Also known as Taurus A, NGC 1952, Sharpless 244, LBN 833, 3C 144, the Crab Nebula is an expanding supernova remnant and pulsar wind nebula located in the northern constellation Taurus (the Bull), at right ascension 05h 34m 31.94s and declination +22°00’52.2”. It is 1 degree northwest of the bright star Zeta Tauri. The star can easily be found by first locating Aldebaran, the brightest star in Taurus, and then following the line of the V-shape that Aldebaran is part of, to Zeta Tauri. Aldebaran is seen by following the line formed by the three stars of Orion’s Belt. It is the first bright star that appears on that imaginary line. M1 has an apparent magnitude of 8.4 with a total luminosity 75,000 times that of the Sun and lies at a distance of 6,500 light years from Earth.
The Nebula is part of the Perseus Arm of the Milky Way galaxy and has a diameter of 11 light years, and expanding at a rate of about 1,500 kilometres per second. The Crab Nebula is the only supernova remnant listed in Messier’s catalogue. The supernova remnant contains the Crab Pulsar, a rapidly rotating neutron star that spins at a rate of 30.2 times per second. The pulsar, also catalogued as PSR 0531+21, is the youngest one observed, emitting radiation in optical, radio, ultraviolet, X-ray and gamma-ray wavelengths. The best time to observe Messier 1 in the northern hemisphere is in late autumn and early winter, during the months of November, December, and January.
In July or August of 1054, Chinese astronomers saw and recorded the star’s demise; the spectacular explosion of a supernova (i.e., the violent death of a star) that may have been as much as 10 times more massive than our Sun. Appearing in the sky above the southern horn of the constellation Taurus, it was described as six times brighter than Venus and about as brilliant as the full Moon. The remains of this star were later christened the Crab Nebula, a cloudy, glowing mass of gas and dust. This “guest star,” as the Chinese called it, was so bright, blazing with the light of about 400 million suns, that people saw it in the sky during the day for almost a month, and it remained visible in the evening sky for 653 days after its discovery. In two separate accounts, Chinese astronomers described the star as having pointed rays in all four directions and a reddish-white colour. If the blast had occurred 50 light-years from Earth, astronomers believe that all living things on Earth could have been destroyed by radiation. In the nine centuries since, astronomers have witnessed only two comparable cataclysms in our Galaxy: the supernova explosions of 1572 and 1604. The American Indians in northern Arizona may have been so inspired by the event that they drew pictures of it. Two pictographs have been found: one in a cave at White Mesa and the other on a wall of Navajo Canyon. Both show a crescent moon with a large star nearby. Scientists have calculated that on the morning of July 5, 1054, the Moon was located just 2 degrees north of the Crab Nebula’s current position. The supernova was also recorded by Arabic and Japanese observers.
The supernova was forgotten for more than 600 years until the invention of telescopes, which revealed fainter celestial details than the human eye can detect. In 1731, English physicist and amateur astronomer John Bevis observed the strings of gas and dust that form the nebula. While hunting for comets in 1758, Charles Messier spotted the nebula, noting that it had no apparent motion. The nebula became the first entry in his famous “Catalogue of Nebulae and Star Clusters,” published in 1774. Lord Rosse named the nebula the “Crab” in 1844 because its tentacle-like structure resembled the legs of the crustacean. In the decades following Lord Rosse’s work, astronomers continued to study the Crab because of their fascination for the strange object. In 1939, astronomer John Duncan concluded that the nebula was expanding and probably originated from a point source about 766 years earlier. Astronomer Walter Baade probed deeper into the nebula, observing in 1942 that a prominent star near the nebula’s centre might be related to its origin.
Six years later, scientists discovered that the Crab was emitting among the strongest radio waves of any celestial object. Baade noticed in 1954 that the Crab possessed powerful magnetic fields, and in 1963, a high-altitude rocket detected X-ray energy from the nebula. Scientists knew that the Crab Nebula was a powerful source of radiation, but what was its origin? They discovered it in 1968: an object in the nebula’s centre – Baade’s prominent star – that emitted bursts of radio waves 30 times per second. Called the Crab Pulsar, it is among the first pulsars discovered and is the fastest and most energetic pulsar formed from a supernova explosion. But scientists wondered why the pulsar was spinning so fast. They concluded that the pulsar was a neutron star because theory suggested that these stars existed at the centres of supernova remnants. Neutron stars also are the only stars that can rotate rapidly without breaking apart. An extremely dense, compact object, a neutron star forms from the matter of a collapsed star. The Crab Pulsar acts as a celestial power station, generating enough energy to keep the entire nebula radiating over almost the whole electromagnetic spectrum. Because of the pulsar’s power, the nebula shines brighter than 75,000 suns.
