Observation of a supernova explosion and measurement of neutrinos reaching Earth confirmed astrophysicists’ theories of star structure and evolution.
Supernova 1987A appeared as a bright point of light in the Large Magellanic Cloud,
Supernovas are extremely rare, in large measure because of the physical conditions required to produce one. When gravitational forces in a gaseous nebula become sufficient to trigger thermonuclear reactions at the core, a star begins life in what astrophysicists call the “main sequence.” This enormous generation of energy balances the tendency of the nebula to collapse under the weight of its own mass. The core reactions begin with the lightest element, hydrogen, fusing hydrogen nuclei to form helium nuclei and releasing energy. The core of a star, thus, is a long-term exploding hydrogen bomb of gargantuan size. Eventually, when the hydrogen in the core has been consumed, the thermonuclear furnace will turn to helium, fusing its nuclei into still heavier elements. Outer portions of the star, still hydrogen-rich, will expand as core temperatures increase, and the star will become a giant.
A supernova is born at the end of a massive star’s life.
At this point in the evolution of a star, mass becomes a critical factor. A star with mass comparable to or less than that of the Sun—which is to say the great majority of stars—will expand to become a huge, tenuous, and hot nebula, with the outer layers still consuming light elements, while the core, now contracting and thus generating ever higher temperatures, turns to still heavier ones. By the time the star reaches a stage where the core begins to consume oxygen or carbon, energy generated by thermonuclear reactions will no longer be sufficient to maintain the expanding outer layers against gravitational collapse. The star will then experience a lingering death. It will become a white dwarf, with its core matter packed so densely together that its diameter will be only about that of Earth, but it will end its life as a cool, dark, carbonized “cinder.” The evolutionary cycle of an “average” star such as our Sun covers 10-15 billion years.
Stars that begin life with masses greater than about 1.4 times that of the Sun have a foreshortened life cycle and a far more dramatic denouement. Their core reactions proceed at much higher temperatures, and they consume hydrogen more quickly as a result. Stars up to about 10 solar masses may drive off huge amounts of matter into stellar nebulas in their late stages of life, thus reducing their mass. For more massive stars, however, the climax is the unparalleled fury of a supernova explosion.
In its final throes, a massive star develops a core of iron. At this point, thermonuclear reactions, rather than generating energy and fusing nuclei into still heavier elements, merely shatter the nuclear structure of iron and drain energy from the star. Surrounding shells of gas, where lighter elements continue to generate thermonuclear energy, collapse upon the core. Ever-increasing gravitational force actually squeezes electrons into nuclei, forming an almost unimaginably dense neutron core. Neutrinos, weakly interactive and possibly massless particles, are driven away from the star by this process, carrying energy with them and thus hastening gravitational collapse. The supernova explosion occurs as shells of lighter elements carom off the neutron core and collide with other, slower shells of gas collapsing upon the core.
Inasmuch as nearly all theory in nuclear physics and astrophysics developed in the twentieth century, the first sighting of a supernova in four hundred years offered researchers an unprecedented opportunity to put these models to the test. Within hours of the first sighting of Supernova 1987A (SN1987A), scientists all over the world turned their attention to it.
The first problem was to identify which star was involved. Given that supenovas are extremely hot in their early stages, they should radiate strongly in ultraviolet wavelengths. The International Ultraviolet Explorer satellite, already in orbit, confirmed quickly that the supernova had been discovered in an early stage but was cooling rapidly. Astronomers identified the exploding star as a blue supergiant of about 50 solar masses. Therefore, it was an extremely strong supernova that had not been able to eject matter into a surrounding nebula before collapsing. It was somewhat unexpected that a blue supergiant should become a supernova. Outer layers of the gas nebula presumably would exist at lower temperatures than required for a bluish luminosity and would show up instead in a reddish color. Red supergiants, in fact, were the common models for supernova events.
