For instance, a very heavy star might explode in a supernova and then become a black hole. A lighter star will eventually run out of fuel, cool down, and become a black dwarf. Stars usually occur in groups called clusters, which are gravitationally bound. Double and multiple stars are linked by gravity, so that they orbit one another in some way. In this respect, our own sun is somewhat unusual in being single, because about three quarters of all stars are in groups.
Open clusters are groups of dozens to hundreds of stars which are loosely bound by gravitation.
Open clusters generally lie in the spiral arms of the Milky Way. Globular clusters are usually spherically shaped, and contain hundreds of thousands of old stars. These clusters lie around our galaxy in a halo. A nebula is a cloud of gas and dust where new stars are being formed. A nebula is created from the remnants of a star in the last stage of its life.