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Spitzer Captures Echo of Dead Star’s Rumblings

June 9, 2005

JPL — An enormous light echo etched in the sky by a fitful dead star was spotted by the infrared eyes of NASA’s Spitzer Space Telescope.

The surprising finding indicates Cassiopeia A, the remnant of a star that died in a supernova explosion 325 years ago, is not resting peacefully. Instead, this dead star likely shot out at least one burst of energy as recently as 50 years ago.

“We had thought the stellar remains inside Cassiopeia A were just fading away,” said Dr. Oliver Krause, University of Arizona, Tucson. “Spitzer came along and showed us this exploded star, one of the most intensively studied objects in the sky, is still undergoing death throes before heading to its final grave.”

Infrared echoes trace the dusty journeys of light waves blasted away from supernova or erupting stars. As the light waves move outward, they heat up clumps of surrounding dust, causing them to glow in infrared light. The echo from Cassiopeia A is the first witnessed around a long-dead star and the largest ever seen. It was discovered by accident during a Spitzer instrument test.

“We had no idea that Spitzer would ever see light echoes,” said Dr. George Rieke of the University of Arizona. “Sometimes you just trip over the biggest discoveries.”

A supernova remnant like Cassiopeia A typically consists of an outer, shimmering shell of expelled material and a core skeleton of a once-massive star, called a neutron star. Neutron stars come in several varieties, ranging from intensely active to silent. Typically, a star that has recently died will continue to act up. Consequently, astronomers were puzzled that the star responsible for Cassiopeia A appeared to be silent so soon after its death.

The new infrared echo indicates the Cassiopeia A neutron star is active and may even be an exotic, spastic type of object called a magnetar. Magnetars are like screaming dead stars, with eruptive surfaces that rupture and quake, pouring out tremendous amounts of high-energy gamma rays. Spitzer may have captured the “shriek” of such a star in the form of light zipping away through space and heating up its surroundings.

“Magnetars are very rare and hard to study, especially if they are no longer associated with their place of origin. If we have indeed uncovered one, then it will be just about the only one for which we know what kind of star it came from and when,” Rieke said.

Astronomers first saw hints of the infrared echo in strange, tangled dust features that showed up in the Spitzer test image. When they looked at the same dust features again a few months later using ground-based telescopes, the dust appeared to be moving outward at the speed of light. Follow-up Spitzer observations taken one year later revealed the dust was not moving, but was being lit up by passing light.

A close inspection of the Spitzer pictures revealed a blend of at least two light echoes around Cassiopeia A, one from its supernova explosion, and one from the hiccup of activity that occurred around 1953. Additional Spitzer observations of these light echoes may help pin down their enigmatic source.

Krause was lead author with Rieke of a study about the discovery appearing this week in the journal Science.

JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate. Science operations are conducted at the Spitzer Science Center, California Institute of Technology, Pasadena, Calif. JPL is a division of Caltech. Spitzer’s multiband imaging photometer, which made the new observations, was built by Ball Aerospace Corporation, Boulder, Colo.; the University of Arizona; and Boeing North America, Canoga Park, Calif. Its development was led by Rieke.

Video Animation: The Cry of Cassiopeia A — This animation begins with a stunning false-color picture of the supernova remnant Cassiopeia A. It is made up of images taken by three of NASA’s Great Observatories, using three different wavebands of light. Infrared data from the Spitzer Space Telescope are colored red; visible data from the Hubble Space Telescope are yellow; and X-ray data from the Chandra X-ray Observatory are green and blue.

Located 10,000 light-years away in the northern constellation Cassiopeia, Cassiopeia A is the remnant of a once massive star that died in a violent supernova explosion 325 years ago. It consists of a dead star, called a neutron star, and a surrounding shell of material that was blasted off as the star died. The neutron star can be seen in the Chandra data as a sharp turquoise dot in the center of the shimmering shell.

