When you look at a photo of a galaxy or nebula, do you wonder what it would look like in 3-D? Astrophotos show objects in just two dimensions on the plane of the sky, but everything out there has depth. On Tuesday, astronomers presented new 3-D animations that blew my mind. The technique, developed from medical imaging technology, will open new doors to scientific discovery.The images reveal unprecedented detail in one of the nearest and youngest supernova remnants: Cassiopeia A. This nebula, X-ray hot, is the tattered, expanding remains of a massive star that exploded about 330 years ago (not including light-travel time; Cas A is about 10,000 light-years distant). As the blast wave expands outward, it sweeps up surrounding gas and dust in a shock wave, the outer blue shell at right. This violent interaction generates interesting physics, including the magnetic acceleration of charged particles to nearly the speed of light the creation of cosmic rays.
But astronomers are also keenly interested in using the remnant the way a police detective uses a crime scene: to deduce what happened. The expanding debris reveals vital clues about how the star exploded, and what the star was like before.
Using observations from NASA’s infrared Spitzer Space Telescope and Chandra X-ray Observatory, along with ground-based facilities, Tracey DeLaney (MIT Kavli Institute) and her colleagues were able to reconstruct the 3-D structure of Cas A, leading to movies and animations that let you see how the remnant would appear from different viewing angles. It’s as if we could scoot around in a starship millions of times faster than light (with no pesky effects of Einstein's relativity) to inspect Cas A from all sides, or plunge right through it.
To deduce depth as well as seeing width and breadth, the team used various observations of the clouds, knots, and filaments of different chemical compositions, along with information about the velocity of these structures as they expand outward from the explosion's center.
The resulting 3-D imagery makes it clear that the explosion had two components. The star’s outer layers, which contained most of its mass, were ejected in a spherical fashion, creating a round blast wave. But the star’s inner core was blasted out in a flattened plane, especially as high-velocity jets that may have been directed along the progenitor’s rotation axis.
“Now we have to turn this data over to the theorists who simulate supernova explosions and say, “Make this!” said DeLaney. “We don’t understand how we get both the round and flat parts.”
The technology that made this research possible was originally developed at Brigham and Women’s Hospital in Boston to help doctors create and roam around 3-D images inside the human body. Alyssa Goodman (Harvard-Smithsonian Center for Astrophysics) acquired the software and helped adapt it for astronomical purposes. She now leads a group known as the Astronomical Medicine Project. The group has created a reconstruction of a star-forming gas cloud as the first manipulable 3-D image published in a scientific journal; watch their demonstration on YouTube.
Using four Chandra images taken in 2000, 2002, 2004, and 2007, a team led by Daniel Patnaude (Harvard-Smithsonian Center for Astrophysics) made an animation showing the small but obvious expansion of Cas A from year to year. (Here are all versions, including zoom-ins.)
The series of images show the outer shock wave to be traveling 4,900 kilometers per second (11 million miles per hour). As fast as this sounds, it’s actually a bit slower than the predicted speed of 6,300 km/sec based on the explosion’s total deduced energy. Patnaude thinks that the blast wave has been dissipating energy along the way as embedded magnetic fields accelerate charged particles (cosmic rays) to nearly to the speed of light.
“These results suggest that about 35% of the energy of the supernova remnant has to go into the acceleration of cosmic rays,” says Patnaude. This deduction, along with other X-ray observations from other satellites, indicates that most of the low-energy cosmic rays that ceaselessly bombard Earth come from supernova remnants.
“This is really exciting stuff,” says Robert Petre (NASA/Goddard Space Flight Center), an expert in supernova remnants who is not part of the team. Goodman adds, “This is Harry Potteresque to me, we’re bringing still pictures to life. This is the future of electronic publishing.”
For more, see this Chandra press release.