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All about SN 1987A
by investigating astronomer Nino Panagia

Supernova 1987A exploded on 1987 February 23, in the Large Magellanic Cloud. Because of its relative proximity to us (a mere 168,000 light years) SN 1987A is by far the best-studied supernova of all time. Immediately after the discovery was announced, literally every telescope in the southern hemisphere started observing this exciting new object.

In addition to light, particle emission was detected from the supernova. "Kamiokande II" is a neutrino telescope whose heart is a huge cylindrical tub, 52 feet in diameter and 53 feet high, containing about 3000 metric tons of water; it is located in the Kamioka mine in Japan, 3,300 feet underground. On February 23, around 7:36 am Greenwich time, the Kamiokande II recorded the arrival of 9 neutrinos within an interval of 2 seconds, followed by 3 more neutrinos 9 to 13 seconds later. Simultaneously, the same event was revealed by the IMB detector (located in the Morton-Thiokol salt mine near Faiport, Ohio), counted 8 neutrinos within about 6 seconds. A third neutrino telescope (the "Baksan" telescope, located in the North Caucasus Mountains of Russia, under Mount Andyrchi) also recorded the arrival of 5 neutrinos within 5 seconds from each other. This makes a total of 25 neutrinos detected on Earth, out of the 10 billions of billions of billions of billions of billions of billions of them produced in the explosion! Neutrinos are elusive particles of very small (possibly zero) mass and very high energy, which are produced in huge quantities in the supernova explosion of a massive star. They interact so infrequently with ordinary matter that almost all of them of them can travel through the entire diameter of the Earth without being stopped; so they are extremely difficult to detect. Nevertheless, a little more than two dozen neutrinos was more than enough to understand what was going on. And, in fact, the detection of those neutrinos was a perfect confirmation of the theoretical expectations for the core collapse of a massive star. The core-collapse process is believed to be the cause of the explosions of massive stars at the end of their lives, and SN 1987A provided strong experimental confirmation of this idea.

Unfortunately, the Hubble Space Telescope was not yet in operation when the supernova exploded, since it was not launced until April 1990. The first images of SN 1987A, taken with the ESA Faint Object Camera on August 23-24, 1990, revealed the inner circumstellar ring in all its "glory" and detail.

close up of rings
As shown in the Heritage image, the supernova remnant (the almost star-like object in the center) is surrounded by inner and outer ring structures, with the whole object set within huge clouds of diffuse emitting gas. This three-color image was constructed from several Hubble images of the supernova and its surroundings, taken over several years and through five different color filters (B, V, R, ionized oxygen, and hydrogen-alpha) that blend together for a breathtaking view.

to the image display page
Space Telescope Science Institute astronomers Nino Panagia and Martino Romaniello (who is currently at the European Southern Observatory), with cooperation from the principal investigator of the Hubble program on SN 1987A, Robert Kirshner (Harvard-Smithsonian Center for Astrophysics), graciously donated their black-and-white mosaics in the individual filters to the Hubble Heritage Project. From these the Heritage team constructed the color image which are released this month in honor of the 12th anniversary of the explosion.

Since 1990, Hubble has kept an attentive eye on SN 1987A, obtaining both imaging and spectrographic observations at least once a year. The results of this ongoing study include:

  • The sequence of images obtained over more than 8 years has allowed astronomers to measure the expansion of the supernova material directly: this the first time this has been possible, and it is leading to new understanding of the explosion phenomenon.

    See press release Pun & Kirshner

  • The origin and the nature of the beautiful circumstellar rings are still a mystery. They have been measured to expand rather slowly, "only" 70,000-100,000 miles per hour (this is considered slow because the supernova material in the center is expanding outward at speeds that are 100-2000 times higher!). Spectroscopic observations show that the rings are enriched in the element nitrogen. Both the slow speeds and the unusual composition show that the rings were expelled from the progenitor star when it was a red supergiant, more than 20,000 years before that star exploded as a supernova. However, one would have expected such a star to eject material in a more regular fashion, steadily expelling material in all directions, rather than puffing rings like a pipe smoker.

    Another puzzle is that the observations of the star just prior to the explosion show that it was a blue supergiant. This was a puzzle in 1987, because up to that time theorists had believed that only red supergiants could explode as a supernova. Apparently the star was, until relatively recently, indeed a red supergiant, but over the millennia before the explosion, it shrank in size and its surface heated up gradually.

    See press release Burrows et. al.

  • The superb resolution of HST has provided an accurate measurement of the apparent angular size of the inner circumstellar ring. The absolute size was determined by studying the observations taken one of Hubble's smaller predecessor satellites, the International Ultraviolet Explorer. IUE measured the time interval between the supernova explosion and the time the inner ring brightened up to be 0.66 years. This means that the diameter of the ring is 1.32 light years. By comparing the angular and true sizes, we find the distance to SN 1987A (and thus to the Large Magellanic Cloud) to be 168,000 light years. This result is fundamental because it permits astronomers to calibrate the luminosity of the Cepheid variable stars in the LMC. Then, knowing how bright Cepheids are, we can measure the distances to many other galaxies, and thus measure the size, expansion rate, and age of the Universe.

SN 1987A is located at the edge of the Tarantula Nebula in the LMC,

a region in which stars have been forming very actively over the last 20 million years, and up through the present. The study of the neighborhood of SN 1987A thus offers a unique opportunity to place the supernova explosion in the context of stellar evolution and the evolution of entire populations of stars.

to the Ango-Australian Observatory pageTaking advantage of the observations made with the Hubble's Wide Field and Planetary Camera 2 (WFPC2) over the past several years, Panagia, Romaniello and collaborators have studied the stellar population within 90 light years of SN 1987A. Within that volume they have detected more than 20,000 stars, which reveal this volume of space has been forming stars in several episodes between 1 and 150 million years ago. These stellar youngsters are superposed on a fainter stratum of older stars, whose ages are between 0.5 and six billion years.

The dozen bright blue stars around the supernova are massive stars, each weighing more than six times as much as our Sun. With an age of about 12 million years, they are members of the same generation of stars that gave birth to the supernova progenitor.

In this region there are also more than 500 fainter stars of approximately the same mass as the Sun, created at about the same time as the supernova progenitor. The whole region also contains brightly gas clouds, from which stars are still actively being formed. As an illustration, we have identified various types of young stars in the three areas marked in the following figure.

leads to ~200 Kb star maps

(Clicking on each region leads to a large, ~200 Kilobyte, file.)

In each of these regions, the hottest and most massive (larger than six times our Sun) stars have bluish colors and have been enclosed in blue pentagons, stars with intermediate masses (say, between 2 and 6 times the Sun) are enclosed with green squares, and some of even less massive stars (less than twice the mass of the Sun) of the same generation are recognizable within the red circles used to denote them.

  • As time goes on, the high-velocity material emerging from the supernova explosion will overtake and crash into the slower-moving surrounding rings. Recent observations with Hubble's Space Telescope Imaging Spectrograph (STIS) show that this collision is starting to occur already.

  • In less than a decade the full force of the supernova fast material will hit the inner ring, heating and exciting its gas and producing a new series of cosmic fireworks that will offer a spectacular view for several years thereafter. But this will be the "beginning of the end" because in about another century most, if not all, the material in the rings will be swept away and disappear, loosing their identities and merging into the general interstellar medium of the Large Magellanic Cloud. This is not a complete loss, however, because by studying this destructive process, astronomers of the 21st century will be able to probe the ring material with a detail and accuracy never before possible.