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By UA News Services
March 20, 2003

In January 2002, a moderately dim star in the obscure constellation Monoceros suddenly became 600,000 times more luminous than our sun, temporarily making it the brightest star in our Milky Way galaxy.

The mysterious star has since faded back to obscurity. But between late April and late October, astronomers using NASA's Hubble Space Telescope observed the most spectacular "light echo" ever seen.

Light from the explosively brightened star lit up surrounding circumstellar dust like a flashbulb lights up fog. Astronomers got detailed, color CAT scan-like views of the three-dimensional structure of dust shells surrounding the star.

They report their results tomorrow in the journal Nature.

Light echoing off circumstellar dust in our Milky Way galaxy was last seen in 1936, long before Hubble was available to study the tidal wave of light and reveal the netherworld of dusty black interstellar space.

"The light echo gives us a recording of the star's unusual eruption and allows us to follow a detective hunt to figure out how this star is evolving. Hubble's view is so sharp that we for the first time can do 'astronomical tomography' of the space around the star," says the lead observer, astronomer Howard Bond of the Space Telescope Science Institute in Baltimore.

Other authors on the report are the Zoltan G. Levay, Nino Panagia and William B. Sparks of the Space Telescope Science Institute; Arne Henden of the U.S. Naval Observatory; Sumner Starrfield of Arizona State University; R. Mark Wagner with the Large Binocular Telescope Observatory at the University of Arizona; Romano L.M. Corradi from the Isaac Newton Group of Telescopes in Canarias, Spain; and Ulisse Munari from the INAF-Osservatorio Astronomico de Padova in Asiago, Italy.

They used the Hubble images to determine that the star, called V838 Monocerotis (V838 Mon), is about 20,000 light-years from Earth. In spite of this large distance, the star released enough energy in a brief flash to illuminate surrounding dust, like a spelunker taking flash pictures of cave walls.

The star presumably ejected the illuminated dust shells in previous outbursts. Some of the light from the latest outburst travels to the dust and then is reflected to Earth. Because of this indirect path, the light arrives at Earth long after light coming directly toward Earth from the star itself.

V838 Mon did not explosively expel its outer layers like a typical nova outburst, where a normal star dumps hydrogen onto a compact white-dwarf companion star. In that case, the piled-up hydrogen spontaneously explodes by nuclear fusion -- like a titanic hydrogen bomb. Instead, V838 Mon slowly grew to an enormous size, with its surface temperature dropping to temperatures not much hotter than a light bulb.

"We don't have any other objects that really compare to this event," UA astronomer R. Mark Wagner said. "It is unique both in its evolution and the presence of the spectacular light echo."

He and ASU Regents' Professor Sumner Starrfield, a scientist with the UA's Theoretical Astrophysics Program, have found by ground-based spectroscopy that V838 Mon is actually a binary system. Only the cooler of the two stars underwent the outburst.

At its peak brightness, the star was relatively blue in color, Wagner noted. But several months later, it was much redder. That's why the rings of the light echoes in the Hubble photos appear blue at the outer edge and red on the inner edge.

Eventually, astronomers will see the light echo close entirely, as light reflected from dust behind the star finally reaches Earth, Wagner predicts.

The astronomers hope to use the light echo to more precisely determine how far away the star is, which is a key to understanding the physics of what happened, Wagner said.

"The distance is hard to estimate," he said. "But we need to pin down how far away this is to determine how much energy was involved in its outburst. Once we know how bright the outburst really was, we may discover what triggered it, amd what caused the slow ejection of the star's envelope."

The circular light-echo feature has now expanded to twice the angular size of Jupiter on the sky. Astronomers expect it to continue expanding as reflected light from farther out in the dust envelope finally arrives at Earth. Bond predicts that the echo will be observable for the rest of this decade.