Orwell Astronomical Society (Ipswich)
Supernova 1987A
At 07:35 UT on 23 February 1987, one of the components of a magnitude 12 star in the constellation Doradus, catalogued as Sanduleak -69d 202, in the Large Magellanic Cloud (LMC) at position RA 05h 35.1, dec -69° 16' 50", erupted to produce supernova 1987A. The supernova was widely seen in the southern hemisphere and was the first to be visible to the naked eye since those observed by Johannes Kepler in 1604 and Tycho Brahe in 1572.
When a supernova explodes, the material involved goes through an enormous nuclear synthesis, forming new elements and material that can ultimately be accreted into other stars. Studies of supernovae can therefore be very important in understanding a great and fundamental mechanism in our Universe. The study of supernova 1987A with modern astronomical instruments revolutionised our knowledge of the phenomenon.
It is best to start our story with two astronomers at Las Companas Observatory in Chile, on the evening of 23 February 1987. One, Oscar du Halde, settled down (with his favourite music playing in the background) to a routine programme of observation after checking the controls of the one metre telescope. Further down the hill, Ian Shelton, a young Canadian astronomer from Toronto, struggled with an old 26 cm telescope. The roof on its dome had jammed and he had to move it by hand. In spite of this, he persevered and made a test plate of the nearest galaxy to the Earth, the LMC (which appears as a faint patch of stars in the Southern skies). For this, he had to hand-guide the telescope for three hours.
At 2.00 am, Oscar decided to take a break and looked outside, during which the LMC attracted his attention. There was a new object there! But he had to go back to continue the work with the one metre telescope. He became engrossed in his work, and temporarily forgot about the new object. Meanwhile, Ian had been forced to stop for the night - the telescope had blown over in the wind and the roof had slammed shut! He anxiously developed the test plate. By then, it was after 4.00am. Ian had intended going to bed after checking the plate but it appeared to show a companion to the star 30 Doradus. He checked on another plate from the previous night's work: it showed no companion. Ian went outside, to see if he could see the new star by naked eye. Yes, it was visible and, in fact, was spectacular! He realised that he was seeing a supernova, and rushed to tell Oscar. They had to act quickly, to alert the world and claim their discovery.
Ian had no idea how to do this to best effect, and looked for the necessary information in a popular astronomy magazine! It was daybreak before Bill Kunkle, the scientist in charge at the office in La Serena, 250 km away, received news of the supernova. How to get the message out? The local telex was unreliable so Bill used the one machine that he felt he could trust, and from which he could receive incoming messages, to register the discovery officially. Already it was 10.15am in Chile. In New Zealand, the supernova was visible. But the primary job was done: the news was out and the discovery had been claimed.
People who picked up news of the discovery included veteran supernova theorist, Stirling Colgate, formerly chief researcher on a hydrogen bomb project many years before. After dealing with the H bomb, he wanted to go on to bigger things, and the field of supernova research was where the biggest explosions are to be found! Next to the possible Big Bang of an initial Creation, Stirling believes that there is nothing to touch a supernova explosion, even if there are so many scattered throughout the Universe that there is about one such explosion every second. In New Mexico, Stirling had built from spare parts a special telescope, linked to a computer, to continuously search for and photograph supernovae. He wanted to know why stars shine, why many eventually blow up and what the distribution of matter is in the processes involved.
Early computer models showed that stars should simply collapse without producing a supernova explosion. For over 20 years therefore, Stirling (with the aid of others) had been developing new computer models to study supernova behaviour, using information from observations of distant exploding stars and nearby supernova remnants. Shortly before the supernova in the LMC, Stirling had produced a detailed theory to explain supernova explosions.
Another theorist, Stan Woodsley, explained how a star can explode. Our Sun would not become a supernova, in fact a star needs at least eight times the mass of the Sun to explode as a supernova. Such massive stars expend their energy very rapidly and so have a much shorter lifetime (circa 10 million years compared with 10,000 million years for the Sun). A star with the potential to become a supernova is much brighter (in absolute terms) than the Sun. Normal nuclear processes occur within the star: hydrogen fusion forms helium; helium becomes carbon and oxygen. But, in a very massive star, the temperature in the core is so high that nucleosynthesis continues to produce heavier elements such as sulphur, argon, calcium and, finally, iron. Iron does not engage in nuclear fusion to produce energy and generate still heavier elements so, as the star ages, an iron core grows which means that the region of the star producing energy decreases. Eventually, gravitational forces overcome the structural forces within the atoms of the star and the iron core collapses almost instantaneously (in approximately 200 ms) and an enormous shock-wave travels back out through the envelope of the star (in about 20 ms). The collapse and shock-wave create pressures and temperatures of such a high order as to be found nowhere else. A burst of neutrinos (atomic particles) is generated, which intensifies the explosion. The entire death of the star is surprisingly fast, occurring in a fraction of a second at the end of a stellar lifetime of about 10 million years.
