Spectral Classes.

Spectral
Class		 Approximate
Temperature (K)		 Other Spectral
Features		 Example
O 40,000 Ionized helium zeta Orionis
B 20,000 Neutral helium Rigel, Spica
A 10,000 Ionized calcium weak Sirius, Deneb
F 7,500 Ionized calcium weak Procyon, Canopus
G 5,500 Ionized calcium medium Sun, Capella
K 4,500 Ionized calcium strong Aldebaran, Arcturus
M 3,000 Titanium oxide strong Antares, Betelgeuse
Note that : 

The Earth's orbit around the Sun fits
within Betelgeuse. 
(What if Betelgeuse was our sun?)

Rigel's light is 60,000 times brighter 
than our sun.

(Both Betelgeuse and Rigel are in the 
constellation of Orion)

Our Sun has been going on for about
4,600,000,000 years and will 
continue for about another 
5,000,000,000 years.
spectral colors.gif (2945 bytes)

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