The Crab Nebula, also known as Messier 1 and Taurus A, is a supernova remnant located in the direction of Taurus constellation, at a distance of 6,500 light years from Earth. The nebula is an expanding remnant of a star that exploded in a supernova event which was observed by Chinese astronomers in the year 1054 AD. Its designation in the New General Catalogue is NGC 1952.
The Crab Nebula was created as a result of an extremely bright supernova explosion, one that was visible to the naked eye for almost two years.
The supernova remnant was later observed by the English astronomer John Bevis, who discovered it in 1731. Charles Messier discovered it independently in 1758 while looking for a comet that Edmond Halley said would return that year.
Messier 1 is also classified as a pulsar wind nebula, which means that the nebula is made of the material ejected by a pulsating neutron star interacting with interstellar gas and the magnetic field of the pulsar itself.
The Crab Nebula expands at a rate of 1,500 kilometres per second. It contains two dim stars at its centre, one of which is the neutron star.
The nebula is notable for its intricate structure, with filaments of dust that can be seen in visible wavelengths. Its contains enough dust, made of carbon and silicate materials, to make 30,000-40,000 Earths.
Study of the nebula provided definitive proof that supernova events created the raw materials for the first solid particles.
Messier 1 is the strongest known persistent source of X-ray and gamma-ray radiation in the sky. It contains the Crab Pulsar at its centre, a neutron star about 28-30 kilometres across. The dead star’s core emits pulses of radiation from gamma rays to radio waves as it spins at a rate of 30.2 per second.
This is useful for studying the objects that pass in front of the nebula, blocking its radiation, like the Sun or the moons orbiting the planets in the solar system. In 2003, scientists used it to measure the thickness of Titan’s (Saturn’s moon) atmosphere and in the mid-20th century, they used it to map the Sun’s corona.
The Crab Nebula has an apparent magnitude of 8.4 and cannot be seen without binoculars or a telescope. Observing it with binoculars requires good viewing conditions, and it only appears as a faint patch of light in 7×50 and 10×50 binoculars.
In 4-inch telescopes, one can see a hint of the nebula’s streak structure in the central region, and in most smaller telescopes the nebula looks similar to a comet without a tail. The filaments and details of the nebula’s structure are only visible in 16-inch or larger telescopes in good viewing conditions.
The Crab Nebula occasionally flares up. The most recent flare was recorded in March 2013, when the nebula exhibited a spike in gamma-ray emission three times more intense than its usual output. In April 2011, a particularly bright outburst was recorded that made the nebula about 30 times brighter than it normally is. The flares are suspected to be the result of the pulsar’s wind.
M1 is about 11 light years in diameter, which translates into 7 arc minutes of apparent diameter. It is part of the Perseus Arm of our galaxy, the Milky Way.
The Crab Nebula was the first object to be entered in Messier’s catalogue and remains one of the most studied objects in the night sky. It helps that the view of the nebula is not obscured by dust and other interstellar material, which makes it easy to observe and study.
The nebula was also the first object associated with a historical supernova event.
The supernova remnant was discovered by John Bevis in 1731, who included it in his sky atlas, Uranographia Britannica. Charles Messier observed the object independently on August 28, 1758, and thought it was a comet. Once he had realized that the object did not have apparent proper motion, he entered it into his catalogue on September 12, 1758. Messier’s catalogue was created to list objects that sky watchers confused for comets, and the Crab Nebula was the first deep sky object on his list.
The supernova explosion in 1054 was so bright that it was visible in the daytime sky for 23 days. The event was documented by astronomers and sky watchers throughout the Far East.
British astronomer William Parsons, Lord Rosse, sketched the nebula around 1844 after observing it though a 36-inch telescope. The image of the object resembled a crab, which is how the nebula got its name. The name stuck even though Lord Rosse later said his sketch was not representative of the nebula’s appearance and structure after observing it through a 72-inch telescope in 1848.
The nebula was first photographed n 1892 using a 20-inch telescope.
It wasn’t until the early 20th century that scientists determined that the nebula was expanding and matched its origination date to the historical supernova explosion.
In 1918, the American astronomer Heber D. Curtis classified the object as a planetary nebula. It was not until 1933 that this classification was disproved. In 1921, the American astronomer Carl Otto Lampland used photographs of the nebula taken using a 42-inch reflector and discovered motions inside the nebula. He saw that individual components changed in brightness, especially those in the vicinity of the central pair of stars.
1921 was also the year the American astronomer John Charles Duncan discovered that the nebula was expanding at a rate of about 0.2” per year after comparing photographs taken 11.5 years apart. It was the Swedish astronomer Knut Lundmark who first noted the proximity of the Crab Nebula to the historical supernova of 1054, also in 1921. In 1928, the American astronomer Edwin Hubble suggested that the object be associated with the historical supernova.
