Forty years ago, in February 1987, a cosmic explosion took place in the southern sky. It was first discovered by astronomers Ian Shelton and Oscar Duhalde, at an observatory in Chile. But this was not the ALMA observatory. Back then, ALMA hadn’t been built yet. No one even had serious plans to construct a huge array of antennas to observe millimeter waves from space. Still, the remains of the stellar explosion have now been studied by ALMA in much detail.
The exploding star was called Supernova 1987A. A supernova explosion marks the death of a very massive star. The core of the star contracts into an extremely compact object – a so-called neutron star, or even a black hole. Meanwhile, most of the star is blown into smithereens. The result: an expanding shell of gas that can be visible for centuries.
During the explosion, the ill-fated star shone with the light of a hundred million suns. It was easily visible with the naked eye, despite its huge distance of 163,000 light-years (over 1.5 quintillion kilometers!). Stellar gas was blown into space at velocities of some 30 million kilometers per hour. It was the best observed supernova explosion ever.
Over the years, many telescopes have observed the expanding remnant of Supernova 1987A, including the Hubble Space Telescope and NASA’s Chandra X-ray Observatory. And when ALMA was completed in 2012, it also became possible to observe the millimeter and submillimeter radio waves from the supernova.
On the supernova’s 40th anniversary, astronomers have released many new images and movie animations of the exploding star. The ALMA observation reveals the submillimeter radiation from dust particles in the supernova remnant. It turns out that Supernova 1987A is an efficient dust factory. The same is almost certainly true for other supernovas.
During the life of the doomed star, new elements (including carbon, nitrogen, oxygen and even iron) were produced in its interior. When the star exploded, these elements were blown into space, together with large amounts of lighter gases. When the blazingly hot material started to cool down, the heavy elements bound up into larger molecules, and condensed into dust particles.
In the future, these dust particles may form the stuff from which new planets are born. Likewise, our home planet Earth probably formed from dust that was produced in another supernova explosion, billions of years ago. Thus, studying Supernova 1987A teaches us about the birth of our own planet.
Supernova 1987A was the terminal explosion of a massive star, called Sanduleak –69° 202. The star was a blue supergiant star in the Large Magellanic Cloud, a small companion galaxy to our own Milky Way galaxy. The supernova explosion occurred on 23 February 1987. Or, to be more precise, it occurred some 163,000 years ago: the light of the supernova took that long to reach Earth. Astronomers have studied the expanding remnant of Supernova 1987A ever since, but they have not yet discovered whether the core of the exploding star collapsed into a very dense neutron star, or into a black hole.
On Supernova 1987A’s 40th anniversary, many new observations of the supernova have been released. These include measurements collected by the Hubble Space Telescope and by NASA’s Chandra X-ray Observatory. The ALMA observations of Supernova 1987A were carried out by a large group of astronomers led by Remy Indebetouw of the University of Virginia in Charlottesville. Remy and his colleagues have published their results in Astrophysical Journal Letters.