A MEETING WITH THE UNIVERSE

Chapter 4-5

The End of Stars:
Death and Transfiguration

White dwarfs

Perhaps the greatest surprise of the Space Age has been the realization that "dead" stars that have used all their nuclear fuel can sometimes produce more energy than they did when "alive". We have discovered that there are three possible ends for a burnt-out star. If the star has about the mass of the Sun, it will collapse under its own gravity until the collective resistance of the electrons within it finally halts the process. The star has become a white dwarf and may be comparable in size to the Earth. A star with a mass of about 1.5 to 2 or 3 times that of our Sun will collapse even further, ending up as a neutron star, perhaps 20 kilometers in diameter. In neutron stars, the force of gravity has overwhelmed the resistance of electrons to compression and has forced them to combine with protons to form neutrons. Even the nuclei of atoms are obliterated in this process, and finally the collective resistance of neutrons to compression halts the collapse. At this point, the star's matter is so dense that each cubic centimeter weighs several billion tons. For stars that end their life weighing more than a few times the mass of the Sun, even the resistance of neutrons is not enough to stop the inexorable gravitational collapse. The star ultimately becomes a black hole, a region in space so massive that no light or matter can ever escape from it.

The existence of white dwarfs has been known for some time, and many have been detected with ground-based telescopes. However, neutron stars and black holes existed only in much-disputed theory until the Space Age.

Strange remains of a shattered star.
Result of a supernova explosion seen in the year 1054 A.D., the Crab Nebula is now about 10 lightyears in diameter. The Crab is shown in visible light; filamentary structures are shreds of the disrupted star, while the smooth while glow is radiation from high-speed electrons streaming through a magnetic field in the nebula.

Strange remains of a shattered star. (cont.)
Two X-ray images from HEAO-2 show the pulsar at the heart of the nebula as it seems to blink on and off. Actually, the pulsar is a neutron star (the surviving core of the exploded star), rotating 30 times per second, each of its twin "searchlight" beams sweeping past the Earth at like intervals. Each sweep corresponds to an observed pulse of X-rays, gamma rays, visible light, and radio waves. The spinning core is gradually slowing as it supplies energy to the fast electrons that make the smooth part of the nebula shine.

photo of the expanding matter of a collapsed star

Strange remains of a shattered star. (cont.)
Two black-and-white photographs from the 5-meter (200-inch) Hale reflector on Mt. Palomar are combined to reveal the motion of the filaments thrown out in the 1054 A.D. explosion. A photo made in 1950 is printed as a positive (bright regions are white), while one made in 1964 is printed as a negative (bright regions are dark). Note that each small white structure has a black rim on the outer side, indicating that expansion from the center persists.

Neutron stars and supernovae

The discovery and understanding of neutron stars involve studies of two poorly understood types of space objects, supernovae and pulsars. Supernovae are extremely violent explosions, in which a star suddenly detonates, pouring out so much energy that for a few days it may outshine all the other stars in its galaxy put together. Pulsars, first detected by radio astronomers in 1967, are sources of very accurately spaced bursts of radio waves. These bursts were so regular, in fact, that the scientists who detected them wondered briefly if they had found artificially generated signals from an interstellar civilization.

The discovery of a pulsar in the Crab Nebula supernova remnant led to a great synthesis in our understand ing of pulsars and supernovae. Supernovae occur at the end of a massive star's life, when it is a red supergiant, with its nuclear fuel almost spent. When the central core becomes so dense that electrons and protons begin to form neutrons, it collapses catastrophically to form a neutron star. In the process, more energy is released than the star ever generated from its nuclear fuel, producing an explosion in which every atom in the outer parts of the star is heated to well over a million degrees. The star is literally destroyed in an instant, but the debris from the explosion shines briefly with the energy of a billion suns.

