A peek into the life of a star

Spotting supernova: Since its first sighting 30 years ago, Supernova 1987A has been fascinating astronomers.

An explosion took place 1,60,000 years ago in the Large Magellanic Cloud, a nearby dwarf galaxy in the southern skies.

Its light, which reached Earth on February 23, 1987, was noticed by a Chile-based astronomer as an unexpected bright star. It had the apparent magnitude of about +5. Thus, the first supernova visible to the naked eyes in modern times was officially designated as SN 1987A. Supernova, the appearance of a bright star, is a regular phenomenon in the universe, with one expected in each galaxy in around 50 years. 

Many civilisations have kept records of such guest stars. Lupus in 1006, Crab in 1054, Tycho in 1572 and Kepler in 1604 are some of the supernovae that were visible to the naked eye in the last thousand years. The 1572 supernova, studied in detail by Tycho Brahe, showed that this was not a terrestrial phenomenon as hypothesised by the Greeks and was indeed located much farther than the moon. This, along with the 1604 supernova observed by Johannes Kepler, and Copernicus’ Sun-centric model were instrumental in the gradual downfall of Greek ideas of
astronomy.

The first systematic theory of supernovae was given by Walter Baade and Fritz Zwicky in 1931. There are two major types of supernovae: Type I, due to the runaway explosion of a white dwarf star accreting matter, and Type II, a result of the gravitational collapse of medium and massive stars. Type I is much brighter and fades away sooner than Type II, which typically shows a plateau in their light curves. The very presence of life on Earth is due to the production of essential elements from the previous generations of stars that were injected into the galaxy before our solar system was formed.

Detecting neutrinos
In 1966, S A Colgate and J C Wheeler had postulated that neutrinos carry away most of the gravitational energy during the formation of supernovae. When the news of the SN 1987A broke out, they immediately pointed out that a neutrino burst from the collapsing star should have reached Earth and that, if detected, it would provide “a unique opportunity to test the theory of neutron star formation in Type II supernova explosions.” It was fortuitous that several underground experiments had underground detectors which had been built to test for possible proton decays predicted by the Grand Unification Theory.

When they looked back at their data, it was found that a total of 25 neutrinos were detected on Earth, out of the 10 billions of billions produced in the explosion! The neutrino burst is expected to reach Earth even before light and, in fact, the experiments recorded the neutrino signal two hours before the optical signal was seen. It is also important to note that several interesting insights into the physics of neutrinos were obtained by these observations. Thus, this proof for the connect between the micro and macro has been one of the outstanding results from the study of this supernova.

SN 1987A was also unusual since its brightness reached the maximum (apparent magnitude of + 2.9) three months after the initial discovery. It was attributed to another source of light: the chain decays of several radio active nuclei with release of large amounts of energy in the form of highly energetic gamma-ray photons. These were discovered by the Compton Gamma Ray Observatory, a space observatory, in the 1990s. Researchers tried looking for very high energy gamma rays from SN 1987A in an experiment in New Zealand, but they could not detect them. Thus, it provided the first opportunity for modern theories of supernova formation to be tested against observations.

Aided by modern telescopes
The supernova has, of course, become dimmer by a factor of 10 million. But, as it was the first supernova to have occurred nearby, after the advent of modern telescopes, the object has been studied almost continuously in all wavelengths. Since the Hubble telescope came in 1990, it has been monitoring the supernova continuously. The earliest data showed a central ring and two fainter rings above and below. With time, the central ring had many hot spots, like ‘a necklace of pearls’. Now the main ring is about a light-year in diameter.

The central structure has now grown to roughly half a light-year across and there are two blobs of debris in the centre racing away from each other at roughly 20 million miles an hour. While the blobs have not been understood, there are predictions that the ring will be blown away in the next decade. NASA’s Space based Chandra X-ray Observatory has also monitored this supernova for a long time.

Initially, the supernova shock wave collided with the ring of gas, heated it up and produced a ring of X-rays.

Over the next 14 years, this expanding ring was seen to be steadily getting brighter. However, in the past few years, the ring has stopped getting brighter.

Furthermore, the bottom left part of the ring has also started to fade. These changes reveal that the explosion’s blast wave has moved into a region which has gases that are less dense. This signifies the end of an era for SN 1987A. The submillimetre observations of Atacama Large Millimeter Array (ALMA), Chile show that the remnant is actually forging vast amounts of new dust from the new elements created in the progenitor star. A portion of this dust will make its way into interstellar space and may become the building blocks of future stars and planets in another system. As for higher energy photons, several experiments have searched for either low or high energy gamma rays but with no success.

This supernova has shown atypical behaviour from the beginning. The progenitor was expected to be a super red giant but it turned out to be a blue giant. Further, one expects to see a black hole or a neutron star left behind by the blast, but repeated searches have not found such a compact object at the centre. There is, however, a possibility of dust covering the compact object but one has to wait. Continued observations of the object are necessary to understand further stages of this object.