Science

# What is it? Stellar Classification

Stars light up the night sky. But as we saw a couple of weeks ago, not all stars are the same.

Thanks to the Hertzsprung-Russell diagram, we can easily see that there is a relationship between the surface temperature of a star and the star’s luminosity. This relationship can tell us a lot about stellar populations, but there’s even more information on that diagram. Let’s look at it again.

Source: European Space Agency

OK, so there’s luminosity on the vertical axis and surface temperature on the horizontal axis. But there’s something else on the horizontal axis. Nonsense letters.

OR ARE THEY? (Hint: No, they are not.)

These letters – O, B, A, F, G, K, and M – represent stellar spectral classes. Specifically, they represent measurements of absorption lines in a star’s spectrum.

Wait, back up. What’s an absorption line? Great question. Let’s pretend for a second that you’re looking at a bright, hot object behind a thin film of gas that is much cooler than the bright, hot object. The gas absorbs some of the heat and light from the bright, hot object, so if we tried to measure the spectrum of our bright, hot object, we would see features that indicate that the gas is taking some energy away at certain wavelengths.

Different stars have different absorption lines, depending on the types of elements the star is made of. We can use these differences to classify stars into spectral classes. Annie Jump Cannon of the Harvard College Observatory is credited with developing this particular stellar classification system.

So…what do the letters mean. As you can see from the H-R diagram, stars get cooler as you move from O-type stars to M-type. The biggest and shortest-lived stars are O- and B-type stars. A-type stars are kind of a blue-white and make up most of the stars we can see with the naked eye. F-type stars are more white. K-type stars are more orange and slightly cooler than the sun. The most common type of star, though, is the M-type star, which is also the smallest and least luminous.

G-type stars like our sun are yellow and, to paraphrase They Might Be Giants, middle-sized. These stars are interesting. There is something called the Yellow Evolutionary Void. Through their evolution, supergiant stars will swing between O- and B-type stars and K- and M-type stars. They don’t spend very much time as a yellow G-type.

Of course, the universe doesn’t necessarily like to pack things into such categories so neatly. There are extended categories for stars smaller and less luminous than M-type stars and for stars made of carbon. White dwarf stars have their own spectral classification system. Even within the original Harvard classification outlined here, there are subclasses that encompass more variation.

It’s amazing that these miasmas of incandescent plasma can vary in size, temperature, and chemical makeup. This is barely scratching the surface. It’s even more amazing that humans, stuck on our own little planet, managed to figure all of this out.

Featured image credit: Wikipedia