Astrophysical theory requires that a supernova event featuring a blue supergiant be particularly energetic, as a much larger mass of matter should be contained in the star. Astrophysicists describe supernovas that are not preceded by ejection of matter into a nebula as Type II events. These are particular rarities among observed supernovas and are of extraordinary importance. (Type I events require that a star receive matter from other stars in a multiple-star system.)
Supernovas release nearly all of their explosive energy in the form of neutrinos, weakly interactive particles possessing little or no mass. The upsurge of neutrinos from SN1987A should have been detectable on Earth. Because of their physical characteristics, almost all neutrinos reaching Earth actually pass through the planet as if it does not exist. A few, however, collide with particles. These collisions may be detected in a number of large tanks scientists have constructed deep underground, to protect their experiments from cosmic radiation, and filled with water or chlorine-rich liquids. Occasional neutrino collisions with atoms in the water generate minute flashes of light, or, in the case of chlorinated liquids, neutrino impact with a chlorine atom creates radioactive argon. Although these detectors originally were constructed merely to ascertain the existence of neutrinos in the universe, they were excellent facilities for detecting incoming neutrino bursts from SN1987A.
Neutrino detectors installed deep in an abandoned South Dakota gold mine, in a salt mine near the southern shore of Lake Erie in Ohio, in Europe, and in Japan all detected an upsurge in these rare neutrino collisions, confirming crucial aspects of astrophysical theory. Measurements of the energy possessed by neutrinos detected in these underground tanks permitted astrophysicists to reconstruct in considerable detail events in the final stages of the collapse and explosion of the blue supergiant. SN1987A is estimated to have given off more energy when it exploded than an entire galaxy of average size emits in a year. Temperatures in the core at the time of explosion must have exceeded 10 billion degrees.
As the debris of the explosion thinned in its outward rush, intense gamma radiation should have become detectable from the supernova core. Again, theory proved accurate when, in December, 1987, the gamma radiation detector on the Solar Maximum Mission satellite began to register an increase. Theory also predicts that a neutron star will remain at the center of SN1987A.
A Hubble Space Telescope view of Supernova 1987A.
As a phenomenon that confirmed a large and critically important matrix of astrophysical theories developed over the preceding century, SN1987A was an extremely significant and reassuring event. Physical models of the universe that depended heavily on these theories could be called on now as explanatory paradigms with much greater confidence.
SN1987A represented an unusual opportunity for scientists to determine whether the neutrino possesses mass. This question illustrates the many interconnections between theories of stellar evolution and cosmology. Scientists already realized that neutrinos were nearly without mass; an electron carries at least twenty-five thousand times the mass of a neutrino. Nevertheless, theory predicts that neutrinos are about 100 million times as abundant as electrons in the universe, so that if they prove to have any mass at all, neutrinos become a significant portion of the overall mass of the universe. Determining this overall mass could help to settle the most basic debate in cosmology: Is the universe destined to expand forever, or does it contain sufficient mass for gravitational forces eventually to halt the expansion and cause contraction, presumably ending in another singularity or beginning similar to the big bang event thought by most cosmologists to have given birth to the present universe?
Unfortunately, neutrino measurement experiments did not yield conclusive results. Theoretically, if the neutrino does not possess mass, all neutrinos should travel at the speed of light and, according to relativity theory, arrive at a given observation point at the same time. If they possess even a minute mass, those with more energy should arrive slightly ahead of others. All neutrino detections from SN1987A occurred within a span of less than ten seconds. Discrepancies in time of arrival could be caused by differing energy levels for particles with mass, but they could also be caused by processes of neutrino emission from the supernova, which, argues the theory, probably takes about ten seconds. Smaller differences also could have been caused by differing locations of the detectors. SN1987A added greatly to the store of data on neutrinos, but it did not enable physicists to solve the mystery of neutrino mass.
