The remnant of a star seen by Chinese astronomers to explode spectacularly on 4 July 1054 (unrecorded in the West). The nebula itself was named in 1848 by the 3rd Earl of Rosse. Photographs show a tangled web of filaments threading a luminous nebula. The explosion which triggered the nebula was a supernova, which expelled its outer layers and left a dense neutron star at the centre, now observed as a pulsar rotating 30 times a second.
Crab Nebula| Diffuse nebula | Lists of nebulae |
|---|---|
|
M1, the Crab Nebula. |
|
|
Observation data (Epoch J2000.0) |
|
| Type | Supernova Remnant |
| Right ascension | 05 31.97 |
| Declination | +22° 00′ 52.1″ |
| Distance | 6,300 ly |
| Apparent magnitude (V) | +8.4 |
| Apparent dimensions (V) | 6 × 4 arcmin |
| Constellation | Taurus |
| Physical characteristics | |
| Radius | 3 ly |
| Absolute magnitude (V) | −3 |
| Notable features | Optical pulsar |
| Other designations | M1, NGC 1952 |
| edit | |
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant in the constellation of Taurus.
The nebula contains a pulsar in its centre which rotates thirty times per second, emitting pulses of radiation from gamma rays to radio waves.
The nebula acts as a source of radiation for studying celestial bodies that occult it. In the 1950s and 1960s, the Sun's corona was mapped from observations of the Crab's radio waves passing through it, and more recently, the thickness of the atmosphere of Saturn's moon Titan was measured as it blocked out X-rays from the nebula.
Origins
First observed in 1731 by John Bevis, the nebula was independently rediscovered in 1758 by Charles Messier as he was observing a bright comet. The Earl of Rosse observed the nebula at Birr Castle in the 1840s, and referred to the object as the Crab Nebula because a drawing he made of it looked like a crab.
In the early 20th century, the analysis of early photographs of the nebula taken several years apart revealed that it was expanding.
Recent analyses of historical records have found that the supernova that created the Crab Nebula probably occurred in April or early May, rising to its maximum brightness of between apparent magnitude −7 and −4.5 (brighter than everything in the night sky except the Moon) by July. Thanks to the recorded observations of oriental astronomers of 1054, Crab Nebula became the first astronomical object recognized as supernova explosion connected.
Physical conditions
In visible light, the Crab Nebula consists of a broadly oval-shaped mass of filaments, about 6 arcminutes long and 4 arcminutes wide, surrounding a diffuse blue central region (by comparison, the full moon is 30 arcminutes across). In the 1960s it was found that the source of the electron curved paths was the strong magnetic field produced by a neutron star at the centre of the nebula.
The Crab Nebula is currently expanding outwards at about 1,500 km/s. Images taken several years apart reveal the slow expansion of the nebula, and by comparing this angular expansion with its spectroscopically-determined expansion velocity, the nebula's distance can be estimated.
Tracing back its expansion consistently yields a date for the creation of the nebula several decades after 1054, implying that its outward velocity has accelerated since the supernova explosion. This acceleration is believed to be caused by energy from the pulsar that feeds into the nebula's magnetic field, which expands and forces the nebula's filaments outwards.
Estimates of the total mass of the nebula are important for estimating the mass of the supernova's progenitor star. Estimates of the amount of matter contained in the filaments of the Crab Nebula range from about 1–5 solar masses;
Central star
At the centre of the Crab Nebula are two faint stars, one of which is the star responsible for existence of the nebula. However, the discovery of a pulsating radio source in the centre of the Crab Nebula was strong evidence that pulsars were formed by supernova explosions. The energy released as the pulsar slows down is enormous, and it powers the emission of the synchrotron radiation of the Crab Nebula, which has a total luminosity about 75,000 times greater than that of the Sun.
The pulsar's extreme energy output creates a unusually dynamic region at the centre of the Crab Nebula. The most dynamic feature in the inner part of the nebula is the point where the pulsar's equatorial wind slams into the bulk of the nebula, forming a shock front.
Theoretical models of supernova explosions suggest that the star that exploded to produce the Crab Nebula must have had a mass of between 8 and 12 solar masses. Stars with masses lower than 8 solar masses are thought to be too small to produce supernova explosions, and end their lives by producing a planetary nebula instead, while a star heavier than 12 solar masses would have produced a nebula with a different chemical composition to that observed in the Crab.
A significant problem in studies of the Crab Nebula is that the combined mass of the nebula and the pulsar add up to considerably less than the predicted mass of the progenitor star, and the question of where the 'missing mass' is remains unresolved. Estimates of the mass of the nebula are made by measuring the total amount of light emitted, and calculating the mass required, given the measured temperature and density of the nebula.
Transits by solar system bodies
The Crab Nebula lies roughly 1½ ° away from the ecliptic—the plane of Earth's orbit around the Sun. These transits and occultations can be used to analyse both the nebula and the object passing in front of it, by observing how radiation from the nebula is altered by the transiting body.
Lunar transits have been used to map X-ray emissions from the nebula. Before the launch of X-ray-observing satellites, such as the Chandra X-ray Observatory, X-ray observations generally had quite low angular resolution, but when the Moon passes in front of the nebula, its position is very accurately known, and so the variations in the nebula's brightness can be used to create maps of X-ray emission.
Very rarely, Saturn transits the Crab Nebula. Observers used the Chandra X-ray Observatory to observe Saturn's moon Titan as it crossed the nebula, and found that Titan's X-ray 'shadow' was larger than its solid surface, due to absorption of X-rays in its atmosphere.
User Comments Add a comment…