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# 13.28 Supernova 1987A

Supernova 1987A Timelapse. Click here for original source URL

Supernova 1987A was the first supernova bright enough to be seen by the naked eye in nearly 400 years. A lucky few astronomers on Earth witnessed the the 170,000 year old collapse of the core of a 20 solar mass star. Within a second, the central iron plasma core, about the size of Mars, had collapsed at a quarter of the speed of light down to a size of about 100 kilometers. As temperatures reached 30 billion Kelvin, iron nuclei were fragmented, and the star exploded in a blast of neutrinos and a flash of ultraviolet light brighter than any other stars in the whole galaxy. Although this event was some 52,000 parsecs away, it was easily detected from the Earth. There are records of supernovae seen from Earth in the 1000 years before this event, and hundreds of supernovae observed since. What made supernova 1987A special was that it was the first supernova seen on Earth since modern physics and modern astronomical instruments had been invented — there were large telescopes that observed the event in many different wavelengths, as well as neutrino detectors that could help confirm theories of massive stellar death.

Three rings of glowing gas around supernova 1987a. Click here for original source URL

The first trace of the supernova was not the dazzling light, however, but an invisible wave of neutrinos that passed through the Earth. The unseen wave contained so many neutrinos that about 10 billion passed through the body of every person on Earth (in the United States these neutrinos passed up through our feet, since the Large Magellanic Cloud is in the southern hemisphere). At the distance of the Large Magellanic Cloud, only 1 in 10,000 people suffered even a single (harmless) neutrino interaction. Sensitive detectors in Japan and Ohio detected a few of the elusive neutrinos, marking the first experimental confirmation of a 50-year-old theoretical prediction — and the first application of neutrino astronomy outside the Solar System.

In Japan, researchers at a particle detector 3000 feet underground sifted through their data and found 11 events caused by interactions between neutrinos and the 2100 tons of highly purified water that filled their detector. The detection of these ghostly particles from the death throes of an object beyond our own galaxy ironically marked the birth of a new field of astronomy.

Light from the explosion began to arrive a few hours later. As predicated by supernova theories, the extra time was required for the shock wave to climb from the center of the dying star to its surface. As brilliant as the sight is, the light from a dying star is less than 0.01% of the total energy of the event, much of which is carried off in the form of neutrinos. For many days it was bright enough to see with the naked eye, but only at equatorial and southern latitudes. Telescopes on Earth, on the Russian Mir space station, and on robotic satellites were pointed at the supernova in a coordinated observational effort. The remnants of this event continue to be observed as they expand into space and interact with the surrounding material, giving us a constantly changing picture of one of the ways that star matter is recycled back into interstellar space.

Detective work has allowed us to recreate the history of the doomed star. The star was a B3 super giant, which had a mass of about 20 solar masses when it left the main sequence. Calculations by Stan Woosley and collaborators at Lick Observatory suggest that the star had an iron core of 1.5 solar masses at the time of the detonation, and a temperature of 1010 K. As the first nearby supernova in nearly four centuries, Supernova 1987A has given astronomers a ringside seat at a stellar explosion that can be studied with the full array of modern astronomical techniques. The Hubble Space Telescope has followed the expanding cloud of debris for over 25 years, and has shown that the blast wave is powered by the decay of radioactive nickel-56 and cobalt-56. The Chandra X-ray Observatory has watched the debris heat up enough to emit X-rays, the start of the glowing gas of a supernova remnant. It is all but certain that the collapsed core of the supernova is one of the most bizarre objects in the universe, a neutron star.