One very significant aspect of SN1987A tended to be overlooked in the excitement of the event: The sighting was in the southern celestial hemisphere. Only in the last quarter of the twentieth century have large telescopes been installed in the Southern Hemisphere in sufficient numbers to allow comprehensive observation programs. SN1987A was only about 1 percent as bright as the supernova named for Tycho Brahe in 1572. Had it exploded only one human generation earlier, instrumentation for studying the phenomenon would have been scarce and far less sophisticated. The event might have been missed altogether by astronomers in high latitudes of the Northern Hemisphere. Cave paintings and rock inscriptions by indigenous groups in the Southern Hemisphere suggest a number of important events in the southern skies in recent centuries—some perhaps supernovas—for which there is no record of observation in Europe or North America.
SN1987A quickly became one of the most intensively studied astronomical phenomena in history and will provide an additional wealth of information as later phases of the event unfold. It remained a focus of considerable research into the twenty-first century.
-
Arnett, David W., John N. Bahcall, Robert P. Kirshner, and Stanford E. Woosley. “Supernova 1987A.” Annual Review of Astronomy and Astrophysics 27 (1989): 629-700. Provides thorough scientific coverage of the event. Aimed at readers with background in astronomy, but summaries and interpretations are valuable for general readers. Includes an exhaustive scientific bibliography. -
Asimov, Isaac. The Exploding Suns: The Secrets of the Supernovas. Rev. ed. New York: Plume Books, 1996. Standard popular treatment of supernovas discusses the theory of their development and their place in the history of astronomy prior to the 1987 event. -
Genet, Russell, Donald Hayes, Donald Hall, and David Genet. Supernova 1987A: Astronomy’s Explosive Enigma. Mesa, Ariz.: Fairborn Press, 1988. Somewhat technical account organized from papers delivered at the June, 1987, meeting of the American Astronomical Society, which was devoted largely to the supernova event. Assumes some scientific background on the part of the reader. -
Kafatos, Minas C., and A. G. Michalitsianos, eds. Supernova 1987A in the Large Magellanic Cloud. New York: Cambridge University Press, 1988. Highly technical but exceptionally well-organized material from a conference at George Mason University eight months after the event. Contains papers predicting gamma-ray emissions and the light echo phenomenon in advance of their observation. -
Mann, Alfred K. Shadow of a Star: The Neutrino Story of Supernova 1987A. New York: W. H. Freeman, 1997. Recounts scientists’ search for evidence of neutrinos expelled during the supernova. Includes illustrations. -
Marschall, Laurence A. The Supernova Story. New York: Plenum Press, 1988. Highly readable account of the history of supernova observation and theory provides clear explanations for lay readers. Covers the story of Supernova 1987A in several chapters. Includes an extensive glossary of astronomical and astrophysical terms and a helpful bibliography. One of the best general works on the topic available. -
Schorn, Ronald A. “Neutrinos from Hell.” Sky and Telescope 73 (May, 1987): 477-479. Presents a synopsis of strategies for measuring neutrinos from SN1987A and their theoretical significance in reconstructing conditions in the core of the exploding star. -
_______. “Supernova 1987A After 200 Days.” Sky and Telescope 74 (November, 1987): 477-479. Provides an update on supernova observations in the first six months of 1987A. Exemplary of numerous articles on the phenomenon in this widely circulated publication. -
Verschuur, L. “The Peculiar Pulsar in Supernova 1987A.” Astronomy 17 (September, 1989): 20-26. Discussion of the vigil for the emergence of a pulsar at SN1987A and the equivocal early data showing that a pulsar may exist but with highly unusual properties. -
Woosley, Stanford E., and Mark M. Phillips. “Supernova 1987A!” Science 240 (May 6, 1988): 750-759. Excellent article provides an early summary of observational data and compares those data with established theory. Also discusses the process of identification of a specific star and describes phenomena inconsistent with or not accounted for by theory.
Organic Molecules Are Discovered in Comet Kohoutek
Inflationary Theory Explains the Early Universe
Cassinelli and Associates Discover the Most Massive Star Known
Kamiokande Neutrino Telescope Begins Operation
Tully Discovers the Pisces-Cetus Supercluster Complex
Oldest Known Galaxy Is Discovered