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Supernova + 20 years: Instructive explosion
1 MARCH 2007

Ultra-bang turns 20
Twenty years ago, or more accurately 163,020 years ago, a star 163,000 light years distant went ka-boom. For several months it released the energy of a hundred million suns, and then it fatigued and faded.

Photograph from deep space shows a brilliant, dotted pink circle enclosed in an orange and red ringThe stellar blast Supernova 1987A was first spotted 20 years ago. The explosion is one of the brightest supernova blasts in more than 400 years. Hubble began watching the blast's aftermath shortly after it was launched in 1990. Spot at lower right is a star not part of the supernova. Photo: NASA

On Feb. 23, 1987, astronomers saw a dot of light in the Large Magellanic Cloud, a southern-Hemisphere constellation. The supernova was the first visible from Earth without a telescope since 1604, and when the Hubble Space Telescope went to work in 1990, it began to ogle the remains of the explosion, called the nebula.

The Supernova was named "1987A" to reflect its year of appearance. And even if giant explosions bore you, supernovas matter because they form all atoms heavier than helium. The calcium in your bones, the carbon in an orchid blossom, the titanium in a jet wing -- all were formed in supernovas. 1987A made an amount of iron equal to the mass of 10 Earths.

Courtesy of Hubble and a scad of other telescopes that see non-visible light, 1987A has become the best-studied supernova in history. And, 20 years later, the results of that study are typical for modern astronomy.

The closer you look, the weirder it gets.

Simple - and wrong
Before 1987A, astronomers had a "simplified, idealized model of a supernova," explained Robert Kirshner, professor of astronomy at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "We thought the explosions were spherical. The actual shreds of the star in supernova 1987A are elongated -- more like a jellybean than a gumball, and the fastest-moving debris is slamming into the gas that was already out there from previous millennia. Who would have guessed?"

1987A, with those three rings and the "dumbbell" at the center, could not have resulted from of a giant, spherical bang that spread energy and junk in every direction.

Kirshner, who heads an international study of the nebula at 1987A, suggests the project has been humbling, riddled with surprises. 1987A formed from a giant blue star that was much hotter than the star that was the supposed source of these stupendous stellar suicides. "If you opened a textbook in 1986 and read about the progenitor of a supernova, it would say, 'These are formed when a red supergiant has a core collapse.'"

Another singularity of 1987A is that center ring, where a terrific shock wave powered by the explosion is smashing into gas that departed the star's surface about 20,000 years before the explosion.

Series of six photos of a purple light surrounded by red circle of light that glows more in more recent photos
Images from Hubble Space Telescope shows a ring of gas beginning to glow around an exploded star. The ring is composed of gas that left the star before the explosion. Photos were taken over 12 years. The dumbbell at the center contains post-explosion star trash. Photos: NASA

A real, three-ring circus
The pair of rings lurking in the background also beg for explanation. One recent calculation (see #1, below), suggests that this three-ring circus grew from the collision of two stars (a binary star) long before the supernova. But it ain't necessarily so, says Kirshner. He notes that Nathan Smith, a post-doctorarl student at the University of California at Berkeley, recently "showed four examples of stars in our galaxy for which there is no evidence that they are binaries, that have very similar nebulae. These are very massive stars that burped out some gas and formed three-ring systems not so different from what we see in 1987A."

 Arrows illustrate the items of interest in the center of a supernova
The supernova's inner debris includes remnants of the star -- and presumably a black hole or neutron star. Diagram show one theoretical explanation for the formation of the outer rings. Graphic: NASA

The standard picture of a supernova was a giant, spherical explosion, Kirshner says, and thus we come to the mother of 1987A's many eccentricities -- its failure to be spherical. Before the space telescope photos, he says, "We were not daring enough to think it could be anything but a sphere; we didn't have any reason to think it could be more complicated than the simplest shape."

Now, with new telescopes on the ground and in orbit, he says, "The technology has gotten so much better, there are so many more kinds of emissions that we are able to sense and put together into a sophisticated picture of this supernova."

Mysteries remain. Mocking the voice of authority heard in planetarium shows, Kirshner intones, "Everybody knows that the neutrinos from 1987a show that it formed a neutron star," an astonishingly dense star that is essentially one big nucleus. But astronomers have failed to find a neutron star using visible-light, X-ray and radio telescopes, he says. "Chances are pretty good there is a spinning neutron star there, but where the heck is it?"

Twenty years after its big bang, 1987A remains problematic, he adds. "It's still unfinished business. You have not checked all the boxes on how 1987A happened until you check the box, 'Formed neutron star,' and we haven't checked that box."

— David Tenenbaum


Bibliography
1 The Triple-Ring Nebula Around SN 1987A: Fingerprint of a Binary Merger, Science 315, 1103 (2007); Thomas Morris, et al.

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