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Why did the supernova "morph"? Scientists using NASA's Advanced X-ray Astrophysics Facility, launching in January, '99, think they can discover why.

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Sept. 9, 1998: Most objects in the sky can be pigeonholed into a few of the hundreds of categories that classify stars, galaxies, and other bodies. Every now and then, you get one that changes its colors - literally - and seems to beg for closer examination. That's the case with a five-year-old supernova known as SN 1993J. "It started out as a classic Type II supernova," said Dr. Doug Swartz of NASA's Marshall Space Flight Center, "with hydrogen lines in its spectrum. These weakened in a few weeks and helium lines appeared, more like a Type Ib supernova." In effect, SN 1993J changed its appearance from a supernova caused by an ordinary massive star to a supernova caused by a dense helium stellar core. To help resolve the apparent conflict, Swartz was recently selected as a guest observer who will use the Advanced X-ray Astrophysics Facility (AXAF), NASA's next Great Observatory for space astrophysics. The first two have been the Hubble Space Telescope and the Compton Gamma Ray Observatory. Both have revolutionized astronomy with their observations in the visible and highest-energy portions of the spectrum. AXAF, managed by NASA's Marshall Space Flight Center, is designed to do the same in the X-ray portion of the spectrum. AXAF is scheduled for launch on the Space Shuttle in January 1999. After a checkout phase and initial observations by the principal investigators, NASA will allocate observing time to guest investigators like Swartz. Swartz worked on the AXAF calibration, with AXAF project scientist Dr. Martin Weisskopf of NASA/Marshall, when the telescope and instruments were at Marshall's X-ray Calibration Facility. Those tests measured the instrument's sensitivities under a range of conditions so scientists will be able to adjust the data to produce a truer understanding of objects under study. NASA's NEXT Great Observatory The world's largest and finest X-ray telescope - the Advanced X-ray Astrophysics Facility (AXAF) - is scheduled for launch aboard Space Shuttle Columbia in January 1999. With AXAF, astrophysicists at NASA's Marshall Space Flight Center and around the world will observe energetic bodies ranging from quasars down to dust clouds in a quest to understand more of how and why the universe operates. To help the public understand the purpose and value of AXAF, we are running a series of stories that describe the science that AXAF will support, and the investigations that will be carried out by scientists at NASA/Marshall. Other stories in the series: How hot is the Crab?: NASA's next Great Observatory takes aim at the Crab Nebula pulsar Why did the supernova change colors? SN 1993J was seen to be one kind of massive explosion, but then seemed to morph into a distinctly different kind. Scientists using NASA's Advanced X-ray Astrophysics Facility, launching in January, 1999, think they can discover why. Looking for Pulsars in the Fast Lane Scientists are looking for bizarre, short-lived, powerhouse stars that burst with some of the brightest energy in the universe. Using AXAF, they hope to find some of the few that may exist. (this story) Left: Before-and-after visible light images of galaxy M81, taken by David McDavid of the D. Nelson Limber Memorial Observatory, show the appearance of SNR 1993J Swartz also has been studying carbon monoxide emissions from SN 1987A, the supernova that made headlines in early 1987. Carbon monoxide is the deadly gas produced from inefficient combustion in furnaces and car engines. It is also produced by stars as carbon and oxygen atoms - the "ash" from helium fusion in aging stars - which are blown into space, cool, and then collide with each other. With the rest of the astrophysics community, Swartz's interest has been drawn to SN 1993J, the 10th supernova to be discovered in 1993. It was found on March 28, 1993 by Francisco Garcia Diaz, an amateur astronomer in Lugo, Spain, in an arm of a beautiful spiral galaxy, M81, about 11 million light-years from Earth. SN 1993J is the second brightest supernova discovered since telescopes were invented; No. 1 is SN 1987A (shown at right by the Hubble Space Telescope). Sign up for our EXPRESS SCIENCE NEWS delivery 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.

SN 1993J links Neutron stars are described in a series of pages at NASA/Marshall's X-ray Astrophysics web site. Images of SN 1993J taken by the D. Nelson Limber Observatory. M81 and SN 1993J are described in an extensive web site maintained by the Students for the Exploration and Development of Space at the University of Arizona. X-ray spectral features of SN 1993J are discussed at the Johns Hopkins University. The International Supernova Network has additional details on SN 1993J The first image of SN 1993J was taken by the NF/Observatory automated telescope, but not examined until after the discovery was by Diaz. The Jodrell Bank radio observatory, which discovered pulsars in 1967, offers fact sheets on supernovae and pulsars.

AXAF links AXAF Project Science at NASA's Marshall Space Flight Center.. AXAF Public Information Server at the Harvard Smithsonian Center for Astrophysics. The AXAF Science Center will operate AXAF for the science community. NASA's AXAF web site. includes descriptions of science plans and mission operations. The ACIS instrument has two web sites, one at MIT and the other at Pennsylvania State University. TRW is building AXAF.

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