The Crab Nebula
from the book, "Our Fascinating Earth"
The Crab Nebula
A supernova is a cataclysmic star explosion during which the star may increase a million or more times in brightness. The star literally blows itself apart; more than half of its mass is lost to space, and the star retains very little of its former nature. Astronomers estimate that supernovas are rare, with fewer than three occurrences in 1,000 years in any given galaxy. The most recent supernova in the Milky Way galaxy was in 1604.
Within just a few days after a supernova occurs, the exploding star brightens by many thousands or millions of times. At its brightest a supernova can rival the combined light output of all the stars in a galaxy such as the Milky Way. At least half the star's original mass is thrown off at speeds of up to 6,000 miles per second. The core of the destroyed star, without the internal energy source of its nuclear fires to sustain it, collapses to form a tiny compressed star, smaller and denser than a white dwarf. The strong inward pull due to the tremendous gravity of the star's core, aided by the tremendous pressures of the supernova explosion in the layers above, compresses the electrons and protons of the core's atoms, forming electrically neutral particles called neutrons. Scientists refer to such a celestial object as a neutron star.
A neutron star could contain as much matter as two of our suns compressed into a single sphere no more than twelve miles across. The density of such an object is so great that a thimbleful of material from a neutron star would weigh at least one billion tons.
The presence of neutron stars was theorized as long ago as 1939, but there seemed little chance of detecting them. In August 1967 astronomers at Cambridge University made recordings of regularly beating radio waves coming from a star. It soon became clear that it was a tiny star that rotated rapidly, sending out shafts of energy each time it turned. Such stars are referred to as pulsars.
Approximately 150 pulsars are now known to astronomers. The periods of their pulses range from thirty per second to one every four seconds. The fastest—flashing pulsar lies in the heart of the Crab Nebula, the collapsed core of a star that exploded in 1054. Its presence in that nebula underscores the theory that pulsars are small, rapidly spinning neutron stars that discharge energy in pulses of electromagnetic radiation.
In the astronomically fateful year of 1054, Chinese astronomers recorded the appearance of a bright star in the constellation Taurus where none had existed before. They identified the visitor as K'o—hsing, or "Guest Star." The guest star shone as brightly as the planet Venus and was easily seen in the daylight for the next twenty—three days. It remained visible after dark for more than 650 days before finally fading from view.
The star certainly must have been visible in Europe and the Middle East as well, but no account of it exists. While these advanced civilizations inexplicably ignored one of the more spectacular phenomena of the universe, the creation of the Crab Nebula, prehistoric Native Americans did not. Scientists believe that two rock drawings found in the 1950s in northern Arizona depict the event. The star appears near a crescent moon, the position in which it would have been seen in 1054. Since then at least nine more rock drawings have been found, all depicting the same event.
The remains of the supernova, now known as the Crab Nebula, consist of dust and gas that expand, due to the momentum of the original explosion, at the rate of 800 miles per second. After nine centuries of travel the Crab Nebula is now more than forty—two light—years across!
At present the Crab Nebula is a course of strong radio waves. Two faint stars are at its center, one of which, the neutron star, is the collapsed core of the star that exploded. As small as it is, about 18 miles in diameter, this center star is bright enough to show up on photographic plates, and scientists have long suspected that it was the remnant of a supernova explosion. In 1968, once astronomers knew what to look for, they found that this star was flashing optically thirty times per second, at the same rate as the pulsating radio waves they had detected for some time.
Only one other pulsar in the southern constellation has been detected optically. It is the star Vela, which is the faintest star ever detected. Vela is ten million times too dim to be seen with the naked eye.
The final outcome of the supernova that created the Crab Nebula can be witnessed in the remnant of another exploded star, the Veil Nebula in Cygnus. The tattered remains of this star, which exploded some 20,000 years ago, are wisps of gas dispersing in space. In about 100,000 years they will disappear from the sight of earthbound telescopes. But then we must imagine that, 1,000 centuries from now, astronomers from earth will not be dependent on telescopes attached to our mountaintops. The space telescope Hubble is an excellent example.