Until Supernova 1987A, no test of the above theory had been made by observation. Now here was the chance! The first thing to do was to identify the star that had blown up and determine its size, age and colour. Graham White was the search leader at the Anglo-Australian 3.9 metre optical telescope at Siding Spring in Australia. He found plates of the nebulous region near 30 Doradus, showing, to a positional accuracy within 0.05", a magnitude 12 star where the supernova now shone; its catalogue number was Sanduleak -69d 202. It seemed clear that the positional match identified the progenitor star of the supernova as Sanduleak -69d 202 - but things were not to be quite so easy!
Bob Kirschner from Harvard University was interested in studying the supernova in ultraviolet (UV) light (using a satellite controlled from Maryland). The supernova should still be bright in UV light but fading as the very hot gas of the explosion cooled. However, as the UV glare faded in successive photographic exposures, the satellite gradually revealed unexpectedly that Sanduleak -69d 202 was still present! The Space Telescope Science Institute in Baltimore solved the problem: computer enhancement by Barry Laska and Nolan Wellborne indicated that there were three stars almost in line of sight, and that the one which had exploded and disappeared was indeed Sanduleak -69d 202.
But, although the question of the star which had blown up had been answered, part of the theory didn't fit. Sanduleak -69d 202 had been a blue star - this was contrary to theory, which stated that blue stars are too young and too dense and only giant red stars blew up as supernovae. Efforts were made to test this and validate other parts of the theory as follows.
The model of the composition of the star in its final hours as a succession of layers of different types of atoms appeared to be correct, as it was confirmed by observations of the different materials expelled by the star in rarefied form following the explosion. Theory indicated that, in the supernova explosion, electrons and protons would fuse to become neutrons, producing a burst of sub-atomic particles known as neutrinos. Some neutrinos would aid the explosion while the rest escaped into space; approximately only 1% of the neutrino energy was thought to contribute to the explosion. Following the explosion, there should be an almost inconceivably dense remnant of the progenitor star left, resulting from the gravitational collapse of the remaining atoms into a degenerate form where electrons, neutrons and protons are compressed together. Such a star, called a neutron star, would have a diameter of circa 15 km and its matter would weigh about 100 million tonnes per cubic centimetre!
Anyway, if the neutrino-emission theory were correct, neutrino detectors already built, although few in number, had a chance of detecting a burst of neutrinos. Such a burst would be expected just before the optical display of the supernova occurred. The scene therefore moves to what is believed to be the most sensitive neutrino detector yet built, deep in a zinc mine at Kamiokande in western Japan. There, Yasuwa Totsuka and his American collaborator Al Mann are in charge of an enormous tank of pure water four storeys high, around the sides of which are 948 large photo-multipliers covering 20% of the tank's surface area. Being so deep beneath the rock, other radiation is screened, but neutrinos can penetrate all the covering rock (and a lot more); in fact, the neutrinos from supernova 1987A would have to pass through a few thousand kilometres of Earth-mass before being detected in Kamiokande.
The hope in using detectors such as the one at Kamiokande was that a few neutrinos would collide with electrons in the water to produce charged particles moving faster than the speed of light in the liquid; the charged particles, on decelerating, would produce a "bow-wave" of Cherenkov radiation, visible as a cone of blue light which could be detected by the photo-multipliers. (The photo-multipliers in Kamiokande are in tube form, designed by a Professor Koshiba, and are believed to be the largest in the world.) A computer feeds the signals from the photo-multiplier tubes to magnetic tapes.
The tapes for 23 February 1987 were retrieved. Almost a day before Ian Shelton noticed the supernova, they showed a sharp spike in the neutrino signals - from the billions of neutrinos generated by the explosion 170 000 light years away passing through the tank, 11 had collided with electrons and produced Cherenkov radiation which had been recorded. In fact, the team at Kamiokande had been very lucky! Every hour, the system is turned off for 105 seconds for recalibration. The neutrino burst occurred over only 13 seconds and, had it arrived two minutes later, it would have passed while the detector was switched off! Had this happened, the 3000 tons of water would have been for nothing in this particular avenue of research and Mann, who had spent 40 years in the business, would have missed his big opportunity.
Another test of theory was concerned with the idea that blue stars like Sanduleak -69d 202 are too young and dense to explode. The star's recent behaviour had been unusual from the start. Rob McNought, a young Scotsman working for the University of Aston in Birmingham, England, taking photographs of Earth-orbiting satellites using an old Schmidt telescope at Siding Spring Observatory now enters the arena. He is a keen amateur astronomer in his spare time, using a camera to search for strange objects such as new stars nearly every night. He had been out taking pictures of the LMC on 23 February, but had not developed his work before going to bed. It turned out that he had the first picture of the explosion of the star. He feels sorry that his evidence had not been worked on and reported earlier - it would have helped others to know and immediately concentrate on getting spectra in the earliest possible stages of the explosion.
