Stars more massive than 40 solar masses have very strong stellar winds, and lose mass so rapidly they can’t expand into red giants. If the remaining core is no more than 1.4 solar masses, the fusion process continues as in less massive stars, except that the temperature is sufficient to fuse some of the neon into magnesium. The star eventually becomes a white dwarf.
If the core is more than about 2.5 solar masses, the oxygen begins to fuse into sulfur and silicon. As the core continues to collapse, the temperature becomes high enough to break down any nucleus. This forms alpha particles that fuse with other nuclei to form aluminum and sodium. If the star is too massive to form a white dwarf but not sufficiently massive to maintain the conversion of neon to oxygen and magnesium, it will collapse completely and explode as a supernova.
In even more massive stars, the fusion process continues until it produces iron. At this point, more energy is consumed producing iron than is generated, and the core undergoes a sudden, catastrophic collapse that overcomes the forces keeping nuclei apart and forces the electrons in the matter into the protons to form neutrons. The star becomes a very dense sphere of neutrons with a thin outer layer of iron.
It is a neutron star. It is only about 10 km in radius, and the compressed matter of which it is composed weighs 1015 grams per cubic cm. In the Hubble photo below of the Crab Nebula (M1), the bottom of the two stars, which are in a vertical line, slightly above and right of center, is the neutron star.
If the star is even more massive, the collapse is even more catastrophic, and the remaining protons and electrons in the collapsing outer layers also are compressed into neutrons. For an astronomical moment, the star releases the energy created by this transformation as a burst of neutrinos. The neutrinos bombard the iron core and create elements heavier than iron, up to and possibly beyond uranium.
They also force the star into one final expansion—an explosive expansion. It becomes a supernova.
There are four types of supernova—1a,1b, 1c and II. 1a supernova are rare, as they are white dwarfs that cannot maintain their plasma outer layers. These usually are in a binary system and pull off material from their companion. In their explosion, they reach an absolute magnitude of -19.5. Compare this with the magnitude of Venus, the brightest object in the sky, at -4.6. (Note that the minus in front of the magnitude number indicates a brighter object.) 1b and 1c supernova are more massive stars that have run out of fuel at their centers and have previously lost all their outer layers due to strong stellar winds. Their absolute magnitude is about -15.
In type II supernova the process is the same as described above, but the neutron core is about 30 km in diameter. It pulsates in a series of collapses and rebounds until it explodes in one final burst. Some of the material often falls back and coalesces. In stars originally of about 20 solar masses, the resultant remnant is a neutron star. If the original star was more than 20 solar masses, the remnant is a black hole.
It is possible for even more massive stars to go supernova without collapsing to a black hole, but that is rare. Generally, they collapse directly to a black hole without going supernova.