Crab Nebula

Crab Nebula or M1, gas cloud that is the remnant of a supernova, or exploding star, in the constellation Taurus. The supernova was noted by Chinese astronomers as a brilliant “new star” on the morning of July 4, ad 1054. It was perhaps as bright as the full moon and visible even in daylight for as long as three weeks. Figures appearing in rock art of the Anasazi people of the Chaco Canyon region of modern-day New Mexico and Arizona may also represent the supernova.

In 1731 the nebulous remains of the explosion were discovered telescopically by John Bevis. The French astronomer Charles Messier independently spotted it in 1758, while searching for comets. At first he believed the object to be a comet, but, after studying it over a two-week period, he realized that it had no independent motion against the background of stars. It was with this discovery that Messier began his Catalogue of Nebulae and Star Clusters on September 12, 1758. The Messier Catalogue was designed to show comet-seekers the locations of this and other such nebulous patches and to prevent them from being misled by non-cometary objects. Not until the mid-19th century was this object, labelled M1 in Messier’s catalogue, dubbed the “Crab Nebula”, after its crablike appearance in a sketch made by the Irish observer Lord Rosse.

The Crab Nebula consists of the gaseous matter making up the former star, blasted outward when it exploded, and now about 10 light years across. Astronomers have measured its expansion velocity at about 1,800 km/s (1,100 mi/s). Even at its distance of about 6,300 light years from Earth, the Crab Nebula appears as the most conspicuous supernova remnant in the sky. With a visual magnitude of 8.2, it is easily visible with small telescopes in dark skies, with an apparent diameter of about 6 × 4 minutes of arc. Modern telescopes reveal two types of radiation emanating from the region: a reddish light, which comes from the diffuse gaseous cloud itself, and bluish synchrotron radiation, emitted by electrons whirling around in a strong magnetic field. In 1948 the nebula was found to be a strong source of radio emissions, and, 16 years later, a source of X-radiation 100 times more energetic than the visible light emitted. In 1967 the astronomers Jocelyn Bell Burnell and Antony Hewish found at its core a pulsating radio source, or pulsar. It is now known to be an extremely dense neutron star about 30 km (19 mi) across, spinning about 30 times per second and sending out powerful beams of radio waves and light that sweep round the sky like the beams of a lighthouse.

See also Nebula.