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SYFY WIRE Bad Astronomy

Watch the Expansion of Debris Hurled Into Space by a Supernova

By Phil Plait

When you look at the image above, you may be reminded of a cell undergoing mitosis. Certainly, even if you knew it was an astronomical object, you’d be excused if you missed the idea that it’s actually one of the most catastrophic events in the Universe: a supernova.

The violence of a supernova is almost too huge to overstate. When a star explodes (an entire star! Exploding!), the energies involved crush our human perspective into dust. There are two general types of supernovae; one where the core of a massive star collapses, generates ridiculous amounts of energy, and the outer layers explode outward. The other — the kind we are concerned with here — is when a white dwarf (the dead, dense core of normal star) steals matter from a nearby companion, compresses it, and eventually explodes. In both cases, a vast amount of material, as much as an octillion tons of vaporized star-matter, is hurled outward at a significant fraction of the speed of light. This debris covers millions of kilometers in seconds, billions in hours, detonated by a blast that’s equivalent to the entire lifetime’s supply of energy from a star ignited all at once*.

We have observed literally thousands of these events, but, even for the closest, the fantastic speeds of their motions are dwarfed by their distance from us, seemingly frozen in time when you see their images.

Only, that is, if you aren’t patient. In a single image that motion is invisible, but wait a few years, and even the chilling remoteness of a galactic supernova cannot erase the motion of its debris.

And we do have the sharp eyes and glacial endurance of telescopes. In the case of the image above, the Chandra X-ray Observatory (together with radio observations from the Very Large Array in New Mexico) observed a supernova remnant over the course of several years, and when those images are put together in an animation, the expansion of the vast cloud of matter is visible. Behold!

Let that animation repeat a few times; the motion is most apparent in the outer blue ring, the glow from electrons heated to 10 million degrees Celsius by the exploded star’s shock wave. The debris itself is turbulent, bubbling away from the center, and its motion too can be seen over the decade and a half of observations.

As the animation plays, let this thought run through your brain: These observations indicate that in some places in the cloud, the debris is expanding at a numbing 5,000 kilometers per second. In the time it takes you read this paragraph, the gas will have traveled comfortably farther than the diameter of the Earth.

The star that gave up its life for these observations lies between 6,000 and 9,000 light-years from us—60,000 to 90,000 trillion kilometers—and when its light reached Earth in 1572, it was bright enough to outshine every other star in the sky, and even be visible during broad daylight. Astronomer Tycho Brahe was captivated by it, documenting his detailed observations made before telescopes were commonly used to peer into the sky. Had he been able to see its motion, he may have guessed what it was.

To me, this is thrilling. Astronomical objects are so distant and so vast that change in them seems impossible; it feels as if they will appear now as they always have, and always will. But the Universe changes at its own pace, and that evolution is perceivable by humans due to our own curiosity and sense of exploration. Despite its appearance over the puny duration of a human life span, the cosmos is neither eternal nor static. But we only notice if we’re paying attention.

P.S. This type of animation has also been done for the famous Crab Nebula, showing the debris expansion over time as well. It can even be used to determine when the star actually exploded!

*Correction, May 19, 2016: I originally described only a core collapse supernova, but this particular event was from a Type Ia, where a white dwarf explodes. My thanks to Peter Edmonds for pointing this out to me!

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