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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10 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.

  5. I think something is about to happen in the Universe and will effect us on Earth. What are your thoughts. Thank you.

  6. Something is pretty much always happening in the universe that has an impact on us. The Sun’s converting millions of tons of matter into energy every second has an impact. The Moon’s daily tide’s have an impact. Last year above Chelyabinsk, Russia we had a genuine “impact.” We live in the constantly changing universe. We, as a part of that universe, also experience change. The universe is not static, it’s dynamic, and we’re a part of it.

  7. Hi Robert. Was there any consensus on the difference in neutrino arrival times between the three detectors. In particular, didn’t the Japanese detector observer a burst well off the timeline expected?

  8. Steve,
    The neutrino detections by Kamiokande in Japan, IMB in Ohio, and Baksan in Russia, all occurred within 13 seconds of each other (see Fig. 3 in this article from CERN). Nearly 5 hours earlier there were 5 neutrinos detected within 7 seconds of each other at the Mont Blanc Underground Neutrino Observatory in Italy. While many think these neutrinos were not associated with SN 1987A, others have proposed models in which the core collapse occurs in two stages over several hours. Each stage would produce neutrinos with different energies. In these models, the collapsing star first breaks under rotation in various pieces. The neutrinos that result from this would have a range of energies that would have a greater likelihood of being observed by the particular detector at Mont Blanc. The final collapse produces a second burst of neutrinos with a different energy range that were observed by the other three detectors. We will probably need another nearby supernova to test this idea. I am waiting.

  9. The detection of neutrino oscillation provides powerful support for the argument that neutrinos have a small but finite mass and thus must travel at velocities less than c. So how is it that neutrinos from a supernova explosion are detected before photons from the same event? If we are to accept relativity then the neutrino, because it has mass, must travel at a speed below that of the companion photons that were generated in the same event. An yet they do not appear to do so. And I think I have a possible explanation why and if this reasoning is valid it may suggest some additional properties of both neutrinos and photons. This argument makes one critical yet seemingly reasonable assumption; the observed neutrinos and photons were generated simultaneously by the SN event.

    Optical and radio astronomy observations of SN84 reveal Faraday rotation and wavelength dependent time dispersion which is interpreted to mean that the radio and optical photons passed though a birefringent media on their way to the observer. The index of refraction of gas clouds is dependent on particle charge and density, magnetic field, wavelength of the photons and the polarization of the photons. This results in differing propagation velocities for photons generated by the same event. This is the underlying mechanism for time dispersion and Faraday rotation. The important point is that all of the photons are delayed to some degree, some more so than others, by the refractive properties of the media through which they pass. In other words the light from the event is actually traveling slower than c at least for part of its journey. The magnitude of time dispersion and Faraday rotation reveals a great deal about the media responsible for the phenomenon.

    Now consider the flight of the neutrino. SN event neutrinos must pass through the same refractive media as the photons. One possible explanation is that neutrinos do not experience refraction and so even though they travel at less than c they still outpace their companion photons which are slowed by the refractive properties of the media to a velocity less than that of the neutrinos. As a result over great distances the neutrinos arrive first.

    Assuming that the above is valid it reveals some interesting additional information about neutrinos.

  10. Charles,
    While evidence for neutrino flavor oscillation does indicate that neutrinos have mass, a very small mass would still allow them to travel fast enough to arrive before the photons, given a head start of several hours. That allowed physicists, based on the time-of-flight, to set an upper limit for the neutrino mass. Mass limits calculated from SN 1987A data are consistent with neutrino mass limits from beta decay experiments (See Neutrino Properties from the Particle Data Group). While you are correct that interactions of photons with any intervening diffuse matter would slightly delay their arrival time (neutrinos are highly unlikely to interact with any intervening matter) that is not needed to explain what was observed.

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