Spectral Types Of Stars

Temperature

A close approximation of a star's temperature can be found by just looking at its color, but can you identify the color of the Sun? Most people would identify the Sun as yellow, but it is in reality a white star. This misrepresentation most likely comes from the Sun being colored yellow in popular media and entertainment. The color of stars range from red/orange being the coolest, yellow in the middle, and white/blue the hottest.
Looking at a star to determine temperature is not a very scientific way to go about things, so scientists turn to something called a spectrum. A spectrum is created when a star's light is passed through a prism and separated into it's different colors (Mother Nature calls this process a rainbow.). Every chemical element present in a star produces a line in the spectrum and by studying these lines scientists can determine a star's temperature and what the star is made of. Once a star's temperature is precisely determined, it is given a letter identification from one of the following : O B A F G K M with O being the hottest and M the "coolest" (Our Sun is type G). Below is the list of letters corresponding to their temperature range :

Star Type Color Approximate Surface Temperature Average Mass (The Sun = 1) Average Radius (The Sun = 1) Average Luminosity (The Sun = 1) Main Characteristics Examples
O Blue over 25,000 K 60 15 1,400,000 Singly ionized helium lines (H I) either in emission or absorption. Strong UV continuum. 10 Lacertra
B Blue 11,000 - 25,000 K 18 7 20,000 Neutral helium lines (H II) in absorption. Rigel
Spica
A Blue 7,500 - 11,000 K 3.2 2.5 80 Hydrogen (H) lines strongest for A0 stars, decreasing for other A's. Sirius, Vega
F Blue to White 6,000 - 7,500 K 1.7 1.3 6 Ca II absorption. Metallic lines become noticeable. Canopus, Procyon
G White to Yellow 5,000 - 6,000 K 1.1 1.1 1.2 Absorption lines of neutral metallic atoms and ions (e.g. once-ionized calcium). Sun, Capella
K Orange to Red 3,500 - 5,000 K 0.8 0.9 0.4 Metallic lines, some blue continuum. Arcturus, Aldebaran
M Red under 3,500 K 0.3 0.4 0.04
(very faint)		 Some molecular bands of titanium oxide. Betelgeuse, Antares

Brightness

Anyone can tell that the star Sirius appears brighter than Rigel in Orion, but did you know that Rigel is thousands of times more luminous than Sirius? Everyone knows that stars are at different distances, therefore because Sirius is so much closer to us than Rigel, it appears brighter. Scientists needed a level playing field in order to compare the "absolute" brightness of many stars, so they decided to determine what a star's magnitude would be if it were placed 32.6 light-years (10 parsecs) away from Earth. After placing the stars at this distance we find a range from -8 to +15 magnitudes with Sirius being at +2, Rigel equal to -7 and the Sun at +5.

Once again we have shown the Sun to be near the middle of the chart. We have now discovered how to determine temperature and luminosity, but is there a way to combine the two? and are there any other variables among stars?

H-R Diagram

The Hertzsprung-Russell Diagram is named after the astronomers Einar Hertzsprung and Henry Russell, who in the 1910's began to see a relationship between a star's temperature and brightness. The H-R Diagram is simply a graph of stars with absolute magnitude plotted on the y-axis and temperature on the x-axis. After plotting thousands of stars on the H-R Diagram, scientists began to see four main groups of stars, which are : Dwarfs, Main Sequence, Giants, and Super-Giants.

Main Sequence

As you can see, main sequence stars cover a large area of the H-R Diagram, from bright, hot stars to dim, cool ones. So what sets them apart? Equilibrium. These stars are almost identical to the state that they were born in. They neither expand nor contract great amounts, and will die fairly non-explosive deaths. Famous examples are our Sun, Sirius, Spica, and Vega.

Giants and Super-Giants

As the Sun, and others in the main sequence, grow older they begin to expand. While expanding, more light is given off and the Sun will climb the H-R diagram until it reaches giant status. (Don't worry, it will not be for at least five million years.) Stars that start off much more massive than the Sun will be able to expand that much more and reach super-giant status. Famous examples are B