The European Space Agency's Faint Object Camera on board NASA's Hubble Space Telescope has provided a fascinating close-up view of Supernova 1987A in the Large Magellanic Cloud.
Since they first saw the initial explosion in February 1987, astronomers world- wide have been monitoring closely the evolution of SN 1987A using both ground- based and space instrumentation. During the three and a half years following its discovery, the supernova initially increased its brightness by a factor of 100 and has since dimmed one million times in brightness from its peak intensity. This dimming has enabled astronomers to take a closer look at the supernova proper and its surroundings.
From theoretical and spectroscopic evidence, SN 1987A is known to be expanding rapidly with the outer regions of the exploding star being ejected at speeds between 2,000 and 30,000 kilometers (1,245 to 18,640 miles) per second. Hence, in the time span of three and a half years, the supernova should have reached a size of approximately one-tenth of a light year in diameter - about 100 times larger than the Solar System. Since SN 1987A is located 170,000 light-years away, its current angular size as seen from Earth is expected to be about 0.1 - 0.2 arcseconds in diameter – too small to resolve with ground-based telescopes.
A close-up of one of the first images of SN 1987A taken with the Faint Object Camera on board the Hubble Space Telescope is shown in the upper left hand frame. This image was taken in ultraviolet light at a wavelength of 275 nanometres. By comparing it with the corresponding image of an unresolved comparison star from the same exposure shown in the upper right hand frame, it can be seen that the image of SN1987A is significantly broader and clearly extended in size. Although even the resolution of the Faint Object Camera is still too coarse to reveal clearly much detailed structure, this is the first time that the exploding outer envelope of SN1987A has been photographed directly - a feat that clearly demonstrates the power of the Hubble Space Telescope, the spherical aberration of the telescope notwithstanding. The angular diameter of SN1987A measured from the images is about 0.15 arc seconds – a value very close to expectations.
A second, less magnified, image of SN1987A and its surroundings taken with the Faint Object Camera is shown in the lower frame. The central object is SN 1987A itself, and the bright objects on each side are relatively bright companion stars. The faint structures, or "halos," surrounding these brighter stars are not real, but because of the spherical aberration. This image – taken in light of twice ionised oxygen at a wavelength of 501 nanometres – reveals a curious luminescent ring surrounding SN 1987A.
The existence of this ring had been gleaned previously from both ground-based and ultraviolet space observations, but the Faint Object Camera images have provided a far clearer view of its structure. The angular separation between the ring and SN 1987A is found to average about 0.8 arc seconds, which at the distance of the Large Magellanic Cloud corresponds to three quarters of a light year. Because this distance is too large for the ring to be material ejected by the supernova explosion, astronomers speculate that it must have existed prior to the explosion in the form of a gas ring ejected and shaped by the "stellar winds" from the progenitor supergiant star in the course of some 10,000 years prior to the supernova explosion. The ring material was then ionized and heated by the intense flash of ionizing radiation emanating from the supernova. This radiation reached the ring within the first year after the explosion, and is still glowing today two and a half years later.
The presence of this ring thus provides astronomers with an important clue for determining the nature and history of the progenitor star that exploded as SN 1987A.