Supernova 1987A: The Three Ring Circus

Week of March 6, 2000

Last week's Snack talked about why the star that was to become Supernova 1987a (what astronomers call the precursor) exploded. I concentrated on the core of the star, because for supernovae that's where the action is. I mentioned the outer layers a few times because they play their role in the explosion too: they provide the pressure for the core so it can fuse elements, and in the end it's the ejection of those outer layers that creates the supernova event.

I didn't talk much about what those outer layers are doing while the core is going through its evolutionary gyrations. As it happens, for me, this is the most interesting part. It has also led to all sorts of new ideas about supernovae and how they behave long after the explosion.

Again, my apologies. This snack is longer than usual. But these topics beg for a somewhat more detailed explanation than usual. Please, again, bear with me!

Once upon a time, say a million years or so ago, there was a star that would later be known as Sanduleak -69 202 (named after the man who catalogued stars in that region, and appended with its rough coordinates). The star was a normal enough star, much like the Sun, but with perhaps 20 or so times the Sun's mass. A star like the Sun can go for billions of years merrily fusing hydrogen into helium, but a star with more mass fuses hydrogen faster, and uses up its fuel even faster. After only a short life, the hydrogen in the core ran out, and helium fusion began. Sanduleak -69 202 didn't know it, but it was already doomed.

As helium piles up in the core, it gets very hot. That heat gets dumped into the outer parts of the star. Remember, a star is gas, and when you heat a gas it expands. So the star suddenly gets much bigger. It swells up to a hundred times its previous size, becoming a supergiant. Even though more heat is being generated in the core, the outer layers actually cool, because when they expand so much they have a much larger surface area. If the radius expands by a 100 times, the surface area expands by 10,000 times! Even if the core generates 1000 times as much energy as it did before, the star surface has expanded so much that each square centimeter actually only needs to give off one-tenth as much energy as before. So to an outside observer, the star cools and is what we call a red supergiant.

Nothing last forever, especially a star. When enough helium piles up in the core, it fuses suddenly, which ironically generates less heat. The outer layers aren't getting as much energy from the core, and they contract. Since the surface area has now decreased, the surface heats up again and the star becomes a blue supergiant.

This cycle may repeat itself, but as I said last week, as heavier elements fuse, the cycle repeats faster, and the outer layers may not have time to keep up. Usually a star becomes a red, then a blue, then a red supergiant again when iron fusion starts and the star explodes.

When a star is a red supergiant, it doesn't just sit there. It gives off a slow, dense wind of material, like a super solar wind. This wind is usually not visible from Earth because the star isn't bright enough to light it up well; you need ultraviolet light to do that, and red stars don't give off much UV. The wind may have a shape, too. If the star is alone, or not spinning quickly, the wind may be a giant sphere. If the star spins, or perhaps has a companion star that it orbits, the wind may look more like a squashed sphere, like a basketball with someone sitting on it (this is called an oblate spheroid, for those keeping track).

When the star becomes a blue supergiant, the wind becomes much less dense, but may get a boost in speed by a factor of ten. The fast wind catches up and slams into the slow wind, and compresses it, like a snow shovel scooping up snow. Depending on the geometry, you can get all sorts of fascinating and beautiful shapes from this. This type of object is called a planetary nebula (or PN for short), not because they have planets, but because they look disk-like and green through small telescopes, like the planet Uranus. You can find a gallery of PN pictures at the Hubble planetary nebula picture archive. You'll see that many of the objects are round, and some have intricately detailed structures.

This happened too with Sanduleak -69 202. The old red supergiant wind, being oblate, was less dense along the poles and more so along the equator. The fast wind penetrated the slow more easily along the poles, and the resulting shape was an hourglass figure. The winds met most fiercely at the equator of the hourglass, and the pileup there was the densest. For reasons still unknown, two more rings were formed along the equator of each lobe of the hourglass. They are larger but less dense then the middle ring.

So the situation stayed for ten or perhaps 20,000 years. Then-- bang! Sanduleak -69 202 exploded, giving birth to Supernova 1987A. A torrent of ultraviolet photons sleeted out from the catastrophe, lighting up the rings. Hubble made the first high-resolution images of the event, and clearly showed the inner ring. The outer rings were too faint to be easily seen (remember, this was when Hubble's optics had not yet been corrected to their present clarity), but after a few years we were able to attain enough images to see the dimmer rings. The image to the left is the early Hubble image, and the one on the right is a later image from after the fix that shows the rings more clearly.

That was in 1990. I had just finished my Masters degree studying-- surprise!-- planetary nebulae, and was eager to begin my PhD work. Roger Chevalier, a supernova astrophysicist, took me on to work on the images of the ring around SN1987A. I learned quite a bit about them, and I will be happy to share what I learned with you, my reader... next week. ;-)

For more detailed info (if you really need it!) take a look at Nick Strobel's Astronomy Notes website. That link takes you directly to his page about stellar evolution and planetary nebulae.