A supernova explosion is a staggering event to imagine. At the core of an evolved star, runaway nuclear processes release a tremendous amount of energy in a fraction of a second. The mass of the star is blown apart at speeds of millions of miles per hour. A vast shock wave streams across interstellar space, followed by the blast wave of stellar material heated to millions of degrees. Within hundreds to thousands of years, the remains of the dense, compact star spreads into a vast nebula spanning tens of light-years: a supernova remnant.
Because the energy of a supernova is created in one place, the general shape of the explosion should be roughly spherical. However, given its energetic nature, one also expects that it will involve considerable chaotic turbulence. There are asymmetries in the star due to rotation. The star's environment may involve a companion star or an encompassing disk of gas and dust. The material in interstellar space has widely varying density and pressure. All of these factors will subtly, but distinctly, distort the motion, shape, and density of the gas in a supernova remnant.
The standard idea of a supernova remnant is a rounded shape, with strong bubbly distortions, that is filled with fractured ribbons of dense gas. Pictured above is SNR N49, which illustrates the type of structure astronomers generally expect.
That expectation makes it all the more amazing to find an almost spherical supernova remnant. The picture at the top of this blog post (click the image to enlarge it) is SNR 0509-67.5. We call it the "red bubble" because it has such a thin, soap-bubble-like appearance. The red color is from glowing hydrogen gas. The fact that it shows only small ripple-like distortions from being spherical is remarkable. The bubble has been expanding for about 400 years at more than 11 million miles per hour, and now stretches 23 light-years across. To maintain such symmetry over these vast length and time scales is a rare cosmic occurrence.
Our visualization of the red bubble is a soft expression of its shape and setting. The bubble was modelled as a semi-transparent sphere, with several thousand stars dispersed around it. The camera move is a gentle fly-in with a small shift to the left. Our intention was simply to remind the public that Hubble images are not two-dimensional postcards, but representations of a three-dimensional universe. That added depth is conveyed mainly by the stars in the visualization. Each one was cut out of the Hubble image and placed on its own small, floating billboard in the model. Having thousands of stars as parallax references allows your brain to interpret the depth in the scene effortlessly.
Finally, not to burst anoyone's bubble, but I'd like to note that the distances within this visualization are both statistical and greatly compressed. First, we don't measure each star, but assign distances randomly from a model that adequately represents a stellar distribution. Second, linear distances in the universe are vast, and using true distances creates rather sparse visuals. We employ a more logarithmic model to make the visualization more compact and enjoyable. The goal here is an expression of three dimensions for the public, and these distance adjustments are designed to enhance that message. After all, the near-spherical shape of this supernova remnant has done enough to explode our expectations.