to the sun by a small margin.

The first thing we need to realize about stars is that they are not just pinpricks of light in the night sky. Shooting stars are about as close to being stars as are sea stars (more recent name for what were formerly “starfish.”)

We need to redefine the way we think about stars.

Think of stars as much like the sun. They are huge, exceedingly hot balls of gas whose gravity is so strong they drive their atoms to a breakneck frenzy.

I’ll talk more about stars and the way they produce heat in later posts—but for now, let’s just say that the hydrogen and helium gases that make up a star is so compressed and so excited that their atoms literally tear themselves apart.

That’s why stars are bright. They produce a ton of energy.

Anyway…because astronomers love labeling things in space, we have a way to describe the brightness of stars. Keep in mind that this only describes their apparent brightness—how they look to the human eye. Like I said, a star’s apparent brightness really has no bearing on its real brightness.

We use the magnitude scale.

This is a system that first appeared in the writings of the ancient astronomer Claudius Ptolemy. It’s likely the system actually originated earlier. Most astronomers actually attribute it to Hipparchus, a Greek astronomer.

According to the magnitude scale, stars are divided into six classes. First-class stars were the best and the brightest, just like the treatment you receive in the first class section of an airplane. And, naturally, sixth-class stars were the dimmest.

Well, actually, they were known as “first-magnitude” through “sixth-magnitude.” But I thought the class explanation made the whole backwards lay of the scale a bit simpler to understand.

I looked up first-magnitude stars on the Internet, and whadya know—I found a chart detailing the names and brightness of each one. It even cooperatively copied and pasted for me. So here you go!

Okay, so I’m guessing first-magnitude can also refer to stars whose brightness goes off the charts of Hipparchus’s system. That would explain the negative numbers. See, the fact is, there’s just too much variation in brightness in the sky to neatly group all stars under six classes.

In the modern day, astronomers use a scale numbered from -30 to 30. On this scale, the sun is -27. The full moon is at -13. When Venus is at its brightest, it measures at -5. And Sirius, the brightest star in the sky, measures at -1.47.

This is the apparent visual magnitude scale. Naturally, there’s an abbreviation for that, since I imagine it would get tedious to write out that the naked eye limit is about 6 apparent visual magnitude. We shorten it to “mv.”

So Polaris, the north star, measures at about 3 mv. The dimmest the naked eye can make out—the naked eye limit—is about 6 mv. The limit for the Hubble Space Telescope is 30 mv.

Naturally, there are dimmer stars than that, and the scale goes beyond 30 for the dim stars. But we won’t really need to talk about anything beyond 30. And there’s honestly nothing brighter than -30. You don’t get brighter than our own nearby sun.

Knowing the brightness of stars—at least their apparent brightness—helps us locate them in the sky. So, we already know how constellations help map out the sky. But how on earth can we keep all the stars straight? There’s trillions and trillions of them.

Easy. All we need to do is rank the stars by brightness within their constellations. We don’t need to worry about naming all of them. A lot of the brighter stars have Arabic names, such as Betelgeuse, Sirius, and Aldebaran. But there’s no way we can name them all.

And now you know why the stars of Orion have those peculiar little squiggles next to them in this diagram. Those are Greek letters. The stars are designated from brightest to dimmest in Greek.

So, according to the diagram above, Betelgeuse is ⍺, or alpha—meaning it’s the brightest star. Close behind is Rigel, β—or beta.

Even if you’re not familiar with the Greek alphabet—I had to learn it in sixth grade, but I never memorized the symbols—you can easily figure it out by ranking the size of the stars. This star chart has a magnitude key at the bottom that abides by the magnitude scale.

Now we know how to identify the apparent brightness of stars. But how do we refer to them by name?

Again, easy. Start with the Greek letter—you can spell it out or you can use the symbols. I thought I’d include a chart of them all for you here.

Keep in mind, when using the symbols for star designations, use the lowercase version.

After the Greek letter comes the possessive form of the constellation name. If that sounds complicated—especially given that they’re mostly in Latin—never fear, just look it up. For example, Canis Major becomes Canis Majoris.

So what would we call the brightest star of Canis Major? Alpha Canis Majoris, or ⍺ Canis Majoris.

Next up in my astronomy posts is the magnitude and intensity of stars. It’s going to be a little bit mathy, so feel free to skip it—I’m planning on releasing a post on the celestial sphere on the same day, and that’s going to be a lot more of just visual stuff.

See you guys around the galaxy!