Types of Stars

There are a LOT of stars.  Go outside on a clear night in a city that's not L.A. and see how many you can count, and it won't be long until you realize that it's a totally pointless endeavor.  There are a lot of stars.  Naturally, as with anything that there's that much of, there are many different types of stars, and our astronomers on earth have been able to discover a several different types of stars.  Usually, the type of the star depends on two things:  the size of the star and the stage in that stars life.  Stars are formed by floating clouds of gas and debris, which begin to collapse in on each other and eventually created a solar system which, at its center, has a star powered by nuclear fusion reactions.  Nuclear fusion causes the heat and the light that you see coming from all of the stars, and it is the process of converting hydrogen into helium.  Then they begin their life cycles:

• Main Sequence Star - This type of star is where a star spends the majority of its life.  Our sun is a main sequence star.  It is when the star has reached an equilibrium that keeps fusion happening and keeps the gravity from making it collapse in on itself.  These stars are relatively stable and, as long as they can continue with nuclear fusion, will stay in the main sequence.  Most of the stars you see in the sky are main sequence stars.
  
  
• Red Giant - When a star runs out of hydrogen on its surface for fusion, a hydrogen core near the center of it catches fire and suddenly expands to much larger than its original size.  Astronomers predict that when the Sun runs out of hydrogen, it will become a red giant and will engulf the entire inner solar system, including the earth.   This type of star doesn't last particularly long, in star terms, because it doesn't have much fuel.
  
  
• White Dwarf - When the red giant runs out of fuel, it collapses in on itself into a hot white dwarf, which slowly radiates the rest of its heat off into space over a period of billions of years.  This type of star eventually becomes a black dwarf, theoretically.

• Neutron Star - If a star is a certain mass (slightly larger than our sun), rather than expand into a Red Giant, it explodes in a catastrophic, huge explosion called a supernova.  If the closest stars to our sun were to explode in a supernova, though they are light years away, it would destroy our solar system.   Fortunately, this is not going to happen, because none of the closest stars to us are large enough to explode into supernovae.  When a supernova is finished, the gravitational pull at the center is so great that it crushes all protons and electrons together, making them into nothing but neutrons.  The density of a neutron star is so great that a single spoon full of material from a neutron star would weigh millions of tons.

• Black Hole - If a dying star is significantly larger than a that of one that makes a neutron star, it collapses even further into a material so dense that the gravitational pull prevents light itself from escaping.  This type of star is called a black hole.  There is a line that separates the black hole from the rest of the universe called the event horizon.  This is the point where there is no possible way to escape the gravitational pull of the black hole.  If, from the outside, you were to watch an object falling towards a black hole, though the object would be speeding up as it approached, it would appear to you to slow down, and, upon hitting the event horizon, it would appear to hover in mid air directly above the black hole, and slowly dim into nothing.  This is because the gravity around the black hole is actually slowing the speed of light.  Once you go beyond the event horizon is a singularity, which is a mathematical concept that can't be properly explained.  Most galaxies - such as ours - rotate around a supermassive black hole.

• Red Dwarf - A red dwarf is a type of star that is somewhat smaller than our sun that is dimmer and cooler.  Because of its properties, it conserves its fuel extremely efficiently, and lasts significantly longer than most regular stars.

• Supergiants - A supergiant star is a massive type of star that is essentially the exact opposite of a red dwarf.  They are huge and unstable.  They use up their fuel incredibly quickly, so they only last a few million years before exploding into supernovae.

• Pulsars - Pulsars are rapidly rotating neutron stars that emit massive amounts of energy at regular intervals.

• Brown Dwarf - Brown dwarfs are not technically considered stars, but, like stars, they have a nuclear fusion core at their center.  However, on the outside, there is no nuclear fusion so they aren't particularly bright, and somehow straddle the line between stars and large planets.

• Black Dwarf - a black dwarf is the remains of a white dwarf once all of the heat has worn off.  This takes an extremely long time to happen - longer than that age of the universe - so technically, there can't be such thing as a black dwarf yet.  Their existence is theoretical.

From what we know now, these are all the types of stars that exist.  Since our knowledge of the outside universe is extremely limited by the natural restraints of our technology, we can be relatively certain that there are many, many more types of stars in existence that we haven't discovered yet.  There are some systems where there are two or more stars rotating around each other, known as binary systems, and there are evidence of galaxies far off in the universe that are emitting high frequency radio waves, known as quasars.  Theories have been developed by astronomers and scientists, but, as with something like a black hole, the amount which we can know and theorize is limited.