Ask Praise Science: Episode 2, Attack of the Kelvins

For our second edition of Ask Praise Science, reader Stu submits the following question:

How do astromomers / scientists / temperature-reading obsessed maniacs determine surface temperatures of stars? For instance, a blue-white star is known to be hotter than 30,000 degrees F on the surface. I know this has something to do with light waves and their color, but how can scientists be so accurate? And, on a similar tip, how about surafce temperature of planets we have not fully explored (i.e. Saturn's surface (well, its clouds) is quite cold, about -220 degrees F)?

Very good question. It definitely is pretty easy to take accept the information we receive about the universe as a whole from astrophysicists as given because, well, they are astrophysicists after all. It's not like they are making this shit up. As it is within all Scientific disciplines, standards and best practices have been developed in order to normalize research and findings. This is especially true for the physics of stars, formally known as stellar astrophysics, because all of our findings are based on the observation of light that in many cases is millions or billions of years old (except in the case of our own Sun and other local stars).

So how do Scientists "know" the temperature of distant stars (and other celestial bodies)? They use something known as the Stefan-Boltzman Law, which determines the effective temperature of a 'black body'... errr... those links are getting dangerously mathy and physicsy. Basically what this means is creating an estimate for the overall (or maybe 'average') temperature for a celestial object, since temperature can vary widely across the surface of said object. This is done by measuring the luminosity, or brightness, of the object and its surface area, and then calculating the temperature of an imaginary 'black body' with the same attributes according to the law mentioned above.

So the stellar "temperatures" that we read in all the interesting astrophysics articles that everyone surely read everyday are more like working estimates based on the purely observable data we can get by looking at stars through telescopes (size, brightness, etc.). This temperature in turn becomes another characteristic that we can use to classify stars and is in fact one of the variables used on the Hertzsprung-Russell diagram, or the Cliffs Notes of astrophysics.

As for planets like Jupiter, I'm pretty sure we know the atmospheric temperature because we've been there. Or at least our unmanned space missions have. As for other distant planets, I assume they use the same method as above.

That was long. Hopefully it allows you all, especially Stu, to praise Science in a more efficient fashion.