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The Paths of Stars
Let's start with the most common definition of a star. A star is an astronomical object which at some stage of its life undergoes thermonuclear reactions in its core.
My definition is a little different, but it explains things in a nutshell. A star is a continuous (and eventually doomed) battle with the inexorable force of gravity. From the moment an interstellar cloud of gas and dust begins to collapse under its own gravitational forces, it is doomed to one day become a cold, dense, degenerate object like a white dwarf, a neutron star, or possibly, if it's massive enough, even a black hole.
Every stage of a star's life represents a pause in this battle,
as the star finds some energy source which can temporarily halt its
inevitable collapse. These stages include:
Protostars, which form deeply embedded within
molecular clouds and cannot be seen at optical wavelengths.
Stars usually form in groups, and can be seen in
stellar nurseries.
Main sequence stars, giants, and
supergiants.
Dying and dead stars.
Last modified 20 September, 2002. © Gregory C. Sloan.