Which object forms when a supergiant explodes? a red giant a protostar a white dwarf a neutron star

As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. In the core of the red giant, helium fuses into carbon. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take from there.

The illustration above compares the different evolutionary paths low-mass stars (like our Sun) and high-mass stars take after the red giant phase. For low-mass stars (left hand side), after the helium has fused into carbon, the core collapses again. As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed by the outer layers. The core remains as a white dwarf and eventually cools to become a black dwarf.

On the right of the illustration is the life cycle of a massive star (10 times or more the size of our Sun). Like low-mass stars, high-mass stars are born in nebulae and evolve and live in the Main Sequence. However, their life cycles start to differ after the red giant phase. A massive star will undergo a supernova explosion. If the remnant of the explosion is 1.4 to about 3 times as massive as our Sun, it will become a neutron star. The core of a massive star that has more than roughly 3 times the mass of our Sun after the explosion will do something quite different. The force of gravity overcomes the nuclear forces which keep protons and neutrons from combining. The core is thus swallowed by its own gravity. It has now become a black hole which readily attracts any matter and energy that comes near it. What happens between the red giant phase and the supernova explosion is described below.

The two supernovae, one reddish yellow and one blue, form a close pair just below the image center (to the right of the galaxy nucleus) Image Credit: C. Hergenrother, Whipple Observatory, P. Garnavich, P.Berlind, R.Kirshner (CFA).

When the core contains essentially just iron, fusion in the core ceases. This is because iron is the most compact and stable of all the elements. It takes more energy to break up the iron nucleus than that of any other element. Creating heavier elements through fusing of iron thus requires an input of energy rather than the release of energy. Since energy is no longer being radiated from the core, in less than a second, the star begins the final phase of gravitational collapse. The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in a shock wave, which we see as a supernova explosion.

As the shock encounters material in the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes. While many of the more common elements are made through nuclear fusion in the cores of stars, it takes the unstable conditions of the supernova explosion to form many of the heavier elements. The shock wave propels this material out into space. The material that is exploded away from the star is now known as a supernova remnant.

The hot material, the radioactive isotopes, as well as the leftover core of the exploded star, produce X-rays and gamma-rays.

Supernovae
http://imagine.gsfc.nasa.gov/science/objects/supernovae1.html
http://imagine.gsfc.nasa.gov/science/objects/supernovae2.html

Life Cycles of Stars
http://imagine.gsfc.nasa.gov/educators/lifecycles/stars.html

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A number of the objects we see in the sky are not stars, but the remains of stars that have died. Manny of these stellar remnants have unusual properties, making them some of the most interesting and exciting objects for astronomers to study.

Planetary Nebula

White Dwarf

The exposed core of a Red Giant that is left behind after the formation of a planetary nebula is known as a white dwarf. In a white dwarf, the force of gravity compressing the star is balanced by a electron degeneracy pressure. This pressure occurs because subatomic particles like electrons don't like to share the same space. Once gravity has collapsed the atoms of the star so close together that their electrons can't get any closer, this pressure from the electrons is enough to keep the star from collapsing further.

White dwarfs are very hot and bright, but very small. Although they may have about as much mass as our sun, even after ejecting most of their mass as a planetary nebula, they are compressed to a tiny ball about the size of the earth. Since white dwarfs are created from the cores of red giants, they are composed of elements like carbon, oxygen and neon. Fusion has already ceased in White Dwarfs, which means that they will slowly cool until they no longer give off light. Like Red Dwarfs, however, their lifespan far exceeds the age of the universe, which means that no White Dwarfs are old enough to have stopped shining.

Supernova

The tremendous amounts of energy released in a supernova are enough to fuse additional particles into the nuclei of heavy elements such as iron and nickel. Thus, supernovae are responsible for most of the elements heavier than iron, such as tin, gold and lead.

Once a supernova has subsided, the dust and gas forms a large nebula where new stars can form. As with planetary nebulae, supernovae blast heavy elements into the interstellar medium that will eventually form into new stars and planets.

Neutron Stars

Although the blast from a supernova sometimes destroys a star completely, usually the core of the star remains. Since the electrons were stripped from their atoms during the core collapse, most of the protons in the star end up absorbing an electron. The positive charge of the proton and the negative charge of the electron cancel each other out, and the resulting particle becomes a neutron. Since most of the matter in the core is converted into neutrons, they are known as neutron stars.

Neutron stars spin very rapidly. Some of these stars emit beams of electromagnetic radiation out of their magnetic poles. Like the earth's magnetic poles, they don't always line up with the star's axis. This causes their beams of radiation to spin, much like the beam of light from a lighthouse. If these beams pass over the earth, they appear as a constant pulse of x-rays and radio waves. These neutron stars are known as pulsars.

Black Holes

If the core left behind after a supernova is massive enough (probably about two or three times the mass of the sun), even the star's neutrons aren't strong enough to resist the force of gravity. These stars collapse into black holes. A Black hole is so small and so dense that not even light can escape from its gravity. Even though we can't see black holes, since they neither produce, nor reflect light, astronomers can still detect them. The gravity from a black hole can bend the light from other stars, like looking at light through a lens.

Black holes can also be spotted if there are other nearby stars from which the black hole can pull matter. As this matter is drawn into the black hole, it begins to spiral around it like a whirlpool. This spinning matter is called an accretion disk, which heats up and glows brightly. Many of the brightest objects in the universe are thought to have formed in this way, with a black hole in the center of a huge mass of spinning matter.

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