A Quarter-Century Perspective on 1987a

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.

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.

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.

To an Ocean of Questions

Nature is a great thing to contemplate when you’re floating on the ocean in the wake of a hurricane. That’s where I was last week, in my role as an astronomy speaker on board a cruise ship. I do these trips several times a year, and usually the worst weather we get is a squall that rocks and rolls the ship for a few hours.  If I’m lucky, it happens when I’m NOT “on duty”, but occasionally I have been known to rock and roll across the stage while sharing the latest and greatest astronomy news with my fellow shipboard travelers.  In the case of this last cruise, long swells rocked the ship, and so I spent one of my talks hanging onto the podium up on the show stage.  Everybody else got to sit safely in the comfort of the show lounge seats as I took them on a tour of the cosmos.

One of the fun parts of doing these trips is answering questions from people about astronomy. Often those questions come at the end of my presentations, but they also pop up at dinner, or when I’m on a walk around the deck.

While I was on board, astronomers announced the discovery of a supernova in the galaxy M101, which lies about 21 million light-years away from Earth. I had a quick look at the early images and noticed that the galaxy’s shape had a  kind of a neat similarity to the shape of the hurricane we were avoiding. It had nothing to do with the supernova, of course, but I often notice these things because I’m just a visual type of person, I guess.

Of course, the supernova didn’t make the news we were seeing aboard the ship, since the hurricane was rocking the ratings on all the news channels we could get. But, I did have a few people ask me about the supernova as I sat down in the Internet Cafe to check mail one day.  Such questions are one reason I always spring for the Internet package aboard the ship—to check for news so that I’m not totally surprised by a fellow passenger who happens to read of an astronomy discovery before I do.

Anyway, one person asked me what the big news was about the supernova, since I had mentioned in one of my talks that supernovae have been observed not just in our own galaxy, but in others. After all, she wanted to know, don’t stars blow up all the time? I liked that she’d kept the idea of star death fresh in her mind.

This particular stellar death event was what astronomers call a Type 1a supernova. It flared up very quickly, and was as bright as a billion Suns right after its explosion began.  One of the astronomers who is working on observations of this supernova, called the stars that are involved in flareups like this one “zombie stars.”  They’re dead stars (called white dwarfs), but their cores contain the ashes of materials they’ve “burned through” earlier in their lives.  They have a gravitational pull on material from a companion star, and as they suck that material in, they come back to life and flare up in brightness, as astronomers (both professional and amateur) watched the star in M101 do.  Over the past 50 years, astrophysicists have discovered that Type Ia supernovae are part of binary systems, which are made up of two stars orbiting a common center of gravity. The one experiencing the explosive activity is the white dwarf star.

There are other types of supernovae—the Type II variety—that result from the deaths of massive stars that collapse rapidly and then explode outward. We talked a lot about those on the ship, mostly because their remnants look very cool. Think of the Crab Nebula, for example. It’s the result of a hugely massive star essentially imploding on itself and then bursting outward, flinging its material to space.  Those kinds of images really captivate people.  The Type 1a explosion took a little more explanation, but when we got through talking about it, my fellow passengers walked away with a bit of wonderment in their eyes—and satisfaction at being able to understand a little bit about the really neat things that astronomers get to study in the ocean of space.

To a large extent, that’s why I talk about astronomy to public audiences when I can, not just on cruise ships, but through this blog and my videos and other work.  It’s just plain fun to explore the ocean of space and seek to answer those big (and little) questions that pique our curiosity about the cosmos.