What do the numbers in a star's classification represent?

First is the spectral class:
OBAFGKM ranging from hottest to coolest and from bluest to reddest.

Between each of those classifications are numbers 0-9

So there are ten gradients between a B and an O type star, thus a B0 is very similar in temperature and colour to an O9. So you can think of the 0-9 simply as ten subdivisions of the OBAFGKM system.

This is known as the Harvard spectral classification and as you rightly pointed out our Sun is a type G2.

When it comes to luminosity though there is another classification called the Yerkes spectral classification.

There are numerous suffixes, ranging from largest to smallest.

0 hypergiants
Ia luminous supergiants
Iab intermediate luminous supergiants
Ib less luminous supergiants
II bright giants.
III normal giants.
IV subgiants.
V main-sequence stars or dwarf stars.

And then these prefixes for stars smaller than normal main sequence dwarf stars.

sd subdwarfs.
D white dwarfs.

Hope this helps but for full information check out the wiki page.

https://en.wikipedia.org/wiki/Stellar_classification

It's a transitional index, showing the extent to which the spectral characteristics of the star are morphing from those of late F type to those of early K type.

G0 is almost the same as F9.
G9 is almost the same as K0.

c2v password.