Tuesday, 21 November, 2017

The Crab Nebula

The Crab Nebula (otherwise known as M1 or NGC 1952) is a supernova remnant and pulsar wind nebula in the constellation of Taurus. Corresponding to a bright supernova recorded by Chinese astronomers in 1054, the nebula was observed later by English astronomer John Bevis in 1731. The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844. At an apparent magnitude of 8.4, comparable to that of the largest moon of Saturn, it is not visible to the naked eye.

M1 Crab NebulaLocated at a distance of about 6,500 light-years from Earth, the nebula has a diameter of 11 light years and expands at a rate of about 1,500 kilometers per second. It is part of the Perseus Arm of the Milky Way galaxy.

Northern Constellations







At the center of the nebula lies the Crab Pulsar, a neutron star 28–30 km across with a spin rate of 30.2 times per second, which emits pulses of radiation from gamma rays to radio waves. The nebula was the first astronomical object identified with a historical supernova explosion.

Crab_Nebula_NGC_1952_(composite_from_Chandra,_Hubble_and_Spitzer)Data from Chandra X-ray Observatory provide significant clues to the workings of this mighty cosmic “generator,” which is producing energy at the rate of 100,000 suns. This composite image uses data from three of NASA’s Great Observatories. The Chandra X-ray image is shown in blue, the Hubble Space Telescope optical image is in red and yellow, and the Spitzer Space Telescope’s infrared image is in purple.

Crab Nebula and PulsarThe red and yellow filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula’s eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. Like a lighthouse, it ejects twin beams of radiation that appear to pulse over 30 times a second due to the star’s rotation.

Crab Nebula PulsarThe photo shows an image of the center of the Crab Nebula obtained using the Hubble Space Telescope’s Wide Field Planetary Camera 2, installed during the repair mission in December 1993. The pulsar, a rapidly spinning “neutron star” that is so dense that a single cubic inch of its material would weigh 6 billion tons, is left over from the explosion of a massive star seen by Chinese astronomers over 900 years ago.

Based on the new WFPC-2 data, we report two discoveries that help to clarify the structure of the nebula surrounding the pulsar and provide clues about the workings of the pulsar and the nebula. The first discovery is a small knot of bright emission located only 1500 AU (= 1500 times the distance from the Earth to the Sun) from the pulsar. This knot has gone undetected up until now because even at the best ground-based resolution it is lost in the glare of the adjacent pulsar. The knot and the pulsar line up with the direction of a jet of X-ray emission. We believe that the knot may be a “shock” in the jet – a location where the wind streaming away from the pole of the pulsar piles up. The second discovery is that in the direction opposite the knot, the Crab pulsar is capped by a ring-like “halo” of emission tipped at about 20 degrees to our line of sight. In this geometry the polar jet flows right through the center of the halo. The newly discovered ring may mark the boundary between the polar wind and jet, and an equatorial wind that powers a larger torus of emission surrounding the pulsar.

Taken together, these discoveries paint a different and more detailed picture of the environment near the pulsar than astronomers previously had. Dr. Paul Scowen, research associate at Arizona State University, states, “The new data have provided a clearer glimpse at the pulsar environment; a glimpse that should have theoretical astronomers scratching their heads for some time to come.”

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