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Almost every dot in the night sky that we see are stars. All of those stars exist within our Milky Way Galaxy. Very rarely will a lone star actually exist in the spaces between galaxies, it is the norm for stars to only exist within galaxies.

There are two main groups of stars:

  • Population II Stars - old, metal poor stars
  • Population I stars - new, metal rich stars

In addition, there are two main endings of a stars life:

Lifetime of a normal star:

  • Dust cloud forms a Main Sequence star that burns for about 10 billion years
  • Star ends Main Sequence life and swells to a Red Giant (about the size of Earth's orbit) and burns for 100 million years
  • Star sheds is layers as a Planetary Nebula lasting 100,000 years
  • Only the core of the star remains as a White Dwarf

Lifetime of a large star:

  • Dust cloud forms a large star that burns on the Main Sequence for 50 million years
  • Star ends its Main Sequence life by swelling to a Red Supergiant (about the size of Mars' orbit) and burns for a million years
  • Core collapse can occur anytime after the million year Red Supergiant phase, and can go supernova
  • All that is left is a supernova remnant (a wispy looking nebula) and a compact object - Neutron Star or Black Hole

Stars can be classified as living in groups as there are no "stray" stars existing in the Universe. There are actually three types of stellar populations:

Open clusters reside mostly within the disk of a galaxy while globular clusters exist outside the galaxy filling a space called the halo. This halo is actually part of the galaxy and it surrounds the entire galaxy.

Open clusters and globular clusters will be discussed in greater detail in their own sections.

Of course, a star does not have to be in an open or globular cluster but almost always a star will be a part of a galaxy.

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