How a Supernova Gave Birth to Neutrino Astronomy 25 Years Ago Today

On February 23, 1987, the astronomy world was abuzz. Astronomers Ian Shelton and Oscar Duhalde at the Las Campanas Observatory in Chile noticed that a bright new point of light had appeared in the Large Megellanic Cloud. It did not take long for other astronomers to verify the discovery of a new supernova, dubbed SN 1987A.1 For members of that ancient profession, it was the observational opportunity of a lifetime. A supernova had erupted practically next door. (The Large Megellanic Cloud is a satellite star cluster to our own Milky Way galaxy, and the last time a supernova had been observed within our own galaxy had been in 1604.)

But, roughly three hours before the first light from SN 1987A had reached us, something perhaps more significant had taken place. At three neutrino detection experiments around the world, (Japan’s Kamiokande II, America’s IMB, and Russia’s Baksan), a flood of neutrino signals was detected at 7:36 GMT. And reconstruction of the trajectories of the incoming neutrinos made it clear that they had come from SN 1987A.2,3

I said a flood. The fact is, 24 distinct neutrino events were detected. But in the world of neutrino physics, that is huge. It is estimated that for about 13 seconds, each square foot of the Earth was bombarded by approximately 100 trillion neutrinos from the supernova. (Yes, that is “trillion” with a “t.”) So, why were so few neutrinos detected?

The simple truth of the matter is that neutrinos are VERY difficult to detect. They do not interact with other matter via electromagnetic or strong nuclear interactions, but rather only via the weak nuclear force (responsible for beta decay) and gravity. Since the weak interaction has an extremely short range (about 10-17m – an order of magnitude smaller than protons and neutrons), the odds of a neutrino hitting something that it can interact with are infinitesimal. Neutrinos can thus pass effortlessly through an entire planet or star with little chance of hitting anything.

And it is precisely this property of neutrinos which explains why the neutrinos from SN 1987A arrived roughly three hours before light did. These neutrinos were able to zip right out from the core of the exploding star and on out into space completely unimpeded, whereas light from the explosion had to bounce around inside the exploding star for a bit, repeatedly getting captured and re-emitted by the steller plasma. Even light, with its vast speed, takes a while to get anywhere when it is constantly being intercepted.

And it is also this same property of neutrinos which makes neutrinos so difficult to detect and study. Most neutrino detectors (but not all) consist of LARGE tanks of highly purified water, frequently located deep underground to screen out other types of particles that might produce false signals. Every once in a while, a neutrino will happen to slam into and interact with one of the particles in the tank, resulting in a cone of light being emitted, which is in turn detected by light sensors lining the walls of the tank. The shape, direction, and intensity of the light cone are used to reconstruct the properties of the detected neutrino, such as its energy and trajectory. The larger the tank, the more mass there is for incoming neutrinos to interact with, thus the greater odds that such an interaction will take place within the detector. The Kamiokande II detector had 3 kilotonnes of reaction mass.

Since the discovery of SN 1987A, larger and larger neutrino detectors have been constructed to improve our ability to study neutrinos. One of the largest in the world, IceCube, uses one million kilotonnes of the Antarctic ice sheet as its detection medium.

So what knowledge has been gleaned from these colossal detectors? For one thing, the study of solar neutrinos has revealed that that they change form. Electron neutrinos, muon neutrinos, and tau neutrinos can all transform into one another. This process, known as neutrino oscillation, implies that neutrinos have some mass, albeit a tiny amount. (Until this discovery, neutrinos were thought to be completely massless, as photons are. I’m currently working on a more detailed posting about neutrinos which digs into this topic a bit more deeply, as well as touching upon sterile neutrinos and Majorana neutrinos, and the overall history of neutrino research.) The study of solar neutrinos has also provided clues for astrophysicists regarding the fusion reactions which fuel the sun.

But the greatest insights to be gleaned from neutrino astronomy are still ahead – by improving our ability to peer into the past!

Using electromagnetic phenomena (light, IR, radio waves, UV, whatever), the Cosmic Microwave Background constitutes the greatest distance that astronomers can view. This great wall represents the point in our universe’s history some 360,000 years after the Big Bang marking the start of the Recombination Epoch. Prior to this, the universe was a dense, hot plasma which was opaque to electromagnetic radiation. The Recombination Epoch marks the creation of the first neutral atoms, bringing about a reduction in the opacity of the universe.

So, if the universe prior to this era was opaque to all EM phenomena, how can astronomers ever hope to observe beyond the CMB? Currently, there are two potential options: gravitational waves, and neutrinos.

Unfortunately, we don’t really know how to detect gravitational waves just yet. For that matter, we aren’t entirely certain that they exist. They are predicted by General Relativity, which has not let us down thus far, and we have indirect observational evidence for their existence through observations of the orbital behavior of binary neutron stars. ( Russell Hulse and Joe Taylor were awarded the Nobel Prize in Physics in 1993 for this.) Currently, several major experiments are underway to attempt to detect gravitational waves directly, including LIGOLISA and DECIGO. But, it will take some time for such efforts to bear fruit. Even once we get to the point of detecting these waves, the equipment will have to be refined to have greater sensitivity and resolution.

The other option, neutrinos, which is much further along experimentally, represents the possibility of measuring the Cosmic neutrino background, a glimpse of the universe a mere 2 seconds after the Big Bang. Pulling off such measurements will require larger and more sensitive neutrino detectors with better angular resolution. (For more on the CNB, see here and here.)

I can hardly wait to see what fruits are born by both approaches.

  1. “IAUC4316: 1987A, N. Cen. 1986”. 24 February 1987.
  2. Nomoto, Ken’ichi; Shigeyama, Toshikazu (1987). “Supernova 1987A: Constraints on the Theoretical Model”. In Minas Kafatos. Supernova 1987a in the Large Magellanic Cloud. Cambridge University Press. section 3.2 Shock propagation time. ISBN 0-521-35575-3
  3. Improved analysis of SN1987A antineutrino events. G. Pagliaroli, F. Vissani, M.L. Costantini, A. Ianni, Astropart.Phys.31:163-176,2009.

For more information, see the following:


About Glen Mark Martin

MCSE-Messaging. Exchange Administrator at the University of Texas at Austin. Unrepentant armchair physicist.
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