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?
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.
Contrary to popular belief, space is not empty. The space between the stars is filled with clouds of dust and gas. And this space—the interstellar medium—is incredibly beautiful and fascinating.
I often refer to the interstellar medium as the galaxy’s “backstage.” Why? Because it’s not the part of the universe that astronomy enthusiasts usually think about. And yet, there are whole studies devoted to studying this natural wonder of the universe.
Also, the interstellar medium is largely hidden from us. There are ways we can detect it—when light from a distant star passes through it, for example. And with our eyes, we can see nebulae, the visible evidence of this interstellar expanse.
The backstage of a theater is similar—it’s not the main part of the show, but you sometimes see evidence of it in the forms of new costumes donned as the play progresses and new props brought into play. The audience often forgets about it entirely.
Nevertheless, it’s beautiful. Stars are born out of giant molecular clouds, triggered by compression from expanding bubbles of coronal gas. The interstellar medium spells our beginning.
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?
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.
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?
Can you find a horse in this image of the night sky?
Yeah, me neither. I’m lost. I see the Great Square of Pegasus because I know what to look for, but I still don’t see a horse.
Okay, so now I see half a horse. Where’s the rest?
Your guess is as good as mine, guys. Truth is, constellations very rarely look like what they’re named after. Constellations are more like relics of our ancient past than actual descriptors of what we see up there in the sky. But they do serve a purpose, even if that horse up there is missing his back legs.
I really find myself wondering if whoever made up Pegasus was concerned with animal rights… No animals were harmed in the making of this sky map…
Okay, yeah, never mind.
Where was I? Ah, that’s right. The purpose of the constellations, and the reason why it really doesn’t matter if they don’t look like their names say they do.