In 1987, astronomers were treated to a spectacular supernova in the Large Magellanic Cloud (LMC), a small satellite galaxy orbiting our own.  (Sec. 24.1) Observers in Chile first saw the explosion on February 24, and within a few hours, nearly all Southern Hemisphere telescopes and every available orbiting spacecraft were focused on the object. It was officially named SN1987A. (The SN stands for "supernova," 1987 gives the year, and A identifies it as the first supernova seen that year.) This was one of the most dramatic changes observed in the universe in nearly 400 years. A 15 solar-mass B supergiant star with the catalog name SK-69° 202 exploded, outshining for a few weeks all the other stars in the LMC combined, as shown in the "before" and "after" images of Figure 21.6.
Because the LMC is relatively close to Earth and because the explosion was detected so soon after it occurred, SN1987A has provided astronomers with a wealth of detailed information on supernovae, allowing them to make key comparisons between theoretical models and observational reality. By and large, the theory of stellar evolution described in the text has held up very well. Still, SN1987A did hold some surprises.
According to its hydrogen-rich spectrum, the supernova was of Type II—the core-collapse type—as expected for a high-mass parent star such as SK-69° 202. But according to Figure 20.16 (which was computed for stars in our own galaxy), the parent star should have been a red supergiant at the time of the explosion—not a blue supergiant, as was actually observed. This unexpected finding caused theorists to scramble in search of an explanation. It now seems that, relative to young stars in the Milky Way, the parent star's envelope was deficient in heavy elements. This deficiency had little effect on the evolution of the core and on the supernova explosion, but it did change the star's evolutionary track on the H—R diagram. Unlike a Milky Way star of the same mass, once helium ignited in the core of SK-69° 202, the star shrank and looped back toward the main sequence. The star had just begun to return to the right on the H—R diagram following the ignition of carbon, with a surface temperature of around 20,000 K, when the rapid chain of events leading to the supernova occurred.
The light curve of SN1987A, shown here, also differed somewhat from the "standard" Type II shape (see Figure 21.7). The peak brightness was less than 1/10 the expected value.
For a few days after its initial detection, the supernova faded as it expanded and cooled rapidly. After about a week the surface temperature had dropped to about 5000 K, at which point electrons and protons near the expanding surface recombined into atomic hydrogen, making the surface layers less opaque and allowing more radiation from the interior to leak out. As a result, the supernova brightened rapidly as it grew. The temperature of the expanding layers reached a peak in late May, by which point the radius of the expanding photosphere was about 2  1010 km—a little larger than our solar system. Subsequently, the photosphere cooled as it expanded, and the luminosity dropped as the internal supply of heat from the explosion leaked away into space.
Much of the preceding description would apply equally well to a Type II supernova in our own Galaxy. The differences between the SN1987A light curve shown here and the Type II light curve in Figure 21.7 are mainly the result of the (relatively) small size of SN1987A's parent star. The peak luminosity of SN1987A was less than that of a "normal" Type II supernova because SK-69° 202 was small and quite tightly bound by gravity. A lot of the energy emitted in the form of |
visible radiation (and evident in Figure 21.7) was used up in expanding SN1987A's stellar envelope, so far less was left over to be radiated into space. Thus, SN1987A's luminosity during the first few months was lower than expected, and the early peak evident in Figure 21.7 did not occur. The peak in the SN1987A light curve at about 80 days actually corresponds to the "plateau" in the Type II light curve in Figure 21.7.
About 20 hours before the supernova was detected optically, a brief (13-second) burst of neutrinos was simultaneously recorded by underground detectors in Japan and the United States. As discussed in the text, the neutrinos are predicted to arise when electrons and protons in the star's collapsing core merge to form neutrons. The neutrinos preceded the light because they escaped during the collapse, whereas the first light of the explosion was emitted only after the supernova shock had plowed through the body of the star to the surface. In fact, theoretical models consistent with these observations suggest that vastly more energy was emitted in the form of neutrinos than in any other form. The supernova's neutrino luminosity was many tens of thousands of times greater than its optical energy output.
Despite some unresolved details in SN1987A's behavior, detection of this neutrino pulse is considered to be a brilliant confirmation of theory. This singular event—the detection of neutrinos—may well herald a new age of astronomy. For the first time, astronomers have received information from beyond the solar system by radiation outside the electromagnetic spectrum.
Theory predicts that the expanding remnant of SN1987A will be large enough to be resolvable by optical telescopes in a few years. The accompanying photograph was taken by the Hubble Space Telescope in late 1996. It shows the unresolved remnant (at center) surrounded by a much larger shell of glowing gas (in yellow). Scientists reason that the progenitor star expelled this shell during its red-giant phase, some 40,000 years before the explosion. The image we see results from the initial flash of ultraviolet light from the supernova hitting the ring and causing it to glow brightly. In about 10 years the debris from the explosion itself will strike the ring, making it a temporary but intense source of X-rays. In 1998, the fastest-moving ejecta already began doing so.
This overexposed photo also shows the core debris moving outward toward the ring. The four insets, taken over a 24-month period, directly resolve material expanding at nearly 3000 km/s. These images also revealed, to everyone's surprise, two additional faint rings (in red) that might be radiation sweeping across the hourglass-shaped bubble of gas. Why the gas should exhibit this odd structure is unclear.
Buoyed by the success of stellar-evolution theory and armed with firm theoretical predictions of what should happen next, astronomers eagerly await future developments in the story of this remarkable object.
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