Our telescopes show the Milky Way galaxy only as it appears from one vantage point: our solar system. Now, using a simple but powerful technique, a group of astronomers led by Armin Rest of Harvard University has seen an exploding star or supernova from several angles.

Image right: In this Chandra X-ray Observatory image of the supernova remnant Cassiopeia A , the red, green, and blue regions in this image show where the intensity of low, medium, and high-energy X-rays, respectively, is greatest.

“The same event looks different from different places in the Milky Way,” Rest says. ”For the first time, we can see a supernova from an alien perspective.”

The supernova left behind the gaseous remnant Cassiopeia A. The supernova’s light washed over the Earth about 330 years ago. But light that took a longer path, reflecting off clouds of interstellar dust, is just now reaching us. This faint, reflected light is what the astronomers have detected.

Image left: Three light echoes from the  Cassiopeia A supernova are shown here in three rows. For each row, the left panel is a reference image while the middle panel shows the same field of view at a later time. Right panels show the difference between the two previous shots, highlighting the (changing) light echo. The position and size of the spectroscopy slit is indicated by the rectangular overlay. For all images, north is up and east is to the left.

The technique is based on the familiar concept of an echo, but applied to light instead of sound. If you yell, “Echo!” in a cave, sound waves bounce off the walls and reflect back to your ears, creating echoes. Similarly, light from the supernova reflects off interstellar dust to the Earth. The dust cloud acts like a mirror, creating light echoes that come 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,” Rest explains.

Moreover, an audible echo is delayed since it takes time for the sound waves to bounce around the cave and back. Light echoes also are delayed by the time it takes for light to travel to the dust and reflect back. As a result, light echoing from the supernova can reach us hundreds of years after the supernova itself has faded away.

Not only do light echoes give astronomers a chance to directly study historical supernovae, they also provide a 3-D perspective since each echo comes from a spot with a different view of the explosion.

Most people think a supernova is like a powerful fireworks blast, expanding outward in a round shell that looks the same from every angle. But by studying the light echoes, the team discovered that one direction in particular looked significantly different than the others.

They found signs of gas from the stellar explosion streaming toward one point at a speed almost 9 million miles per hour (2,500 miles per second) faster than any other observed direction.

“This supernova was two-faced!” said Smithsonian co-author and Clay Fellow Ryan Foley. “In one direction the exploding star was blasted to a much higher speed.”

By combining the new light-echo measurements and the movement of the neutron star with X-ray data on the supernova remnant, astronomers have assembled a 3-D perspective, giving them new insight into the Cas A supernova.

“Now we can connect the dots from the explosion itself, to the supernova’s light, to the supernova remnant,” said Foley.

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These supernovae, called Type Ia, serve as cosmic mile markers to measure expansion of the universe because they can be seen at large distances, and they follow a reliable pattern of brightness. However, until now, scientists have been unsure what actually causes the explosions.

This NASA/Chandra X-ray Observatory animation shows two white dwarf stars merging into a supernova.

“These are such critical objects in understanding the universe,” said Marat Gilfanov of the Max Planck Institute for Astrophysics in Germany and lead author of the study that appears in the Feb. 18 edition of the journal Nature. “It was a major embarrassment that we did not know how they worked. Now we are beginning to understand what lights the fuse of these explosions.”

Most scientists agree a Type Ia supernova occurs when a white dwarf star—a collapsed remnant of an elderly star—exceeds its weight limit, becomes unstable and explodes. Scientists have identified two main possibilities for pushing the white dwarf over the edge: two white dwarfs merging or accretion, a process in which the white dwarf pulls material from a sun-like companion star until it exceeds its weight limit.

Image left: X-ray, optical and infrared composite image of galaxy M32, one of six galaxies used in a study to examine properties of Type Ia supernovas

Because these two scenarios would generate different amounts of X-ray emission, Gilfanov and Bogdan used Chandra to observe five nearby elliptical galaxies and the central region of the Andromeda galaxy. A Type 1a supernova caused by accreting material produces significant X-ray emission prior to the explosion. A supernova from a merger of two white dwarfs, on the other hand, would create significantly less X-ray emission than the accretion scenario.

The scientists found the observed X-ray emission was a factor of 30 to 50 times smaller than expected from the accretion scenario, effectively ruling it out. This implies that white dwarf mergers dominate in these galaxies.

“Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging,” said co-author Akos Bogdan, also of Max Planck. “This is probably not what many astronomers would expect.”

“To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appeared to exist,” said Gilfanov. “Now this path to supernovae will have to be investigated in more detail.”

NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

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“We think that radio observations will soon be a more powerful tool for finding this kind of supernova in the nearby Universe than gamma-ray satellites,” said Alicia Soderberg of the Harvard-Smithsonian Center for Astrophysics.

Image Right: This artist’s conception shows an “engine-driven” supernova explosion with accretion disk and high-velocity jets.

The telltale clue came when the radio observations showed material expelled from the supernova explosion, dubbed SN2009bb, at speeds approaching that of light. This characterized the supernova, first seen last March, as the type thought to produce one kind of gamma-ray burst.

“It is remarkable that very low-energy radiation, radio waves, can signal a very high-energy event,” said Roger Chevalier of the University of Virginia.

When the nuclear fusion reactions at the cores of very massive stars no longer provide the energy needed to hold the core up against the weight of the rest of the star, the core collapses catastrophically into a superdense neutron star or black hole. The rest of the star’s material is blasted into space in a supernova explosion. For the past decade or so, astronomers have identified one particular type of such a “core-collapse supernova” as the cause of one kind of gamma-ray burst.

Not all supernovae of this type, however, produce gamma-ray bursts. “Only about one out of a hundred do this,” according to Soderberg.

Image left: This artist’s conception shows a normal core-collapse supernova explosion expelling a nearly-spherical debris shell. (Artwork by Bill Saxton)

In the more-common type of such a supernova, the explosion blasts the star’s material outward in a roughly spherical pattern at speeds that, while fast, are only about 3 percent of the speed of light. In the supernovae that produce gamma-ray bursts, some, but not all, of the ejected material is accelerated to nearly the speed of light.

The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. Material falling toward the core enters a swirling disk surrounding the new neutron star or black hole. This accretion disk produces jets of material boosted at tremendous speeds from the poles of the disk.

Image right: Initial radio telescope data showing the supernova SN 2007gr.

“This is the only way we know that a supernova explosion could accelerate material to such speeds,” Soderberg said.

Until now, no such “engine-driven” supernova had been found any way other than by detecting gamma rays emitted by it.

Soderberg and Chevalier worked with Alak Ray and Sayan Chakrabarti of the Tata Institute of Fundamental Research in India; Poonam Chandra of the Royal Military College of Canada; and a large group of collaborators at the Harvard-Smithsonian Center for Astrophysics. The scientists reported their findings in the January 28 issue of the journal Nature. The supernova was first discovered in optical images by Guiliano Pignata of the Chilean Automatic Supernova Search (CHASE).

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