What do you think it would mean for a star to be in a specific luminosity class? I mean…does that mean they go to school to learn how to be bright?

(Ha, ha…yeah, I know, bad astronomy pun.)

Well…not quite.

Stars can be sorted in a lot of ways—and a good thing, too, because there are literally trillions upon trillions of them. Astronomers would be lost if we couldn’t sort them into groups to study.

They can be sorted according to spectral type (composition and temperature), apparent visual magnitude (how bright they look to the naked eye from Earth), and absolute visual magnitude (how bright they would look to the naked eye from ten parsecs away).

They can also be sorted according to their absolute bolometric magnitude (how bright they would look from ten parsecs away if the human eye could see all types of radiation).

And…they can even be sorted according to their luminosity.

But before we dive into luminosity, let’s take a look at spectral types again.

The main spectral types are, in order from hottest to coolest, O, B, A, F, G, K, and M.

Before you ask me why the heck astronomers decided on the most illogical order possible for those letters…that just happened over time, as the system of spectral classification was perfected. Some classes were reordered, removed, added, etc, leaving us with this fine arrangement of letters.

Astronomy students have made the best of it, coming up with all sorts of creative mnemonics—my favorite is Only Bad Astronomers Forget Generally Known Mnemonics, but the most memorable one is probably Oh, Be A Fine Girl/Guy, Kiss Me.

Anyway…have you noticed the numbers written on the right side of the diagram above? Those are temperatures in Kelvins, which are basically Celsius degrees plus 273. That tells you that the spectral classes are in order of temperature.

But what do you also notice about the spectral classes? Take a close look at the rainbow-like strips for each one.

If you noticed that those weird black lines fade smoothly in and out of existence as you run your eyes up and down the spectra, you’re right. And there’s a reason for that.

I’ve already talked a bit about how spectra work. Actually, I’ve talked a lot about it—see the “Atoms, Starlight, and Spectra” heading on my Astronomy page. But for the bare bones, just click on over to this post I wrote a while back.

Basically, the whole universe is made up of beyond-microscopic particles called atoms. Just as letters in different combinations make up words in a language, atoms in different combinations make up everything you’ve ever perceived with your senses…and then some.

Very conveniently for astronomers, each atom has its own unique signature. If you use a prism to separate white starlight into a rainbow, atoms in a star’s atmosphere will block certain wavelengths—and you can tell which atoms they are by which wavelengths are blocked.

You get something like this:

This spectrum most likely came from a B-class star. You’ll see what I mean if you match it up to the spectra in the spectral class diagram up above—this spectrum most closely resembles that of the B-class, at 20,000 Kelvins.

How do we know how hot these stars are, anyway?

Well, like I explained in my previous posts on spectra, certain atoms can only be found in stars of certain temperatures. Heavier atoms are more likely to be stripped apart in the extreme temperatures of very hot stars, so they’re more likely to be found in cooler stars.

There are a few specific elements—molecules that contain only one type of atom, bonded together—that help astronomers identify a star’s temperature. But I’m not going to go into that here.

The point is, if you have a spectrum, you have a star’s temperature, and thus its spectral class—O, B, A, F, G, K, or M. But that’s not all you have.

Notice what happened to the spectral lines?

What’s happening here? Well, most obviously, the spectral lines are getting broadened. That means that the energy levels within the stars’ atoms are getting distorted. And that means that the atoms are colliding more often than normal.

So, basically, broader spectral lines will appear in stars whose atoms collide faster.

How do we get atoms to collide faster?

Well, the reason atoms collide at hyper-speed in stars in the first place is because of the absolutely insane temperatures found there. Our 20,000 K B-class stars are 19,727℃—way hotter than an oven.

If you flew into a star, you wouldn’t just be vaporized. You would instantly be torn apart at the atomic level. The carbon that’s essential to life on Earth can’t exist in the cores of hot stars—it’s a heavier element, and it couldn’t survive.

So…how do we get stars to such insane temperatures?

The simple answer is their density. Their atoms zip around like crazy because it’s essentially like putting hundreds of people in an auditorium and making them run around at freeway speeds. It’s a recipe for disaster.

What we get is atoms colliding all the time, which cranks up the heat. Stars are not dense because of their heat—they’re hot because of their density.

But stars don’t all have the same density. Larger stars—like giants and supergiants—have less dense atmospheres, something I’ll elaborate on in future posts.

So…larger stars have less dense atmospheres, and thus fewer atomic collisions. This means their spectral lines will not be as broad as those of smaller stars.

And thus, we have another way to identify stars: by the width of their spectral lines.

Here’s that spectral line comparison image again:

And here is the H-R diagram, which I’ve been describing in recent posts.

You can easily identify both giants and supergiants on the H-R diagram. They are defined by both their size and their luminosity, and differ slightly in their spectral classes (see O, B, A, F, G, K, and M on the horizontal axis).

It’s easy to calculate a star’s size and luminosity based on visual magnitude and spectral class; it just takes a few simple steps. That way we can find out what type of star it is. But we don’t necessarily have to.

If we can identify a star as main sequence, giant, or supergiant by the width of its spectral lines, we now have yet another simple, easy way to classify it: by the luminosity classes.

The luminosity classes are as follows:

• Ⅰa − bright supergiant
• Ib − supergiant
• Ⅱ − bright giant
• Ⅲ − giant
• Ⅳ − subgiant
• Ⅴ − main-sequence star

The luminosity classes are useful because we can use them with spectral classes to clarify more about a star.

For example, the sun is a G2 class main-sequence star, which would be written as G2Ⅴ. Rigel, the bright blue star of the constellation Orion, is a B8-class supergiant—or a class B8Ⅰa star. Arcturus, on the northern horizon near the Big Dipper, is a class K1Ⅲ star.

Wait…but what about the white dwarfs? How do we classify them?

Well, they don’t really fit on the luminosity classification system. Stars may conform to certain patterns, but the ways astronomers choose to sort stars are arbitrary.

The thing is, we can’t expect all stars to conform to our sorting systems, and white dwarfs aren’t the only rebels. There are also L, T, and Y dwarfs, which I’ll talk about much later.

But for now, that’s enough on spectral and luminosity classes. Next up, we’ll take a quick look at some other ways stellar spectra are helpful in studying stars.