Death of a massive star

What is a supernova?
What is a neutron star?

Quick explanation

Stars more massive than 8 Solar masses are referred to as "massive stars".

A supernova is the violent explosion of a massive star ending its life. For a brief period, a supernova may outshine the remaining billions of stars in its galaxy.

For a particular type of supernovae ("type Ia"), we know exactly how bright they shine. This information can be used to calculate exact distances to them, and hence to their host galaxies. In that way we're able to map the Universe.

The explosion leaves behind an extremely compact and rapidly spinning neutron star, or, in some cases, a black hole.

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Stars shine by burning hydrogen to helium. The energy thus produced keeps the star from collapsing under its own gravity. When there is no more fuel left, after a relatively short period of burning heavier elements, the star collapses.

While the core of a low-mass star, i.e. weighing less than 8 Solar masses, contracts to a white dwarf, for a more massive star nothing can prevent the catastrophe: The extreme pressure forces the electrons to be squeezed into the protons, making the inner core collapse even further to a neutron star. While this star has a radius of but 10 km, it contains as much mass, or even more, as our Sun.

The remaining mass of the original star rebounces on the compact surface of the neutron star and is ejected in a tremendous explosion, causing it to brighten up enormously and then gradually fade. At peak light output, supernova explosions can outshine a galaxy.

The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant.

If the neutron core is more massive than 2 – 3 Solar masses, not even the pressure of the neutrons can hold it up, and it collapses to a black hole.

Historically, supernovae were referred to as "novae" or "new stars", while the Chinese referred to them as "guest stars" (and the Icelandics call them "exploder stars").

Important supernova in the past were:

  • in 134 B.C., observed by Hipparchus, which led him to make a catalog of stellar positions;
  • in 1054 A.D. in the constellation of Taurus, leaving behind the supernova remnant now known as the Crab Nebula. It was observed for several days in the daytime by the Chinese and Pueblo Indians in New Mexico, though not a single European recording exists;
  • in 1572, observed by Tycho Brahe who found the object had to be at a distance comparable to the stars, thus demonstrating that the starry sky was not unchangable.
  • in 1987,