Earth's observatories hope to detect neutrinos and gravitational waves
T. Melson, H.-T. Janka and A. Marek/Astrophys. J. Lett. 2015
Almost every night that the constellation Orion is visible, physicist Mark Vagins steps outside to peer at a reddish star at the right shoulder of the mythical figure. “You can see the color of Betelgeuse with the naked eye. It’s very striking, this red, red star,” he says. “It may not be in my lifetime, but one of these days, that star is going to explode.”
With a radius about 900 times that of the sun, Betelgeuse is a monstrous star that is nearing its end. Eventually, it will no longer be able to support its own weight, and its core will collapse. A shock wave from that collapse will speed outward, violently expelling the star’s outer layers in a massive explosion known as a supernova. When Betelgeuse detonates, its cosmic kaboom will create a light show brighter than the full moon, visible even during the daytime. It could happen tomorrow, or a million years from now.
Countless stars like Betelgeuse — any of which could soon explode — litter the Milky Way. Scientists estimate that a supernova occurs in our galaxy a few times a century. While brilliantly gleaming supernovas in far-flung galaxies are regularly spotted with telescopes trained on the heavens, scientists eagerly hope to capture two elusive signatures that are detectable only from a supernova closer to home. These signatures are neutrinos (subatomic particles that stream out of a collapsing star’s center) and gravitational waves (subtle vibrations of spacetime that will also ripple out from the convulsing star).
A star’s last moments
Over millions of years, a large star fuses nuclei into increasingly heavy elements. Once iron forms in the core, fusion stops and gravity overwhelms the star, collapsing its core and setting up a shock wave (yellow) that travels outward. Neutrinos emanating from the core push the shock wave toward the star’s surface. The star explodes, flinging elements into space.
1. Star's core collapses. t=0 seconds
2. The core rebounds, producing a shock wave.
3. Neutrinos give the shock wave a boost, keeping it moving as outer material falls inward. t=0.3 seconds
4. Shock wave travels through the layers, expelling material outward. t>0.3 seconds
5. Shock wave reaches surface and star explodes.
t> 2 hours
“These two signals, directly from the interior of the supernova, are the ones we are really longing for,” says Hans-Thomas Janka, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany. Unlike light, which is released from the star’s surface, stealthy neutrinos and gravitational waves would give scientists a glimpse of the processes that occur deep inside a collapsing star.
Supernovas offer more than awe-inspiring explosions. When they erupt, the stars spew out their guts, seeding the cosmos with chemical elements necessary for life to exist. “We clearly wouldn’t be here without supernovas,” says Vagins, of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo. But the processes that occur within the churning mess are still not fully understood. Computer simulations have revealed much of the physics of how stars explode, but models are no substitute for a real-life nearby blast.
One inspiration for scientists is supernova 1987A, which appeared in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, 30 years ago (SN: 2/18/17, p. 20). That flare-up hinted at the unparalleled information nearby supernovas could provide. With the detectors available at the time, scientists managed to nab just two dozen of 1987A’s neutrinos (SN: 3/21/87, p. 180). Hundreds of papers have been written analyzing that precious handful of particles. Calculations based on those detections confirmed scientists’ hunch that unfathomably large numbers of neutrinos are released after a star’s core collapses in a supernova. In total, 1987A emitted about 1058 neutrinos. To put that in perspective, there are about 1024 stars in the observable universe — a vastly smaller number.
Since 1987, neutrino detectors have proliferated, installed in exotic locales that are ideal for neutrino snagging, from the Antarctic ice sheet to deep mines across the globe. If a supernova went off in the Milky Way today, scientists could potentially nab thousands or even a million neutrinos. Gravitational wave detection has likewise come on the scene, ready to pick up a slight shift in spacetime stirred up by the blast. Detecting such gravitational waves or a surfeit of supernova neutrinos would lead to a distinct leap in scientists’ knowledge, and provide new windows into supernovas. All that’s needed now is the explosion.
Despite estimates that a few stars explode in the Milky Way every century, no one has glimpsed one since the early 1600s. It’s possible the explosions have simply gone unnoticed, says physicist John Beacom of Ohio State University in Columbus; light can be lost in the mess of gas and dust in the galaxy. A burst of neutrinos from a supernova would provide a surefire signal.
