The Universe—Some Secrets Unlocked
ON THE 4th of July, in the year 1054, Yang Wei Te gazed up at the early morning sky. As official astronomer of China’s Imperial Court, he was meticulously observing the movement of the stars when suddenly a bright light near the constellation of Orion attracted his attention.
A “guest star”—the name the ancient Chinese gave to such a rare occurrence—had made its appearance. After dutifully reporting to his emperor, Yang noted that the “guest star” had become so bright that it even outshone Venus and could be seen in broad daylight for several weeks.
Nine hundred years were to pass before this spectacle could be adequately explained. It is now believed that the Chinese astronomer was witnessing a supernova, the cataclysmic death throes of a massive star. The whys and wherefores of such an extraordinary phenomenon are just some of the secrets that astronomy is trying to unlock. The following is one explanation that astronomers have painstakingly pieced together.
Although stars like our sun may have immensely long and stable lives, their formation and demise give rise to the most spectacular sights in the skies. Scientists believe that the life story of a star begins inside a nebula.
Nebula. This is the name given to an interstellar cloud of gas and dust. Nebulas are among the most beautiful objects in the night sky. The one seen on the cover of this magazine is called the Trifid Nebula (or nebula with three clefts). Inside this nebula new stars have been born, which cause the nebula to give off a reddish glow.
Apparently, stars form inside a nebula when the diffuse matter condenses under the force of gravity into contracting regions of gas. These huge balls of gas stabilize when they reach the temperature at which nuclear reactions begin in the core of the cloud, preventing further contraction. Thus a star is born, often in conjunction with others, with which it makes up a star cluster.
Star clusters. In the photograph on page 8, we see a small cluster called the Jewel Box, thought to have been formed just a few million years ago. Its name was coined from the graphic description by 19th-century astronomer John Herschel: “a casket of variously coloured precious stones.” Our galaxy alone is known to have over a thousand similar clusters.
The star’s energy. A nascent, or developing, star stabilizes as a nuclear furnace is fired in its interior. It starts converting hydrogen to helium by a fusion process somewhat like that which occurs in a hydrogen bomb. Such is the mass of a typical star, like the sun, that it can burn its nuclear fuel for billions of years without exhausting the supply.
But what happens when such a star eventually uses up its hydrogen fuel? The core contracts, and the temperature rises as the star exhausts the hydrogen in the central regions. Meanwhile, the outer layers expand enormously, increasing the star’s radius 50 or more times, and it becomes a red giant.
Red giants. A red giant is a star with a surface temperature that is relatively cool; its color therefore appears red, rather than white or yellow. This phase in a star’s life is relatively short, and it ends—when most of the helium supply runs out—with a celestial fireworks display. The star, still burning helium, ejects its outer layers, which form a planetary nebula, glowing because of energy received from its mother star. Eventually, the star contracts dramatically to become a faintly shining white dwarf.
If the original star is massive enough, however, the final outcome is that the star itself explodes. That is a supernova.
Supernovas. A supernova is the explosion that ends the life of a star that was originally much more massive than the sun. Huge amounts of dust and gas are spewed into space by violent shock waves at speeds of over 6,000 miles a second [10,000 km/sec]. The intense light of the explosion is so bright that it outshines a billion suns, appearing as a sparkling diamond in the sky. The energy liberated in a single supernova explosion corresponds to the total energy the sun would radiate in nine billion years.
Nine hundred years after Yang observed his supernova, astronomers can still see the scattered debris of that explosion, a structure called the Crab Nebula. But something more than the nebula was left behind. At its center they discovered something else—a tiny object, rotating 33 times a second, called a pulsar.
Pulsars and neutron stars. A pulsar is understood to be a superdense, spinning core of matter left over after a supernova explosion of a star no more than three times as massive as the sun. Having diameters of less than 20 miles [30 km], they are rarely detected by optical telescopes. But they can be identified by radio telescopes, which detect the radio signals that are produced by their rapid rotation. A beam of radio waves rotates with the star, like the beam of a lighthouse, appearing as a pulse to an observer, giving rise to the name pulsar. Pulsars are also called neutron stars because they are principally composed of tightly packed neutrons. This accounts for their incredible density—over a billion tons per cubic inch [over a hundred million tons per cubic centimeter].
But what would happen if a really massive star went supernova? According to astronomers’ calculations, the core could continue its collapse beyond the neutron-star stage. Theoretically, the force of gravity compressing the core would be so great that a so-called black hole would result.
Black holes. These are said to be like gigantic cosmic whirlpools from which nothing can escape. The inward pull of gravity is so strong that both light and matter that get too close are inexorably sucked into them.
No black hole has ever been observed directly—by definition that is impossible—although physicists hope to demonstrate the existence of them by the effect they have on neighboring objects. New observing techniques may be needed to unlock this particular secret.
Secrets of the Galaxies
A galaxy is a cosmic structure made up of billions of stars. In 1920 it was discovered that the sun was not the center of our galaxy, as had previously been assumed. Soon afterward, powerful telescopes revealed a host of other galaxies, and man began to comprehend the immensity of the universe.
The misty tapestry we call the Milky Way is really an edge view of our own galaxy. If we could see it from afar, it would look much like a giant pinwheel. Its shape has been likened to two fried eggs placed back to back but, of course, on a far grander scale. Traveling at the speed of light, it would take 100,000 years to cross our galaxy. The sun, situated toward the outer edge of the galaxy, takes 200 million years to complete its orbit around the galactic center.
Galaxies, like stars, still hold many secrets that intrigue the scientific community.
Quasars. In the 1960’s, strong radio signals were picked up from objects far, far beyond our local group of galaxies. They were called quasars—short for “quasi-stellar radio sources”—because of their similarity to stars. But astronomers were perplexed by the prodigious energy quasars emitted. The more luminous one is some ten thousand times as bright as the Milky Way, and the most distant ones detected are over ten billion light-years away.
After two decades of intensive study, astronomers have come to the conclusion that these distant quasars are very active nuclei of outlying galaxies. But what goes on in the nucleus of these galaxies to generate such enormous energy? Some scientists suggest that the energy is released by gravitational processes rather than by nuclear fusion as in stars. Current theory associates quasars with gigantic black holes. Whether this is correct or not remains uncertain at present.
Quasars and black holes are just two of the puzzles that are yet to be solved. In fact, some of the secrets of the universe may be forever beyond our grasp. Nevertheless, those that have been unlocked can teach us some profound lessons, lessons that have implications far beyond the realm of astronomy.
[Picture on page 7]
Spiral galaxy M83
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Photo: D. F. Malin, courtesy of Anglo-Australian Telescope Board
[Pictures on page 8]
The Jewel Box
Open star cluster, the Pleiades in Taurus, M45
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Photo: D. F. Malin, courtesy of Anglo-Australian Telescope Board
[Pictures on page 8]
Orion nebula, with inset showing Horsehead nebula
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Photo: D. F. Malin, courtesy of Anglo-Australian Telescope Board