Discovery Space: Dying Stars
The nature of the last phase in the life of a dying star is largely determined by its mass. The star’s last energy reserves are expended, which can produce stunning visual effects.
When all the hydrogen in the center of a star has been used up and fused to form helium, the core contracts and begins to heat up. The outer layers of the star also begin to undergo fusion. This consequently causes the star to inflate massively and become what is known as a red giant. Planets orbiting near this star will be swallowed up. The star glows red because its energy is distributed over a large surface
area. The temperature in the center of a red giant can easily reach a remarkable 180 million°F (100 million°C).
Meanwhile, at the center of a red dwarf, helium fuses to form carbon and oxygen. The star casts off its outer gaseous envelope to form planetary nebulae and the car-bon-oxygen core remains to form a white dwarf. A white dwarf, however, no longer undergoes heat-generating reactions, so it collapses to be extremely dense.
The fate of massive stars
Stars above a certain mass (approximately ten times the mass of our sun) become so dense and hot that heavier elements undergo fusion. Carbon, neon, oxygen, and silicon undergo fusion. The formation of elements heavier than iron consumes rather than produces energy, thus hastening the star’s eventual collapse.
When the energy source of the star finally dries up, it collapses and explodes in a giant supernova. For a few days, it shines brighter than its entire home galaxy. Within the remaining core of the star, the atomic components are com-pressed to such an extreme that electrons and protons are converted to neutrons. One teaspoonful of this matter from such a neutron star has the mass of about one billion small cars. The neutron star revolves at high speed and emits so- called pulsar radio waves; for instance, in the Crab Nebula one pulsar revolves about 30 times per second.
If the neutron star has a mass between 3 and 15 times the size of the sun, it undergoes a more radical gravitational collapse and becomes a black hole: an area of space with such enormous gravitational pull that neither matter nor light can escape from it.
ACCRETION DISKS IN DOUBLE STAR SYSTEMS
An accretion disk consists of gas or dust. In a close double star system, the gas from a red giant can form an accretion disk around a white dwarf, which slowly gathers the gas.
If the white dwarf accumulates too much gas, it will collapse under its own weight and be torn apart by a supernova explosion. Before the final supernova, there can be novae outbreaks along its surface, similar to hydrogen bomb explosions.
BASICS
DYING STARS produce heavy chemical elements.
MASSIVE STARS explode as supernovae.
AFTER A STAR DIES, white dwarfs, neutron stars, or black holes remain, depending on the mass of the original star A white dwarf can explode as a supernova if it picks up matter from a nearby star.
RADIATION from a supernova can damage organisms on a planet, even at a distance of several light-years away.