Pan-STARRS Observatory captures cosmic event
On the evening of Aug. 17, astronomers atop Haleakala were preparing to train the world’s largest digital camera onto a spot in the sky where, hours earlier, teams on the U.S. Mainland and in Chile had detected strange bursts of energy.
The Pan-STARRS Observatory on Haleakala possesses the world’s most complete survey of the sky. So when astronomers snapped images of the spot and compared them to past files, they knew — this was out of the ordinary.
“We were able to see that it was fading rapidly, faster than any supernova ever fades,” said Ken Chambers, director of the Pan-STARRS Observatory. “And that told us this is something new.”
The observation that would capture the astronomy world’s attention for two weeks turned out to be the remnants of a neutron star collision — a cosmic event never before seen from Earth, and one that astronomers believe will change their understanding of how elements like gold are created.
“This is something they’re going to be teaching in basic science classes for years to come,” University of Hawaii spokesman Dan Meisenzahl said Monday.
Astronomers believe the crash actually happened 143 million years ago, and because it was so far out — 143 million light years away — the signal only reached Earth recently, Chambers said.
“In this galaxy far, far away, there were two neutron stars orbiting each other,” he explained. “Neutron stars are the ashes of massive stars. These are stars that had burned up all their fuel.”
Bigger than the sun, these two stars had collapsed and were starting to circle toward each other in a “death spiral.” When they finally lost their energy, they collided in a spectacular crash that sent cosmic debris flying through the galaxy.
“These neutron stars are the hardest things in the universe,” Chambers said. “They’re just made out of neutrons. It’s much harder than pure diamond. . . . So when they hit, they hit hard.”
The neutrons emitted by the crash “become the building blocks” of other elements, Chambers said.
At 2:41 a.m. HST Aug. 17, observatories on Earth and in space sensed the crash. First, there was a burst of gravitational waves — invisible ripples that cause space to expand and contract slightly — picked up by detectors at the Laser Interferometer Gravitational-Wave Observatory in Washington state, Louisiana and Italy. Then, 1.7 seconds later, NASA’s space-based Fermi telescope caught a burst of high-energy gamma rays.
But neither observatory could pinpoint exactly where it had come from. Global alerts sent astronomers into a scramble to find the source. In Chile, a small team — including Benjamin Shappee, now an astronomer with the UH Institute for Astronomy — located the galaxy and the bright object left over from the crash.
When Pan-STARRS senior observer Angie Schultz got the early-morning notification on Maui, it was too close to daylight for her to take a look. So, later that day, around 7:30 p.m. HST, she and another astronomer pointed the Pan-STARRS telescope to the heavens.
“We could only observe for a few minutes each time,” Schultz said. “It was also very difficult because it was near the sun, so the sky background is really bright.”
Using Pan-STARRS surveys of the sky, and collaborating with the nearby ATLAS telescope, an asteroid warning system developed by UH, astronomers were able to conclude that the object had the signature of a kilonova, the glowing radioactive remnants of a neutron star crash.
“ATLAS surveyed the sky much more recently,” Chambers said. “They had additional information that this was probably not a supernova. A supernova takes awhile to fade, weeks to fade. ATLAS had surveyed the sky within a week and was able to say there was nothing there before. That turned out to be important.”
A supernova is the explosion of a star, caused by a change in its core or center.
Over the next two weeks, astronomers around the globe fixed their telescopes on the kilonova, scrambling to gather as much data as possible as it went from bluish to red and faded from view. What they found “changes the way we think about the elements in the periodic table,” Chambers said.
“We used to believe that elements heavier than iron are produced in supernovae explosions,” Schultz said. “But now they think they may be produced in these neutron star mergers.”
Astronomers on Maui were able to detect elements heavier than iron in the remnants of the crash.
“Stars just going through their natural lifestyle can make everything up to iron, but nothing higher than iron,” Chambers said. “The question is, ‘How in the world do you make iron up to uranium?’ What this shows is this happens in this collision of neutron stars. There aren’t many elements heavier than iron in your body. Iodine is one. Iodine is made in this process. That’s pretty cool. We each have an intimate connection to two neutron stars.”
Chambers said that the collision and merger of neutron stars could help astronomers better understand how fast the universe is expanding.
Based on the crash in August, astronomers calculated a figure of how fast the universe is expanding that Chambers said “is in pretty good agreement with the value that we found by other astronomical means. Someday when we have a hundred of these things, then it will start to become a very accurate way to measure (the expansion of) the universe.”
* Colleen Uechi can be reached at firstname.lastname@example.org.