Where Do Old Stars Go When They Die?

The specifics of what happens when a star leaves the main sequence depend on its mass. If it is less than half the mass of the sun, it just trundles along without sufficient mass to fuse helium, gradually running out of energy as it depletes its hydrogen stores; since they’re so light, their fusion rate is very low and their theoretical lifetimes can range into the hundreds of billions of years. If it happens to be greater than half but less than three times the mass of the Sun, the helium-burning phase causes it to leave the main sequence and become a red giant when hydrogen fusion shifts to a shell around a mostly helium core. Eventually, it exhausts its helium resources and, with insufficient mass/pressure to fuse elements past carbon, it throws off its outer layers as a planetary nebula and becomes a white dwarf. At this stage, the star is literally a cinder that eventually fades to a black dwarf.

If the star has greater mass, it can repeat this process, turning into a dwarf if it fails to fuse the next element; the only exception is that the internal pressure causes a more intense fusion reaction - as a result, the star becomes even larger - a supergiant, rather than just a giant. Given an initial mass 10 or more times that of the sun, their death will result in a supernova; the mass of the star is so great that it can successfully fuse elements up to iron, but at iron, exothermic fusion (fusion with a net energy surplus) ceases. The star then collapses until the pressure is high enough to fuse iron, absorbing a significant amount of energy in the process but creating a tremendous amount as well. Consider that stars spend millions or billions of years burning hydrogen and helium, while the time remaining after switching to carbon may be thousands of years and the fusion of silica into iron takes fractions of a year at most. A supernova can release on the order of 10⁴⁴ joules, about 10¹⁸ times the sun’s energy output in any given second, or approximately equivalent to its energy output for eight and a quarter billion years, three billion years longer than it has been around.

At this point, the remnant of that explosion becomes a neutron star (if its initial mass was less than 20 solar masses) or a black hole, but that's a story for another article...

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