Type II supernova

7.9 Type II supernova

This sequence of events is called a Type II supernova . The peak absolute mag- nitude of about –18 then drops by around six to eight magnitudes per year so that it gradually fades from view. We believe that such supernovae will occur in our Galaxy on average about once every 44 years. Sadly, the dust in the plane of the galaxy only allows us to see about 10–20% of these and so they are not often seen.

Stellar Evolution – The Life and Death of Stars

7.9.1 The Crab Nebula

On July 4, 1054 AD a court astrologer during the Sung dynasty, Yang Wei-T’e, observed a supernova in the constellation Taurus. The gas shell thrown out in the supernova explosion was fi rst discovered in modern times by John Bevis in 1731, who included it in his sky atlas, Uranographia Britannica. Later, in 1758,

it was independently discovered by Charles Messier whilst he was searching for the return of Halley’s Comet. It became the fi rst object in the Messier cata- logue with the name M1. The Third Earl of Rosse, who drew its form using his

72 in. telescope in Ireland, thought that it appeared similar to a horseshoe crab and so he called it the ‘Crab Nebula’, the name by which it is usually known (Figure 7.9).

The Crab Nebula is still, nearly 1000 years after it was fi rst observed, expand- ing at a rate of 1500 km s ⫺1 and its luminosity is about 10 000 times brighter than our Sun. Much of this radiation appears to be the result of electrons, moving close to the speed of light (called relativistic electrons), spiralling around magnetic fi eld lines in the nebula. The fact that the nebula still appears so energetic remained a puzzle until a neutron star (which is the remnant of the stellar core) was discov- ered in 1969 at the centre of the nebula. This will be described in detail below. The gas shell, now of order 6 ⫻ 4 arcmin in size and shining at 8.4 magnitudes, can still be observed with a small telescope.

The Crab Nebula is thought to be the remains of a Type II supernova. A super- nova, (1987A) that was observed in the nearby galaxy, the Large Magellanic Cloud, in 1987 is also thought to have been a Type II supernova, but those observed by Tycho Brahe in 1572 and Johannes Kepler in 1604 are thought to

Introduction to Astronomy and Cosmology

have been caused by a different mechanism and are termed Type I supernovae . These have proved very valuable for extending the cosmic distance scale out to distant galaxies and will be discussed in Chapter 9.

7.9.2 Supernova 1987A

In Febuary 1987, a supernova was observed in the Large Magellanic Cloud, a gal- axy close to our own Milky Way (Figure 7.10). Visible for a while to the unaided eye, it became the closest observable supernova since that of 1604.

Supernova 1987A has played an important role in determining the cosmic distance scale as will be discussed in Chapter 8. However, there is a further aspect of its explosion that merits mention which was the result of a wonderful piece of

serendipitous timing. In the late 1970s a particle physics model called the Grand

Unifi ed Theory (GUT) suggested that protons would decay with a half-life of 10 31 years. This means that if one observed a number of protons for 10 31 years half would have decayed. This is obviously not an experiment that can be mounted, but the possible proton decay could be detected if one observed a very large num- ber of protons for a relatively short period. The proposed decay process is:

p ⫹ →ε ⫹ ⫹π ο π ο → 2γ ε ⫹ ⫹ε ⫺ → 2γ

where ε ⫹ is a positron and π ο is a neutral pi-meson or pion.

Figure 7.10 Supernova 1987A in the Large Magellanic Cloud. Image: European

Stellar Evolution – The Life and Death of Stars

The proton decays into a positron and neutral pion which then immediately decays into two gamma rays. The positron will annihilate with an electron to form two more gamma rays.

To this end, a number of detectors were built in the 1980s including that at the Kamioka Underground Observatory located 1000 m below ground in Japan. To provide the protons, 3000 tons of pure water was contained in a cylinder 16 m tall and 15.6 m in diameter. The cylinder was surrounded by 1000 photomulti- plier tubes attached to its inner surface which would be able to detect the gamma rays produced in the proton decay. It came into operation in 1983 and was given greater sensitivity in 1985. To date, even with a new detector containing 50 000 tons of water, no convincing proton decays have been detected and later versions

of GUT suggest that the decay half-life might be nearer to 10 35 years. However, what is critically important was that the detector, which came into full operation at the end of 1986 after its upgrade in 1985, could also detect neutrinos.

Relativity states that no particle can travel at the speed of light in a vacuum. However, in dense media, like water, light travels at lower speeds. It is thus possible for a particle to travel through water faster than the speed of light. If the particle is charged, it will emit light radiation called Cherenkov radiation. The process is analogous to the formation of a sonic boom when an airplane exceeds the speed of sound. Neutrino interactions with the electrons in the water can transfer almost all the neutrino momentum to an electron which then moves at relativistic speeds in the same direction.

The relativistic electron produces Cherenkov radiation which can be detected by the photomultiplier tubes around the tank. The expanding light cone will trigger a ring of photomultiplier tubes whose position gives an indication of the

direction from which the neutrino has travelled. This makes it more than just a detector – it forms a very crude telescope!

When Supernova 1987A was seen to explode just a few months later (this being the serendipitous timing referred to earlier) the Kamiokande experiment detected

11 neutrinos within the space of 15 s. A similar facility in Ohio detected a further

8 neutrinos within just 6 s and a detector in Russia recorded a burst of 5 neutrinos within 5 s. These 24 neutrinos are the only ones ever to have been detected from

a supernova explosion. Perhaps surprisingly, the neutrinos were detected some

3 h before the supernova was detected optically. This is not because they had trav- elled faster than light! They, of course, travelled out directly from the collapsing core of the star, whereas the visible light was not emitted until later when the shock wave reached the surface of the star. The detection of those 24 neutrinos was perfect confi rmation of the theoretical models that had been developed for the core collapse of a massive star and consistent with theoretical prediction that

∼10 58 neutrinos would be produced in such an event. The Kamiokande observa- tions also allowed an upper limit to be placed on the neutrino mass. If one assumes

Introduction to Astronomy and Cosmology

that the neutrinos began their trip somewhat ahead of the light from the super- nova and given the fact that they arrived before the light having travelled through space for ∼169 000 years means that they must have been travelling very close

(within one part in 10 8 ) to the speed of light. This, together with the fact that the higher and lower energy neutrinos arrived at the same time allows an upper limit to be put on the mass of a neutrino. It cannot be greater than 16 eV which is about three- millionths the mass of an electron. (Other evaluations of the data give a somewhat lower value of ∼7 eV.)

Electronvolt

The unit, called an electronvolt (eV), is often used as an indicator of mass when talking about objects like protons, neutrons and electrons. An electronvolt is a mea- surement of energy, but since mass and energy are related by Einstein’s famous

equation, E ⫽ mc 2 , the energy equivalent of mass may be used instead. In terms of MeV (1 MeV ⫽ 1 000 000 eV), the masses are:

Neutron ⫽ 939.56563 MeV Proton ⫽ 938.27231 MeV

Electron ⫽ 0.51099906 MeV