Star Properties

Two major properties of a star:

1 - Brightness
2 - Surface temperature
 

Brightness:

2,000 BC Greek Hipparchus called the brightest stars "first magnitude" and dimmest visible with the unaided eyes "sixth magnitude".  In 19th century quantified so that each step is 2.512 times.  A star of the 1st magnitude is therefore exactly 100 times brighter than a star of the sixth magnitude. (2.512**6)

apparent magnitude - as we see it, as it appears from earth.
absolute magnitude - intrinsic brightness, as it would appear at a distance of 10 parsecs (32.6 light years)  Normal stars range from about -6 to 20 on this scale.  The sun is a 5.
 

Surface temperature:

Like the temperature of a hot piece of metal, it can be deduced from the color.  Red stars were coolest, like Antares, near 2000 degrees K. Yellow stars, such as our sun, are medium hot, about 5,500K.  White stars, like Procyon, are tens of thousands of degrees Kelvin.  The very hottest blue-white stars are more than 50,000 degrees K.

Spectral Types

Type  Color   Temperature (degrees K)    Example
O    Blue        28,000-45,000           Mintaka (O9.5)
B    Blue        10,000-28,000           Rigel (B8)
A    Blue-white  10,000-7,500            Sirius (A1 V)
F    White        7,400-6,000            Procyon (F5)
G    Yellow-white 6,000-5,000            Sol (G2)
K    Orange       5,000-3,500            Arcturus (K2 III)
M    Orange-red   3,500-2,000            Antares (M1)
L    Infrared     1,500-2,000            2MASS J1146+2230

Note: Class L was added in 1998.  Other classes are over 100 years old.

Subtypes divide each class into more specific types to better classify the full range of stars.