Astronomy Picture of the Day 3-27-03

Explanation: Nominated for most mysterious star in the Milky Way, V838 Monocerotis briefly became one of the brightest stars in our galaxy. Its outburst discovered in January 2002, observations have indicated that V838 Mon somehow transformed itself over a period of months from a small under-luminous star a little hotter than the Sun, to a highly-luminous, cool supergiant star -- defying the conventional understanding of erupting stars and stellar life cycles. In this spectacular image, one of a series recorded last year by the Hubble Space Telescope, the dimmed V838 Mon is surrounded by an "expanding" nebula. But the nebula is actually a series of light echoes from formerly unseen shells of dust up to light-years in diameter. Previously ejected, the intricate shells progressively reflect light as it reaches them from V838 Mon's outbursts. Astronomers anticipate that light echoes from farther out in the dust envelope will continue to be visible until about 2010. Some estimates now place V838 Mon about 20,000 light-years from planet Earth.

The phenomenon of the "light-echo" is one of the stranger artifacts of the extreme distances in interstellar space. The reason for this is that the light leaving the star had to travel a certain distance to reach Earth on a straight path. But the light which radiated out and reflected off the dust rings took a longer path to reach Earth. Thus the appearance of nebular expansion is really an illusion in this case.

Here's multiple images of V838 Monocerotis. You can see the change in the light echo over the intervening months.

V838 Mon is not the only example. See below for the light echoes from Supernova 1987A.

Top left is NE. Image width is about 5.5 arc min
Image and text © 1989-2002, Anglo-Australian Observatory, photograph by David Malin.

When supernova 1987A was seen to explode in the Large Magellanic Cloud, the Milky Way's nearest companion galaxy, the brilliant flash of light from the self-destructing star had taken about 170,000 years to arrive. Some light was deflected by two sheets of dust near the supernova, and is seen after the star has faded away because it covers a longer path to reach us. The dust responsible for the rings seen here lies in two distinct sheets, about 470 and 1300 light years from the supernova. The colour picture, made by subtracting images on plates taken before and after the supernova, is an accurate reproduction of the colour of the extremely faint light echo, which in turn reflects the yellow colour of the supernova when it was at its brightest, in May, 1987.

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