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M 1 - Crab Nebula
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M 1 - Crab Nebula

February 18, 2005
Supernova Remnant M1 (NGC 1952) in Taurus

Right Ascension: 05 : 34.5 (h:m)
Declination: +22 : 01 (deg:m)
Distance: 6.3 (kly)
Visual Brightness: 8.4 (mag)
Apparent Dimension: 6x4 (arc min)

Discovered 1731 by British amateur astronomer John Bevis.

The Crab Nebula is the most famous and conspicuous known supernova remnant, an cloud of gas created in the explosion of a star as supernova.

The supernova was noted on July 4, 1054 A.D. by Chinese astronomers, and was about four times brighter than Venus, or about mag -6. According to the records, it was visible in daylight for 23 days, and 653 days to the naked eye in the night sky. 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 Anasazi art online. In addition, Ralph R. Robbins of the University of Texas has found Mimbres Indian art from New Mexico, possibly depicting the supernova.

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, who added it to his sky atlas, Uranographia Britannica. Charles Messier independently found it on August 28, 1758, when he was looking for comet Halley on its first predicted return, 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 the discovery of this object which caused Charles Messier to begin with the compilation of his catalog. It was also the discovery of this object, which closely resembled a comet (1758 De la Nux, C/1758 K1) in his small refracting telescope, which brought him to the idea to search for comets with telescopes (see his note). Messier acknowledged the prior, original discovery by Bevis when he learned of it in a letter of June 10, 1771.

Although Messier's catalog was primarily compiled for preventing confusion of these objects with comets, M1 was again confused with comet Halley on the occasion of that comet's second predicted return in 1835.

This nebula was christened the "Crab Nebula" on the ground of a drawing made by Lord Rosse about 1844. Of the early observers, Messier, Bode and William Herschel correctly remarked that this nebula is not resolvable into stars, but William Herschel thought that it was a stellar system which should be resolvable by larger telescopes. John Herschel and Lord Rosse, erroneously, thought it is "barely resolvable" into stars. They and others, including Lassell in the 1850s, apparently mistook filamentary structures as indication for resolvability.

Early spectroscopic observations, e.g. by Winlock, revealed the gaseous nature of this object in the later 19th century. The first photo of M1 was obtained in 1892 with a 20-inch telescope. First serious investigations of its spectrum were performed in 1913-15 by Vesto Slipher; he found that the spectral emission lines were split; it was later recognised that the true reason for this is Doppler shift, as parts of the nebula are approaching us (thus their lines are blueshifted) and others receding from us (lines redshifted). Heber D. Curtis, in his description of this object based on Lick Observatory photographs, tentatively classified it as a planetary nebula (Curtis 1918), a view which was disproved only in 1933; this mis-classification can still be found in much newer handbooks.

In 1921, C.O. Lampland of Lowell Observatory, when comparing excellent photographs of the nebula obtained with their 42-inch reflector, found notable motions and changes, also in brightness, of individual components of the nebula, including dramatic changes of some patches near the central pair of stars (Lampland 1921). The same year, J.C. Duncan of Mt. Wilson Observatory compared photographic plates taken 11.5 years apart, and found that the Crab Nebula was expanding at an average of about 0.2" per year; backtracing of this motion showed that this expansion must have begun about 900 years ago (Duncan 1921). Also the same year, Knut Lundmark noted the proximity of the nebula to the 1054 supernova (Lundmark 1921).

In 1942, based on investigations with the 100-inch Hooker telescope on Mt. Wilson, Walter Baade computed a more acurate figure of 760 years age from the expansion, which yields a starting date around 1180 (Baade 1942); later investigations improved this value to about 1140. The actual 1054 occurrance of the supernova shows that the expansion must have been accelerated.

