A very familiar Object of the Day
On February 23, 1987 a Canadian astronomer, Ian Shelton, was working at the Las Campanas Observatory
in Chile taking photos of the Large Magellanic Cloud. On one of his photographic plates he noticed a previously undetected bright light so he went outside and found that the new star was visible to the naked eye. His discovery turned out to be the first supernova visible to the naked eye since Johannes Kepler observed SN 1604
nearly 383 years previously.
At the same time in another part of the Las Campanas Observatory, Oscar Duhalde, another telescope operator, was having a break outside the dome looking up at the Large Magellanic Cloud. He noticed the same new star but unfortunately, he didn't report it - which was a shame as he was actually the first person to have seen it. Meanwhile, Ian Shelton had developed the photographic plates he had been taking of the Large Magellanic Cloud and had evidence of the new star. He had been looking for novae
but realised that what he had found was much too bright to be a nova and was a much rarer supernova
. He reported his findings to the other astronomers at the Observatory that night and only then did Duhalde mention that he had also seen it earlier.
They sent a telegram to Cambridge, Massachusetts, the clearing house for astronomical discoveries, reporting the new supernova. Half an hour later Albert Jones in New Zealand, also sent a telegram reporting the supernova. Several other sightings followed. Having got there first, Ian Shelton was credited with its discovery. On March 4-12, 1987 it was observed from space by Astron,
the largest ultraviolet space telescope of that time.
SN 1987A was a supernova on the edge of the Tarantula Nebula in the Large Magellanic Cloud. At 168,000 light years it was the closest observed supernova since SN 1604, which occurred in the Milky Way itself. Catalogued as "1987A" as it was the first supernova to be discovered in 1987, its brightness peaked in May at an apparent magnitude of about 3 and slowly faded in visible light over the following months. X-ray and radio emissions from 1987A actually grew brighter as the shock wave crashed into a dense cloud of gas and dust previously shed by the giant star thousands of years earlier.
The star that went supernova was Sanduleak -69° 202a, a blue supergiant – not the type of star usually considered a candidate for a supernova event. Current thinking is that the blue giant was the product of a merged binary system.
SN 1987A appears to be a Type II core-collapse supernova
(only possible in stars with at least 9 times the mass of the Sun). If the progenitor star is less than 20 solar masses the remnant of a core collapse is a neutron star. At 20 or more solar masses the remnant collapses to produce a black hole. The search for the remnant of SN1987A is still continuing.
These giant stars possess the mass needed to fuse elements that are heavier than hydrogen and helium. The star evolves to accommodate the fusion of these heavier elements, until a core of iron is produced. However, the nuclear fusion of iron produces no net energy to sustain the star, so the core becomes an inert mass. When the mass of the iron core exceeds 1.44 solar masses (the Chandrasekhar limit
), an implosion is triggered. The rapidly shrinking core heats up, causing rapid nuclear reactions that produce neutrons and neutrinos.
Neutrino and antineutrino bursts were detected
The core collapse is halted by forces between these neutrons, causing the implosion to then bounce outward as a shock wave. The energy of this expanding shock wave is sufficient to rip the star apart in a massive supernova explosion which produces a burst of radiation that often outshines the host galaxy. A shell of illuminated gas and dust is left behind as a supernova remnant enriching the surrounding Interstellar Medium with elements such as carbon and iron which in turn contribute to a new generation of stars and galaxies.
A shock wave of material unleashed by the blast from SN1987A is crashing into gas and dust previously shed by the central star around 20,000 years ago heating it up, and causing it to glow in a ring of hot spots about a light-year across. Astronomers detected the first bright spot in 1997.
Only Hubble can see the individual bright spots. In the next few years, the appearance of the bright ring is likely to change as it absorbs the full force of the shock wave. The pinkish object in the center of the ring is debris from the blast and will continue to glow for many years. The origin of the faint outer red rings is still a mystery.
So.... a star that blew up around 166,000 BC in a neighbouring dwarf irregular galaxy whose light didn’t reach Earth until 1987 AD was chosen 20 years later to be the perfect “O” in GALAXY ZOO.
Isn’t astronomy great?
With thanks to Zookeeper Chris for casting an eye over the fizzicks.
UPDATE: More information and a cool video here