Supernova!
A Quarter-Century Perspective on 1987a
Supernova 1987A, in the Large Magellanic Cloud, a nearby galaxy. Astronomers in the Southern hemisphere witnessed the brilliant explosion of this star on Feb. 23, 1987. Shown in this NASA/ESA Hubble Space Telescope image, the supernova remnant, surrounded by inner and outer rings of material, is set in a forest of ethereal, diffuse clouds of gas. This three-color image is composed of several pictures of the supernova and its neighboring region taken with the Wide Field and Planetary Camera 2 in Sept. 1994, Feb. 1996 and July 1997. Courtesy Hubble Heritage Team (AURA/STScI/NASA/ESA).
It had to have been quite an exciting thing for Ian Shelton and Oscar Duhalde when they first saw a brightening star on a photographic plate that hadn’t been there night before. Or, for Albert Jones of New Zealand, and Rob McNaught in Australia, who saw the same brightening and must have wondered “What??!”. In Chile, Ian stepped outside the Las Campanas Observatory in Chile to visually check that area of the sky. Sure enough, there was a hugely bright star in the Large Magellanic Cloud that wasn’t that bright the night before. All three observers had discovered the supernova of the century, named Supernova 197a. It was the last explosive gasp of the dying blue supergiant star Sanduleak -69° 202 (called the “progenitor star”), and an eye-opener for scientists studying supernovae, particularly a type called “core collapse” or Type II.
When massive stars like the one that died to form Supernova 1987a come to the ends of their lives, they have basically run out of fuel to consume in their cores. Stars begin by fusing hydrogen to helium in their cores. The result is heat and light. Eventually the star runs out of hydrogen as fuel, so it begins to fuse helium, then carbon, and so forth, until it gets to iron. At that point, fusing iron takes more energy than the process can put out, and that’s when the fusion action stops. Dead. And, there’s no way that the core can support the mass of the layers above it. So, it collapses. The outer layers collapse, too, and when they hit the core, they rebound out, forming a huge shock waves that blows everything but the core out into space. That’s what we detect as a supernova.
Hubble images show the sequence of ring expansion around Supernova 1987a. Courtesy Mark McDonald via Creative Commons Share-Alike License.
Supernova 1987a was immediately surrounded by an expanding ring of debris. Astronomers immediately began looking for that ring, and eventually the Hubble Space Telescope took images and data of it a few years later. Today, 25 years after the first detection, astronomers are still watching the debris expand. As it does, it collides with material (gas and dust clouds) that the star shed earlier in its death process. When the shock wave and expanding debris make contact with that material, everything lights up.
Supernova 1987a has given astronomers new insight into the types of stars that become Type II supernovae. For one thing at the time of Supernova 1987a’s discovery, blue supergiants were not considered likely supernova candidates for a variety of reasons. Yet, here was one exploding in a supernova. So, astronomers had to go back and re-examine their ideas and theories about these kinds of high-mass stars.
For one thing, the progenitor star, Sanduleak -69° 202, just wasn’t on people’s radar as a possible supernova candidate. It didn’t show any hints that it was about to blow itself up. That raises a lot of questions about what we know of high-mass stars and their death cycles.
A composite image of supernova 1987a taken 20 years after the explosion was first detected. Data came from NASA's Chandra X-ray Observatory and Hubble Space Telescope. The outburst was visible to the naked eye, and is the brightest known supernova in almost 400 years. This shows the effects of a powerful shock wave moving away from the explosion. Bright spots of X-ray and optical emission arise where the shock collides with structures in the surrounding gas. These structures were carved out by the wind from the destroyed star. Hot-spots in the Hubble image (pink-white) now encircle Supernova 1987A like a necklace of incandescent diamonds. The Chandra data (blue-purple) reveals multimillion-degree gas at the location of the optical hot-spots. These data give valuable insight into the behavior of the doomed star in the years before it exploded. Credit: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis
The progenitor star was a very compact and blue; not the kind of star to explode like this. So, there had to be another influence. It turns out there was more than one star involved; this system was a binary. One idea is that both the progenitor star and its companion were engulfed in an envelope of material. The companion may have dissolved in some way, and that affected the progenitor star, and helped send it down the road to supernova-hood. There are other explanations, and current and ongoing studies of the supernova remmants and the immediate neighborhood may help solve the mystery of why a blue supergiant exploded as it did.
Once the explosion DID occur, aside from the shock wave and light, there was also a huge burst of neutrinos — fast-moving particles that whiz across space. One expert estimated that 1057neutrinos were generated by the explosion, speeding away in all directions. A few of them hit Earth and were detected by the Kamioka experiment in Japan, and by detectors in Cleveland and the former Soviet Union.
All in all, only 19 neutrinos were detected from 1987a, but they told astronomers a story of core-collapse inside a massive star. They also suggest that a neutron star formed in the wake of the core collapse of the supernova 1987a progenitor star. As of today, that neutron star has yet to be observed. There are a number of reasons for that, including the formation of a black hole at the same site. Astronomers are still looking.
So, 25 years after the appearance of Supernova 1987a, there’s still something to study. The continued expansion of the shock waves and debris rings into the surrounding material in interstellar space will provide much data about the material and those interactions. The search for the neutron star (or whatever’s left of the progenitor star), continues. And, astronomers continue to use this event to bolster and tweak theories about massive stars and their ultimate ends. It’s been a fascinating quarter-century, and the data continues to flow. No doubt Hubble Space Telescope and ESA’s Herschel Space Observatory will continue to watch this object, as will the other facilities (such as Gemini Observatory) around the world. It will likely be a target for James Webb Space Telescope. So, stay tuned for new images and data to mark the 25-year mark of this cosmic event. Supernova 1987a might have exploded, but it’s not dead yet.
