Neutrinos and Supernova 1987A

Robert Bigelow

Twenty-three years ago, on 23 February 1987 at 12:35 a.m. MST, detectors in the US, Japan and Russia observed a burst of 24 neutrinos. They came from a supernova in the Large Magellanic Cloud, a small satellite galaxy to the Milky Way. About 2 hours later, an experienced New Zealand amateur astronomer observed the area of the supernova. He noted nothing unusual.

Supernova 1987A appears as a very bright object near the center of this image. The photograph was taken by Marcelo Bass at the National Optical Astronomy Observatories’ Cerro-Tololo Inter-American Observatory, on March 2nd 1987. Image Credit: Marcelo Bass, CTIO/NOAO/AURA/NSF

Supernova 1987A appears as a very bright star near the center of this image. The photograph was taken by Marcelo Bass at the National Optical Astronomy Observatories’ Cerro-Tololo Inter-American Observatory, on March 2nd 1987. Image Credit: Marcelo Bass, CTIO/NOAO/AURA/NSF

The first visible sign of the supernova was captured on a photograph taken at a telescope in Australia about 3 hours after the neutrino burst. Since the neutrinos arrived two to three hours earlier than the light, does that mean that neutrinos travel faster than light? No, it means the neutrinos got a head start.

The processes inside stars and the events that lead to a supernova are detailed and complex. While the following description omits many important and interesting details, it has enough information to explain the neutrino head start.

A supernova is the explosive end of a massive star (the Sun is not big enough to explode as a supernova). Normal stars produce energy by fusing lighter elements into heavier ones deep in their cores. Energy is produced in the fusion process. The energy moves outward and eventually reaches the surface of the star, causing it to shine. This energy production also results in an outward pressure that balances the inward force of gravity. A supernova occurs when a star runs out of fuel in its core and the fusion reactions suddenly shut down. With the loss of outward pressure, gravity takes over and the core of the star collapses in a fraction of a second. The core of a massive star has enough gravity to squeeze the matter in it so tightly that protons and electrons combine to form neutrons. This transformation also produces an enormous number of neutrinos. The neutrinos are able to pass through the star’s outer layers and escape into space before the star shows any outward sign of trouble.

Meanwhile, deep within the star, the core collapse triggers a shock wave that moves rapidly outward. The shock wave takes several hours to reach the surface. When it does, the radiation released in the explosion can briefly outshine a galaxy. Astronomers predicted that neutrinos from a supernova would arrive before its light. So, the early arrival of neutrinos from supernova 1987A was evidence that astronomers have a correct understanding of what causes a massive star to go supernova.

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4 thoughts on “Neutrinos and Supernova 1987A

  1. In the light of the 2011 publication of the OPERA experiment, how confident are you of the head start theory?

  2. To quote from the press release announcing the OPERA findings, “independent measurements are needed before the effect can either be refuted or firmly established”. In the OPERA experiment, time of flight of individual neutrinos is not measured directly. Instead, statistical analysis of the data is used to infer the flight time. So, I suspect that a flaw will be found in the analysis and/or other experiments will eventually refute the effect. However, even if verified in future experiments, the OPERA results cannot explain the observed difference in arrival times between neutrinos and light from supernova 1987A. (Verification would require a revision to the fundamental laws of physics).

    The claim in the OPERA experiment is that the neutrinos reached the detectors 730.5 km away 60.7 billionths of a second earlier than would be expected if they traveled at the speed of light in vacuum. If we apply the same ratio of early arrival time to distance traveled to neutrinos from supernova 1987A (at an estimated distance of 186,000 light years), the neutrinos would have arrived at Earth 4.7 years earlier than light from the supernova, not several hours. Based on our current understanding of supernovae, neutrinos are predicted to arrive a few hours before the light. That is what was observed. Therefore, I am still confident in that model . . . for now.

    Note: It has since been determined that an equipment problem was responsible for the apparent “faster than light” neutrinos.

  3. I have read that trillions of neutrinos pass through our bodies every second, presumably generated in the sun. What is so significant about the 24 neutrinos that we know they were not generated in the sun?

  4. Supernova 1987A is the only nearby supernova during the last several hundred years. The significance of this event is that it was the first (and to date the only) opportunity to test astronomer’s theoretical model of this type of supernova against observations. All other supernovae are too far away for enough neutrinos to reach Earth to be detected.

    While there are enormous numbers of neutrinos passing through our bodies every second, neutrinos interact so rarely that it is likely that only one or two will interact with a subatomic particle in our body during our lifetime. The Irvine-Michigan-Brookhaven (IMB) detector, located in an Ohio salt mine was filled with 2.5 million gallons of ultra-pure water. On average it recorded about two neutrino interactions per day. The fact that a total of 24 neutrinos were detected within 13 seconds at three different detectors across the globe (8 within 6 seconds at IMB) indicates that there was a brief but substantial increase in the number of neutrinos. This has been interpreted as a burst of neutrinos from the supernova that passed by Earth.

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