McNought's picture, taken within three hours of the neutrino burst, showed that the supernova had brightened immensely in that time. Previous supernovae had shown a usual pattern of taking days rather than hours to brighten. But then the light of the new supernova suddenly stopped brightening, whilst still only a tenth as bright as it would be if it were a red super-giant star that had gone supernova, which can take weeks to reach visual maximum. (Compared to the Sun, red super-giants are thousands of times brighter and hundreds of times larger.) Sanduleak -69d 202 had been as bright as a red super-giant but being a blue giant was only one tenth the diameter and thus a lot more dense. This meant that more energy of the explosion was needed to drive the expansion of the star, leaving less energy to be converted into light. With a blue giant star going supernova, ejection velocities are higher so the plateau in brightness is reached sooner than with a red super-giant. In the case of Sanduleak -69d 202, matter had been flung into space at up to 80 million km/hour.
To see what remnant of the progenitor star was left behind (in addition to the expanding gas layers), NASA brought into play its deep-space radio telescope at Canberra, although the operators were caught somewhat unprepared. In order to obtain high-resolution radio images, David Johntsey and his colleagues at Canberra decided to link their Australian radio dishes with one in South Africa, 8000 km away, to provide effectively the resolution of a radio telescope of the size of the distance between the two. In applying this technique, both telescopes must use precisely the same radio frequency. In their haste, they nearly slipped-up over this. After sending the tapes from the respective stations to Bob Preston in the USA for analysis, it seemed for a while that the telescopes had not achieved sync, registration being 2 MHz off. When informed of this, David really felt bad and endured a very restless night. Then came inspiration: just move one set of signals down the width of one recording channel and, with a little modification, the data should be totally recoverable by the correlators in the USA. David slept again!
The supernova grew in extent so fast that it was too large for the radio telescopes to see in full. The initial burst of radio signals then disappeared as dust and gas obscured them. But, as the months went by, it was expected that the signals would reappear. David Johntsey at Canberra with a smaller, VLBI array and other researchers at Molonglo, South Africa are meanwhile checking two or three times weekly for the reappearance, hoping eventually to map the continuing expansion of the supernova.
By May 1987, much routine observation had been fed into the databanks around the world, but then came another new discovery, made at Cerro Tololo Observatory in Chile, just 150 km south of where the supernova was discovered. Two scientists from Harvard University, Costas Papallerlios and Peter Niesensen had arrived there in March, eager to carry out a delicate experiment in speckle interferometry, in which they were experts. They intended to combine thousands of images of the object to see what was happening in the expanding shell, using a computer to process the images and overcome irregularities caused by the Earth's atmosphere. At the end of May, Peter returned to Cerro Tololo with the news that their work had revealed a new object in the field of the supernova. It was as much as one-tenth the brightness of the supernova and had characteristics of a close double star. It was 100 times brighter than Sanduleak -69d 202 or other nearby stars. But what was it? Various suggestions were put forward: from a black hole to another supernova. Indeed, throughout August, there were arguments as to whether the new object was real.
Meanwhile, British observers detected X-rays of increasing intensity coming from an object of about 1.3 solar masses. These could have been generated by radioactive decay of a remnant core of the progenitor star or from a hot, young pulsar. Theorists came in with their predictions: a neutron star? (It was not massive enough to be a black hole.) Is it rotating fast enough to be a pulsar? Does it have a strong magnetic field? We should know in about a year. We may be able to see pulsed emissions, X- and gamma-rays, additional to those produced by radioactivity. But then again, perhaps what we see will give rise to even further questions and controversy....
The supernova in Taurus that produced the Crab Nebula was undoubtedly awesome to stargazers when it burst forth into the heavens on 04 July 1054, achieving a peak magnitude of approximately -5 and remaining visible to the naked eye until April 1056. The explosion of the Crab supernova appears to have been approximately 100 times more energetic than that of a typical supernova, and Horizon [1] quoted its energy as about 1046 J, the equivalent of the energy dissipation of the whole Universe (as far as we know it) for one second. Here are some other examples of energy dissipation, to put this in context:
The Crab Nebula was first seen in modern times in 1731 by Bevis, forgotten and then rediscovered by Messier in 1758, inspiring him to compile his catalogue of over 100 objects that could be mistaken for comets. The nebula has grown so that it is now some 13 light years in diameter.
[1]
BBC2 Horizon programme Death of a Star, broadcast 11 January 1998.
[2]
The Elements Rage by Frank W Lane, Vol 1, Sphere Books Edition, 1968, pages 161, 165 and 166.
Roy Adams