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What do stellar classifications mean?

I am 14 years old and love astronomy. I need to get some specific information for a project on the big dipper. I'm mapping the big dipper's bowl and I want to know everything possible, but I can get most of that through simple research. My computer gave me some spectral type information on the stars in the area but I don't know what it means. If you could explain what A51v means I would be very grateful. Thank you.

Sure. Spectral type gives you a good idea about the color nad temperature of a star, as well as where it is in its life cycle. The "A" means that the star's surface is about 10,000 degrees Kelvin and the color is white. The "A5" is a subclass of "A," pinning down the temperature to a more exact value, but I wouldn't worry about that. The "1v" should really be the roman numeral "IV" which tells you that the star is a subgiant.

From hot (50,000 K) to cold (2,500 K), blue to red, the spectral types are:

O B A F G K M.

You can remember this with the mneumonic, "O Be A Fine Girl/uy, Kiss Me."

The roman numerals represent:
V: Main Sequence stars, like the sun, which are burning
hydrogen in their cores.
IV: Subgiant stars which have just recently run out of core
hydrogen and are burning hydrogen in a shell around
the core.
III: Red giant stars which have exhausted their supplies of
hydrogen and which are larger and brighter than IV
stars. RGs also burn hydrogen in a shell.
II: Asymptotic Giants and Horizontal Branch stars which are
burning elements other than hydrogen in shells (AGs) and
cores (HBs).
I: Supergiant stars are huge, very massive stars at the end of
their lifetimes.

The sun is a G2V-class star.

March 1999, Dave Kornreich (more by Dave Kornreich) (Like this Answer)

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