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SYFY WIRE Bad Astronomy

30 years after a star blew itself to shreds

By Phil Plait

170,000 years ago, a titanic star detonated.

It started out its life with a mass about 20 times our Sun’s, but it was so hot and energetic that, over its lifetime, it blew off quite a bit of its mass. Around 20,000 years before its demise, it started blowing fierce amounts of matter into space, creating a weirdly shaped hourglass nebula around it: Two huge expanding lobes of gas with a denser ring of material around the waist.

Then, finally, its core ran out of fuel. It had fused hydrogen into helium, then helium into carbon, and so on, creating heavier and heavier elements in an ever-accelerating pace. When it got to iron, though, it was doomed. Iron doesn’t generate energy when it fuses, and in a complicated series of processes that lasted mere seconds, the core collapsed. Under the huge pressures and temperatures created, the result was a fantastic release of energy, enough to dwarf the combined might of all the stars in a galaxy.

The star exploded.

170,000 years later, the light from this colossal event reached Earth. On February 23/24, 1987, Ian Shelton, an astronomer in Australia, happened to spot a star in the Large Magellanic Cloud (or LMC), a companion satellite galaxy of our Milky Way. The star was not there the night before, so he contacted fellow astronomers. They followed up and confirmed it: There was a supernova in the LMC (you should really watch this wonderful Claymation reconstruction of that fateful night). The star’s light had been dimmed by the fantastic distance, but the import of the event was no less explosive to astronomers. Supernova 1987A had been born in the death of a massive star, the nearest such event in centuries, and the first in the modern era of space telescopes and digital detectors.

That night was just 30 years ago; the anniversary was last week.

To celebrate, several groups have released new images of the supernova, and they’re glorious. Before I go on, I urge you to read an article I wrote five years ago on the occasion of the 25th anniversary of this event. I explain how and why the star blew up, with more links to how it created the bizarre rings of material we see now, and how this event changed the way we looked at supernovae.

the inner ring of SN87A

The image above shows the inner dense ring of material surrounding the star that exploded. That ring, mind you, existed for 20,000 years before the star blew up! It was formed from winds blown out by the star, but it was too dim to see. When the star exploded, it released a huge flash of ultraviolet light that energized the gas in the ring, causing it to glow brightly. The gas then faded over time — I studied that fading for my Ph.D. thesis, in fact, and used it to determine how big the ring is, how dense it is, and other important physical characteristics about it.

Here's a very cool animation showing a simulation of the actual structure of the ring:

The thing about this object, though, is that it changes over time. In the images above, you can see the ring from 1994 to 2016. The supernova is the bright object in the middle. The bright spot on the ring at the lower right is a star coincidentally superimposed on the ring seen from our viewpoint. The ring itself is clumpy; when I studied it from 1990 to 1994, those clumps were fading. But then, just a few years later, they started to brighten again as fast-moving debris from the explosion reached them, slamming into them and energizing them.

You can also see the supernova debris expanding in the middle of the ring. That’s a huge amount of material, more than the mass of the Sun, expanding outward at tens of millions of kilometers per hour.

Supernovae are terrifyingly huge events.

Supernova 1987A ring in visible, radio, and X-rays

At some point, that debris will smash into the ring, destroying it over the course of a few years. In fact, the latest observations indicate some big news: The shock wave from the supernova has finally passed the inner ring, and is starting to light up the material just beyond it. This stuff has been dark since the explosion, and we know very little about it. As it brightens, we’ll learn more about it. In a sense, it’s like a time machine running in reverse; the farther away the material is, the longer ago it was ejected by the star, so as the shock wave lights it up, we’ll see farther into the star’s past.

Speaking of which ... it’s fun to see new observations of 87A. I remember when it happened; I was an undergrad at Michigan, and we were all abuzz about it. I had always been interested in supernovae, and when I got to graduate school at the University of Virginia my first research paper was on the very first supernova ever seen in another galaxy (SN 1885A, in the Andromeda Galaxy). When I signed up for my Ph.D. work to look at 87A, I was terribly excited, which took a bad turn when we got our first observations back and realized Hubble wasn’t focusing correctly.

Two years later, we had better figured out how to clean the data up, compensating somewhat for Hubble’s badly ground mirror, and new observations of the ring showed it had faded significantly. I was able to tease out very strong hints of the outer bipolar hourglass nebula from the data, barely, and then a year or so later new optics were installed on Hubble, fixing its vision. New observations of the supernova showed my work was correct; there was a huge set of rings near the star, and they were a mystery. Since that time, more objects like this have been found, though it’s still not clear how they form.

That was in the early 1990s, over 25 years ago. And now, here we are, still learning about the supernova, still pointing our most advanced telescopes at it, using our most sophisticated techniques to study it, and calling on what we’ve learned since those early days to better understand it. I’m proud to have played a small role in this, and I hope that this supernova will continue to delight and amaze astronomers -- everyone -- for many, many more decades to come.

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