Click on the image for the movie
For the first time, a multiwavelength three-dimensional reconstruction of
a supernova remnant has been created. This stunning visualization of
Cassiopeia A, or Cas A, the result of an explosion approximately 330 years
ago, uses data from several telescopes: X-ray data from NASA's Chandra
X-ray Observatory, infrared data from NASA's Spitzer Space Telescope and
optical data from the National Optical Astronomy Observatory 4-meter
telescope at Kitt Peak, Ariz., and the Michigan-Dartmouth-MIT 2.4-meter
telescope, also at Kitt Peak. In this visualization, the green region is
mostly iron observed in X-rays. The yellow region is a combination of
argon and silicon seen in X-rays, optical, and infrared—including
jets of silicon—plus outer debris seen in the optical. The red
region is cold debris seen in the infrared. Finally, the blue reveals the
outer blast wave, most prominently detected in X-rays.
Most of the material shown in this visualization is debris from the
explosion that has been heated by a shock moving inwards. The red material
interior to the yellow/orange ring has not yet encountered the inward
moving shock and so has not yet been heated. These unshocked debris were
known to exist because they absorb background radio light, but they were
only recently discovered in infrared emission with Spitzer. The blue
region is composed of gas surrounding the explosion that was heated when
it was struck by the outgoing blast wave, as clearly seen in Chandra
images.
To create this visualization, scientists took advantage of both a
previously known phenomenon—the Doppler effect—and a new
technology that bridges astronomy and medicine. When elements created
inside a supernova, such as iron, silicon and argon, are heated they emit
light at certain wavelengths. Material moving towards the observer will
have shorter wavelengths and material moving away will have longer
wavelengths. Since the amount of the wavelength shift is related to the
speed of motion, one can determine how fast the debris are moving in
either direction. Because Cas A is the result of an explosion, the stellar
debris is expanding radially outwards from the explosion center. Using
simple geometry, the scientists were able to construct a 3-D model using
all of this information. A program called 3-D Slicer—modified for
astronomical use by the Astronomical Medicine Project at Harvard
University in Cambridge, Mass.—was used to display and manipulate
the 3-D model. Commercial software was then used to create the 3-D
fly-through.
The blue filaments defining the blast wave were not mapped using the
Doppler effect because they emit a different kind of
light—synchrotron radiation—that does not emit light at
discrete wavelengths, but rather in a broad continuum. The blue filaments
are only a representation of the actual filaments observed at the blast
wave.
This visualization shows that there are two main components to this
supernova remnant: a spherical component in the outer parts of the remnant
and a flattened (disk-like) component in the inner region. The spherical
component consists of the outer layer of the star that exploded, probably
made of helium and carbon. These layers drove a spherical blast wave
into the diffuse gas surrounding the star. The flattened
component—that astronomers were unable to map into 3-D prior to
these Spitzer observations—consists of the inner layers of the star.
It is made from various heavier elements, not all shown in the
visualization, such as oxygen, neon, silicon, sulphur, argon and iron.
High-velocity plumes, or jets, of this material are shooting out from the
explosion in the plane of the disk-like component mentioned above. Plumes
of silicon appear in the northeast and southwest, while those of iron are
seen in the southeast and north. These jets were already known and Doppler
velocity measurements have been made for these structures, but their
orientation and position with respect to the rest of the debris field had
never been mapped before now.
This new insight into the structure of Cas A gained from this 3-D
visualization is important for astronomers who build models of supernova
explosions. Now, they must consider that the outer layers of the star come
off spherically, but the inner layers come out more disk-like with
high-velocity jets in multiple directions.