Crab Nebula

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Crab Nebula
Hubble Crab Nebula.JPG
Observational Data
Designation Messier 1
NGC 1952
Right ascension 05h 34m 31.97s
Declination +22o 00′ 52.1"
Constellation Taurus
Type of object Nebula
Dimensions 6x4 (arc min)[1]
Magnitude Apparent Mag: +8.4[1]
Absolute Mag: −3.1±0.5[1]
Distance from Earth 6,500 ly[1]

The Crab Nebula, also known as M1 and NGC 1952, is an expanding gas cloud surrounding a neutron star located in the constellation of Taurus near the star Zeta Tauri, the south horn of the bull. Possibly the most famous nebula, it is the result of a supernova event that was first observed on Earth in 1054 A.D. Since then, the nebula has been expanding at a rate of 1500 km/s and is now on average some 11 light years in diameter.[2]

Supernova Event

The Crab Nebula is the result of the supernova event called SN1054, which was first recorded by Chinese and Arab astronomers on July 4, 1054 A.D. as a "guest star". The event was possibly also the inspiration for astronomy-related art created by Anasazi Indians.[3] Initially the supernova had an apparent magnitude of -6, making it brighter then even Venus. The star then began to fade, but remained subsequently bright enough to be observed during the day for the next 23 days, and at night for the next 653 days before finally fading from view.[4] The supernova has been classified since with the variable star designation CM Tauri.

Based on theoretical models of supernova explosions, the progenitor star for the explosion is calculated to have been between 9 and 11 solar masses.[5]

History of Observation

This image from the Chandra X-Ray Observatory shows the central pulsar of the Crab Nebula surrounded by tilted rings of high-energy particles.

The first person to discover the nebula remnant was English astronomer John Bevis in 1731, who recorded it in his book, Uranographia Britannica. Later in 1758, Charles Messier discovered the nebula independently while he was searching for Halley's Comet, and initially mistook the nebula for another comet. He soon realized it wasn't, when it became apparent it lacked any proper motion, and it became the first astronomical object recorded in his famous Catalogue des Nébuleuses et des Amas d'Étoiles ("Catalogue of Nebulae and Star Clusters").

The name "Crab Nebula" itself originated from a drawing of the nebula made by Lord Rosse in 1844, where the supernova remnant looked like a crab.[6]

In 1892 the first photograph was taken of the nebula using a 20-inch telescope. Later in 1921 it was discovered that the nebula was expanding at a rate that allowed for the tracing of its origin to about 900 years before.[7] In the same year it was independently realized that the nebula was in the proximity of the supernova SN1054. These two observations led to the revelation that the nebula was the remnant of that same supernova event.[8]

Later in 1949, the nebula was discovered to be a strong source of radio waves.[9] In 1963 it was discovered that the nebula was also a major source of x-ray, emitting x-ray radiation at 100 times the energy the nebula emits energy in the visual spectrum.[10] Eventually the source of this energy was discovered in 1968, when a radio pulsar, later revealed to be a neutron star, was found.


Despite being one of the best known nebulae, the Crab Nebula is invisible to the unaided eye, with an apparent magnitude of only +8.4 The exact distance of the nebula itself is unknown. It is roughly 11 light years across overall, extending some 13 light years across at its longest. Right now the nebula's gasses are expanding at around 1500 km/s, and it is believed that this velocity is increasing due to energy emitted from the pulsar. The amount of matter in the nebula is estimated to be around 4.6 ± 1.8 solar masses, and is used to help estimate the total mass of the progenitor star.[11]

Crab Pulsar

The Crab Pulsar is a rapidly rotating neutron star some 28-30 kilometers in diameter, and has a mass twice that of the Sun.[12] The pulsar rotates once every 33 milliseconds, emitting radiation towards us once per rotation. This allows us to measure its rotational speed at 6.4 million km/h. The pulsar is slowing down though, rotating 15 microseconds slower each year. The radiation emitted though from the remnant pulsar is enough to illuminate the entire Crab Nebula with a total luminosity some 75,000 time greater than our Sun.[13]