The movie then pans out to show a Spitzer view of Cassiopeia A (yellow ball) and surrounding clouds of dust (reddish orange). Here, the animation flips back and forth between two Spitzer images taken one year apart. A blast of light from Cassiopeia A is seen waltzing through the dusty skies. Called an “infrared echo,” this dance began when the remnant’s dead star erupted, or “turned in its grave,” about 50 years ago.

Infrared echoes are created when a star explodes or erupts, flashing light into surrounding clumps of dust. As the light zips through the dust clumps, it heats them up, causing them to glow successively in infrared, like a chain of Christmas bulbs lighting up one by one. The result is an optical illusion, in which the dust appears to be flying outward at the speed of light. Echoes are distinct from supernova shockwaves, which are made up material that is swept up and hurled outward by exploding stars.

This infrared echo is the largest ever seen, stretching more than 50 light-years away from Cassiopeia A. If viewed from Earth, the entire movie frame would take up the same amount of space as two full moons.

Hints of an older infrared echo from Cassiopeia A’s supernova explosion hundreds of years ago can also be seen.

The earlier Spitzer image was taken on November 30, 2003, and the later, on December 2, 2004.

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Image 1 (above): Cassiopeia A: Death Becomes Her — This stunning false-color picture shows off the many sides of the supernova remnant Cassiopeia A. It is made up of images taken by three of NASA’s Great Observatories, using three different wavebands of light. Infrared data from the Spitzer Space Telescope are colored red; visible data from the Hubble Space Telescope are yellow; and X-ray data from the Chandra X-ray Observatory are green and blue.

Located 10,000 light-years away in the northern constellation Cassiopeia, Cassiopeia A is the remnant of a once massive star that died in a violent supernova explosion 325 years ago. It consists of a dead star, called a neutron star, and a surrounding shell of material that was blasted off as the star died. This remnant marks the most recent supernova in our Milky Way galaxy, and is one of the most studied objects in the sky.

Each Great Observatory highlights different characteristics of this celestial orb. While Spitzer reveals warm dust in the outer shell about a few hundred degrees Kelvin (80 degrees Fahrenheit) in temperature, Hubble sees the delicate filamentary structures of hot gases about 10,000 degrees Kelvin (18,000 degrees Fahrenheit). Chandra probes unimaginably hot gases, up to about 10 million degrees Kelvin (18 million degrees Fahrenheit). These extremely hot gases were created when ejected material from Cassiopeia A smashed into surrounding gas and dust. Chandra can also see Cassiopeia A’s neutron star (turquoise dot at center of shell).

Blue Chandra data were acquired using broadband X-rays (low to high energies); green Chandra data correspond to intermediate energy X-rays; yellow Hubble data were taken using a 900 nanometer-wavelength filter, and red Spitzer data are from the telescope’s 24-micron detector.

Image 2: A Year in the Life of an Infrared Echo — These Spitzer Space Telescope images, taken one year apart, show the supernova remnant Cassiopeia A (yellow ball) and surrounding clouds of dust (reddish orange). The pictures illustrate that a blast of light from Cassiopeia A is waltzing outward through the dusty skies. This dance, called an “infrared echo,” began when the remnant erupted about 50 years ago.

Cassiopeia A is the remnant of a once massive star that died in a violent supernova explosion 325 years ago. It consists of a dead star, called a neutron star, and a surrounding shell of material that was blasted off as the star died. This remnant is located 10,000 light-years away in the northern constellation Cassiopeia.

Infrared echoes are created when a star explodes or erupts, flashing light into surrounding clumps of dust. As the light zips through the dust clumps, it heats them up, causing them to glow successively in infrared, like a chain of Christmas bulbs lighting up one by one. The result is an optical illusion, in which the dust appears to be flying outward at the speed of light. Echoes are distinct from supernova shockwaves, which are made up material that is swept up and hurled outward by exploding stars.

This infrared echo is the largest ever seen, stretching more than 50 light-years away from Cassiopeia A. If viewed from Earth, the entire movie frame would take up the same amount of space as two full moons.

