Monday, May 23, 2011

The Unstable Crab Nebula


On the 4th of July 1054 or maybe even earlier, in April or May that year, a new bright star near the Sun was observed in the constellation of Taurus by witnesses in Italy, Armenia, Iraq, China, Japan, and North America. The unusual object appeared with a magnitude between –4 and –7.5 and was visible to the naked eye, even in the daytime sky. Apparently, maximum brightness coincided with the solar conjunction. Chinese astronomers observed the star in daylight until the 27th of July 1054, and they were able to see it in the night sky until the 17th of April 1056, before it faded from naked-eye visibility. In Europe, sightings of the supernova were probably censored, since the catholic church saw this celestial event as a bad omen in connection with the split from the orthodox church in the same year.

In 1731, the English physician and self-taught astronomer John Bevis, without any knowledge of the related historic observations, discovered a nebula at the position of the supernova. Independently,Charles Messier found the nebula on the 28th of August 1758, while following a comet for the first time. In fact, at first he took M 1for the comet. Messier wrote: "Nebula, contains no star; it is a whitish light, elongated in the shape of a candle's light." Only later did Messier learn of Bevis' observation and recognized the English man's priority on the discovery.

William Herschel described the object as "very bright, of irregular figure, full 5' in longest direction," and he speculated that "it consists of stars." His son John described M 1 as a "fine object, very large, extended,very gradually brightening a little toward the middle, mottled,4' long, 3' broad." Admiral Smith spoke of a "pearly-white nebula" and added: "It is of oval form, with a major axis from northwest to southeast, and the brightest portion toward the south. "In 1844, Lord Rosse devoted an extended study to the "Crab Nebula, "so called after an earlier observing report by him.

Because of its fine-structure, he mistook it for a star cluster: "We see resolvable filaments singularly disposed, springing principally from its southern extremity, and not, as is usual in clusters, irregularly in all directions. Probably greater power would bring out other filaments, and it would then assume the ordinary form of a cluster. It is studded with stars, mixed however with a nebulosity probably consisting of stars too minute to be recognized."

A few years later, William Lassall, too, mentioned the filaments, after his observations from Malta: "Long filaments run out from all sides, there appears to be a number of very minute and faint stars scattered over it. The brightest parts are 2' long, while its extensions reach 6'. "On the eve of the 19th century, Leo Brenner wrote about the popular name of the nebula: "So called, because Rosse pictured it in his fantastic drawing like a crab, with which it shows no resemblance, however. It rather likens a sponge. A broad, deep bulge, not containing any nebular masses, is located on its north-eastern side; a smaller bay is partially filled with nebulosity. "A first photo of M 1 was obtained in 1892, and Slipher's spectroscopic work of 1913–1915 showed split spectral lines in the vicinity of the outer filaments.

Later, this was explained by means of the Doppler effect in combination with a fast expansion of the nebula. Not without some doubt, Curtis characterized the Crab Nebula in 1918 as a planetary nebula – a contemporary assumption, but a mistake, which reappeared in some catalogs until the 1960s. On this occasion, however, the central star got mentioned for the first time: "Two stars of mag 16 are close together near the center but it is not certain that either of them is a central star. This very complex and interesting object is nearly 6'×4' in P.A. about 125°. It is not a typical planetary in form and it is doubtful whether it is properly to be included as a member of the class.""

Lampland, in 1921, found displacements of filamentary detail when comparing photos of different age. The same year, Duncan found an expansion rate of 0.2'' per year from such a comparison, and dated the possible creation of the nebula to 900 years back. Lundmark, also in 1921, then suggested the historic supernova of1054 as the cause of the nebula, which nowadays is a commonly accepted fact. In 1968, the central pulsar was discovered by the radio telescope in Arecibo.

The Unstable Crab Nebula

Astrophysicists have long assumed that the Crab Nebula(M1), the strongest permanent X-ray source in the sky, is a steady "standard candle" at these wavelengths, and they have used it to calibrate X-ray telescopes accordingly. The brightnesses of other X-ray sources are often stated in "milli-crabs". But a NASA team now confirms what others have been unwilling to admit: this "standard candle" varies by at least several percent.

The group compared observations from four separate X-ray satellites and found the same variations over the years. These include a steady decline totaling 7% in the past two years in four high-energy X-ray bands ranging from 12,000 to 500,000 electron volts. Checking further back, the group found that the Crab brightened and dimmed in X-rays as much as 3.5% from year to year since 1999. Previous observers had assumed that these variations were in their equipment. "Since we haven't gone back yet and reanalyzed previous results, we can't say for sure what the impact will be," said team member Colleen Wilson-Hodge (NASA/Marshall Space Flight Center). "For some instruments and observations the effect would be slight, but there may be instruments for which re-analysis could change conclusions of some studies." The Crab Nebula is the remnant of a supernova seen in 1054 A.D.

(Above Image) The Chandra images in this collage were made over a span of several months (ordered left to right, except for the close-up). They provide a stunning view of the activity in the inner region around the Crab Nebula pulsar, a rapidly rotating neutron star seen as a bright white dot near the center of the images.A wisp can be seen moving outward at half the speed of light from the upper right of the inner ring around the pulsar. The wisp appears to merge with a larger outer ring that is visible in both X-ray and optical images.

The inner X-ray ring consists of about two dozen knots that form, brighten and fade. As a high-speed wind of matter and antimatter particles from the pulsar plows into the surrounding nebula, it creates a shock wave and forms the inner ring. Energetic shocked particles move outward to brighten the outer ring and produce an extended X-ray glow.Enormous electrical voltages generated by the rotating, highly magnetized neutron star accelerate particles outward along its equator to produce the pulsar wind. These pulsar voltages also produce the polar jets seen spewing X-ray emitting matter and antimatter particles perpendicular to the rings.

The exploding star's core became a pulsar, a hyper dense neutron star now spinning 30 times per second. The pulsar is slowing down by magnetic braking, and the lost spin energy ends up exciting the nebula to glow. The nebula's inner portion "is dominated by the pulsar's magnetic field, which we suspect is organized precariously,"says Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology. "The X-ray changes may involve some rearrangement of the magnetic field, but just where this happens is a mystery."

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