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The supernova of a lifetime: thirty years later, supernova 1987A continues to develop and surprise.

On the morning of February 23, 1987, the phone on my desk at the Harvard-Smithsonian Center for Astrophysics started ringing. Word was filtering in of the brightest supernova since Kepler's in 1604. I thought it might be a practical joke; my friends had fooled me before. In that pre-internet era the teletype was the gold standard of instant communication, so I hustled down the hall to the quaintly named Central Bureau for Astronomical Telegrams in Brian Marsden's office.

Brian was on his phone, and the teletype next to his desk was clunking away with reports from Chile, New Zealand, and around the world. A bright new star had erupted in the Large Magellanic Cloud, the largest satellite galaxy of the Milky Way, 165,000 light-years distant. Three months later I saw the supernova in person as it peaked at magnitude 2.9. It was the most distant naked-eye star in recorded history.

Technology has changed our lives since 1987, but it has changed astronomy more. The instruments we used in those first heady days were primitive compared to the ones we use now. And a good thing, too: today SN 1987A is ten million times fainter than at its peak, but better tools let us study it across the electromagnetic spectrum. For example, in 1987 the space telescope available to our team was the International Ultraviolet Explorer, with an aperture of 45 centimeters (18 inches). I had been using 1UE to study extragalactic supernovae and had mentioned in passing that we would also like to observe any supernova that might appear in the Local Group of galaxies. The Large Magellanic Cloud certainly qualified, and in a phone call that morning, Yoji Kondo at the IUE proudly told me that observations had already begun.

Since the Hubble Space Telescope was launched, we've been able to use its 2.4-meter aperture with a succession of ever more powerful visible-light, infrared, and ultraviolet instruments installed by Space Shuttle astronauts. At radio wavelengths, the Australian Compact Telescope Array (ACTA) has tracked the rising radio emission from SN 1987A. The new ALMA observatory is creating millimeter-wave images of it that are as sharp as our HST images, to tease apart the ongoing chemistry and physics of the explosion. Far-infrared observations from the Spitzer and Herschel missions have probed the cold dust that formed from the expanding debris. X-ray observatories, such as the durable Chandra and the recent NUSTAR, measure emission from million-degree gas where the debris is currently colliding with surrounding clouds. These tools give us a rich view, as the supernova of 30 years ago morphs into the supernova remnant of the future.

The Cry of a Collapsing Star

There were some real surprises in 1987. To start with, the star that exploded turned out to be number 202 in the -69[degrees] band of Nick Sanduleak's catalog of bright stars in the Large Magellanic Cloud. It had been a 12th-magnitude blue supergiant, spectral type B3Ia. Oops. In giving astronomy exams at Harvard, the answer that I marked correct was that core-collapse supernovae happen only in red supergiants at a different stage of development. Nature hadn't read the textbook.

Stars in the Large Magellanic Cloud have less than half the heavy-element abundance of in stars in the Milky Way's disk, and people calculated the effect that this chemical difference would have on the life of a massive star. If it began with 18 times the mass of the Sun, a likely estimate, it would shed about 4 solar masses as its core stepped through the final, unstable stages of nuclear burning--fusing helium to carbon and oxygen, then step by step up to silicon, ending with a hot and brief episode of fusing silicon into iron. Iron is nearly the minimum-energy atomic nucleus: it's unburnable, the "ash" at the end of the line for releasing energy by fusion. As the core of Sanduleak -69[degrees] 202 accumulated iron and could produce no further heat to hold it up, it shrank, teetered on the edge of gravitational collapse, and finally fell over the brink.

Computer models had shown that the headlong collapse of a star's core would stop only when the inner few solar masses became as dense as an atomic nucleus--forming an incredibly dense neutron star about the size of a city on Earth. In those last seconds, as the neutron star forms, the material crashing down heats to about 100 billion kelvin. It should be hot and dense enough to produce an immense pulse of neutrinos, amounting to about 10% of the star's rest mass. Most of the neutrinos fly clean out of the star, carrying off the vast majority of the energy released in the entire disaster.

One of the great physics events of 1987 was the first detection, in two giant underground neutrino detectors, of this surge of ghostly neutrinos. It began with a burst and trailed out in the next 13 seconds.

