Supernovae can rarely be seen in our galaxy

As a massive star ages, iron in its core might begin to act as a sponge — not a source — of energy. At that point, the star collapses — and explodes outward as a supernova. Scientists want to understand how stars explode as supernovae.

But they’ve never observed the whole process. Supernovae generally don’t become bright enough to be noticed until after the explosion is well underway. Astronomers see supernovae in our galaxy only rarely. Kate Scholberg is a physicist at MIT.

Kate Scholberg: I think that’s one of the big challenges. When something happens only every 30 years, it becomes sometimes hard to maintain enthusiasm for waiting for one. On the other hand, the rarity of it makes it all the more valuable to actually get information from it…

When they first erupt, supernovae emit a burst of particles called neutrinos. Scholberg and others have linked several underground neutrino detectors — in different parts of the world — with a computer network. Now, at the first sign of a flood of neutrinos, electronic messages will go out to astronomers — who will try to locate the supernova as quickly as possible.

Kate Scholberg: … the Hubble Space Telescope can’t just swing around and scan for something. It has to know where to point. And amateurs on the other hand have wonderful wide angle viewing capability and they may be the first ones to find it.

Throughout human history, people have looked up and noticed what seem to be bright new stars appearing in the night sky. (The stars were actually there already, they were just much fainter.) Such a “new” star tends to reach peak brightness in a few days or weeks, then slowly dim over the next few weeks. These new stars are actually existing stars that collapse and violently explode — they’re called supernova. But by the time people first notice this bright “new” star, its already minutes or even days after the explosion began.

Such explosions only happen in our galaxy about every 30 years and no one knows exactly when or where the next one will occur. So they’re extremely difficult to study.

When a star collapses and then explodes as a supernova, most of the energy of the explosion is in the form of particles known as neutrinos that last for just a few seconds. Then anywhere from a few days to a few minutes later, visible light is emitted. And it appears that a bright new star appears in the sky.

Scholberg’s main interest lies with understanding neutrinos themselves. But she thinks that the thousands of neutrinos that would be detected from a supernova in a few seconds at her main site — Super Kamiokande — will keep her busy analyzing for a lot time.

Scholberg says the amateur astronomers who get SNEWS (SuperNova Early Warning System) alerts “also may be able to gather a lot of information — a lot of them have pretty sophisticated tools and may be able to take spectra and actually get some of the first valuable observations with real scientific information in it.”

Scholberg to walk through what would happen if a real supernova began. She responded:

“Well, the star at the end of its life can no longer support the outlying mass, and the energy generation on the inside can’t support the weight of the star. It starts to collapse. At this point, gigantic numbers of neutrinos start to stream out and over a time scale of about 10 seconds, almost all of the energy of the supernova — 99 percent of the energy in fact — streams out in the form of neutrinos. A supernova is a gigantically bright event in optical radiation and in other forms of energy — it can outshine a galaxy. But that’s only one percent of the total energy of a supernova. 99 percent of it is actually in neutrinos.

“Of course, neutrinos are hard to detect, but what we’ll see on Earth at this time scale, 10 seconds after collapse, is a huge burst in all the detectors around the world. For instance, the detector which I’m a part of is Super Kamiokande, which is a giant water detector in a mountain in Japan — that will basically light up like a Christmas tree, will get thousands of events, thousands of neutrinos, in 10 seconds … and that’s to be compared with a dribble of perhaps a dozen per day from the sun or of the atmosphere. So this will be a really bright and extremely obvious signal.

“Other detectors around the world — for instance there’s the Sudbury neutrino detector in Canada that’s a heavy water detector and that will also light up with hundreds of events. A detector which is made of liquid scintilator will also light up, this is in Italy, with hundreds of events as well. Perhaps detectors at the south pole. There are other detectors in Japan as well. These will all at the same time, all within the same 10 seconds, all light up.

“Each detector individually will be looking at its data in real time. If each detector respectively finds a burst in its data stream, then each detector will send a message to the central computer — actually we have two central computers — one’s in Japan and one’s in Italy. And this central computer, if it finds messages within 10 seconds — it determines that there is a coincidence. Under normal circumstances, it’s extremely rare for two detectors to go off at the same time — if both two of them go off at the same time, it’s got to be something interesting like a supernova and it sends out a message to the participating experiments and the astronomers.”

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