This continues until the star finally blows its outer layers off in a gust of superwind. This leaves behind a white dwarf surrounded by an expanding shell of gas in an object known as planetary nebula . They are called this because early observers thought they looked like the planets Uranus and Neptune. There are some planetary nebulae that can be viewed through a backyard telescope. In about half of them, the central white dwarf can be seen using a moderate sized telescope.
Planetary nebulae seem to mark the transition of a medium mass star from red giant to white dwarf.
Stars that are comparable in mass to our Sun will become white dwarfs within 75,000 years of blowing their envelopes. Eventually they, like our Sun, will cool down, radiating heat into space, fading into black lumps of carbon. It has taken 10 billion years, but our Sun has reached the end of the line and quietly become a black dwarf. White dwarfs can tell us important things about the age of the universe. If we can estimate the time it takes for a white dwarf to cool into a black dwarf, that would give us a lower limit on the age of the universe and our galaxy .