Monday Bites: Supernova SOS

Supernova SOS: Neutrinos and SN 1987A

Credit: ESA/Hubble & NASA

Before a star is to die, it sends out an SOS message in the form of a ghostly kind of subatomic particle known as neutrinos. 

In the particle zoo, neutrinos are the strangest species. They aren't exactly massless but their rest mass is so small that they can effectively be treated as such and since they interact only very very weakly —billions of them are passing through you right here right now as they are passing through Earth, even through the Sun. The fact that they carry no electric charge makes them hard to detect. 

When a massive star explodes, it emits a brilliant flash of light; rivaling the surface brightness of an entire galaxy of stars, shining brighter than the combined light of its hundred billion cousins. In a core-collapse type supernova, hours before the star shatters itself to its very atoms, it produces an enormous burst of neutrinos — of the order of 10⁵² while its core transforms into a neutron star. Almost massless, neutrinos are the first to exit the imploding star. The explosion by which the star unbinds itself takes place afterwards and the optical signature comes usually an hour or so later, sometimes even a day after. Detecting the neutrinos gives us a head start to train our optical telescopes toward the exact location and record the shattering bang in real-time. 

The only time we succeeded in this scheme was in February 1987. Two to three hours prior to visible (optical) detection of the SN 1987A event, neutrino observatories across Japan, Russia, and the one in the Alps registered an unusual spike lasting less than 13 seconds. On February 23, two hours before optical detection was possible, twenty-five neutrinos brought us an SOS message of a star meeting its demise in one of our sister galaxies, the Large Magellanic Cloud, under 186,000 light years away. 

At first, the sudden burst of neutrinos and the SN 1987A supernova looked like disjoint events. But since nothing could explain the emissary of neutrinos coming out of nowhere, coincidence became fact.