In 1942, the German astronomer Walter Baade used a 100-inch Hooker telescope to study the nebula and traced back its year of origin to 1180. Later calculations changed the estimated date to 1140. As the supernova occurred in 1054, the expansion rate of the nebula is believed to have accelerated. The nebula was linked to the Supernova 1054 beyond reasonable doubt in 1942 by the Dutch astronomer Jan Oort.
Crab Nebula – Messier 1 (NGC 1952, Sharpless 244)
Coordinates: 05h34m31.94s (right ascension), +22°00’52.2” (declination)
Visual magnitude: 8.4
Absolute magnitude: -3.1
Distance: 6,500 light years
Apparent dimensions: 420” x 290”
Radius: 5.5 light years
The pulsar inside the Crab Nebula is the rapidly spinning, relatively young neutron star that powers the nebula. A neutron star is created in the aftermath of a supernova. Stars explode when they run out of fuel to keep from collapsing into themselves; their outer layers are ejected into space in a supernova explosion, creating a nebula, while the inner core contracts to form a neutron star.
The neutron star that remains is very small, normally just 10 to 15 miles across, but it is extremely dense. The star within the Crab Nebula rotates very rapidly, and completes about 30 revolutions each second. Rotating neutron stars like this one are called pulsars. The word is derived from the contraction of the term “pulsating star.”
The Crab Pulsar is one of the most powerful pulsars known. It lights the nebula in every wavelength, from radio to gamma-rays. The optical pulsar has a diameter of about 20 kilometres.
The X-rays emitted by the nebula were discovered in April 1963, and the source was subsequently named Taurus X-1. After further observation, scientists concluded that the X-rays come from an area which is at least 2 arcminutes in size, and that the nebula’s X-ray emission is roughly 100 times more intense that the energy emitted in visible light. The nebula’s luminosity in visible light translates into an absolute magnitude of about -3.2, which makes the Crab Nebula more than 1,000 times more luminous than the Sun. Its luminosity across all spectral ranges is estimated to be 100,000 times that of the Sun.
The Italian astrophysicist Franco Pacini predicted the existence of a neutron star in the 1960s to account for the nebula’s brightness.
The Crab Pulsar (PSR B0531+21) was discovered in Messier 1 on November 9, 1968. The pulsating radio source was detected at the Arecibo Observatory in Puerto Rico by astronomers using a 300-meter radio telescope.
On January 15, 1969, astronomers at the Steward Observatory in Tucson, Arizona, observed the nebula using a 36-inch telescope and discovered that the Crab Pulsar was flashing with a period of 33.085 miliseconds. The knowledge of the pulsar’s exact age helped astronomers in the study of these objects and their physical properties. The only other pulsar connected to a known supernova is located in 3C58 in Cassiopeia constellation, the remnant of the supernova SN 1181, but the connection between the two has been contested. The discovery of the pulsar within the Crab Nebula confirmed the theory that neutron stars were formed in certain types of supernova events.
The pulsar was really first detected in the summer of 1967 by Charles Schisler, a U.S. Air Force officer, who detected a radio source and determined its position, which corresponded to that of the Crab Nebula. Schisler later discovered a number of other pulsars, but the Air Force never published his discoveries because they did not think they were of any concern to them.
The Crab Pulsar emits pulses in every part of the electromagnetic spectrum and powers the nebula. Its spin rate, 30 times per second, is slowly decelerating, by 38 nanoseconds per day, because the pulsar wind carries away a lot of its energy. The spinning neutron star has a visual magnitude of 16 in visible light and an absolute magnitude of 4.5, roughly the same as the Sun in visible light.
SN 1054, the supernova that created the Crab Nebula was originally seen by Chinese astronomers on July 4, 1054 AD. They documented it as a “guest star,” a new star in the sky that appeared four times brighter than the planet Venus.
At its peak, the supernova had a visual magnitude of about -6, and could be seen during the day for 23 days. The event was visible to the naked eye for 653 days at night. It was the brightest object in the sky next to the Moon for a few months, and is the best known supernova in the history of astronomy. Supernova 1054 was given the variable star designation CM Tauri.
The star that exploded was 12 to 15 times more massive than the Sun, turned by the supernova event into a small neutron star that spins rapidly, ejecting stellar material.
The Crab Nebula is located near the southern horn of the celestial bull, represented by Taurus constellation.
The southern horn is marked by the 3rd magnitude star Zeta Tauri, located to the east-northeast of the bright Aldebaran, Alpha Tauri.
The nebula can be found approximately 1 degree to the north and 1 degree to the west of Zeta Tauri. It is best seen in the evenings in January.
Messier 1 lies only a degree and a half from the ecliptic and is frequently eclipsed by planets and the Moon.