Sorting Things In the Sky: Stars

A basic part of astronomy (and pretty much any science) is taxonomy:  classifying things according to similar characteristics. If you’ve ever gotten into rock collecting, you probably remember picking up rocks that looked alike and putting them in order of color or size or mineral composition. You can do similar things with plants or animals.

Astronomers do it with planets, stars, and galaxies. Although, it’s not like they can sit out there and collect stars and put them on a bookshelf somewhere to be admired.

The first stellar taxonomy was pretty easy. Blog, the caveman stepped outside with his friend Ogga and pointed up to those little lights in the sky and said the caveman equivalent of “Look at those shiny things Ogga.”  And she probably said something like, “Yes, look at them. What do you suppose they are?”

That question didn’t get answered for thousands of years. But, Blog and Ogga, being inquisitive thinking beings probably set out to try and answer it anyway. The first thing they noticed was that there were two types of stars: bright ones and dim ones.  Voila!  The first stellar classification was made.

That probably worked well for a while, since Blog and Ogga and all their cavemates were also busy just trying to survive the last bits of the Ice Age glaciation or onslaughts of attacks by saber-toothed tigers, or storms destroying their crops. There wasn’t a lot of time for stargazing, but at least they had “bright” and “dim” sorting part down.

Star classification has come a long ways since then. The next steps were to classify “bright” and “dim” into levels of “brightness” or what we call “magnitude.”  From there, scientists started noting stellar characteristics like color (notice the colors in this HST image at left — our eyes don’t see such bright colors, but specially filtered telescopes do).

Now we have stars lumped into classes depending on their temperature and spectra (the properties of light they give off).  You can read more about the intricacies of the classifications here or here.

Essentially, however, you can lump stars into categories by colors (which are determined by spectral observations), with blue and blue-white stars being the hottest and brightest and yellow-orange and orange-red stars being the dimmest and coolest. And, then there are things that are dimmer but aren’t quite planets, and those get set aside as dwarfs and dwarf objects.

Those classifications don’t just tell you about the star’s color and luminosity and chemical makeup; embedded in that taxonomy is a lot of back story about how each type of star forms and what its future history is likely to be. That’s the essence of stellar astronomy and astrophysics, and it all began with classification. There aren’t just two types of stars anymore–there are many types, and astronomy grows richer each time scientists uncover a new type or tease out the details of stars classified in existing types.