What do stellar classifications mean? (Beginner)

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, "OBe AFine 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.


This page updated on June 27, 2015

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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