20-7 In 1987 a nearby supernova gave us a close-up look at the death of a massive star

Typical supernovae have peak luminosities of visible light as great as 10⁹ L_⊙, rivaling the light output of an entire galaxy. These large luminosities make it possible to see supernovae in galaxies far beyond our own Milky Way Galaxy, and indeed hundreds of these distant supernovae are observed each year. In fact, the vast majority of supernovae occur so far beyond our Galaxy that astronomers cannot tell from earlier images the type of star that exploded. However, in a handful of nearby cases, images made before the explosion have allowed astronomers to identify the star that subsequently “went supernova,” called the progenitor star. One significant case occurred in 1987.

A Supernova in the Galaxy Next Door

On February 23, 1987, a supernova was discovered in the Large Magellanic Cloud (LMC), a companion galaxy to our Milky Way some 51,500 pc (168,000 ly) from Earth. The supernova, designated SN 1987A (because it was the first discovered that year) was so bright that observers in the southern hemisphere could see it without a telescope (Figure 20-16).

Figure 20-16: R I V U X G
SN 1987A—Before and After (a) This photograph shows a small section of the Large Magellanic Cloud as it appeared before the explosion of SN 1987A. The supernova’s progenitor star was a B3 blue supergiant. (b) This image shows a somewhat larger region of the sky a few days after the supernova exploded into brightness.

(Australian Astronomical Observatory/David Malin Images)

Such a bright supernova is a rare event. In the past thousand years, only five other supernovae—in 1006, 1054, 1181, 1572, and 1604—have been bright enough to be seen with the naked eye, and all of these occurred in the Milky Way Galaxy. As the brightest supernova with modern telescopes in place, SN 1987A has given astronomers an unprecedented view of the violent death of a massive star.

The light from a supernova such as SN 1987A does not all come in a single brief flash; the outer layers continue to glow as they expand into space. For the first 20 days after the detonation of SN 1987A, its glow was powered primarily by the explosion’s tremendous heat. As the expanding gases cooled, the light energy began to be provided by a different source—the decay of radioactive atoms that were produced during the supernova explosion.

Why SN 1987A Was Unusual

Ideally, SN 1987A would have confirmed the theories of astronomers about typical supernovae. But SN 1987A was not typical. Its luminosity peaked at roughly 10⁸ L_⊙, only one-tenth of the maximum luminosity observed for typical supernovae. Fortunately, the doomed star was close enough to be observed prior to becoming a supernova, and these observations helped explain why SN 1987A was an exceptional case.

Figure 20-13 indicates that a massive star is usually in a red supergiant stage when the iron core collapses and the star becomes a supernova. The progenitor star of SN 1987A was indeed a high-mass star: Its estimated main-sequence mass was about 20 M_⊙, although by the time it exploded—some 10⁷ years after it first formed—it probably had shed a few solar masses. However, this progenitor star was identified not as a red supergiant, but as a blue B3 I supergiant.

The explanation of this seeming contradiction is that stars in the Large Magellanic Cloud, including the progenitor of SN 1987A, are Population II stars with a very low percentage of metals—that is, elements heavier than hydrogen and helium (see Section 19-5). A small difference in the amount of metals present can cause a star to follow a somewhat different evolutionary track on the H-R diagram. Apparently, when the progenitor star of SN 1987A developed an iron core and became a supernova, it was in the blue supergiant stage.

When in a blue supergiant phase, a star’s radius may be less than one-tenth as large as when it is in a red supergiant phase. Hence, for its large mass, the progenitor star was relatively small when its core collapsed (though its radius was still more than 10 times that of our Sun). This means that the star’s outer layers were held more strongly by the core’s gravitational attraction. When the detonation occurred, a relatively large fraction of the supernova’s energy had to be used against this gravitational attraction to push the outer layers into space. Hence, the amount of energy available to be converted into light was smaller than for most supernovae. This explains why SN 1987A was only one-tenth as bright as an exploding red supergiant would have been.

The Aftermath of SN 1987A

Three and a half years after SN 1987A exploded, astronomers used the newly launched Hubble Space Telescope to obtain a picture of the supernova. To their surprise, the image showed a ring of glowing gas around the exploded star. After the optics of the Hubble Space Telescope were repaired in 1994 (see Section 6-7), SN 1987A was observed again and a set of three glowing rings was revealed (Figure 20-17a).

Figure 20-17: R I V U X G
SN 1987A and Its “Three-Ring Circus” (a) This true-color view from the Hubble Space Telescope shows three bright rings around SN 1987A. (b) This drawing shows the probable origin of the rings. A wind from the progenitor star, shown in Figure 20-16a, formed an hourglass-shaped shell surrounding the star. (Compare with Figure 20-8.) Ultraviolet light from the supernova explosion ionized ring-shaped regions in the shell, causing them to glow.

(Robert Kirshner and Peter Challis, Harvard-Smithsonian Center for Astrophysics; STScI)

These rings are relics of a hydrogen-rich outer atmosphere that was ejected by gentle stellar winds from the progenitor star when it was a red supergiant, about 20,000 years ago. This diffuse gas expanded in a hourglass shape (Figure 20-17b), because it was blocked from expanding around the star’s equator either by a preexisting ring of gas or by the orbit of an as yet unseen companion star. (Figure 20-8 shows a similar model used to explain the shapes of certain planetary nebulae.) The outer rings in Figure 20-17a are parts of the hourglass that were ionized by the initial flash of ultraviolet radiation from the supernova; as electrons recombine with the ions, the rings emit visible-light photons. (We described this process of recombination in Section 18-2.)

By the early years of the twenty-first century, the explosion debris from the supernova was beginning to collide with the “waist” of the hourglass shown in Figure 20-17b. This collision is making the hourglass glow more brightly in visible wavelengths—though not enough, unfortunately, to make the supernova again visible to the naked eye—and emit copious radiation at X-ray and ultraviolet wavelengths. Because SN 1987A provides a unique laboratory for studying the evolution of a supernova, astronomers will monitor it carefully for decades to come.