Wanted: amateur stargazers
and World Science staff
Scientists have thought of a new way to solve an astronomical mystery, and their plan relies on a network of amateur stargazers and a very elusive subatomic particle.
To understand what happens inside exploding stars, or supernovae, scientists need to study particles called neutrinos, said John Beacom, assistant professor of physics and astronomy at Ohio State University in Columbus, Ohio.
Neutrinos are formed in the nuclear reactions that make stars like our sun shine. Exploding stars overflow with the particles, and flood the universe with them.
Neutrinos should be everywhere, but they are very hard to detect – so hard to detect, in fact, that even though countless neutrinos burrow through our planet every second, scientists only capture a few of them each day.
Scientists know that most neutrinos they do detect probably come from our own sun, from nuclear reactors in terrestrial power plants, or from cosmic radiation interacting with our atmosphere. There has been no way to distinguish whether a particular neutrino came from elsewhere, until now, according to Beacom.
But Beacom and his team have calculated that each year, one or two of the neutrinos detected on Earth can probably be matched to the exploding star that made them. They can thus be used to probe the physics of that blast.
The discovery comes at a special time, Beacom said. The method will fully exploit the capabilities of the next generation of neutrino detectors, which are now being planned. The finding also comes as a growing number of amateur astronomers are capable of discovering supernovae. They, too, can contribute to the neutrino research, by scanning the sky for supernovae.
“Even with all our modern telescopes, the professionals can’t look at the whole sky at once,” Beacom said. “But the amateurs are everywhere. With relatively small telescopes, they can see these nearby supernovae, which are very bright – often brighter than their host galaxies.”
For a study appearing in a recent issue of the research journal Physical Review Letters, Beacom and his coauthors developed a kind of litmus test for finding supernova neutrinos: If a detector on Earth registers two of the particles within ten seconds, odds are high that they came from a supernova in a nearby galaxy.
Also, if an astronomer – amateur or not – spots a supernova, scientists at neutrino detectors can look back through their records to see if they captured a neutrino around that time.
Given that a few supernovae occur in nearby galaxies every year, and given the sensitivity of neutrino detectors on Earth, they’ve determined that at least one of those scenarios – the two-in-ten-seconds event or the identification of a supernova neutrino after the fact – could happen about once a year.
The professionals need amateur astronomers to help spot new supernovae fast, so scientists can quickly match captured neutrinos with the exploding stars that made them. But amateurs would need telescopes somewhat larger than the average backyard telescopes for the job, he added.
Coauthor Hasan Yüksel, a postdoctoral researcher at Ohio State , explained that many of today’s so-called amateur astronomers aren’t really so amateur. “You can think of them more as ‘professional amateurs,’“ he said.
These are the semi-pro players of the hobby set – skilled folks who build custom telescopes, Yüksel explained. They have day jobs, but they scan the skies at night, and share their findings with other amateurs over the Internet. Often, they have ties to professional astronomers. They often know of major discoveries as soon as the professionals do.
Yüksel also pointed out that since 2002, at least nine supernovae were identified in galaxies within about 30 million light years of our Milky Way, and amateurs discovered more than half of those. A light year is the distance light travels in a year.
But the last supernova that astronomers were able to match with neutrinos was an event designated 1987A, which was sighted in that year in a galaxy that is a very close companion to the Milky Way. Because detectors on Earth captured 20 neutrinos in only a few seconds during that event, astronomers knew for sure that they came from 1987A, Beacom said.
But since then the neutrino matchings have been “a big fat zero,” he said. “What if using this technique, we could have been identifying one additional supernova neutrino per year? By now, we would have collected a sample as big as that burst in 1987.” With the much larger neutrino detectors that are now being devised, and with the large number of supernovae being spotted these days, it could be done, he added.
Galaxies up to 200 times farther away than the one that spawned Supernova 1987A are still considered near by astronomical standards, he added. Amateurs would be able to spot supernovae in them. Those galaxies may give us only one or two neutrinos per year, but that’s still more than scientists would be able to study otherwise.
“Since a supernova expends 99 percent of its energy in neutrinos, those neutrinos tell the story of how the explosion works, and therefore we have to find them,” he added.
One sign that scientists don’t understand supernovae yet, said Shin'ichiro Ando, a visiting scholar Ohio State, is that computer simulations of the events always go wrong. The explosion starts, and then it fizzles. “If we can’t make a supernova blow up on the computer, that means we’re missing something. We need clues. We need to find those neutrinos.”
Beacom envisions that scientists at neutrino detectors could sound an alarm whenever they detect two particles in ten seconds. Since supernovae emit neutrinos at the very start of the explosion, the particles would reach Earth hours before the supernovae would be visible in telescopes, and the announcement would amount to a supernova forecast.
Alternatively, when astronomers spot a nearby supernova, they could ask the scientists at the detectors to look back through their data from previous hours to find any particle events.
At Beacom’s suggestion, scientists working at the Japanese neutrino detector Super-Kamiokande plan to search their records for events that could be linked to nearby supernovae in past years.
“While this detector is smaller than those envisioned for the future, it’s been in operation for a decade or two, so it actually stands a good chance of having detected the first neutrino from an identified supernova beyond the Milky Way and its closest companions,” Beacom said.
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