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Its change from Type II to Type Ib emissions is strange because the two supernova types are distinctly different. A Type II supernova happens when a lone, massive star has burned everything in its core. The furnace inside turns off and the star collapses to become a massive piston that blows most of its mass into space and compresses the core into a neutron star. A Type Ib is believed to result from a star that has somehow lost its entire hydrogen envelope, probably as a result of mass transfer in a binary system, before collapse. Because the supernovae have different origins, they emit light differently as they explode. But SN 1993J is a transition object which had lost most, but not all, of its hydrogen envelope. "The idea is that supernovae, when they go off, are surrounded by whatever materials the star emitted in the last 10,000 to 100,000 years of its life," Swartz explained. All stars have stellar winds - just as our sun has a solar wind - that carry away parts of the star's mass. Stars in binary systems can lose material to a companion. A star destined to become a Type IIb supernova will start off with perhaps 15 times as much mass as our Sun, and lose about 9 solar masses over the course of its life. When it finally explodes, the outrushing blast wave - about 2.5 solar masses' worth in the case of SN 1993J - will run into and energize the hydrogen cloak, or circumstellar medium, around the star and emit x-rays. Left: Visible and ultraviolet views of galaxy M81 (several years before the supernova) help illustrate how objects present different faces in different parts of the spectrum. This image was taken with the Ultraviolet Imaging Telescope during the Astro mission on the Space Shuttle. "Supernova 1993J is one of the few that has made the transition from one type to another," Swartz explained. Only one other supernova, SN 1987K, has been seen making such a change. To help see this "missing" hydrogen envelope, and to study its composition and shape, Swartz has been allocated 50,000 seconds (13.9 hours) of observing time with the AXAF CCD Imaging Spectrometer (ACIS), one of two principal instruments aboard AXAF (the other is a high-resolution camera). AXAF also carries two spectral gratings that will spread incoming X-rays in much the same way that a prism breaks white light into colors. Supernova 1993J is one of the few that has made the transition from one type to another," Swartz explained. Only one other supernova, SN 1987K, has been seen making such a change. Obviously, the star did not suddenly develop a white dwarf at its former core. But some other mechanism is at work. Right: The National Radio Astronomy Observatory produced this series of images showing SN1993J as it expands to a diameter of 1/10th of a light year in 18 months. ACIS actually is a two-in-one camera designed to make high-resolution images and moderate-resolution spectra of interesting X-ray sources like galaxies, pulsars, and supernovae. One CCD - a charge-coupled device, similar to those in TV camcorders - will produce images while the other measures energy levels within the objects being studied. The camera will observe an area of sky just 16.9 arc-minutes across (that' a little more than half the apparent diameter of the Moon). The images will be 2,048 by 2,048 pixels in size, thus making highly detailed images. The spectrometer will divide the X-ray spectrum into 8,192 slices - in effect, 8,192 X-ray "colors" - for precise measurements of a source's energy and chemical makeup. With ACIS, Swartz hopes to see more signatures, in X-rays, of the chemical makeup of the outrushing material, including the hydrogen that disappeared a few weeks after the blast. "It's still there," he said. "You just don't see it. Eventually it will run into the circumstellar medium and become visible again. Then the whole thing becomes transparent and just fades away." Already, SN 1993J has stirred interest among astronomers using radio and optical telescopes, and the few X-ray telescopes now in orbit have found a number of intriguing details. With AXAF, Swartz expects to take the closest look yet, in x-rays, at this oddity. |
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