These hermitlike elementary particles shun most interactions with matter. Produced in stars, radioactive decay and other reactions, neutrinos are so intangible that trillions of neutrinos from 1987A’s explosion passed through the body of every human on Earth, yet nobody felt a thing. For supernovas like 1987A, known as type 2 or core-collapse supernovas, about 99 percent of the explosion’s energy goes into the tiny particles. Another, less common kind of supernova, type 1a, occurs when a remnant of a star called a white dwarf steals matter from a companion star until the white dwarf explodes (SN: 4/30/16, p. 20). In type 1a supernovas, there’s no core collapse, so neutrinos from these explosions are much less numerous and are less likely to be detectable on Earth.
For scientists studying supernovas, neutrinos’ reluctance to interact is an advantage. The particles don’t get bogged down in their exit from the star, so they arrive at Earth several hours or even more than a day before light from the explosion, which is released only after the shock wave travels from the star’s core up to its surface. That means the particles can tip off astronomers that a light burst is imminent, and potentially where it is going to occur, so they can have their telescopes ready.
Most neutrino experiments (there are more than a dozen) weren’t built for the purpose of taking snapshots of unpredictable supernovas; they were built to study neutrinos from reliable sources, like the sun, nuclear reactors or particle accelerators. Nevertheless, seven neutrino experiments have joined forces to create the SuperNova Early Warning System, SNEWS. If neutrino detectors in at least two locations report an unexpectedly large burst of neutrinos, SNEWS will send an e-mail alert to the world’s astronomers. The experiments involved are a weirdly diverse bunch, including IceCube, a detector composed of light sensors frozen deep in the ice of Antarctica (SN: 12/27/14, p. 27); Super-Kamiokande, which boasts a tank filled with 50,000 tons of water stationed in a mine in Hida, Japan; and the Helium and Lead Observatory, or HALO — with the motto “astronomically patient” — made of salvaged lead blocks in a mine in Sudbury, Canada. Their common thread: The experiments are big to provide a lot of material for neutrinos to interact with — such as lead, water or ice.
With light sensors sunk kilometers deep into the ice sheet, IceCube’s detector is so huge that it could pick up traces of a million neutrinos from a Milky Way supernova. Because it was designed to capture only the highest energy neutrinos that are rocketing through space, it’s not sensitive enough to detect individual neutrinos emitted during a supernova. Instead, IceCube’s focus is on the big picture: It catalogs an increase of light in its detectors produced by neutrinos interacting in the ice in time slices of two billionths of a second, says IceCube leader Francis Halzen of the University of Wisconsin–Madison. “We make a movie of the supernova.”
Super-Kamiokande is the neutrino detector that can pinpoint the location of the impending stellar paroxysm. It is a successor to Kamiokande-II, one of a few detectors to spot a handful of neutrinos from 1987A. Shortly after a burst of neutrinos from a nearby supernova, the detector could direct astronomers to zero in on a few degrees of sky. If that happens, says neutrino physicist Kate Scholberg of Duke University, “I expect anybody with the capability will be zooming in.”
Story continues after slideshow
On the lookout
Various neutrino detectors await signals emitted from a supernova, including Super-Kamiokande in Japan, IceCube in Antarctica and HALO in Canada. They are joined by a gravitational wave observatory, LIGO, with detectors in Louisiana (shown) and Washington state.
Languages of a supernova
Light and neutrinos are two of several languages that a supernova speaks. In that sense, supernova 1987A was a “Rosetta stone,” Beacom says. By scrutinizing 1987A’s light and its handful of neutrinos, scientists began piecing together the theoretical physics that explains what happened inside the star. In a future supernova, another language, gravitational waves, could add nuance to the tale. But the explosion has got to be close.
If neutrinos are elusive, gravitational waves border on undetectable. Minute tremors in space itself, predicted by Einstein’s general theory of relativity, are generated when massive objects accelerate. In 2016, scientists with the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, announced the first direct detection of gravitational waves, produced by two merging black holes (SN: 3/5/16, p. 6). That milestone required a pair of detectors so precise that they can sense quivers that squish the detectors’ 4-kilometer-long arms by just a tiny fraction of the diameter of a proton.
Gravitational waves from a supernova should be even harder to tease out than those from merging black holes. The pattern of ripples is less predictable. Surveys of the properties of the many supernovas detected in other galaxies indicate that the explosions vary significantly from one to the next, says astroparticle physicist Shunsaku Horiuchi of Virginia Tech in Blacksburg. “We ask, ‘Is there a standard supernova?’ The answer is ‘No.’ ”
Despite the challenges, finding gravitational waves from supernovas is a possibility because the explosions are chaotic and asymmetrical, producing lumpy, lopsided bursts. An explosion that expands perfectly symmetrically, like an inflating balloon, would produce no gravitational waves. The gravitational wave signature thus can tell scientists how cockeyed the detonation was and how fast the star was spinning.
