# Science Background
A History of the Crab Nebula

Hundreds of years before Americans began celebrating Independence Day by peppering the sky with fireworks, a more powerful celestial explosion brightened a summer sky.

It was the spectacular explosion of a supernova, the violent death of a star that may have been as much as 10 times more massive than our Sun. In July or August of 1054, Chinese astronomers saw and recorded the star's demise. Appearing in the sky above the southern horn of the constellation Taurus was a star the Chinese described as six times brighter than Venus and about as brilliant as the full Moon. The remains of this star were later christened the Crab Nebula, a cloudy, glowing mass of gas and dust about 7,000 light-years away from Earth.

This "guest star," as the Chinese called it, was so bright that people saw it in the sky during the day for almost a month. During that time, the star was blazing with the light of about 400 million suns. The star remained visible in the evening sky for more than a year. In two separate accounts, Chinese astronomers described the star as having pointed rays in all four directions and a reddish-white color.

If the blast had occurred 50 light-years from Earth, astronomers believe that all living things could have been destroyed by radiation. In the nine centuries since, astronomers have witnessed only two comparable cataclysms in our Galaxy: the supernova explosions of 1572 and 1604.

By Chinese accounts, the supernova was a tremendous celestial display. But the Europeans may not have agreed, because astronomers have not found any European records of the event.

The American Indians in northern Arizona, however, may have been so inspired by the event that they drew pictures of it. Two pictographs have been found, one in a cave at White Mesa and the other on a wall of Navajo Canyon. Both show a crescent moon with a large star nearby. Scientists have calculated that on the morning of July 5, 1054, the Moon was located just 2 degrees north of the Crab Nebula's current position.

The supernova was forgotten for more than 600 years until the invention of telescopes, which revealed fainter celestial details than the human eye can detect. In 1731, English physicist and amateur astronomer John Bevis observed the strings of gas and dust that form the nebula. While hunting for comets in 1758, Charles Messier spotted the nebula, noting that it had no apparent motion. The nebula became the first entry in his famous "Catalogue of Nebulae and Star Clusters," first published in 1774. Lord Rosse named the nebula the "Crab" in 1844 because its tentacle-like structure resembled the legs of the crustacean.

In the decades following Lord Rosse's work, astronomers continued to study the Crab because of their fascination for the strange object. In 1939, astronomer John Duncan concluded that the nebula was expanding and probably originated from a point source about 766 years earlier.

Astronomer Walter Baade probed deeper into the nebula, observing in 1942 that a prominent star near the nebula's center might be related to its origin. Six years later, scientists discovered that the Crab was emitting among the strongest radio waves of any celestial object. Baade noticed in 1954 that the Crab possessed powerful magnetic fields, and in 1963, a high-altitude rocket detected X-ray energy from the nebula.

Radio waves. X-rays. Strong magnetic fields. Scientists knew that the Crab Nebula was a powerful source of radiation, but what was its origin? They discovered it in 1968: an object in the nebula's center -- Baade's prominent star -- that emitted bursts of radio waves 30 times per second. Called the Crab Pulsar, it is among the first pulsars discovered, and is the fastest and most energetic pulsar formed from a supernova explosion.

But scientists wondered why the pulsar was spinning so fast. They concluded that the pulsar was a neutron star because theory suggested that these stars existed at the centers of supernova remnants. Neutron stars also are the only stars that can rotate rapidly without breaking apart. An extremely dense, compact object, a neutron star forms from the matter of a collapsed star. The Crab Pulsar acts as a celestial power station, generating enough energy to keep the entire nebula radiating over almost the whole electromagnetic spectrum. Because of the pulsar's power, the nebula shines brighter than 75,000 suns.

#### May 30, 1996