The term was invented in the 1930s by Fritz Zwicky, although it is an unfortunate term - implying a relationship with novas, which are not the same thing.
It is true that a Type 1 Supernova has possible common origins with novas in that the prevailing theories attribute the origin of both to a binary system where matter is streaming from one star onto its companion. But whereas one of the stars in a nova will flare up 'vigorously', the supernova theory envisages a companion White Dwarf going much further than that and actually blowing up, destroying itself and releasing an enormous amount of energy.
A Type 2 Supernova, on the other hand, is definitely something else. It is the result of the death throes of a massive star. The envelope of the star is ejected at speeds of , leaving a remnant which will be either a neutron star (called a pulsar by radio astronomers) or a Black Hole (according to current theories).
Despite supernovas being predicted to occur about once every thirty years on average in our Galaxy, only a limited number of Milky Way supernovas have actually been seen because the view of our galaxy is limited visually by dust. Nowadays new techniques, particularly neutrino detectors, should allow detection of Type 2 supernovas which cannot be seen visually
Type 1 Supernovas
A Type 1 Supernova emits few Hydrogen lines. This is compatible with the theory of a White Dwarf progenitor in a binary system being sent over its Chandrasekhar Limit. This theory also appears to favor a binary system where both stars are White Dwarfs.
Instead of a simple streaming of matter from one star to the other, this theory envisages a loss of energy through emission of gravitational radiation, a consequent reduction of the distance between the stars, tidal disruption and consequent accumulation of matter by one of the stars (this matter not being hydrogen). This theory is compatible with the fact that most Type 1 supernovas occur in regions populated by old stars.
Type 2 Supernovas
This is the death throes of a massive star, i.e. a star which had at least four solar masses while on the Main Sequence. Fusion in the star has developed, stage by stage, until the star has an iron core. It is impossible to derive energy from iron by fusion and the star is thus doomed.
The core will eventually collapse in a fraction of a second. The inner core becomes a Neutron Star if the collapse can be halted, or a Black Hole if it cannot.
The envelope is ejected at a speed of several kilometers per second. The exact cause of this ejection is open to debate. The two candidates are
Neutrinos 99% of the energy given off is given off in the form of neutrinos. Although these particles are notoriously 'unreactive', if 1% of their energy can be imparted to the envelope then this will be enough to cause the observed envelope ejection. Under the extreme conditions prevailing, some theories predict this being possible (although others don't)
The adjacent graph shows the rise and fall of brightness for a Type 2 supernova. The 'almost straight' line just after the peak is a good indicator of radioactive decay (which is predicted by the theory).
Heavy elements are produced during a Type II supernova, during a period of 1/30 to 3 seconds of the actual 'explosion' itself. There is therefore truth in the words of the song which go:'We are stardust...'.
Milky Way Supernovas Observed
Very few supernovae have been observed in our galaxy due to obscuration by dust. They are more regularly seen in other galaxies and, in fact, this is one area in which amateurs have been playing a major role.
After the detection of neutrinos from 1987A, several neutrino detectors are prepared for those galactic supernovas that will occur in the future.
Three 'possibilities' exist prior to 1006.
|1006||Lupus||The brightest of these stars, reaching brightness of -9 or -10.|
|1054||Taurus||The Crab Nebula (M1) is the remnant of the most famous supernova of all, which was observed by Chinese astronomers in 1054 (but not in the West, if the lack of records is anything to go by). See here for further details. The supernova could apparently be seen during the day (for about 23 days, compared with 21 months at night).|
|1572||Cassiopeia||This supernova first appeared in November 1572 and was observed by the Danish astronomer Tycho Brahe. Brighter than the planet Venus, for about 16 months it was visible to the naked eye (even, for some of the time, at noon). The remnant can be observed optically and is also a radio emitter|
|1604||Ophiuchus||Observed by Kepler. Identified nowadays with a radio source (G.4.5+6.8).|
|1987A||Great Magellanic Cloud||visible with the naked eye, which is how one of the discoverers first saw it.|
There is another 'possible' from 1181.
S Andromedae was a supernova in M31 in 1885, which reached just the bottom level of naked-eye visibility.
Tycho's own diagram of the supernova of 1572, marked I on this diagram.
This occured in the Large Magellanic Cloud (the galaxy shown in the adjacent image) and caused great excitement because such a 'close' supernova had not been viewed for over four hundred years. It was first spotted on 23 February 1987.
It demonstrated the problems of relying on supernovae for distance determination - its peak luminosity was lower than expected.
The star that went supernova had been classified as Sanduleak 69° 202, a supergiant.
It was the first time that a star had been studied before it went supernova. Current theory seems to treat it as an exception to the rule. The theory expects the 'explosion' of a red supergiant but Sanduleak was a blue supergiant. At 6 solar massess, it was also below the theoretical eight solar mass lower limit.
Most remnants have been detected via radio telescopes.
Vela this supernova remnant is one of the nearest. It is 11 000 years old. A pulsar can be detected at optical wavelengths. By now the remnant is 2300 light years across.
Cassiopeia A radio source which is the remnant of a supernova from 1680 which was not observed. No pulsar is detected.
SN1320 +/- 650LY is an X-ray remnant from the closest known remnant although the supernova itself does not appear to have been seen (which is a bit of a mystery).
Supernovas in Other Galaxies
Sterling work in this area has been done by amateurs. Notably Bob Evans, from Hazelbrook in Australia, has been regularly checking galaxies and finding supernovas since the 1950s. In order to do this, he has memorized several hundred galxies. Nevertheless, it is not easy work -he has been finding one supernova per several thousand galaxy searches.
99& of the energy given out during a supernova is in the form of neutrinos.
About 20 seemed to have been detected from 1987A.
Neutrino detectors should allow a supernova to be seen visually right 'from the word go' - the neutrinos will arrive significantly ahead of the supernova's visual entrance into the sky and will provide a certain prediction of the coordinates at which it will become visual. Observations of supernovas at such an early stage should provide great insight into theories of stellar evolution, for starters.
Amateurs can get involved in this aspect of astronomy. When a seemingly positive dection is made by the neutrino detectors, the Supernova Neutrino Early Warning System (SNEWS) will swing into action and send an estimate of the upcoming supernova position to AstroAlert, a facility run by 'Sky and Telescope' magazine. AstroAlert will inform all its subscribers - and you can become a subscriber by sending a message to email@example.com with the word 'subscribe' in the body of the message.