The visible brightening got under way in the subsequent hours, as the shock wave from the core hit the star's surface, abruptly heating it to ultraviolet temperatures and blasting it outward to enormous size.

The neutrino event is widely regarded as the yelp of a newborn neutron star, not a black hole. But our careful scrutiny of the wreckage of SN 1987A using the Hubble Space Telescope has failed to show any sign of a point source in the center, even with the view clearing during the last 30 years. Is the neutron star shrouded by dust? Or did infalling debris shove it over the next gravitational-collapse brink, to become a black hole? The case is open, and we will keep looking with ever more sensitive techniques for the presumed hot corpse, as the remaining light from the explosion continues to fade away.

Struck By the Light ...

Other surprises have played out over the past three decades. Our IUE measurements showed a prompt flare of ultraviolet light, denoting the shock wave bursting through the star's surface. In the next 1.1 years, that flare of ultraviolet brilliance travelled 1.1 light-years outward and reached something that was already loitering in the neighborhood: slow-moving, dense gas. We saw new emission as the ultraviolet flash excited it to glow. This was, plausibly, gas that the star had shed tens of thousands of years earlier, toward the unstable end of its fuel-burning life.

Then in 1990 Hubble was launched. Even with its compromised resolution due to the notorious error in its primary mirror, we were surprised and delighted to see that this newly illuminated material was not a spherical shell as we expected, but a beautiful, thin ring. Something had previously sculpted the star's ejecta into this shape. Here was a lesson: nature doesn't always produce the simplest thing we think of.

That lesson was repeated when we obtained the first images of SN 1987A after astronauts installed corrective optics into Hubble in 1993. We saw not just one ring but three, aligned in remarkable symmetry.

The two larger, fainter, outer rings seem to indicate a tilted hourglass-shaped structure around the exploded star, with the inner ring as its neck--like the hourglass shapes often seen among planetary nebulae, where a star with lower mass blows off material near the end of its life. An artist's concept of the side view is at lower left. More recently, astronomers have found other superluminous blue-giant stars with similar three-ring structures. One is pictured on the next page. Are these, too, about to become core-collapse supernovae?

... and Then By the Blast

Meanwhile, the shredded remains of the star itself were hurtling outward behind the ultraviolet pulse at up to about 10% the speed of light. It did not take a supercomputer to calculate that a ring about 1 light-year from the explosion would get whacked in about 10 years. Sure enough, in 1995 Hubble began to see this collision.

But, just as the material around the star was not a shell but a three-ring extravaganza, the collision was more intricate than we expected. Instead of lighting up all at once as the first debris hit, the ring lit up in about 30 "hot spots," spaced around its circumference like a string of pearls.

The necklace of spots was presumably the sign of dense fingers of gas in the surrounding ring pointing toward the star, a little like stalagmites from the floor of a cave. As the shock wave expanded, it hit the tips of the fingers first. Now this preliminary encounter is coming to an end: some of the hot spots are fading and merging, and the destruction of the entire ring by the oncoming blast is under way.

Radioactive Time Release

The interaction of the expanding shock with the ring was not just a spectacle--it represented a fundamental change in the energy source for the light and other emissions we see.

In the initial supernova explosion itself, the nuclear alchemy that changes one element into another produces an array of isotopes. Some of these are stable nuclei of iron and other elements near iron in the periodic table. The iron in your blood came from star-core destruction of this sort, which seeded iron into interstellar clouds before our solar system formed from them. But not all the nuclei produced are long-lived. Some are radioactive, having decay times from moments to years.

The exploding debris was initially heated by the shock wave that blew the star apart. As it expanded and cooled, the recombination of ionized atoms with their lost electrons provided most of the light around the time of peak brightness a few months along. But after that, the debris was kept hot and glowing by the decay of radioactive nuclei.

In the case of SN 1987A, we infer that the heat and pressure of the core collapse created 0.07 solar mass (2,300 Earth masses) of nickel-56: the isotope of nickel with 28 protons and 28 neutrons. It decays with a half-life of 6 days to cobalt-56, which then decays with a half-life of 111 days to iron-56. That was the main energy source lighting the debris for about the first 500 days of its long, slow fade. Along the way an abrupt episode of dust formation partially veiled the view, further dimming the visible light. But when you added up the light, the infrared radiation re-emitted by the warm dust, and the gamma-ray photons coming directly from the decaying nuclei, the total continued to track the decay of the nickel-56 and its cobalt-56 daughter.

