What type of star is the closest start to earth, outside of our own sun?
Proxima Centauri is about 4.2 light-years away. It is a low-mass star known as a red dwarf.
Proxima Centauri is actually the smallest of three stars gravitationally bound to each other.
The two larger stars, collectively known as Alpha Centauri, are closely coupled as a binary star system; each of these stars is about as massive as our Sun.
Proxima Centauri, a much less massive type of star known as a red dwarf, is some distance away from the Alpha Centauri pair and orbits them like a planet would. Nonetheless, Proxima Centauri is a star with planets of its own.
For more about Proxima Centauri, see https://en.wikipedia.org/wiki/Proxima_Centauri.