Scientists size up supernova.

Scientists size up the supernova

The explosion of supernova 1987A in the Large Magellanic Cloud (SN: 8/22/87, p.122) has given astronomers a very rare opportunity to watch the development of a supernova from close by. The detailed shape and dynamics of the exploding volume fascinate both those interested in the supernova as an astrophysical event-- the last stage in a star's existence--and those whose primary interest is in large explosions for their own sake. The nearness of the Large Magellanic Cloud offers the possibility of obtaining actual images of the explosion at various stages.

A project to do just that was undertaken by four astronomers from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., Margarita Karovska, Peter Nisenson, Costas Papaliolios and Clive Standley. The group has just calculated diameters for the explosion on two days last spring, April 2 and June 1. This is the first time a direct imaging method has accomplished this for a supernova. As the project continues, Nisenson told SCIENCE NEWS, they hope to get actual images.

Because of the turbulence of the atmosphere, it is impossible to get an image of something as small as SN 1987A by simply taking a photograph through a telescope. What one sees is a point of light that jumps around. But the Harvard-Smithsonian group's method, known as speckle interferometry, takes a lot of images, sometimes through special masks, as the point of light jumps around. A computer program searches the collection of images, called a speckle pattern, for geometric correlations, out of which it can generate either images or dimensions of the object. In this case about 30,000 frames were used.

For the two days they cite, the calculation produced some surprising diameters at various wavelengths. The diameters differ by wavelength and, of course, by day. At 450 nanometers wavelength on April 2, the supernova was 12 milliarcseconds across, which corresponds to 600 astronomical units. (One AU is the mean redius of the earth's orbit, or about 100 million miles.) At 533 nm on the same day, it measured 11 milliarcseconds or 550 AU. On June 1, at 450 nm, it was 23 milliarcseconds or 1,150 AU; at 533 nm, 18 milliarcseconds or 900 AU; at 656 nm, 8 milliarcseconds or 400 AU.

Astrophysicists would expect different diameters at different wavelengths. They believe that at different wavelengths they are seeing to different depths in the photosphere, the glowing mass of the supernova. However, the theoretical expectation is for the reverse of what's here. At lower wavelengths, the available theoretical models would predict smaller diameters than at higher ones.

At the moment there is no good theoretical explanation of the discrepancy, but Nisenson suggests that at the shorter wavelengths the observer may be seeing not the supernova, but material outside it that is illuminated by it. He believes the 656-nm figure, which represents the Lyman alpha radiation of hydrogen, is a good measure of the actual photosphere of the supernova.

On the two days calculated so far, Nisenson says, the supernova's size was just below the telescope's diffraction limit, the point at which the telescope would be optically incapable of resolving detail even if the atmosphere were not present. Later, as the supernova grows, the group hopes to get actual images. Even if they are very coarse images, only one or two pixels, Nisenson says, they ought to show whether or not the supernova's expansion is symmetrical. On the basis of how the light from the supernova is polarized, some astronomers have suggested that the expansion is asymmetric.