In time, as the cobalt decayed away, a slower time-release of stored energy became dominant: the decay of titanium-44, half-life 1,200 days. This was the main power source for the remnant's dimming emission until about 5,000 days (14 years) after the explosion.

That was more or less what astrophysicists had inferred from the fading light curve. They were able to test these ideas directly for the first time by measuring the energies of gamma-ray photons arriving from the debris. These matched the laboratory energies from decaying 56Ni and 44Ti. It's an iron-clad case that these iron-peak elements powered the remnant's long-lasting glow.

A New Process Takes Over

But now, the energy source for the supernova remnant has shifted again. Radioactivity accounted neatly for the decline, but since about 2001, careful measurement of Hubble images shows that the expanding debris remnant inside the inner ring--the keyhole-shaped nebula seen in the top row on the previous page--is rebrightening. Radioactivity cannot do that. But the violent collision of the blast's outer wave with the ring converts some of the blast's kinetic energy into heat. The gas at the collision sites becomes so hot that it shines in X-rays, which we can see with the Chandra satellite. Chandra has sharp enough vision to show that the X-ray emission is indeed coming from near the ring. The surprising thing is that these X-rays are shining back on the slower debris that has yet to reach the ring, reheating it from the outside.

The collision with the ring is also accelerating electrons there to relativistic energies. When the electrons interact with the magnetic fields in the neighborhood, they emit at radio wavelengths (synchrotron radiation). We've seen increasing radio emission over the past decade, again mostly coming from near the ring.

Nobody alive today will live long enough to see the complete transition of Supernova 1987A into the supernova remnant SNR 1987A. But the balance has tipped in the 30 years we've watched. The physical processes that make the star-debris shine have shifted from the explosion itself, to radioactive decay of what the explosion created, to its kinetic crash into the surrounding interstellar gas.

When we look at remnants of historical supernovae, such as Kepler's of 1604 or Tycho's of 1572 or somewhat older ones in the Milky Way, we see their completed transitions. Their conspicuous features now are radio emission from particles that the shock wave accelerates to nearly the speed of light, and X-ray emission from gas heated to millions of degrees. For SN 1987A, we have a much richer and more complete story that tells us what type of star exploded, what elements the explosion initially produced, and the detailed sequence of events that then unfolded. We can watch as the explosion destroys the evidence of the star's pre-explosion behavior, as embodied in the three-ring circus of surrounding hydrogen. Nature made SN 1987A in a more intricate and interesting way than we imagined. It will be good to remember that lesson of humility.

The Fire Next Time

Tycho had his supernova, Kepler had his. Perhaps there's an astronomer on Earth today whose name will go on the next one in the Milky Way itself. More likely, the first galactic supernova of the telescopic era will be caught by some large cooperative effort. Perhaps it will be an array of survey telescopes staring unblinkingly at the whole sky. Perhaps it will be specks of light produced by neutrinos as they slash through Antarctic ice, or a flare of ultraviolet light detected by a satellite, or the jiggle of the gravitational waves that a messy core collapse should produce. The next Milky Way supernova could outshine every star in the sky, or it might remain hidden from view behind thick interstellar dust. But the statistics are pretty clear: the Milky Way has a supernova about once a century on average. The next could happen at anytime. Keep looking up.

Harvard professor ROBERT P. KIRSHNER specializes in supernovae. He won the 2015 Wolf Prize in Physics, and the U.S. National Academy's Watson medal, for his work using supernovae to measure cosmic expansion. He is the chief program officer for science for the Gordon and Betty Moore Foundation and author of the popular-level book The Extravagant Universe: Exploding Stars, Dark Energy, and the Accelerating Cosmos.

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Title Annotation:SUPERNOVA 1978A TODAY
Author:Kirshner, Robert P.
Publication:Sky & Telescope
Date:Feb 1, 2017
Words:2483
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