Stars are one of the most amazing and awe-inspiring objects in the universe. Stars are living things, and, like all living things, when stars die they contribute to the birth and growth of other things, namely other stars. There are several types of stars, and they are only classified by what part of the “star life spectrum” they currently occupy.
Protostars are “before” stars. These are not actually stars yet, but are gaining enough luminosity and thus temperature to become a main sequence star.
Main sequence stars are the most commonly found in the universe. That is because stars spend most of their life as main sequence stars. Main sequence stars are defined by how they produce energy. All main sequence stars are main sequence stars because they burn hydrogen (or convert hydrogen into helium) for energy. Our Sun is still a main sequence star, and will continue to be so for the next 7 billion years.
Red giants are one of the largest stars in the universe. These stars are more luminous than main sequence stars. They begin to expand and are usually 100 times bigger than the size that they were when they were main sequence stars. Red giants do not stay as red giants for very long. That is because once main sequence stars become red giants, they begin to burn hydrogen at a faster rate.
Once a star leaves the red giant stage, it becomes a white dwarf. We often don’t see many white dwarfs because they are small and not very luminous. They produce energy and are able to emit some light because they are contracting. If you look at an H-R diagram, white dwarfs can be found below the main sequence line, at the bottom left hand corner of the diagram. This is because they have very little luminosity.
Brown dwarfs are slightly bigger than white dwarfs, but they aren’t part of a regular star’s life. Brown dwarfs are formed as is. They aren’t very luminous and they don’t have any fusion in their core because of their low inner pressure and temperature (compare to the main sequence stars).
Black holes and neutron stars are one of the most interesting types of stars. They don’t follow what you would call ordinary star guidelines. Black holes are basically huge masses. Nothing can pass a black hole. Neutron stars, on the other hand, emit radiation (or light) that can either seem to rotate or stay still. Pulsars are examples of neutron stars.
Source:
Freedman, Roger A. “Universe.” Chapters 21-23. New York: W. H. Freeman & Company, 2008.
