Hubble may have caught rare `light echo'

Photos: The space telescope snapped an eerie sequence showing the illumination of a dusty cloud around a star.

Medicine & Science

April 14, 2003|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

The Hubble Space Telescope has snapped some of its eeriest photographs ever - a sequence of images that appears to capture the opening of some kind of pale, alien rosebud.

Astronomers believe the pictures show a rare "light echo" - the gradual illumination of a dusty cloud surrounding a distant star by an expanding ripple of light that erupted in a mysterious outburst seen early last year.

"Only a handful have ever been seen in the Milky Way galaxy," said Howard E. Bond, an astronomer at the Space Telescope Science Institute in Baltimore. "I only know of three or four others in history."

A report on the discovery by an international team led by Bond, Zoltan G. Levay, Nino Panagia and William B. Sparks of the space telescope institute appears in a recent edition of the journal Nature.

The events began in January last year, when an Australian amateur astronomer looking for variable stars spotted a sudden flaring of an obscure, dim star in the constellation Monoceros.

In February, the star - called V838 Monocerotis - brightened again, reaching 10,000 times its original magnitude, almost bright enough to see with the naked eye.

At first, astronomers thought they were seeing an ordinary "nova" - an explosion in which a "white dwarf" star suddenly blasts its outer layers into space, forming a debris cloud called a planetary nebula.

Then, Arne Henden, an astronomer at the Naval Observatory in Flagstaff, Ariz., noticed that a glowing cloud had appeared around the star, and it appeared to be expanding faster than the speed of light.

Since nothing in the universe can move faster than light, scientists realized the expansion was an optical illusion. "We very quickly realized that this was not gas ejected from the star, but pre-existing material being lighted by light from the outburst - what we call a light echo," Bond said.

In a light echo, a flash of light leaves a star, traveling in all directions. A portion of that light travels directly toward Earth. But light traveling in other directions begins to bounce off the dust it encounters, illuminating it, and then beginning its own journey toward Earth.

Viewed in a sequence of photos, the spreading illumination of the dust makes it appear to expand like an inflating balloon.

Word of the rare light echo began to spread. Viewing the first pictures from ground-based telescopes, Bond realized a great deal of structure was in the dust that Hubble could reveal in unparalleled detail.

"It will tell us a lot about the star and its past history, and give us a direct way to estimate the distance to the star," he said.

Knowing the speed of light, Bond's team was able to time the apparent expansion of the light echo and calculate the star's distance from Earth - about 20,000 light-years.

"That's a huge distance, almost as large as the distance from the sun to the center of the Milky Way," Bond said. "And it's in a direction away from the center, way out on the edge of the Milky Way."

Knowing its distance also allows astronomers to compute the star's true brightness. "At that distance, and at almost naked-eye visibility, it was actually, temporarily, the brightest star in the Milky Way," he said, or 600,000 times brighter than the sun.

What remains a mystery, however, is what caused V838 Monocerotis to go off like a flashbulb.

For that, Bond said, "we need to go back to the theoretical drawing board."

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