Introductory Astronomy for Undergraduates
Death of an "Ordinary" Star
After a low mass star like the Sun exhausts the supply of hydrogen in its core, there is no longer any source of heat to support the core against gravity. Hydrogen burning continues in a shell around the core and the star evolves into a red giant. When the Sun becomes a red giant, its atmosphere will envelope the Earth and our planet will be consumed in a fiery death.
Meanwhile, the core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core and the star becomes a red supergiant. At this stage, the Sun will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which lasts only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking "planetary nebula", much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star.
Death of a Massive Star
Massive stars burn brighter and perish more dramatically than most. When a star ten times more massive than Sun exhaust the helium in the core, the nuclear burning cycle continues. The carbon core contracts further and reaches high enough temperature to burn carbon to oxygen, neon, silicon, sulfur and finally to iron. Iron is the most stable form of nuclear matter and there is no energy to be gained by burning it to any heavier element. Without any source of heat to balance the gravity, the iron core collapses until it reaches nuclear densities. This high density core resists further collapse causing the falling matter to "bounce" off the core. This sudden core bounce (which includes the release of energetic neutrinos from the core) produces a supernova explosion. For one brilliant month, a single star burns brighter than a whole galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced. This heavy element enriched gas will be incorporated into future generations of stars and planets. Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible.
White Dwarfs
Low-mass stars and medium-mass stars like the sun take billions of years to use up their nuclear fuel. As they start to run out of fuel, their outer layers expand, and they become red giants. Eventually¡ the outer parts grow larger still and drift out into space, forming a glowing cloud of gas called a planetary nebula. The blue-white core of the star that is left
behind cools and becomes a white dwarf. White dwarfs are only about the size of Earth, but they have about as much mass as the sun. Since a white dwarf has the same mass as the sun but only one millionth the volume, it is
one million times as dense as the sun. A spoonful of material from a white dwarf has as much mass as a large truck. White
dwarfs have no fuel, but they glow faintly from leftover energy. After billions of years, a white dwarf eventually stops glowing. Then it is called a black dwarf.
Neutron Stars
After a supergiant explodes, some of the material from the star is left behind. This material may form a neutron star. Neutron stars are the remains of high-mass stars. They are even smaller and denser than white dwarfs.
A neutron star may contain as much as three times the mass of the sun but be only about 25 kilometers in diameter, the size of a city.
In 1967, Jocelyn Bell, a British astronomy student, detected an object in space that appeared to give off regular pulses of radio waves. Some astronomers hypothesized that the pulses might be a signal from an extraterrestrial civilization. At first,astronomers even named the source LGM, for the "Little Green Men" in early science-fiction stories. Soon, however, astronomers concluded that the source of the radio waves was really a rapidly spinning neutron star. Spinning neutron stars are
called pulsars, short for pulsating radio sources. Some pulsars spin hundreds of times per second!
Black holes
The most massive stars-those having more than 40 times the mass of thesun-may become black holes when they die.
A blackhole is an object with gravity so strongthat nothing, not even light, can escape. After avery massive star dies in a supernova explo-
sion, more than five times the mass of the sunmay be left. The gravity of this mass is so strong that the gas is pulled inward, packing the gas into a smaller and smaller space. Thegas becomes so d ensely packed that its intense gravíty will not allow even light to escape. The remains of the star have become a black hole.No light, radio waves) or any other form of radiation can
ever get out of a black hole, so it is not possible to detect a blackhole directly. But astronomers can detect black holes indirectly. For example, gas near a black hole is pulled so strongly that it revolves faster and faster around the black hole. Friction heats
the gas up. Astronomers can detect X-rays coming from thehot gas and infer that a black hole is present. Similarl¡ if
another star is îear ablack hole, astronomers can calculate themass of the black hole from the effect of its gravity on the star. Scientists have detected dozens of star-size black holes with the Chandra X-ray Observatory. They have also detected huge black holes that are millions or billions of times the sun's mass.