If we look at the info box for Alkaid, one of the stars in the Big Dipper, we see a very strange entry, right next to the (B-V) color index:

It says: "Spectrum: B3V".  It looks like some random collection of letters and roman numerals. What does it mean?

Stars are classified into categories depending on their temperature (color), age, and luminosity, which is related to the size of the star. The first level goes like this:

This classification was developed by the astronomer Annie Jump Cannon, long before astrophysicists actually understood why there were differences between the spectra readings of the stars they were seeing. Now we know that O-type stars are the hottest stars, and M-type are the coolest. Some astronomers recently have proposed an L-type star, even cooler than M stars. This stellar classification is a group popularly known as brown dwarves, and may be added to the official classification scheme sometime in the near future.

There is a mnemonic for the main spectral classes: "Oh, Be A Fine Girl (Guy), Kiss Me." If we introduce a new spectral class, the astronomers will have to invent a new way to memorize them!

The second level of classification divides each letter into 10 subclasses, based on a star's temperature within the class. Our own sun, for instance, is a G2 type star. Its temperature is about 5700 K. A G5 type star would have a temperature of 6000 K.

All stars with the same spectral class have the same temperature, and since temperature is related directly to luminosity, they have the same luminosity as well. In addition, interestingly enough, stellar luminosity and the mass of the star turn out to be related as well. Data from many binary star systems has established this relationship. So, two stars of the same spectral class also have the same mass. Generally, brighter stars are more massive, dimmer ones less massive - however, there are evolutionary stages which prove exceptions, as we will explore in the next spectral classification level.

The third level of classification tells us how old the star is. Not in years - a very hot star gets old faster than a very cool star - but in its evolution. All stars go through basically the same life-cycle. You can think of it as being the same as people, in the classic riddle of the Spinx: What creature first goes on four legs, then two, then three? A human, of course, in different parts of his own life-cycle. Stars go through the same sort of process.

Here is a chart of the third spectral classification, and what each one means:
 

Some of this is a bit misleading: the "dwarfs", spectra class V, are actually normal, "adult" stars, if you will. Most stars will spend most of their life-spans in spectral class V. A few stars, unusually massive stars, will start out in spectral classes I, II, or III. Especially low-mass stars will start out as class VI stars. However, stars which begin at V will tend to age into class II or III, then drop down to class D at the very end of their life-spans.

So, back to Alkaid. Alkaid's spectral class is B3V. So, we can see that Alkaid has a temperature of about 18,000 K, and is still on the main sequence - the "adult" stage of its life-span. Let's consider another star, Rigel. Rigel has a (B-V) color index of -0.03 and a spectrum of B8Ia:e. So Rigel has a temperature of about 25,000, and is an extremely luminous supergiant - meaning it is very big, as well as very hot. Rigel is what is known as a blue giant. Nihal, in the constellation Lepus, is a G5III type star - about 6000 K, and a yellow giant, reaching the end of its life-span.

There are other things which are sometimes noted in spectrum denotations, but most of these convey more complicated information than is necessary at this level.

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