Some 168,000 years ago, in a nearby galaxy called the Large Magellanic Cloud, a massive star collapsed and then spectacularly exploded, releasing 100 times the amount of energy than the sun will release over its lifetime in just one second. The faraway light took so long to reach us that astronomers first saw the explosion in 1987, and, not-so-creatively, named it Supernova 1987A.
In the 23 years since, Robert Kirshner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, has acted as a sort of steward over this supernova, tracking and studying its insides as they emerged from the blast and traveled through space. But in 2004, science grounded to a halt when the spectrometer on the Hubble Space Telescope failed, leaving a five-year gap in the research. Kirshner, co-author of a Science article on the first findings on 1987A since Hubble was repaired, spoke recently with the NewsHour.
Using Spectra to Study Stars
While the Hubble Space Telescope brings to mind dazzling pictures of star births, space storms and spiral galaxies, more than half of the pictures it takes are actually spectra.
In 1665, Isaac Newton famously shone sunlight through a prism and saw a rainbow. What he found led to a series of important discoveries about light: white light contains all the colors of the rainbow, its spectrum. And color is the visible manifestation of light at different wavelengths. Hubble’s spectroscope applies Newton’s findings to stars and distant galaxies by creating a sort of fingerprint of these stellar objects. It does this by measuring the light that radiates from them, and beaming them back as spectra, in the form of rainbow-colored bar codes.
Scientists can glean a lot about Supernova 1987A by studying the spectral blueprint of the debris and shock waves as they travel outward from the explosion toward a system of rings that were thrown out about years before the star’s death.
The spectra carry information on the chemical elements that make up this explosive debris, Kirshner says. “We can also tell the speed, because we can identify a particular chemical element, and if the material is coming toward us, the spectrum gets shifted a little to the blue. If it’s going away from us, it gets shifted a little to the red. So by making these measurements, we can actually understand which chemical elements are present, and how fast they’re moving.”
Watching the Explosion
Watching this explosion over the past 23 years has been like watching a car crash in slow motion, says Richard McCray, an astrophysicist at the University of Colorado and co-author of the Science paper. “We’re seeing this thing play out in detail. It happened, it threw out the debris and then it takes a while for the crash to develop.”
Just after the explosion, radioactive debris emerged from the interior of the supernova. Now scientists are studying gas and other material that is blasting into the star’s outer ring as a shock wave traveling at 20 million mph.
“It’s like a sonic boom,” McCray says. “It gives a sudden compression to the gas in the ring, which heats it up and makes it glow. As it enters the ring and compresses the gas in the ring, it makes it light up.”
Because 1987A is the brightest, most-observable supernova of the last 400 years, it has been a great teacher to scientists. But much remains a mystery. Early spectral measurements detected a pulse of neutrinos, indicating that a neutron star may have formed during the explosion, but no one has ever seen it. Some scientists believe it still exists, veiled in stardust. Also unknown is how the rings formed. One theory is that two stars merged, and in so doing, flung out a disc of star material; the ring that we see in Hubble images may be only the innermost part of that disc, according to McCray.
What science does know about the supernova is like a partially completed jigsaw puzzle, with most of the pieces still missing. But answers may lie in the still-invisible material that is expected to become illuminated over time. Already, accelerating electrons are brightening the “hot spots” in the inner ring, and scientists expect the ring and other material surrounding it will get increasingly brighter in the coming years.
New technology like the ALMA telescope, under development in Chile, and the James Webb Space Telescope — which sees in the infrared, and therefore, through stardust — should allow scientists to see more of 1987A. Knowing more about the debris ejected from the supernova could also provide answers to bigger questions about the formation of planets and the universe, scientists say.
“That’s one of the wonderful things about a supernova,” McCray said. “We know that all the material of which the planets are made, and of which you and I were made, was made in a supernova. Without supernovae, there would be no heavy elements in the universe.”