Astronomers See Historical Supernova From New Angle

MAUNA KEA, HI— By observing visible “light echoes,” astronomers have assembled one of the first 3-D perspectives of a cosmic object. The new view of the supernova remnant Cassiopeia A confirms that it formed during a lopsided explosion.

“Light echoes allow us to conduct forensic studies of stars that exploded long ago,
before modern astronomical instruments became available,” said astronomer Alex Filippenko of the University of California, Berkeley. “It’s kind of like getting photographs of a crime that was committed years ago, before cameras existed.”

Filippenko and his collaborators made the light-echo measurements of Cassiopeia A, which is located about 16,000 light years from Earth, based on the familiar concept of a sound echo. If a person yells “Echo!” in a cave, the sound waves bounce off the walls and reflect back to his ears, or the ears of other people, as echoes. A similar phenomenon occurs with light.

The supernova’s light, for example, reflects off interstellar clouds of dust, creating light echoes that come toward Earth from different directions, depending on where the clouds are located.

“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,” said Armin Rest of Harvard University, the lead investigator of the project.

Most of Cassiopeia A’s light washed over the Earth about 330 years ago and is long gone. But light that took a longer path, reflecting off clouds of interstellar dust, is just now reaching the planet. This faint, reflected light is what the astronomers detected as 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 to obtain high-quality spectra of the light echoes, which are several million times fainter than the faintest stars visible to the unaided eye in dark skies. Keck’s Low Resolution Imaging Spectrometer spread out the light from each echo into its component colors, or spectrum, and from this, the astronomers were able 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 Filippenko, 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.”

Each echo comes from a spot with a different view of the explosion. The Keck spectra ultimately revealed that the gas was streaming away from the remnant in one direction at a speed of almost nine million miles per hour (or 2,500 miles per second) faster than gas moving in the other two observed directions.

Previous studies support the team’s findings. The neutron star, created when the core of the original star in Cassiopeia A collapsed, is zooming through space at nearly 800,000 miles per hour, in the opposite direction of the unique light echo. The explosion may have kicked gas one way and the neutron star out the other side, a consequence of Newton’s third law of motion, which states that every action has an equal and opposite reaction.

“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 new paper. Filippenko noted that theoretical astrophysicists will now definitely need to include asymmetries in their physical models of how massive, dying stars explode.

The results have been submitted for publication in the Astrophysical Journal.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California, and NASA. For more information please call 808.881.3827 or visit