The University Record, February 13, 1996
Crab Nebula feature of February skies in Michigan
Michigan starwatchers who brave February's cold will be rewarded with a look at the Crab Nebula, located between the horns of the constellation Taurus, the Bull. The gaseous debris left over from a supernova or exploding star, the Crab Nebula was first noticed by Chinese astronomers on July 4, 1054, says astronomer Richard Teske.
"According to Chinese records, the brilliant "guest star" created by the cataclysm was visible in daylight for three weeks," Teske says. "Then it grew dimmer, finally fading altogether from the Chinese night sky almost two years later."
The Crab Nebula lies directly north of Orion, and just above the star at the tip of the Bull's southern horn. Although it is 6,300 light-years from us, the Crab Nebula can be seen with small amateur telescopes as a faint and fuzzy "star." Slightly larger telescopes reveal its apparent size on the sky to be the equal of the largest craters on the Moon. "Here are the remains of the detonation of an entire star," Teske explains. "The actual width of the debris cloud is now six light-years, and it continues to expand into the empty space around it at some 50 million miles per day.
"Hidden in the nebula's center and visible only through the largest of amateur telescopes, gleams a pulsar the compact cinder of the original shattered star's innermost core," Teske adds. Time exposure photographs of the pulsar show it shining with a steady light, but special equipment shows that it actually blinks on and off 30 times each second.
Pulsars received their name when first discovered by radio telescopes to emit pulses of radio-frequency waves spaced several seconds apart. More than 800 are now known to astronomers. Some, like the Crab Nebula pulsar, also emit X-ray pulses detected by rockets and satellites.
Scientists are convinced that pulsars like the one in the Crab Nebula are spheres roughly the size of the city of Detroit that weigh about as much as our sun, according to Teske. Theoretical calculations establish that they are composed almost entirely of neutrons, sub-atomic chargeless particles found in the nuclei of atoms of all chemical elements except simple hydrogen. In effect, a pulsar can be likened to a spinning, city-sized atomic nucleus.
"As a pulsar spins, radio energy, X-rays and sometimes visible light are emitted in the form of a narrow beam, something like the beam of a lighthouse or searchlight," Teske explains. "Each time the beam sweeps across Earth we detect a pulse of radiation."
He says presence of a pulsar in the Crab Nebula and the Chinese records of the "guest star" have helped astronomers understand one way stars die. "When a star larger than our sun can no longer generate energy deep in its interior by thermonuclear reactions, it simply falls in upon itself. In the ensuing collapse, the core, containing as much matter as our sun, is squeezed into the cosmically tiny size of a city, and the outer parts are explosively ejected into space. The core becomes a tiny, spinning pulsar whose radio blips can be "seen" for millions of years. The rest of the star becomes a rapidly expanding nebula that dissipates after about 100,000 years."
Astronomers are examining the remains of the Crab Nebula to learn more about the process of collapse and ejection that blasted them into space. Through these studies, they will determine the size, age and chemical composition of the star that exploded and how the pulsar generates the beam of radio waves, light and X-rays that streams from it.
"Scientists expect their work will eventually reveal as much about what happened to the original star as if they had been there on the spot, watching the explosion take place," Teske says.