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Basic Stellar Evolution for Stargazers

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Here's a (very) basic run-down of star formation, life and death:

1. Diffuse Nebulae
Clouds of interstellar gas and dust collapse gravitationally and heat up, becoming star-forming diffuse nebulae.
(Examples: Orion Nebula, Lagoon Nebula, Trifid Nebula)

2. Open Clusters
The young stars have pretty much used up the surrounding gas and dust but are still a gravitationally-bound group of stars traveling through the galaxy together. Open clusters still contain some massive, hot, bright stars (O and B type), giving many clusters a pretty, "jewel-box" appearance. Some nebulosity may still be visible - for example, in the Pleiades.
(Examples: Pleiades; Beehive; Hyades; Wild Duck Cluster; E.T. Cluster)

3. "Main Sequence" Stars

Star Classes: Giants, Supergiants, and "Oh Be A Fine Girl (Guy), Kiss Me"

Astronomers classify stars by the type of light and other radiation they emit - their spectra.

A star's properties - spectral class, temperature, color, as well as its longevity and eventual fate - are determined by its mass at formation. Stars that start out very large burn hot, bright and fast and die out quickly; small stars can burn virtually forever.

Stellar classes & properties of "main-sequence" stars (in the long hydrogen-burning stage).
Class Color Mass Surface
Temp. 		Life
Expectancy
% of stars Examples
O
 violet-white 20 M 30,000 K < 2 my 0.06 % Alnitak & Mintak (in Orion's belt)
B
 blue-white 8-20 M 20,000 K 30 my 0.3 % Rigel
A
 white 4-8 M 10,000 K 400 my 0.8 % Sirius, Vega
F yellow-white 2-4 M 7,000 K 4 by 3 % Polaris, Procyon
G yellow 1-2 M 6,000 K 9 by 8 % Sun, Capella
K orange 0.5-1M 4,000 K 60 by 19 % none naked-eye
M red < 0.5 M 3,000 K 200 by 69 % none naked-eye

Note that almost 90% of the stars actually out there in the cosmos are "red dwarfs" (K and M-class stars smaller than the Sun); however, none of these are visible naked-eye.

Classes O ("blue supergiants") and B ("blue giants") are very large, very hot, and quite rare. They are short-lived and end up exploding as a supernova and leaving a collapsed remnant known as a neutron star.

K-M main-sequence stars won't run out of fuel any time soon...
 
Classes A - G share the same destiny, larger ones going through the sequence more quickly:
  1. the star burns hydrogen for billions of years until a critical amount of helium builds up in its core
  2. it then bloats into a red giant, still burning hydrogen, for another billion years or so
  3. for a short time (about a million years) it then burns helium into oxygen and carbon, expanding again into an even bigger red giant or red supergiant;
  4. it finally becomes unstable and ejects its outer layers, often forming a spectacularly beautiful "planetary nebula"
    the remnant white dwarf slowly cools into a dead "black dwarf."

 

Sources:

SEDS websites:

      diffuse nebulae: www.seds.org/messier/diffuse.html