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.

Before we take a look at eclipsing binary stars, let’s focus on binary stars in general. As you might be aware from my previous three posts, binary stars are pairs of stars that orbit one another.

That’s right. They orbit one another, two stars, just like Earth orbits the sun.

Well, not quite like that. Earth orbits the sun because it’s hundreds of times smaller than the sun, so its gravity isn’t strong enough to drag the sun around in an orbit of its own. A star orbiting another star, though, would be a different story.

As I’ve described before, binary stars orbit around their center of mass, which is the balance point of their system. The more massive star will have an orbit that circles much closer to this point; the smaller star will stay farther out.

Actually, that’s the way any orbit works. But it’s hardly noticeable when we’re talking about a star and a planet, or a planet and a moon. The Earth and sun’s center of mass is deep inside the sun, so the sun just wobbles around in space a little bit. And the same goes for the Earth and the moon.

Anyhow…I’ll return now to my frisbee analogy.

A binary system works the same as any solar system, except, you know, with an extra star in the picture. The orbits of both stars will be on the frisbee, not above or below it. My point: they will be in the same orbital plane. They will be flat with one another.

Now think a little more about that frisbee. The one I showed you above was tilted at a bit of an angle. But what if we looked at a frisbee edge-on?

Here’s a frisbee that’s more or less edge-on. It’s not perfect, but it’s good enough to show you what happens with eclipsing binary stars.

So what are eclipsing binary stars, exactly?

They’re binary stars like any other. There’s nothing special about their solar system in itself. The only thing that grants them a special name is the way their orbit is tilted relative to Earth.

Meaning, absolutely any binary star system out there—be they a red giant and a white dwarf, or two G-type stars, or far apart or close together, is an eclipsing binary system if it’s angled like this frisbee to our line of sight.

It’s just a description of how the star system looks to us, from where Earth happens to be in space. Not a description of anything about the star system itself.

In an eclipsing binary star system, each star regularly moves in front of the other. And when the happens, we see little dips in the light curve.

Light curve?

Think about it. If you see two equally bright stars right next to one another in the sky, they’re both emitting light. The light from both of them reaches your eye. But if one moves in front of the other, that light is cut in half.

The light from one of them reaches your eye. But the light from the other gets stuck behind the one in front of it, and you never see it.

You don’t see this happening. If you were to aim your telescope at an eclipsing binary—provided that they’re near and bright enough to see at all—you’d just see one star.

But if you could measure the total light coming from that star, you’d realize that it’s changing periodically over time. Because you can only make out one star, but there are really two there, too far away and too faint to make out as more than one.

And periodically, one moves in front of the other and blocks half the light.

Except, very rarely will it actually block half.

Binary stars aren’t always equal in brightness—well, actually, it would be more accurate to say luminosity. In astronomy, brightness implies just how bright a star appears from Earth. That’s it’s apparent visual magnitude.

Luminosity is the total amount of energy coming from a star, and I mean all of it, not just the visible light.

In a normal case, we’ll have stars of not only two different luminosities, but two different sizes—as in the case of the diagram above. So let’s take a look at that again.

Let’s call that small red star a red dwarf, and the larger yellow star a G-type star.

Red dwarfs are small, relatively cool stars. They’re faint not only because they’re cool, but also because they don’t have much surface area to emit light. G-type stars are those like our sun. They’re average as far as star sizes go, and pretty average as far as star temperatures go, too.

Now remember, luminosity depends on temperature and surface area. That’ll be important in a second.

Let’s say the two stars start out apart from one another, to our line of sight. But then the red dwarf moves in front of the G-type star. The red dwarf is too small to eclipse the G-type star completely. But it will eclipse some of the G-type star’s light.

Notice what’s happening with the graph below it? That line measures brightness, and as the red dwarf eclipses its larger, brighter companion, the total brightness that reaches our eyes drops dramatically.

Then the two stars move apart again. The brightness is back to normal.

Next, though, the red dwarf will swing around behind the G-type star. And now the red dwarf is completely eclipsed. The brightness will drop once again. When the red dwarf moves in front of the G-type star again, the same thing will happen.

But notice something else. These eclipses are labeled as primary and secondary, and there’s a good reason for that—it looks like the brightness drops a lot more during the primary eclipse.

But why?

Well, let’s consider that luminosity, temperature, and surface area relation. How large are these stars…and how much light is being blocked, exactly?

Notice this: in each eclipse, the same amount of surface area is always being blocked.

The red dwarf can only eclipse an area of the G-type star that’s equal to its own size. And when the red dwarf is eclipsed, it’s entirely eclipsed, and the light we lose is exactly equal to the light the red dwarf itself emits.

The same surface area is being lost each time. But the G-type star is hotter, so it emits more light for the same surface area. When the red dwarf eclipses its more luminous companion, it eclipses the same surface area but more light.

And when the G-type star eclipses the red dwarf, that light is hardly missed. We see the light curve drop a bit, but the red dwarf was hardly emitting that much to begin with. It’s a smaller eclipse.

The smaller star won’t always be the cooler star. If we had a white dwarf—an extremely hot but small star—and a red giant, a much larger, cooler star, then whenever the white dwarf was eclipsed, we’d get a much more significant drop in the light curve than we would whenever the red giant was eclipsed.

Anyway, I think that’s enough on binary stars for now. Next up, let’s take a broader look at stars in general. What is the average star like?