At some point during the supergiant phase, a catastrophic collapse will
be initiated, and the star will go nova, shooting outwards in a gigantic
explosion that spews its metals (the remaining carbon, oxygen, iron, and such
elements) into the surrounding stellar field. The explosion will be so big
that we elevate it from the status of a normal nova to a supernova, the
death of a very massive star. Over time these metals will be recycled into
young forming stars in the neighborhood of the star – such a supernovae
was in fact the source of the carbon and other higher order elements which
make up the planets of our solar system, our planet, and our very selves! The
remnants of the stellar core which are left after the supernovae explosion
will follow one of two paths: neutron star or black hole.
[NMSU, N. Vogt]
For high mass stars, electron degeneracy pressure is not sufficient to
hold off gravitational collapse, and the star continues to contract in size
once it has burned through its hydrogen and helium. As it contracts, the core
becomes hotter and hotter. When the core temperature reaches several hundred
millions of degrees, the carbon and oxygen atoms begin to fuse together to
form even heavier elements such as silicon, calcium, and iron. At the
innermost shell, silicon and sulfur burn to produce iron adding mass to the
central iron core. The central iron core is smaller than the Earth, while the
outer hydrogen envelope has a diameter greater than the Earth's orbit. Since
the fusion of carbon and oxygen into heavier elements does not release very
much energy, these reactions cannot delay the star's inexorable collapse.
If the remaining core of a star is more massive than 1.4 solar masses
(recall that the star began life with more than 8 solar masses, but much of
its mass was blown off to form planetary nebula as the collapse process began)
and is formed mostly of iron, there is no force known that can stop it from
collapsing further. It becomes so hot and so dense that the iron nuclei begin
to melt into helium again, and then into hydrogen. This melting process
consumes energy and temporarily reduces pressure in the core of the star.
Gravity quickly takes over again, however, and the core collapses. In less
than a tenth of a second, the core of the star compresses to a radius of 20
kilometers and its density rises to that of the atomic nucleus. The
hydrogen nuclei (protons) absorb electrons and become neutrons, releasing
neutrinos in the process. These closely packed neutrons form a degenerate sea
of neutrons, and can now exert an outward pressure sufficient to stop further
stellar collapse.
Before the collapse is halted, the core is falling in at 10% of the
speed of light (3 × 104 kilometers per second). The thunderclap
resulting from the sudden stop releases an enormous amount of energy, most of
which escapes in the form of small, virtually massless neutrinos. But about 1%
of the energy is absorbed by infalling gas, causing a tremendous rebound
explosion that we call a core collapse supernova. For a few months, the
star becomes as luminous as a billion Suns!
The Course of a Stellar Supernova
(click on pictures for animation)
Briefly outshines host galaxy
Shock wave throws off gas shells
Remnants shine for thousands of years
[NASA/HST, Z. Levay, B. Preston]
Supernova 1987A, is the brightest core collapse supernova seen since
Kepler's supernova (1604). It occurred in the Large Magellanic Cloud, a
neighboring galaxy. Astronomers saw the burst of neutrinos in underground
detectors, and observed gamma rays from newly formed radioactive elements
within the stellar core. The rings seen around SN1987A by the Hubble Space
Telescope were a big surprise and their origin is still a mystery. We suspect
that the progenitor star was a binary star system that merged some 20,000
years before it exploded, ejecting the rings during the merger. The blast wave
from the supernova explosion is just now beginning to hit the ring, causing a
bright spot to appear. During the next ten years, the ring should become
several hundred times brighter than it is today, giving us an opportunity to
understand the mechanism by which the rings were ejected.
What is left behind after the explosion? The remnant is called a
neutron star – a sphere a million times as massive as the entire
Earth, compressed to a few miles in diameter. Its average density will be
1,000 trillion times that of water: a tablespoon of neutron star material
weighs more than a mountain! Astronomers have discovered evidence of hundreds
of neutron stars in the Universe. They are absolutely real – as real as
the Earth beneath your feet!