20-8 Neutrinos emanate from supernovae like SN 1987A

Observations of SN 1987A confirm that a core-collapse supernova emits most of its energy in the form of neutrinos

In addition to electromagnetic radiation, supernovae also emit a brief but intense burst of neutrinos from their collapsing cores. In fact, theory suggests that most of the energy released by the exploding star is in the form of neutrinos. If it were possible to detect the flood of neutrinos from an exploding star, astronomers would have direct evidence of the nuclear processes that occur within the star during its final seconds before becoming a supernova.

Unfortunately, detecting neutrinos is difficult because under most conditions matter is transparent to neutrinos (see Section 16-4). Consequently, when supernova neutrinos encounter Earth, almost all of them pass completely through the planet as if it were not there. The challenge to scientists is to detect the tiny fraction of neutrinos that do interact with the matter through which they pass.

Neutrino Telescopes

During the 1980s, two “neutrino telescopes” designed uniquely for detecting supernova neutrinos went into operation—the Kamiokande detector in Japan (a joint project of the University of Tokyo and the University of Pennsylvania), and the IMB detector (a collaboration of the University of California at Irvine, the University of Michigan, and Brookhaven National Laboratory). Both detectors consisted of large tanks containing thousands of tons of water. On the rare occasion when a neutrino collided with one of the water molecules, it produced a brief flash of light. Any light flash was recorded by photomultiplier tubes lining the walls of the tank; photomultiplier tubes simply convert any detected light into an electronic signal that can be recorded. Because other types of subatomic particles besides neutrinos could also produce similar light flashes, the detectors were placed deep underground so that hundreds of meters of Earth would screen out almost all particles except neutrinos. (The more recent Sudbury Neutrino Observatory, shown in Figure 16-6, has a similar design.)

What causes the light flashes? And how do we know whether a neutrino comes from a supernova? The key is that a single supernova neutrino carries a relatively large amount of energy, typically 20 MeV or more. (We introduced the unit of energy called the electron volt, or eV, in Section 5-5. One MeV is equal to 106 electron volts. Only the most energetic nuclear reactions produce particles with energies of more than 1 MeV.) If such a high-energy neutrino hits a proton in the water-filled tank of a neutrino telescope, the collision produces a positron (see Box 16-1). The positron is emitted at a speed greater than the speed of light in water, which is 2.3 × 105 km/s. (Such motion does not violate the ultimate speed limit in the universe, 3 × 105 km/s, which is the speed of light in a vacuum.)

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Just as an airplane that flies faster than sound produces a shock wave (a sonic boom), a positron that moves through a substance such as water faster than the speed of light in that substance produces a shock wave of light. This shock wave is called Cherenkov radiation, after the Russian physicist Pavel A. Cherenkov who first observed it in 1934. It is this radiation that is detected by the photomultiplier tubes that line the detector walls.

By measuring the properties of the Cherenkov radiation from an emitted positron, scientists can determine the positron’s energy and, therefore, the energy of the neutrino that created the positron. Determining the energy allows them to tell the difference between the high-energy neutrinos from supernovae and neutrinos from the Sun, which typically have energies of 1 MeV or less.

Neutrinos from SN 1987A

Both the Kamiokande and IMB detectors were operational on February 23, 1987, when SN 1987A was first observed in the Large Magellanic Cloud. Soon afterward, the physicists working with these detectors excitedly reported that they had detected Cherenkov flashes from a 12-second burst of neutrinos that reached Earth 3 hours before astronomers saw the light from the exploding star. Only a few neutrinos were seen: The Kamiokande detector saw flashes from 11 neutrinos at about the same time that 8 were recorded by the IMB detector. But when the physicists factored in the sensitivity of their detectors, they calculated that Kamiokande and IMB had actually been exposed to a torrent of more than 1016 neutrinos.

Given the flux of neutrinos measured by the detectors and the distance of 168,000 ly from SN 1987A to Earth, physicists used the inverse-square law to determine the total number of neutrinos that had been emitted from the supernova. (This law applies to neutrinos just as it does to electromagnetic radiation; see Section 17-2.) They found that over a 10-second period, SN 1987A emitted 1058 neutrinos with a total energy of 1046 joules. This energy is more than 100 times as much energy as the Sun has emitted in its entire history and more than 100 times the amount of energy that the supernova emitted in the form of electromagnetic radiation. Indeed, for a few seconds the supernova’s neutrino luminosity—that is, the rate at which it emitted energy in the form of neutrinos—was 10 times greater than the total luminosity in electromagnetic radiation of all of the stars in the observable universe! Such comparisons give a hint of the incomprehensible violence with which a supernova explodes.

Why did the neutrinos from SN 1987A arrive 3 hours before the first light was seen? As we saw in Section 20-6, neutrinos are produced when nuclear reactions cease in the core of a massive star and the core collapses. The neutrinos that are not absorbed around the core pass easily through the volume of the star. However, the tremendous increase in the star’s light output occurs only when the shock wave reaches the star’s outermost layers (see Figure 20-15a). For SN 1987A, it took 3 hours for this shock wave to travel outward from the star’s core to its surface, by which time the neutrino burst was already billions of kilometers beyond the dying star. Traveling toward Earth for the next 168,000 years, the neutrinos that would eventually produce light flashes in Kamiokande and IMB remained in front of the photons emitted from the star’s surface, and so the neutrinos were detected before the supernova’s light. Thus, the neutrino data from SN 1987A gave astronomers direct confirmation of theoretical ideas about how supernova explosions take place.

Kamiokande and IMB have both been replaced by a new generation of neutrino telescopes (see Section 16-4). While these new detectors are intended primarily to observe neutrinos from the Sun, they are also fully capable of measuring neutrino bursts from nearby supernovae. Astronomers have identified a number of supergiant stars in our Galaxy that are likely to explode into supernovae, among them the bright red supergiant Betelgeuse in the constellation Orion (see Figure 2-2 and Figure 6-27).

Unfortunately, astronomers do not yet know how to predict precisely when such stars will explode into supernovae. It may be many thousands of years before Betelgeuse explodes. Then again, it could happen tomorrow. If it does, neutrino telescopes will be ready to record the collapse of its massive core. However, you will not need a telescope to observe this spectacular event, as it would be about ten times as bright as the full Moon!

CONCEPT CHECK 20-14

How do high-energy neutrinos produce detectable light?