K is for Kelvin, or how we measure temperature in the universe

So yesterday I inflicted math on you and suggested you choose a temperature for a cloud of hydrogen in outer space.  Day before I enforced somewhat easier math on you, but only gave you a temperature in Celsius.

And all the Americans stuck out their lower lips and whined, "But what's that in Fahrenheit?"

Nuh-uh. I am not going to give you any temperature in Fahrenheit, now or ever. Real scientists calculate in Kelvin, as it's one of the base units of Système International. (Degrees Celsius are often used when dealing with relative temperatures and kelvin are used with absolutes.) Astronomers, on the big scale, prefer to measure in Kelvin (K), because we often deal with extremely hot stuff (blue stars at 30,000 kelvin) and extremely cold stuff (hydrogen clouds at 5 kelvin).

What, you say?  Who's this Kelvin dude?

William Thomson, 1st Baron Kelvin was a 19th century scientist who got elevated to the peerage for his marvelous work in thermodynamics. He was quite famous, enjoying the kind of fame and fortune a Kardashian can only dream of. Yep, even Scottish nerds can be meritocratically famous.

He expressed a need for an absolute thermometric scale that went from absolute zero on up. Fahrenheit (imperial) and Celsius (metric) were both based on arbritrary temperatures (like the freezing point of water... but is that pure water or a 50/50 saline solution? and the temperature of the human body, and boiling water... but is that at sea level?).  He wanted a system that was free from such arbitrariness.  The only thing that would be absolute enough for him was the absolute absence of energy, or Absolute Zero.

Thus, the kelvin scale was born and named after Lord Kelvin.

The kelvin scale's good for measuring colour temperature, as there is a correlation between the colour of stars and their temperature.  (Remember a couple of days ago when you calculated your personal peak radiation, and you ended up shining brightly at about 900 nm in the infrared? That's what I'm talking about.)

The human body, at approximately 300 kelvin peaks in the infrared. Our lovely G2-type Sun peaks in the yellow-white visible light range at 5778 kelvin. It's hotter, so that's why it glows so much brighter than we do.

Bellatrix, one of the blue stars in the constellation Orion is a B2-type star glowing at about 22,000 kelvin. But if you go around claiming how hot Bellatrix is, people might wonder if you're interested in Helena Bonham Carter. Bore them with math and science instead.

Wanna go hardcore? Use Planck's law to calculate what temperature you'd have to be to start glowing visible red (about 650 nm).