This February marks the 25th anniversary of the discovery of Supernova 1987A. A star, in the Tarantula Nebula within the Large Magellanic Cloud (LMC), called Sanduleak 69 202, exploded and became a supernova back on 24th February 1987.
It is now 25 years since the light from this cosmic explosion first reached us here on Earth. The star itself actually exploded about 168,000 years before, of course, with the light taking that long to reach us.
SN1987A has become the most studied star remnant in history and has provided great insights into supernovae and their remnants. It has some interesting connections to the Anglo-Australian Observatory and Siding Spring and is still being studied to this day.
SN 1987A was discovered by Ian Shelton and Oscar Duhalde at the Las Campanas Observatory in Chile on February 24, 1987, and within the same 24 hours independently by Albert Jones in New Zealand. On March 4–12, 1987 it was observed from space by Astron, the largest ultraviolet space telescope of that time. Hubble had yet to be launched.
On February 23rd, approximately three hours before the visible light from SN1987A reached the Earth, a burst of neutrinos was observed at separate neutrino observatories around the globe. This is likely due to neutrino emission (which occurs simultaneously with core collapse) preceding the emission of visible light (which occurs only after the shock wave reaches the stellar surface).
At 7:35 a.m. Universal time, Kamiokande II detected 11 antineutrinos, IMB 8 antineutrinos and Baksan 5 antineutrinos, in a burst lasting less than 13 seconds. Even though, the actual neutrino count was only 24, this was a significant rise from the previously-observed background level. This was the first time neutrinos emitted from a supernova had been observed directly, and marked the beginning of what is now ‘neutrino astronomy’.
The Supernova exploded in the Tarantula Nebula area near the edge of the Large Magellan Cloud – a satellite galaxy to our own. Even though its location in the LMC meant it was 10 times more distant than if it had been in our own Milky Way, it also meant that we had a relatively unobscured view of the supernova and its environment; there was never any ambiguity about its distance; and the fact that it lies so far south meant that observations could be done each night throughout the first year as it was always visible at some time during the night.
SN1987A was the brightest and closest supernova that has been seen since the invention of the telescope back in 1604. That supernova was called “Kepler’s Star” and the supernova was in our own galaxy around 20,000 light years away. It was so bright that it outshone Venus and Jupiter and was even visible in daylight for 3 weeks back then. The only other one in our own galaxy was “Tycho’s Star” seen in 1572 and also visible to the naked eye. There have only been a total of 8 naked eye supernova that are known.
SN1987 was also It was able to be seen quite easily with the naked eye, despite not even being in our own Milky Way galaxy. Visible for some months afterwards, it was easy to find and exciting to point out to others this bright new star in the LMC.
SN1987A provided some significant opportunities specific to the decade that followed and which came about as a direct result of the supernova discovery. These include the development of infrared instrumentation and spectropolarimetry which were evolving rapidly at the AAT and elsewhere. At the time of the supernova, photographic film was still in use at observatories and the internet did not yet exist. All discovery communications were done by telex between the various observatories around the world.
Data was shipped on large tapes back across to England and the other observatories to be reduced and analysed.
Even the introduction of CCDs, with their great sensitivity and dynamic range, made a major difference during the first few years. But much more importantly, the Hubble Space Telescope became available not long after the explosion. We would have learned a lot less about SN1987A had it occurred a decade earlier, and there is unfortunately no guarantee that we will have any HST-like capability even 10 years from now.
Pioneer Astrophotographer, David Malin, was working at the Anglo-Australian Telescope at the time of SN 1987A’s first sighting and was able to take several images of the light echoes of the supernova. One of his images is shown below.
Although the AAT observing schedule had been fully assigned for the semester through to June , when SN1987A erupted, data was able to be obtained on almost two thirds of the nights, and those observations took up 16% of the total time available. This was achieved by using up all the engineering, service and director’s time, as well as, requiring each observer to give up one hour per night of their time to allow observations with whatever instrument was on the telescope. The observers willingly co-operated and so much incredible data was able to be collected.
The emphasis was on observations that could best utilise the 3.9m aperture of the AAT or observations which best used the available instrumentation unique to that telescope: for example, speckle interferometry and spectro-polarimetry.
The Anglo-Australian Telescope was ideally situated as it was the only telescope from which the supernova was accessible and which had the necessary instrumentation to make the observations. Even though there were some changes necessary to the existing IPCs based spectro-polarimeter which was optimised for observing much fainter object.
Data was collected by the non-optimised instrument 4 days after the discovery and it was found that the instrument issues meant that data accuracy would be impacted. So over the next 10 days the engineers worked to modify the instrument to use CCDs as detectors and the first observations with were made on March 7th, a mere 2 weeks after the supernova was first seen.
One particular highlight was the very rapid construction, by Peter Gillingham, of a temporary Littrow spectrograph, which had a resolving power of almost one million, which was able to be used to study interstellar lines. It was available within 2 months of the Supernova. It has been called the “wooden spectrograph”.
The new CCD spectropolarimeter – was built up from the RGO spectrograph plus Pockels cell plus IPCS which itself was a well proven set-up. In order to make it all work with incredibly bright object – the instrument was modified to use the CCD detectors. This raised further hurdles which were one by one overcome and they were able to collect over the next 12 months the first ever spectropolarimetry data on a supernova.
A common-user version of this very successful Ultra-High Resolution Spectrograph (UHRF) was later commissioned and operates on the telescope to this day. Peter Gillingham was able to very quickly come up with a temporary version of a coude mounted spectrograph by combining a novel Littrow lens design with one of the gratings that was eventually destined for use on the UCLES instrument. It was the success of this project which meant they were able to obtain funds to later build the new instrument. The original instrument was first housed in a large wooden box.
The AAT was perfectly placed geographically and so was able to get onto observing this supernova very quickly. It has now been observed for many years and the campaign in radio, infra-red and optical continue to this day since 25 years is a long time to us – but rather infinitesimal in the scale of the universe.
So was it just luck? Being in the right place at the right time?
Personally, I don’t think so – I think it was having this amazing instrument up here at Siding Spring and a really innovative group of staff who were able to think outside the box and design instruments and obtain data on this incredible supernova explosion in a very short space of time. Luck may have been the SN going off nearby – but skill and dedication was what made all the difference.
SN1987a before and after by David Malin Anglo-Australian Telescope - used with permission
This photograph shows the field around the site of the supernova in great detail, both before the supernova exploded (right) and about 10 days afterwards, when it was still brightening. The image of the star that exploded to create the supernova is elongated. This does not necessarily indicate any peculiarity or a close companion, rather it is the effect of stars being by chance aligned along the line of sight. Several other examples can be seen in this picture and other, different, blended images are seen in the photograph of the same field taken two weeks after the supernova appeared (left). The pre-supernova plates were taken over about 90 minutes on the night of 1984 February 5, centred on the Tarantula nebula. The post-supernova plates (LHS image) were exposed for a total of about an hour on the night of 1987 March 8.
The difference in image quality (‘seeing’) between these pictures is an effect of the Earth’s atmosphere which was much steadier when the plates used to make the pre-supernova picture were taken. Top left is NE. Width of each image is about 8 arc minutes. Text and Image © 1989-2010, Australian Astronomical Observatory, photograph by David Malin.