 You'll recall that planetary nebula are the result of a star exploding at the end of its life. This is the expected end for the sun, and most stars less than five times the mass of the sun. The star left behind at the end is called a white dwarf. The typical white dwarf is around the mass of the sun, but packed into a star about the size of the earth. It's so dense that a spoonful of it would weigh several tons here on earth. For massive red supergiant stars, a different end is in store. They undergo a core collapse when their nuclear fusion runs out of fuel. At that point, the star can no longer sustain the core's volume against its own gravity. The collapse happens in seconds, reaching velocities of 70,000 km per second. That's over 43,000 miles per second. A rebound causes the violent expulsion of the outer layers of the star, resulting in a supernova. The supernova is billions of times larger than planetary nebula explosions. A supernova may shine with the brightness of 11 billion suns. The total energy output can be as much as the total output of the sun during its entire 13 billion year lifetime. Here's what the daytime sky might look like if and when Betelgeuse, over 400 light years away, supernovas. The star left behind at the end is called a neutron star, because it is made entirely of neutrons. The typical neutron star is around one-and-a-half times the mass of the sun, but packed into a star with a diameter of around 20 kilometers, or 12-and-a-half miles. It's so dense that a spoonful of it would weigh 10 million tons.