Cas A 3-D Model: A Star From the Inside Out
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Cassiopeia A Comes Alive Across Time and Space
For the first time, a multiwavelength three-dimensional (3-D) reconstruction of a supernova remnant has been created. This stunning visualization of Cassiopeia A (Cas A), the result of an explosion approximately 330 years ago, uses X-ray data from Chandra, infrared data from Spitzer and pre-existing optical data from NOAO's 4-meter telescope at Kitt Peak and the Michigan-Dartmouth-MIT 2.4-meter telescope. 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 are 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 -- was used to display and manipulate the 3-D model. Commercial software was then used to create the 3-D fly-through.
Credit: NASA/CXC/MIT/T.Delaney et al.
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Cas A Movie: A Remnant Evolves
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This new movie of X-ray data from Chandra of the supernova remnant Cassiopeia A (Cas A) was made by combining observations taken in January 2000, February 2002, February 2004, and December 2007. In these images, the lowest-energy X-rays Chandra detects are shown in red, intermediate energies in green, and the highest energies in blue. Scientists have used the movie to measure the expansion velocity of the leading edge of the explosion's outer blast wave (shown in blue). The researchers find that the velocity is 11 million miles per hour, which is significantly slower than expected for an explosion with the energy estimated to have been released in Cas A.
This slower velocity is explained by a special type of energy loss by the blast wave. Electrons are accelerated to high energies as they travel backwards and forwards across the shock front produced by the blast wave. As the electrons travel around magnetic fields in the shock they lose energy by producing synchrotron emission and glowing in X-rays. Scientists think heavier particles like protons and ions are accelerated in the same way. The energy lost by these heavier particles can amount to a large fraction of the energy from the supernova explosion, resulting in a slower shock velocity. The accelerated protons and ions that escape from the remnant are known as "cosmic rays", and continually bombard the Earth's atmosphere. Supernova remnants are believed to be one of the main sources of cosmic rays.
The authors have constructed a model that combined the measured expansion velocity, as well as its observed size, with estimates of the explosion energy, the mass of the ejected material in Cas A and efficient particle acceleration. For everything to agree, about 35% of the energy of the Cas A supernova went into accelerating cosmic rays.
Credit: NASA/CXC/SAO/D.Patnaude et al.
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