Spectral types?

Question

I have this assignment and I'm supposed to be graphing the absolute magnitude by the spectral type of certain stars, but it hasn't given me enough information as to how to work out the spectral type of the stars. I know the OBAFGKM and the number ranges that go with them, but I don't understand where the letter 'v' comes into it. This is the website http://www.astro.wisc.edu/~dolan/constellations/extra/nearest.html

Like, it says our sun has a spectral type of G2V, why isn't it just G2? And also, the star below it is M5.5Vc, what does that mean?

Thanks for any help

Answer 1

The number right after the letter is a finer grade of spectral class.  A G4 star is a little cooler than a G2 star.

The V is not a letter and it *is* very important in determining what magnitude the star would have, in a general range.

The V is a 5.  You will also see IV (4), III (3), II (2), and I (1).  These indicate generally what size of star they are and that relates directly to the magnitude.  A G2V star is a small one like ours.  A star designated K3I is a red supergiant.  A K3V is a red dwarf.  They would have the same intrinsic luminosity, but the K3I star would be tens to hundreds of thousands of times brighter because of its size.

I = supergiants

II = bright giants

III = normal giants

IV = sub-giants

V = main sequence dwarfs

VI and VII are rarely used.

Answer 2

BRANT and DOOM SHE get thumbs up from me for great answers. I'll complement those with some additional classification breakdowns just to muddy the water a little more : (

I (supergiants)



Ia-0 (hypergiants or extremely luminous supergiants



Ia (luminous supergiants)



Iab (intermediate luminous supergiants)

Ib (less luminous supergiants)



II bright giants

III normal giants

IV subgiants

V main sequence stars (dwarfs)

VI subdwarfs

VII white dwarfs

Answer 3

The Roman Numerals I through VII are used to designate what size/type the star is:

I Supergiants (divided into a -very luminous and b -less luminous)

II Luminous giants

III Giants

IV Subgiants

V Main sequence stars (dwarf stars)

VI Subdwarf

VII White Dwarf

Yep, big as it is, our sun is called a "dwarf" star.  Bet all the "dwarf" planets don't feel so bad, now.

Answer 4

You can gust ignore all those extra letters and use the letter, or letter and first number if you like. The extra stuff refers to special extra features in the spectrum that you don't need for this exercise.