Chandra X-ray Observatory image of the supernova remnant Cassiopeia A. The red, green, and blue regions in this X-ray image of Cas A show where the intensity of low, medium, and high-energy X-rays, respectively, is greatest. While this photo shows the remains of the exploded star, light echoes show us reflected light from the explosion itself.
Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.
A star that died in a supernova explosion has been resurrected by a team of forensic astronomers that has built a new 3-D view of the long-dead object using echoes of light.
Astronomers were able to assemble one of the first 3-D perspectives of the supernova remnant Cassiopeia A by observing light that is reflected off of interstellar dust scatted throughout the Milky Way.
The supernova explosion that created Cassiopeia A emitted light that reached Earth approximately 330 years ago. But, some of that light encountered celestial traffic, reflecting off clouds of interstellar dust - meaning it is just now reaching us on Earth [how light echoes work].
This faint light is what astronomers detected and used to create the new perspectives of Cassiopeia A from multiple angles.
Echoes of light
Light, like sound, can be delayed depending on the time it
takes for it to travel to the dust and reflect back. As a result, light
echoes from supernovas can reach us hundreds of years after the supernova
itself has faded.
Light, like sound, can be delayed depending on the time it takes for it to travel to the dust and reflect back. As a result, light echoes from supernovas can reach us hundreds of years after the supernova itself has faded.
The dust clouds act like mirrors, creating light echoes from different directions, depending on where the clouds are located, said Armin Rest, an astronomer from Harvard University who led the Cassiopeia A project, in a statement.
"Just like mirrors in a changing room show you a clothing outfit from all sides, interstellar dust clouds act like mirrors to show us different sides of the supernova," Rest said.
For Cassiopeia A, scientists first observed its light echoes using the Mayall 4-meter telescope at Kitt Peak National Observatory in Arizona.
They then used the 10-meter Keck I telescope on Mauna Kea in Hawaii to study the light echoes in depth and compare their component spectrums. These findings allowed the astronomers to measure the expansion speeds of the ejected gases.
"One of the big uncertainties in our understanding of how massive stars explode is whether the explosions are spherically symmetric, the same in all directions," said Alex Filippenko of the University of California, Berkeley, who conducted the supernova echo project at the Keck Observatory. "Up until now, we have had some indirect evidence for asymmetries, but our new Keck observations of light echoes directly reveal them."
Lopsided star explosion
The Keck spectra ultimately revealed that ejected
gas was streaming away from the dying star faster in one particular direction
than in others. In the fastest direction, gas was hurtling away from the dead
star at nearly 9 million miles per hour (or 2,500 miles per second) faster than
in any other direction.
The Keck spectra ultimately revealed that ejected gas was streaming away from the dying star faster in one particular direction than in others. In the fastest direction, gas was hurtling away from the dead star at nearly 9 million miles per hour (or 2,500 miles per second) faster than in any other direction.
This led scientists to believe that the explosion may have kicked gas one way and the neutron star (the shrivelled remains of a star composed entirely of neutrons) out the other side, scientists said. This would be consistent with Newton's third law of motion, that every action has an equal and opposite reaction.
Past observations have already shown that the neutron star at the heart of Cassiopeia A is zooming through space at nearly 800,000 miles per hour in the opposite direction of its light echo, researchers added.
"Now we can connect the dots from the explosion itself, to the supernova's light, to the supernova remnant," said Ryan Foley of the Harvard-Smithsonian Center for Astrophysics and co-author of the study.
Furthermore, Filippenko noted that theoretical astrophysicists will now have reason to include asymmetries in their physical models of how massive, dying stars explode.
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