Supernova yields cosmic yardstick.

Supernova yields cosmic yardstick

Observations of remnants of supernova 1987 A by camera and spectrograph have furnished new clues to the life of a star whose catastrophic explosion -- as a supernova -- astronomers witnessed four years ago. The observations may also lead to an even greater cosmic payoff -- a more accurate determination of the distance between Earth and the Large Magellanic Cloud, the galaxy containing the burned-out star.

Astronomers say their new estimate of the distance to that galaxy represents a first step toward developing a better yardstick for the age and size of the universe, says Nino Panagia of the Space Telescope Science Institute in Baltimore. Attaining such a yardstick, he believes, might also lead to an eventual recalibration with unprecedented accuracy of the Hubble constant -- the change in the expansion velocity of the universe with distance.

Panagia and his colleagues, including scientists from Harvard University and the Space Telescope European Coordinating Facility in Garching, Germany, based their work on observations by the International Ultraviolet Explorer (IUE) satellite and Hubble Space Telescope.

Instruments aboard these craft examined nitrogen-rich gas ejected from 1987A thousands of years before this heavenly body exploded as a supernova. Most of this wispy gas envelope eventually collapsed into a ring. But that ring went undetected until the supernova explosion heated it, causing the gas to radiate ultraviolet light. Finding this ring, Panagia notes, ends a debate about the geometry of the ejected gas.

A Hubble image of 1987A (see photo), taken in August and released last week, shows the supernova core surrounded by a ring of glowing gas tilted 47[degrees] from an imaginary plane cutting through the Earth and the stellar core.

Once they glimpsed the ring, Panagia and his co-workers set about precisely calculating its distance from Earth. From Hubble data, they determined that the ring has an angular diameter of 1.66 arcseconds -- comparable to the separation of two auto headlights viewed from 100 miles away. The astronomers then measured the diameter of the ring, a second factor needed to calculate its distance from Earth.

In analyzing spectroscopic IUE data, they found that light from the edge of the ring closest to Earth reached the satellite 80 days after the first detection of the supernova explosion. Ultraviolet emissions from the ring's farthest edge did not arrive until 340 days later. This time difference allowed the researchers to calculate the ring's diameter of 1.37 light years. Using straightforward geometry, they then determined that the supernova lies 169,000 light years from Earth. Previous estimates ranged from 143,000 to 179,000 light years.

At the Cerro Tololo Inter-American Observatory in La Serena, Chile, other astronomers used a high-resolution spectrograph to deduce the speed at which 1987A's gas ring is moving out from the supernova's core. Based on this velocity and the location of a still-wispy shell of gas about 12 light years from the ring, Arlin P.S. Crotts of Columbia University in New York City and Stephen B. Heathcote of Cerro Tololo conclude that 1987A began ejecting its gas cloud at least 400,000 years before it went supernova. They also estimate that the star ended the ejection and evolved from a red supergiant to a blue supergiant some 20,000 years before it exploded.