The center of the star has now met its demise. During the formation of the planetary nebula the star ceases all nuclear reactions. The star is still very hot, up to several hundred thousand degrees Fahrenheit. Over a few hundred million years, the star cools and becomes a white dwarf. As the star cools more, it becomes dark and barely detectable. It is now known as a black dwarf.
a white dwarf is circled
The white/black dwarf is composed of carbon and oxygen. Surrounding this is a thin layer of helium, sometimes surrounded by hydrogen. The star is very compact. Although only about the size of earth, it's mass can be from a little less than one half a solar mass to a little more than one solar mass.
This iron core now just sits in the center of the star. The reason for this is that iron doesn't burn. Nuclear burning is only possible if an object is releasing energy. In order for iron to go the the fusing process, energy must be added. This leads to the collapse of the star. The addition of energy that the iron needs will only occur during the supernova explosion caused by the collapse of the star.
Because the iron is not
fusing, it does not create any outward pressure do balance the
effects of gravity. As the iron gets a mass of about 1.4 solar
masses, gravity gets the upper hand and the core collapses from a
size of about 5,000 miles to about 12 miles in less than a
second. This sudden crush makes protons and
combine to form neutrons. This expels high-energy subatomic
particles (known as neutrinos.) This huge energy release is
equivalent to 100 of our stars burning for more than 10 billion
years. A small amount of energy is deposited in the lower layers
of the shell surrounding the core, triggering the supernova
explosion. The energy deposited around the core creates a shock
wave that runs outward toward the stars surface. As it is passing
through, it heats up the shell sounding the core, starts nuclear
burning, and throws off the shell faster than 10 million mph.
This is when the iron fuses to create heavier particles. When the
shock wave reaches the surface, it heats them very quickly and
causes them to glow. In a day or 2, the star is brighter than a
billion suns. In a couple of weeks, the explosion diminishes,
although it may remain visible for months or years.
the rings of supernova 1987A
What's left is two distinct parts. There is a rapidly expanding gaseous shell that barges through the surrounding interstellar medium and interacts with it, and there is the compact stellar remnant which is either a neutron star or a black hole.
Neutron stars are super-dense remnants of supernovae. They have about 1.4 solar masses, but only have a diameter of 12 miles. Because they are so small and faint, they can't be seen with visible light. Neutron stars spin very fast. Usually they spin about once every second, but some can spin much faster. For example, a neutron star in the Crab nebula has been found to spin 30 times per second. This rapid rotation creates a large magnetic field. The neutron star begins to emit radiation out of it's poles. This radiation ranges from radio waves to visible light to x-rays to gamma rays.
Neutron stars can lead to pulsars.
Pulsars are short for "Pulsating radio sources".
Because neutron stars emit beams of radiation out of their poles,
if they're positioned right they will sweep across earth and a
pulsating signal will be detectable.
If an object with four or more solar masses remains after a supernova explosion, it will become a black hole. Because there is so much mass and no nuclear reactions inside of the star to compensate for it, the gravity continues to crush the star. Eventually, the gravity gets so strong that it even holds back light. When material first begins to get pulled into a black hole, it swirls around it for awhile. It will heat up and eventually give off visible x-rays that are detectable before finally crossing the event horizon and enter the black hole.