The Crab Pulsar is also catalogued as PSR 0531+21 or NP0532. It is about 28 to 30 kilometres across and, as a result of its high spin rate, it emits pulses of optical, X-ray and radio radiation. It was one of the first pulsars to be discovered and it provided evidence for the theory that pulsars were formed by supernova events. The progenitor star of Messier 1 was identified in 1942 by the German-American astronomer Rudolf Minkowski, who discovered that it had a very unusual optical spectrum. In 1967, the region around the star was identified as one of the brightest gamma-ray sources in the night sky. The mass of the neutron star is believed to be in the range from 1.4 to 2 solar masses. The existence of the Crab Pulsar was first predicted by the Italian astrophysicist Franco Pacini in the 1960s to explain the nebula’s brightness. The neutron star was observed for the first time in 1968. As the remnant was associated with a historical supernova, it played a huge role in helping astronomers understand the nature of supernova remnants and made it possible for them to verify the pulsar’s basic properties, such as age, spin-down luminosity, and the orders of magnitude. The explosion of the progenitor star produced a large shell of filaments that has continued to expand ever since and will eventually disperse and disappear into the surrounding space. The Nebula’s filaments contain ionised gas which is responsible for the nebula’s glow. The electrons found in the gas move at speeds close to the speed of light, emitting synchrotron radiation, which makes the nebula visible in radio wavelengths. The filaments of the Crab Nebula are what is left of the progenitor star’s atmosphere and they mainly consist of ionised hydrogen and helium, along with other elements including oxygen, carbon, iron, nitrogen, sulphur and neon. The temperatures of the filaments are typically in the range from 11,000 to 18,000 K.
Though changes in most astronomical objects are barely perceptible over a human lifetime, Hubble shows that the interior of the nebula “changes its stripes” every few days, according to Jeff Hester of Arizona State University in Tempe, AZ, who leads the team of astronomers that took the Wide Field and Planetary Camera 2 images. “We took the images a few weeks apart because we knew that it might be possible to observe slight differences in the Crab over a short time,” said Hester. “But I don’t think that any of us were prepared for what we saw.” Though ground-based images of the Crab had shown subtle changes in the nebula over months or years, the Hubble movie shows sharp wisp-like features streaming away from the centre of the nebula at half the speed of light. The powerhouse at the centre of the nebula responsible for these changes is a rapidly spinning neutron star – the compact core of the exploded star. As the neutron star spins on its axis 30 times a second, its twin searchlight beams sweep past the Earth, causing the neutron star to blink on and off. Because of this flickering, the neutron star is also called a “pulsar.” In addition to the pulses, the neutron star’s rapid rotation and intense magnetic field act as an immense slingshot, accelerating subatomic particles to close to the speed of light and flinging them off into space. In a dramatic series of images assembled over several months of observation, Hubble shows what happens as this magnetic pulsar “wind” runs into the body of the Crab Nebula. The glowing, eerie shifting patterns of light in the centre of the Crab are created by electrons and positrons (anti-matter electrons) as they spiral around magnetic field lines and radiate away energy. This lights up the interior volume of the nebula, which is more than 10 light-years across. The Hubble team finds that material doesn’t move away from the pulsar in all directions, but instead is concentrated into two polar “jets” and a wind moving out from the star’s equator.
The most dynamical feature in the inner part of the Crab is the point where one of the polar jets runs into the surrounding material forming a shock front. The shape and position of this feature shifts about so rapidly that the astronomers describe it as a “dancing sprite,” or “a cat on a hot plate.” The equatorial wind appears as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula. “Watching the wisps move outward through the nebula is a lot like watching waves crashing on the beach – except that in the Crab the waves are a light-year long and are moving through space at half the speed of light,” said Hester. “You don’t learn about ocean waves by staring at a snapshot. By their nature waves on the ocean are ever-changing. You learn about ocean waves by sitting on the beach and watching as they roll ashore. This Hubble ‘movie’ of the Crab is so significant because for the first time we are watching as these ‘waves’ from the Crab come rolling in.”
Credit: Messier Objects, NASA, Universe Today, Wikipedia.