DSO of the Month :
Messier 1
AKA: Crab Nebula, NGC 1952, Taurus A.
Position: 05 hrs 34 min 32 sec + 22 degrees 00 min 52 sec
Due south at midnight (GMT) on 15/16 December 2021
Messier 1
Image: Martin Gill, HAS member. Used with permission.
Perhaps not surprisingly we have not featured a Supernova Remnant (SNR) as a DSO of the Month before as they are not very common and only two (the Crab Nebula and the Veil Nebula) are visible through amateur telescopes. A supernova is the final explosion of a massive star and is much brighter than the progenitor star, although it does not last for very long. It is reckoned that the Crab Nebula supernova was about -6 at its peak, much brighter than Venus and thus visible in the daytime. It was probably brighter than either Tycho’s supernova (-4) or Kepler’s supernova (-2.5). You might therefore assume that the Crab supernova was closer than the two later supernovae, and this is true. The Crab SNR is 6,300 light years away, whereas Tycho’s SNR is 7,500 light years from us and Kepler’s SNR is far more distant at 20,000 light years. However Tycho’s supernova and Kepler’s supernova were type Ia whereas the Crab supernova was Type II. Type Ia supernovae only occur in binary systems in which one member is a white dwarf which accretes matter from its companion star and undergoes a runaway nuclear fusion process leading to the supernova. As the process and the amount of matter is the same every time, they have a consistent light curve which enables their use as standard candles. By contrast type II supernovae such as the Crab supernova are the result of the rapid collapse and explosion of massive stars at the end of their life.
The Crab supernova occurred in 1054 and was observed by Chinese, Japanese and Arabic astronomers but not by European astronomers. It might be assumed that European astronomers with their belief in the fixity of the heavens might reject the idea of a “new star”, but the even brighter supernova of 1006 was observed in Europe despite its very low declension. However the astronomers of the time regardless of their geographical location had no concept of a supernova and hence no idea that a relic of the supernova survived in the sky after the supernova faded. The Crab Nebula was first observed by the English astronomer John Bevis in 1731 and independently by Charles Messier in 1758. The ghostly patch of light gave Messier the idea of listing similar comet-like objects in the sky which were not comets and hence the Crab Nebula became the first object in his catalogue: Messier 1. The nebula got its familiar name after the Earl of Rosse drew what he saw in his 36 inch telescope in 1844 and his drawing looked like a crab or perhaps more accurately a lobster from above, but the Lobster Nebula does not have the same ring about it. Actually I think it looks like a crab edge on in a small telescope, an oval shape with two brighter features at the front like claws, but this not how it got its name, which was in common use by the 1850s.
For a long time, it was not clear what this crustacean-shaped nebula was. William Herschel thought it was a group of stars, but the arrival of larger telescopes such as the Earl of Rosse’s 72 inch telescope showed that this was not the case. In the early twentieth century, it became clear that it was a supernova remnant and the hunt was on to find the supernova in historical records. Edwin Hubble suggested it was associated with SN 1054 and this was proved by Nicholas Mayall in 1939. As new parts of the electromagnetic spectrum were opened up to astronomy, the Crab Nebula was found to be a powerful emitter of radio waves (1949), X-rays (1963) and even gamma rays (1967). Finally the central star was shown in 1968 to be a pulsar and hence a neutron star. This neutron star has a powerful magnetic field, and much of the luminosity of the Crab Nebula is produced by the relativistic motion of electrons in this field, the so-called synchrotron radiation. More recently the exotic chemical species the argon hydride cation has been found in the Crab Nebula. It is now suspected that the supernova was an electron capture supernova of a super-asymptotic giant branch star of about eight to ten solar masses. In such a supernova, only one of which has actually been observed, electrons are captured by magnesium in the star’s core, leading to the core’s collapse and an explosion fuelled by the fusion of oxygen.
It is a relatively easy business to find the Crab Nebula in the winter sky. Look for the V-shaped “horns” of the bull, Taurus, formed by Alpha Tauri (Aldebaran), Beta Tauri (El Nath or Alnath) and Zeta Tauri. Zeta Tauri, sometimes called Tianguan, is the one we are interested in. M1 lies roughly in the same direction as El Nath but much closer to Zeta Tauri. M1 is just over a degree, two full-moon-widths, from Zeta Tauri and it should be possible to see both of them in a wide-angle eyepiece. Even closer to M1 is STF 472 or HD 37013, a fairly easy binary star with magnitudes 7.1 and 7.5, white and yellowish-white which is to the left (or east) of M1. The Crab Nebula is magnitude 8.4 which makes it fairly dim, but it is easy to see in a four or five inch telescope as it is compact. In a small telescope it will appear as a grayish-white oval with brighter white filaments at the front. A much large telescope is needed to see the central pulsar.
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