The Milky Way–our home galaxy–is a spiral galaxy, a classification I often describe as pinwheel-shaped.
The main difference between a spiral galaxy’s shape and a pinwheel’s shape is that spiral galaxies, like the Milky Way, only have two main arms. For the Milky Way, those are the Scutum-Centaurus arm and the Perseus arm. If you study the image above, you’ll notice that all the other arms are a bit wispier, and most branch off from the main arms.
There’s just one problem, though…
How do we even know that this image is an accurate depiction of our galaxy? How do we know that the Milky Way has spiral arms?
Consider that we can’t really take a photo like this of our galaxy. We’re inside it, and space travel has not advanced to the point where we can leave it just yet. There’s no way we can get a camera out to take a picture from this perspective.
Most things in the universe–like stars, planets, and even other galaxies–can be measured using their angular diameters. That is, we use trigonometry to find their actual sizes based on how large they appear to us in the sky.
But that doesn’t work for an object that we’re inside of.
In order measure the size of our own galaxy, early astronomers had to get a bit creative–with variable stars.
Imagine you’re observing the sky with a radio telescope. Observing the faintest, lowest-energy photons the universe has to offer is your specialty. You study interstellar dust clouds, protostars, and lots more.
One day, though, something interesting pops up in your data. You’re looking at raw data on a computer screen, not an eyepiece of a “typical” (optical) telescope—you get all your data from the giant dish above. Strangely enough, there’s a series of regular pulses.
At first, you think it’s just “noise” from sources on Earth—like static on your car radio. But then you see it, day after day, in the same place in the sky. It’s not static. It’s real.
You wonder if this is perhaps evidence of contact with a distant civilization. Personally, I’d hope for that one. Unfortunately, more research leads to the conclusion that it’s nothing of the sort—within weeks, you find that there are several other objects in completely different parts of the sky, all emitting similar (but different) pulses.
You’ve discovered a pulsar. But…what exactly is a pulsar?
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?