EMBARGOED FOR An eruptive star that brightened to 600,000 times its initial intensity and briefly outshone all others in the Milky Way Galaxy has astronomers amazed and puzzled over what happened.
The star's light bounces off surrounding dust clouds, creating a spectacular "light echo" in a series of new images from the Hubble Space Telescope. The echo is seen to grow over time as the light races out to fresh layers of material, presumed to have been cast into space long ago by one or more eruptions of the star. The light bounces off that dust and is reflected toward Earth.
That is not the strange part.
The star, named V838 Monocerotis, has suddenly grown so big that if placed in the center of our solar system it would engulf Jupiter.
Oddly, it isn't hot and eruptive in the manner of a supernova or nova, both of which toss off outer layers in explosive fits. Instead, V838 Mon, as astronomers call it, achieved remarkable brilliance while swelling to gargantuan size and remaining cool at its surface.
"A supernova would have been much brighter than V838 Mon, so that is ruled out," said Howard Bond, a Space Telescope Science Institute researcher who led the observations. "V838 Mon was roughly as bright as an ordinary nova, but its behavior was very different."
When a nova ejects its outer layers, a hot core is exposed, Bond explained. V838 Mon did not explosively eject its outer envelope, so it remained cool throughout the event, which was observed from April to December 2002.
"In fact, at present it is one of the coolest stars known," Bond told SPACE.com.
The surface of V838 Mon is about 3,600 degrees Fahrenheit (2,000 degrees Celsius), less than half that at the surface of the Sun. The observations will be detailed in the March 27 issue of the journal Nature.
'Eye-popping'
The pictures were publicly released today, but astronomers have been cooing over them for nearly three months.
"These are eye-popping images," said Karen Kwitter, a Williams College astronomer who was not involved in the study. "I couldn't believe it at first. Astronomers are not used to seeing stars change on such a short time scale."
Scientists don't know how long ago or how often the star might have erupted, apparently filling the space around it with dust. This material is thought to be racing away from the star, but not near as fast as the fresh burst of light that illuminates the dust and continually overtakes new regions of it to create the expanding light echo.
The visible structure around V838 Mon grows from 4 to 7 light-years during the sequence [How did this happen in a matter of months?]. Bond said if the dust is expanding at 223,700 mph (100 kilometers per second), then some of it was hurled into space about 20,000 years ago.
"We are still working out the detailed structure of the dust, which will take more observations as the echoes continue to evolve," he said. "So we're not yet sure whether there are multiple shells that indicate multiple outbursts."
The star has now been observed with ground-based observatories, too, but researchers still don't know what sort of celestial animal they are dealing with. They suspect the star might have flung a relatively small portion of its outer shell into space, then expanded and cooled.
"We don't understand the outburst or its cause, Sumner Starrfield, an Arizona State University astronomer, said in a January interview at a meeting of the American Astronomical Society, where the images were previewed.
"This object got bigger and brighter and cooler, but we don't know why," Starrfield said today. "Right now we know the effects and we're trying to use the effects to determine the cause."
Second star
Starrfield and colleague R. Mark Wagner of the University of Arizona, looking at V838 Mon from the ground, did learn that it has a smaller, hotter companion. Because the smaller star is of a common type, the researchers were able to use its brightness to estimate a distance to the pair, which they put at 20,000 light-years or more.
That means the currently observed outburst actually took place 20,000 years ago, and the light from the developing echo is just now arriving here. Because of the great distance, the light diminished to the point that the star was visible from Earth only with binoculars or telescopes.
The light echo is expected to continue growing for another decade or so, Earth time, so further study is planned.
"This research will likely have significant impact on our understanding of the late phases of stellar evolution," said Phil Ianna of the National Science Foundation, which helped fund the research.
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Kwitter, the Williams College astronomer, agreed with the scientists who studied V838 Mon and determined only that it is a strange beast.
"To create an outburst as sudden and as luminous as V838 Mon's, you have to do something pretty significant to the star," Kwitter said. "Right now we have no idea what. There are some interesting theories involving binary companion interactions or planet swallowing that may turn out to be relevant, but the truth is that nobody knows yet why this happened."
Kwitter said the light echo, one of only a few that have ever been examined, may be a relatively common phenomenon that just has not been spotted very often because it doesn't last long.
"It's like a flower that blooms for 1 second," she suggested. "What are the odds that you're looking right at it the moment it opens? If you had blinked or glanced at it 5 seconds earlier or later, you'd have missed it."
Update: The Nature of Light Echoes
A reader asked, "How can the visible structure around the star grow from 4 to 7 light years in 7 months?" Very good question. We turned to Howard Bond of the STScI for an answer. Bond's response, posted here at 3:15 p.m. on March 27:
"You have hit upon the "superluminal" nature of light echoes.
"One of the properties of light echoes is that they appear to violate the limit imposed by the speed of light, but this in only a geometric illusion.
"The echo does indeed appear to expand at greater than the speed of light, due to the peculiar geometry of a light echo. The dust illuminated at the outer edge of the echo is indeed about 5 light years from the star, but this dust is WELL IN FRONT of the star, not at the same distance as the star. So, this light has only gone slightly out of its way off to the side, and was then reflected in the direction toward the Earth. The light did not travel faster than c.
"This illusion that the speed of light has been violated can be understood in more detail if you take a look at the graphic called Anatomy of a Light Echo, and consider one of the paraboloids, such as the one labeled "2". At one particular time, all of the dust along this paraboloid is illuminated, all the way down to the end of the parabola at the lower right. This illuminated material down at the end appears to lie at the largest distance from the star, as seen from Earth, but as you can see it is well in front of the star and only slightly off to the side.
"Now follow the parabola up to the vicinity of the star, and consider in particular the point where the angle from the star and then to the Earth is 90 degrees. The distance from the star to THAT point is how far the light has traveled, at the speed of light, in the plane of the sky, which as you can see is much less than the apparent radius from the star that has been reached down at the lower right, as seen from Earth.
"You could almost say that Nature is fooling us with smoke and mirrors! (Well, with interstellar dust and reflected light, to be more precise.)"
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The first images from 
