Stellar evolution is the study of how a star changes over time. Stars can change very much between when they are first created and when they run out of energy. Because stars can produce light and heat for millions or billions of years, scientists study stellar evolution by studying many different stars in different stages of their life. The stages in a star's life are: protostar, main-sequence star, red giant and compact star: either white dwarf followed by black dwarf, neutron star or black hole. How a star is born A star starts its life as a cloud of dust and gas called a nebula. This is pulled together by gravity which causes it to heat up. It also starts to spin and to look like a ball. When it gets hot enough, it starts to release energy through nuclear fusion, changing hydrogen to helium. This makes it shine very brightly and become what astronomers think of as a main-sequence star. It may stay a main-sequence star, looking about the same, for billions of years. The details of a main-sequence star are based on its mass. The mass is often measured in terms of the Sun's mass, or a solar...