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Black-body Radiation & Spectral Types

Black-body radiation

Everything which gets hot enough glows. As objects are heated up they start to glow – at first only slightly, with a dull red colour like a stove element. As it heats up further it glows a brighter red, then orange, and yellow.

A very thin piece of tungsten wire – a filament – is used in an incandescent light bulb. When a current is passed through the filament, it heats up so much it glows brightly with a yellow-white light.

If an object were to be heated up further it would glow white, then as it got really hot it would start to glow with a blue tint. However, no solid materials can be heated up that much without melting or sublimating, and even tungsten evaporates at 6203 K (5,930 °C).

Spectral types

Stars are classified according to their spectral type, which describes how hot the surface of the star is. (The interiors of stars are much much hotter.) Very hot stars have a blue tint, very cool stars are red. Most stars are relatively cool.

In order of increasing colour temperature (K = kelvin), from red to blue: M, K, G, F, A, B, O.

Spectral Type Surface
Colour Occurrence
M 2,400-3,700 K Called "red", actually light orange red. 76%
K 3,700-5,200 K Called "orange", actually pale yellow orange. 12%
G 5,200-6,000 K Called "yellow", actually yellowish white to white. 8%
F 6,000-7,500 K Called "yellow-white", actually white/slightly blue. 3%
A 7,500-10,000 K Called "white", actually blue white. 0.6%
B 10,000-30,000 K Called "blue white", actually deep blue white. 0.13%
O ≥ 30,000 K Called "blue", actually blue. very rare

There are sub categories within each of these types. Our Sun is a G2V type star. It is white, but appears yellow through our atmosphere because the atmosphere scatters some of the blue light.

Hotter stars are normally bigger, and cooler stars are normally smaller. Big hot stars burn through their nuclear fuel quicker than small "cool" stars, even though they have much more fuel to use.

Within 16.3 light years (5 parsecs) of Sol are a total of 55 hydrogen-fusing stars. 46 of them are red dwarfs. There are also 14 brown dwarfs, and 4 white dwarfs.

Even though they're quite close, only the nine hydrogen-fusing stars that aren't red dwarfs can be observed with the naked eye. All the others are too small and dim to be seen.