Besides splattering stellar debris into space, supernova explosions leave behind a "cinder" - the dense, collapsed core, made of neutrons - where there once was a star. The weak magnetic field of the original star is greatly enhanced in the collapse, and the remnant core - the neutron star - may have a magnetic field trillions of times stronger than the magnetic field of the Earth. The rotation of the star also increases dramatically during collapse, and the resulting neutron star spins many times a second. Beams of radio waves, X-rays, and other radiation, perhaps focused by the powerful magnetic field, sweep through space like the revolving beam of a lighthouse. The neutron star has become a pulsar.

Pulsars were discovered accidently during a study of "twinkling" radio sources in the sky. This twinkling is not due to our atmosphere, as is the twinkling of stars. Instead it is caused by the highly rarefied interstellar gas, which affects the passage of radio waves. As the study went on, the scientists at Cambridge University noticed that in some sources the twinkling was periodic, the signals came at regular intervals of 1 or 2 seconds or less.

Gradually, more pulsars were discovered. The fastest one known so far, which rotates at 30 times a second is in the Crab Nebula, the remnant of a supernova explosion that was observed in 1054 A.D. When this rapid pulsar was found, it was quickly realized that it must be a neutron star. Only a neutron star could remain intact under such rapid rotation with out breaking up. (A rotating black hole would remain intact, but it would not produce a regular signal.)

Now that we can see the universe by the light of X-rays and gamma rays, further unexpected properties of pulsars have been found. The theories that were rather successful in explaining the Crab Nebula pulsar failed to predict or account for phenomena found in the brightest gamma ray pulsar, located in the constellation Vela. New theories are needed to explain how pulsars can create intense radio waves, visible light, X-rays, and gamma rays, all at the same time. Many neutron stars of another kind have been found with orbiting X-ray telescopes. We usually cannot detect the heat left over from their collapse, but instead we detect X-rays from matter that is heated intensely as it falls rapidly towards the surface of the star. The realization that neutron stars suck up surrounding matter came from the discovery in 1971 of an X-ray pulsar, Hercules X-1. Detailed study of this X-ray source revealed very small variations in the 1.2 second period of pulsation. More study proved that these small variations were caused by motion of the neutron star in orbit around another star. We have now learned that most X-ray emitting neutron stars are in orbit around other, otherwise normal stars. In some cases the stars are so close that the intense gravity of the neutron star actually pulls gas away from the atmosphere of its companion.

Even when the stars are farther apart, the neutron stars may collect material from the stellar winds of the companions. As the gas is pulled from the normal star down to the surface of the neutron star, the gravitational energy of the neutron star heats the gas to millions of degrees. The hot gas gives off X-rays that mark for us the location of the otherwise invisible neutron star. X-ray pulsars derive their energy from the accretion of matter; the pulsars discovered by the radio astronomers are mostly single stars that are using up their energy of rotation and thus are gradually slowing down.

Black holes: the end point

When the gravity of a collapsing star is too strong for even neutrons to resist, a black hole may be formed. A black hole is a point mass in space, surrounded by a literally black region in which the gravity is so strong that no matter, nor even light, can escape it. But, just as in the case of a neutron star, matter that falls toward the black hole is intensely heated, producing copious X-rays that can be detected with telescopes flown above the atmosphere.

A few of the brightest X-ray sources in our galaxy are probably black holes orbiting closely with relatively ordinary stars. The X-ray source called Cygnus X-1 is a famous example. In 1971, astronomers learned that Cygnus X-1 was associated with a visible star that also is a radio source. This discovery is an important example of how ground-based optical and radio telescopes work in consort with orbiting X-ray telescopes to solve the problems of Space Age astronomy. The identity of the stellar companion was confirmed when both the radio source and the X-ray source were observed to change dramatically and simultaneously in intensity. Observations of the spectrum of the visible star and its changes in velocity as it and its X-ray source companion followed their orbits led to an estimate of the mass of the X-ray source. This unseen star that does produce X-rays appears to have at least six times the mass of our Sun, much more than can possibly be supported by the resistance of neutrons. Comparing the deduced mass with the theoretical limits on the masses of neutron stars, we conclude that the unseen X-ray source in the Cygnus X-1 binary star system must be a black hole. However, the proof necessarily is limited - you can't see a black hole and further studies of this and other cosmic X-ray sources are needed.