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, first found by Roscoe Frank Sanford in 1919 by spectroscopic investigations, see (Sanford 1919), photographically confirmed by Walter Baade and Rudolph Minkowski in 1930: First, a reddish component which forms a chaotic web of bright filaments, which has an emission line spectrum (including hydrogene lines) like that of diffuse gaseous (or planetary) nebulae. Second a blueish diffuse background which has a continuous spectrum and consists of highly polarised `synchrotron radiation', which is emitted by high-energy (fast moving) electrons in a strong magnetic field, an explanation first proposed by the soviet astronomer J. Shklovsky (1953) and supported by observations of Jan H. Oort and T. Walraven (1956). 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, named and listed as Taurus A and later as 3C 144. X-rays from this object were detected in April 1963 with a high-altitude rocket of type Aerobee with an X-ray detector developed at the Naval Research Laboratory; the X-ray source was named Taurus X-1. Measurements during lunar occultations of the Crab Nebula on July 5, 1964, and repeated in 1974 and 1975, demonstrated that the X-rays come from a region at least 2 arc minutes in size, and 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, by Virginia Trimble (1973)), 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 !

On November 9, 1968, a pulsating radio source, the Crab Pulsar (also cataloged as NP0532, "NP" for NRAO Pulsar, or PSR 0531+21), was discovered in M1 by astronomers of the Arecibo Observatory 300-meter radio telescope in Puerto Rico. This star is the right (south-western) one of the pair visible near the center of the nebula in our photo. This pulsar was the first one which was also verified in the optical part of the spectrum, when W.J. Cocke, M.J. Disney and D.J. Taylor of Steward Observatory, Tucson, Arizona found it flashing at the same period of 33.085 milliseconds as the radio pulsar with the 90-cm (36-inch) telescope on Kitt peak; this discovery happened on January 15, 1969 at 9:30 pm local time (January 16, 1969, 3:30 UT, according to Simon Mitton). This optical pulsar is sometimes also referred to by the supernova's variable star designation, CM Tauri.

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 !

Jeff Hester and Paul Scowen have used the Hubble Space Telescope to investigate the Crab Nebula M1 (see also e.g. Sky & Telescope of January, 1995, p. 40). Their continuous investigations with the HST have provided new insight into the dynamic and changes of the Crab nebula and pulsar. More recently, the Heart of the Crab was investigated by HST astronomers.

This object has attracted so much interest that it was remarked that astronomers can be devided into two fractions of about same size: Those who do work related to the Crab nebula, and those who don't. There was a "Crab Nebula Symposium" in Flagstaff, Arizona in June, 1969 (see PASP Vol. 82, May 1970 for results - Burnham). The IAU symposium No. 46, held at Jodrell Bank (England) in August 1970 was solely devoted to this object. Simon Mitton has written a nice book on the Crab Nebula M1 in 1978, which is still most readable and informative (it is also source for some of the informations here).

The Crab Nebula can be found quite easily from Zeta Tauri (or 123 Tauri), the "Southern Horn" of the Bull, a 3rd-magnitude star which can be easily found ENE of Aldebaran (Alpha Tauri). M1 is about 1 deg N and 1 deg W of Zeta, just slightly south and about 1/2 degree west of a mag-6 star, Struve 742.

The nebula can be easily seen under clear dark skies, but can equally easily get lost in the background illumination under less favorable conditions. M1 is just visible as a dim patch in 7x50 or 10x50 binoculars. With a little more magnification, it is seen as a nebulous oval patch, surrounded by haze. In telescopes starting with 4-inch aperture, some detail in its shape becomes apparent, with some suggestion of mottled or streak structure in the inner part of the nebula; John Mallas reports that under excellent conditions, an experienced observer can see them throughout the inner portion of the nebula. The amateur can verify Messier's impression that M1 looks indeed similar to a faint comet without tail in smaller instruments. Only under excellent conditions and with larger telescopes, starting at about 16 inches aperture, suggestions of the filaments and fine structure may become visible.

As the Crab Nebula is situated only 1 1/2 degrees from the ecliptic, there are frequent conjunctions and occasional transits of planets, as well as occultations by the Moon (some of them mentioned above).

M1 is situated in a nice Milky Way field. The star Zeta Tauri is remarkable as it is a Gamma Cassiopeiae type variable, a rather rapidly rotating star of spectral type B4 III pe which has ejected an expanding gas shell, and has a fainter spectroscopic companion star in an orbit of about 133 days period. Preceding M1 two minutes (or half a degree) in Right Ascension is Struve 742 or ADS 4200, another visual binary star with components A (mag 7.2, spectrum F8, of yellow color) and B (mag 7.8, white) separated by about 3.6" in position angle 272deg, and orbiting each other in about 3000 years.