How Do You Hide a Whole Supernova?

By Phil Plait

SLATE

Posted Monday, June 17, 2013, at 10:30 AM

Supernova remnant G306.3-09, as seen by Chandra, Spitzer, and ATCA. Click to explodenate.
Image credit: X-ray: NASA/CXC/Univ. of Michigan/M. Reynolds et al; Infrared: NASA/JPL-Caltech; Radio: CSIRO/ATNF/ATCA

You’d think it would be hard to hide a whole exploding star, but the galaxy has managed to do just that for 2,500 years.

Supernova remnant G306.3-0.9 somehow managed to elude discovery until 2011, when it was seen during a survey of the galaxy by the orbiting Swift observatory. Swift detects X-rays and gamma-rays, extremely high-energy forms of light emitted by the most powerful and violent events in the Universe—like exploding stars. X-rays from G306 betrayed its existence to Swift, and when astronomers noticed it, they swung other telescopes around to investigate.

The image at the top of this post is a combination of observations from the Chandra X-ray Observatory, Spitzer Space Telescope (which sees infrared light), and the Australian Telescope Compact Array (which detects radio waves).  X-rays are shown in blue, infrared in red and cyan, and radio in purple.

A wide-field view, showing the gas and dust surrounding G306, which is to the lower left. Click to embiggen.
Image credit: X-ray: NASA/CXC/Univ. of Michigan/M. Reynolds et al; Infrared: NASA/JPL-Caltech; Radio: CSIRO/ATNF/ATCA

The circular shape of the object is a big hint to its origin as an exploding star, but the presence of X-rays is the clincher. When a star explodes, it blasts out a brain-crushing amount of matter and energy. The expanding debris can outmass the Sun by a factor of 10 or more, screaming outward at a substantial fraction of the speed of light. The energy emitted in a supernova in just a few weeks is the equivalent of the Sun’s output over its entire lifetime of 10 billion years.

The violence of a supernova is almost too much to comprehend.

This is belied by the beauty and apparent serenity of the image, isn’t it? We see it frozen in time, its vast power diminished by distance. It’s only through our understanding of physics and mathematics that we can even begin to understand the forces at work here.

Swift detected gamma rays from the supernova remnant; each dot is a single gamma ray. The shape of G306 is obvious.
Image credit: NASA/Swift/Stefan Immler

The star exploded about 2,500 years ago (ignoring the time it took for the light to reach us), and the debris has expanded to about 25 light-years in diameter. That means the material was blasted outward to at least 1 percent the speed of light—and probably more, given it slows down over time as it rams through the thin material between the stars. As it slams into that gas, shock waves form in a similar way that sonic booms are created from supersonic aircraft. These shocks bounces around in the material, compressing it, and playing havoc with the magnetic fields inside. This accelerates subatomic particles to fantastic speeds, just under that of light itself, and it’s the interaction of the particles with those magnetic fields that generates the X-rays seen by Chandra and Swift.

Stars explode in our galaxy every century or so on average. We know of roughly 300 supernova remnants in our own galaxy, and G306 is one of the youngest. Some are tens of thousands of years old, and a handful older yet. Many of them are when a massive star explodes after a very short life, so they are still embedded in the gas and dust clouds from which they were born. That absorbs the fierce light, obscuring them from our view. That in turn means there are probably hundreds more supernova remnants in the Milky Way, many as young or younger than G306, that remain undiscovered.

It’s incredible that one of the singular violent events possible in the Universe can be relatively nearby yet be completely invisible. But we’re getting better at this. Our telescopes improve all the time, and we keep a sharp eye on the sky. I expect that over time, many more of these hidden cataclysms will come to light.