Deep-Sky Objects: Crab Nebula (M1)

Crab Nebula

The Crab Nebula (M1) is a supernova remnant in the constellation Taurus, and although its central neutron star was only physically discovered in 1968, it has since been positively identified as the remains of a star that exploded in the year 1054, making it the first celestial object to be positively linked to a historical supernova event. At the time, the supernova attained a peak magnitude of –7, meaning it was easily visible during the daytime, but it has since faded to a magnitude of just +8.4. The supernova remnant was first observed in 1731 by John Bevis, and in 1758 it became the first entry in Charles Messier’s famous astronomical catalogue recording nebulae that could be mistaken for comets. The Crab Nebula is currently expanding at a velocity of roughly 1,500 km/sec, or about 0.5% of the speed of light, which is fast enough to reveal the expansion of the nebula in images taken a few years apart.

Quick Facts

• Constellation: Taurus
• Coordinates: RA 05h 34m 31.94s |Dec. +22° 00′ 52.2″
• Distance: 6,500 ± 1600 light years
• Object type: Supernova remnant
• Apparent dimensions: 420 × 290 seconds of arc
• Effective diameter: 11 light years
• Apparent magnitude: +8.4
• Other designations: Messier 1, NGC 1952, Taurus A, Sh2-244

Visibility

Since the Crab Nebula is only about as bright as Saturn’s moon, Titan, it is not visible without optical aid. However, the Crab Nebula is an easy binocular target under dark skies when seeing conditions are good. In general terms when using telescopes to view the nebula, the bigger the aperture of the telescope, the more detail becomes visible. Note, however, that most published images of the Crab Nebula are composites of images taken in different wavelengths, which means that amateur equipment will generally not reveal the same level of detail and structure.

Physical Properties

Supernova Event

In simple terms, the Crab Nebula is the remains of a giant star that died in either a Type Ib/c or Type II (core-collapse) supernova event; this is because the destruction of the star left behind a neutron star, which does not happen with Type Ia supernova events, which occur in binary systems in which at least one of the stars is a white dwarf. According to recent studies, the progenitor star may have been a highly evolved red giant on the asymptotic giant branch, and which likely had a mass of between 8 and 10 Suns. This particular supernova event produced a neutron star of between 28 and 30 km (17–19 mi) in diameter, and a measured spin rate of 30.2 revolutions per second.

Radiation

The Crab pulsar (aka neutron star) is also a strong emitter of electromagnetic radiation that ranges from radio waves to gamma rays, and at X-ray and gamma energies above 30 Kev (Kilo Electron volt). This makes the nebula the strongest known persistent source of radiation, particularly given the fact that energy spikes of up to 10 Tev (Terra Electron volt) have been observed. These high-energy emissions have proved to be particularly useful tools with which to analyse the properties of objects that occult the nebula. For instance, in the 1950’s and 1960’s the Sun’s corona was mapped by mapping the strength of radio waves from the nebula passing through the corona. Similar methods were used to measure the thickness of Saturn’s moon Titan, when Titan passed in front of the nebula.

Gas

When viewed in visible light, the mass of filaments that make up the oval shape of the nebula consist primarily of ionised helium and hydrogen, along with varying amounts of carbon, oxygen, nitrogen, iron, neon, and sulfur, and represent the remains of the progenitor star’s “atmosphere”. The temperature of various parts of the filaments range between 11,000K and 18,000K, while their average density is about 1 300 particles per cubic centimeter.

The Missing Mass Problem

While much about the Crab Nebula is known for certain significant areas of uncertainty remain, one of which involves the mass of the nebula, which is an important factor in determining the true nature of the nebula’s progenitor star. Essentially, the problem involves the fact that the masses of the nebula and the neutron star do not add up to arrive at the mass the progenitor must have had to produce the nebula as it exists today. For instance, the best estimates put the mass of the nebula at between 2 and 3 solar masses, while the neutron star itself is estimated to have between 1.4 and 2 solar masses.

The total mass is thus much lower than the mass required to produce a supernova in the first place. To date, the issue remains unresolved, although many investigators now believe that the missing mass can be explained if the progenitor star had lost most (or much) of its mass through a strong solar wind, as is commonly seen to be the case in highly evolved asymptotic giant branch and particularly, in Wolf-Rayet stars. However, if the progenitor star had blown off significant amounts of mass through a violent solar wind, this material should have collected in a shell around the nebula, as is also commonly seen. To date, and despite many studies in a wide range of frequencies, no trace of such a shell has been found, which leaves the problem of the Crab Nebula’s missing mass unresolved.

Notable Features

One particularly notable feature of the Crab Nebula is the ring, or torus, consisting mostly of helium that can be seen as a wide band running east/west across the inner regions of the nebula. Studies have shown that the torus contains about 25% of the total amount of visible matter in the nebula, and that it consists of about 95% helium, which is strange considering that most of the helium in the progenitor star either would have been converted into something else, or would have been mixed more completely throughout the nebula. To date, no credible explanations for the presence, structure, or composition of the nebula have been proposed.

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