Much earlier on—probably months ago now—I explained how something called the proton-proton chain generates massive amounts of energy within stars, and enables them to fuel whole solar systems. That’s the battery of a star.
We’ll address the proton-proton chain later, when we start talking about star life cycles. We’ve still got some talk about nebulas and interstellar space to go before we get that far. For now, what’s important is that the proton-proton chain depends on high density.
That is, stars will have the strongest batteries if they have very dense interiors. It doesn’t really matter how dense their middles and atmospheres are. But conditions in their cores must be very dense.
You’ll find, if you study stars closely, that there is a definite relation between their densities, masses, and luminosities.
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?
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?
Consider a solar system far different from our own. A solar system governed by two suns, and consisting of planets we can only dream of.
Would it surprise you to hear that, based on recent discoveries, that might actually be the norm?
The surroundings we grow up in determine our outlook on the world, and this is never more true than with our solar system. Our eight planets (though some would vehemently insist upon nine) and their parent star are all we know.
But what if I told you that most of the stars you see when you look up at the night sky have companions? And often, these companions are impossible to detect by visual means.
Here’s a visual binary that just about stretches the limits of the definition. It’s a star, though you’ll never see it like this with the naked eye. Specifically, this is Sirius, the brightest star in the sky.
But if you look closely on the top left, you’ll see a tiny dot just peeking out from behind Sirius’s brilliance. That’s Sirius B, this bright star’s faint companion. Together, they’re known as Sirius A and Sirius B.
It’s tradition for astronomers to name all the stars in a system the same thing, but it also makes sense. Most of them aren’t obvious. You might look at some ordinary-looking star in the sky, say…Antares. But as it turns out, Antares has a barely-visible companion.
The visibility of visual binaries has a wide range. Consider the famous double star in the Big Dipper, Mizar.
We know how big stars are; they range from the size of the Earth to over a thousand times the size of the sun (which is in itself over one hundred times the size of the Earth). We know they’re huge.
But how massive are they?
Yes, that’s a different thing.
A pingpong ball and a golf ball are close to the same size, but a golf ball is much more massive—in that it has more stuff in it. A pingpong ball is hollow and easily tossed; a golf ball has more matter in it and will hit the ground with a harder thunk.
Stars are similar. They have a wide range of sizes, but nothing I’ve described thus far has told us about their masses. That is, how much stuff is in them? Are they like puffy gaseous balls, or are they more dense, like planets?
The best way to learn about stars’ masses is by studying binary stars. But what exactly are binary stars?
By now, I’ve introduced you to a lot of different ways to classify stars.
Months ago, I talked about the different spectral classes—O, B, A, F, G, K, and M. Even before that, I told you about apparent visual magnitude, our ranking system for how bright stars appear to the naked eye.
More recently, we explored absolute visual magnitude and the related absolute bolometric magnitude and luminosity. All these are related to a star’s actual brightness, not just how bright they seem to be from Earth.
And last but not least, we talked about the H-R diagram and how to rank stars by their luminosity classification.
In short, it may seem like sorting stars is a complicated business. But it’s not really. And here, I intend to give you an overview to put all this together.
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.
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.