Due to crushlng gravita- tlonal forces, the center of the star becomes extremely hot. At this point nuclear reactions occur causing hydrogen nuclei to turn into helium nuclei. This change causes very large amounts of energy to be released in the form of the electro-magnetic spectrum, which includes visible light. It is 4.2 light years away from Earth. These are loose collections of stars with no well defined shape.
Some open clusters are only a few hundred light years from Earth.
Nebulea are large areas of gas and dust containing mostly hydrogen. Some of these nebula glow due to young, hot stars formed from the gas. These stars generate large amounts of ultraviolet radiation that causes the gas in the nebula to glow. One of the finest examples of a bright or emission nebula is M42, the Orion Nebula. M42 is about 1600 light years from Earth.