Hints of an older infrared echo from Cassiopeia A’s supernova explosion hundreds of years ago can also be seen.

The top Spitzer image was taken on November 30, 2003, and the bottom, on December 2, 2004.

Image 3: Illustration of a Light Echo — Imagine yourself standing in a large open chamber like an aircraft hangar. If you clap your hands, you will be rewarded with a series of echoes reverberating through the building. These sound echoes are very much like a phenomenon astronomers have observed called “light echoes.”

A sound echo occurs because of two basic properties of sound: it travels at a limited speed, and it will reflect off of many surfaces. In the case of an aircraft hangar, the walls are far enough away that it takes a sound enough time to reach them, reflect, then return, that the listener can hear a noticeable delay. Often many echoes are heard as the sound reflects off of walls at different distances, returning at different times.

Light possesses the same properties of a limited speed and the tendency to reflect off of surfaces. As such, light can also produce echoes. However, since the speed of light is fantastically greater than the speed of sound, in spaces as tiny as an aircraft hangar it won’t produce a noticeable delay. If a flash bulb goes off in even the largest hangar, only the most sensitive scientific instruments could detect the tiny delay before the reflected light returns from the most distant walls.

In deep space, where distances between objects are measured in light years, astronomers can directly observe echoes from cosmic flashes of light. The sequence of events associated with such an astronomical light echo are sketched out in this illustration.

The stage is set in panel A, where a neutron star emits a brilliant burst of light. This flash travels out in all directions, but will still take years before it reaches the nearest of the surrounding dark, cold clouds of dust. Earth is towards the bottom of the page. Downward-pointing arrows in this figure indicate light rays that are headed toward Earth and the telescopes of astronomers.

In panel B, light from the flash reaches the first, closest dust cloud, heating it up. The cloud, which was too cold to detect directly, begins warmly emitting infrared light. This infrared echo travels towards Earth, though it lags behind the original flash (which got a head start) and will arrive later. This can be seen in the relative positions of the arrows.

Light from the flash continues to pass through and warm the first cloud in panel C. It has also just reached the second dust cloud and started a new echo there. Note that the arrow from the first light echo has progressed as far as the arrow from the new echo. This means the light from both of them will reach the Earth at the same time.

In panel D the flash continues expanding outward, passing through the rest of the second cloud and now warming a third cloud as well. There are several groups of arrows indicating the continuing progress of these light echoes. The numbers on these Earth-bound light rays indicate the order in which they will arrive.

The initial flash (1) will clearly arrive first. The first light echoes from all three clouds (2) will arrive next, all around the same time. Echoes from other parts of the clouds (3) will arrive last.

Panel E shows the astronomer’s view of the light echo event as seen at the three times noted above. Initially, the telescope sees only the flash from the neutron star. At this point, the cold, dark clouds are nearly invisible.

In the next time step, the light echoes begin to brilliantly light up these clouds, making them pop into view. Interestingly, all three echoes appear at the same time, even though the clouds are at different distances from the neutron star and from Earth. This is due to the particular geometry of this example; a viewer watching from some other direction would not see these echoes as simultaneous.

In the third time step, the echoes shift through the dust clouds, now lighting up different parts. This creates the illusion of small, rapidly-moving clumps of dust. In fact, there is no motion of the dust. It is only the expanding flash that brings different parts of stationary clouds into view.

This illustration is a simplified explanation of what astronomers have seen in the space surrounding the Cassiopeia A supernova remnant. Two Sptizer Space Telescope images of the region around this object, taken a year apart, reveal such a light echo. The images appear to show many filaments and blobs moving outward at the speed of light. Astronomers, however, have determined this motion is actually due to a light echo from a recent explosion on the central neutron star dating back to 1953. The original supernova happened 325 years ago.

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Spitzer Captures Echo of Dead Star8217s Rumblings Spitzer Captures Echo of Dead Star8217s Rumblings


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