Gravitational waves might also reveal some of the physics of the strange stew of neutrons that makes up a protoneutron star — the beginnings of an incredibly dense star formed in a supernova. Scientists would like to catalog the compressibility of the neutron-rich material — how it gets squeezed and rebounds in the collapse. “The gravitational wave signature would have an imprint in it of this stiffness or softness,” says computational astrophysicist Tony Mezzacappa of the University of Tennessee.
There’s a chance the supernova would collapse into an even more enigmatic state — a black hole, which has a gravitational field so strong that not even light can escape. When a black hole forms, the flow of neutrinos would abruptly drop, as their exit route is cut off. Detectors would notice. “Seeing the moment that a black hole is born,” says Vagins, “would be a tremendously exciting thing.”
While neutrinos can be oracles of supernovas, a stellar explosion could reveal a lot about neutrinos themselves. There are three types of neutrinos: electron, muon and tau. All are extremely light, with masses less than a millionth that of an electron (SN: 1/26/13, p. 18). But scientists don’t know which of the three neutrinos is the lightest; a nearby supernova could answer that question.
Supernovas, with all the obscure physics at their hearts, have a direct connection to Earth. They are a source of many of the elements from which planets eventually form. As stars age, they fuse together heavier and heavier elements, forging helium from hydrogen, carbon from helium and so on up the periodic table to iron. Those elements, including some considered essential to life, such as carbon and oxygen, spew out from a star’s innards in the explosion.
“All the elements that exist — that are here on Earth — that are heavier than iron were either made in supernovas or other cataclysmic events in astronomy,” says physicist Clarence Virtue of Laurentian University in Sudbury, Canada. Gold, platinum and many other elements heavier than iron are produced in a chain of reactions in which neutrons are rapidly absorbed, known as the r-process (SN: 5/14/16, p. 9). But scientists still argue about whether the r-process occurs in supernovas or when neutron stars merge with one another. Pulling back the curtain on supernovas could help scientists resolve the dispute.
Even the reason supernovas explode and sow their chemical seeds has been vigorously debated. Until recently, computer simulations of supernovas have often fizzled, indicating that something happens in a real explosion that scientists are missing. The shock wave seems to need an extra kick to make it out of the star and produce the luminous explosion. The most recent simulations indicate that the additional oomph is most likely imparted by neutrinos streaming outward. But, says Mezzacappa, “At the end of the day, we’re going to need some observations against which we can check our models.”
Hurry up and wait
Supernovas’ potential to answer such big questions means that scientists are under pressure not to miss a big break. “You better be ready. If it happens and you’re not ready, then you will be sad,” Scholberg says. “We have to be as prepared as possible to gather as much information as we possibly can.”
If a detector isn’t operating at the crucial moment, there’s no second chance. So neutrino experiments are designed to run with little downtime and to skirt potential failure modes — a sudden flood of data from a supernova could crash electronics systems in an ultrasensitive detector, for example. Gravitational wave detectors are so finicky that interference as subtle as waves crashing on a nearby beach can throw them out of whack. And in upcoming years, LIGO is scheduled to have detectors off for months at a time for upgrades. Scientists can only hope that, when a supernova comes, everything is up and running.
Some even hope that neighboring stars hold off a little longer. “It seems like I’m always telling people that I’d like Betelgeuse to go off one year from now,” jokes Bronson Messer, a physicist who works on supernova simulations at Oak Ridge National Laboratory in Tennessee. With each improvement of the simulations, he’s eager for a bit more time to study them.
Messer keeps getting his wish, but he doesn’t want to wait too long. He’d like to see a supernova in the Milky Way during his lifetime. But it could be many decades. Just in case, Vagins, who’s been taking those nightly peeks at Betelgeuse, is doing his part to prepare the next generation. He no longer scans the skies alone. “I’ve already taught my 6-year-old son how to find that star in the sky,” he says. “Maybe I won’t get to see it go, but maybe he’ll get to see it go.”
This article appears in the February 18, 2017, issue of Science News with the headline, "Waiting for a Supernova: When a nearby star explodes, observatories plan to be ready."
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