A white dwarf (or the degenerate core of a very massive star) has an upper limit to its mass of M = 1.4 Msun (the Chandrasekhar limit). If a degenerate compact object exceeds this limit, it does NOT shrink quietly into a black hole. Instead, it explodes as a supernova.
On occasion, throughout the centuries, astronomers have been puzzled by the appearance of ``new stars'' in the sky. For instance, in July 1054 AD, Chinese astronomers noted the appearance of a ``guest star'' (as they called it) in the constellation Taurus. The guest star was visible in broad daylight for three weeks, and was visible at night for two years before it faded into invisibility.
Flash forward: when modern astronomers turn their telescopes to the position of the guest star, they see the Crab Nebula (pictured below):
The Crab Nebula is located 2000 parsecs (6500 light years) from Earth, and is 1.35 parsecs (4.4 light years) in radius. It consists of an expanding filamentary shell of gas. The currently measured expansion velocity of 1400 km/second is sufficient to have carried the shell outward by 1.35 parsecs in the past 943 years, since the ``guest star'' was first noted.
Plausible hypothesis: What the Chinese astronomers saw 943 years ago was a giant explosion. What we see today is the expanding debris from that explosion.
Other bright ``new stars'' have been noted. In November 1572, for instance, the astronomer Tycho Brahe saw a new star in the constellation Cassiopeia. He wrote a book about his observations, entitled, in Latin ``De Nova Stella'' (About the New Star). After the time of Tycho, all ``new stars'' were referred to as novae (the plural of nova). In the 1930's, it was released that there was one particular class of ``new stars'' which consisted of exploding stars. These monumental explosions were given the name of supernovae (or ``supernovas'', if you dislike the Latinate plural).
Supernovae are rare, luminous, and fairly brief events.
A supernova is an explosion, triggered by the collapse of a degenerate object with M > 1.4 Msun. There are two types of supernova explosion in the universe, called (boringly enough) Type I and Type II.
Consider a white dwarf made of carbon and
oxygen. (This is the end state for stars whose mass on the main
sequence is between 0.4 Msun and 6 Msun.)
When such a white dwarf is solitary, it leads a boring and stable
existence. It merely cools down gradually as it radiates its
energy into space.
HOWEVER, if the white dwarf is in a close binary system, its
partner, as it expands into a giant or supergiant, will start to
dump gas onto the white dwarf. If the white dwarf's mass exceeds
the Chandrasekhar limit, it is no longer stable against collapse.
As soon as the white dwarf has M > 1.4 Msun, it collapses rapidly.
At the new higher density and temperature, the fusion of carbon and oxygen into iron occurs in a runaway fashion. The white dwarf is converted into a fusion bomb, and is blown completely apart by the explosion! This represents a triumph of the outward force of pressure over the inward force of gravity.
As the type I supernova expands outward, it spreads 1.4 Msun of iron into the interstellar medium.
Consider a supergiant star. At the very end of
its life as a fusion-powered star, it has a layered look,
something like this:
A layer of hydrogen over a layer of helium over a layer of carbon
over a layer of oxygen over a layer of silicon over an iron core
(roughly speaking). The degenerate iron core steadily grows in
size as fusion proceeds.
When the mass of iron = 1.4 Msun,
When the degenerate iron core is nudged over the Chandrasekhar limit, it collapses in less than a second. After collapse,
Okay, so the core is falling inward at a very rapid rate. However, a supernova, we have stated, is an explosion, in which matter is thrown outward. We need some way to turn around the collapse and fling at least a part of the star's matter outward.
It's very difficult to squeeze matter until it's denser than an atomic nucleus. The core resists further compression at this density, and bounces back outward. The rebounding core sends a shock wave through the outer layers of the star (the silicon, oxygen, carbon, helium, hydrogen layers), and heats them up. The shock heating is assisted by neutrinos (which can actually be absorbed by matter at these insanely high densities) and by convection. The shock-heated gas expands outward to form a {\bf supernova remnant}, such as the Crab Nebula.
A Type II supernova remnant releases a large amount of energy.
For comparison, the total amount of energy radiated by the Sun during its main sequence lifetime will be 1044 joules. The visible light produced by a supernova is only a minor byproduct. Most of the energy is carried off by neutrinos.
The most recent naked-eye supernova was SN1987a, which appeared in the Large Magellanic Cloud, a satellite galaxy which orbits our own galaxy. Of the 1058 neutrinos emitted by SN1987a, precisely 19 were detected here on Earth. They arrived at half past two in the morning (EST) on February 23, 1987. A few hours later, the photons from the supernova began to arrive. (The photons were trapped for a few hours within the expanding outer layers of the supernova, waiting for the material to become low enough in density to be transparent.)
``After'' [left] and ``Before'' [right]
pictures of the type II supernova SN1987a in the Large Magellanic
Cloud. In the right panel, an arrow is pointing to the blue
supergiant star SK-69 202, which was the progenitor of the
supernova. In the left panel, the supernova is shining brightly
as it explodes.
(Image credit: Anglo-Australian Observatory)
SN1987a has provided useful tests of our theories about how Type II supernovae evolve. Astronomers continue to monitor the supernova, watching the evolution of its supernova remnant, and hoping to glimpse the central collapsed core as soon as it is visible within the expanding supernova remnant.
Supernova remnants
At the center of the supernova remnant is the compressed core, which is now either
