Rektascenzija	05 : 34.5 (u:m)
Deklinacija	+22 : 01 (sto:m)
Razdalja	6.3 (*1000 sv.l.)
Vizual. magnituda	8.4 (mag)
Zorni kot	6x4 (loc min)

The most conspicuous known supernova remnant. This object caused Messier to begin his catalog. The supernova was noted on July 4, 1054 A.D. by Chinese astronomers, and was perhaps about as bright as the Full Moon (at least, however, about mag -6), and visible in daylight for 23 days. It was probably also recorded by Anasazi Indian artists (in present-day Arizona and New Mexico), as findings in Navaho Canyon and White Mesa (both AZ) as well as in the Chaco Canyon National Park (NM) indicate; there's a review of the research on the Chaco Canyon Anazasi art online. The Supernova 1054 was also assigned the variable star designation CM Tauri. It is one of few historically observed supernovae in our Milky Way Galaxy.

The nebulous remnant was discovered by John Bevis in 1731, according to Messier, who independently found it on August 28, 1758, and first thought it was a comet. Of course, he soon recognized that it had no apparent proper motion and cataloged it on September 12, 1758. It was christened the "Crab" on the ground of a drawing made by Lord Rosse about 1844.

The nebula consists of the material ejected in the supernova explosion, which has been spread over a volume approximately 10 light years in diameter, and is still expanding at the very high velocity of about 1,800 km/sec. It emits light which consists of two major contributions: A reddish component which forms a chaotic web of bright filaments, which has an emission line spectrum like that of diffuse gaseous (or planetary) nebulae, and a blueish diffuse background of highly polarised `synchrotron radiation', which is emitted by high-energy (fast moving) electrons in a strong magnetic field; synchrotron radiation is also apparent in other "explosive" processes in the cosmos, e.g. in the active core of the irregular galaxy M82 and the peculiar jet of giant elliptical galaxy M87. These striking properties of the Crab Nebula in the visible light are equally conspicuous in the Palomar images post-processed by David Malin of the Anglo Australian Observatory, and in Paul Scowen's image obtained on Mt. Palomar.

In 1948, the Crab nebula was identified as a strong source of radio radiation. X-rays from this object were detected in 1964 with a high-altitude rocket; the energy emitted in X-rays by the Crab nebula is about 100 times more than that emitted in the visual light. Nevertheless, even the luminosity of the nebula in the visible light is enormous: At its distance of 6,300 light years (which is quite well-determined), its apparent brightness corresponds to an absolute magnitude of about -3.2, or more than 1000 solar luminosities. Its overall luminosity in all spectral ranges was estimated at 100,000 solar luminosities or 5*10^38 erg/s !

In 1968, a pulsating radio source, the Crab Pulsar (also cataloged as NP0532), was discovered in M1. This star is the right of the pair visible near the center of the nebula in our photo. It has now been established that this pulsar is a rapidly rotating neutron star: It rotates about 30 times per second! This period is very well investigated because the neutron star emits pulses in virtually every part of the electromagnetic spectrum, from a "hot spot" on its surface. The neutron star is an extremely dense object, denser than an atomic nucleus, concentrating more than one solar mass in a volume of 30 kilometers across. Its rotation is slowly decelerating by magnetic interaction with the nebula; this is now a major energy source which makes the nebula shining; as stated above, this energy source is 100,000 times more energetic than our sun.

In the visible light, the pulsar is of 16th apparent magnitude. This means that this very small star is roughly of absolute magnitude +4.5, or about the same luminosity as our sun in the visible part of the spectrum !

A History of the Crab Nebula from the Space Telescope Science Institute

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Povzeto po:
Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)