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V838 Mon Bursts Into Light

A leading candidate for the most mysterious star found in recent times is variable star V838 Monocerotis. At a distance of about 8,000 light-years, V838 Mon was discovered to be in outburst in January of this year. Initially thought to be a familiar type of classical nova, astronomers quickly realized that instead, V838 Mon may be a totally new addition to the astronomical zoo. Observations indicate that the erupting star 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 undergoing rapid and complex brightness changes. The transformation defies the conventional understanding of stellar life cycles. A most notable feature of V838 Mon is the "expanding" nebula which now appears to surround it. Seen above in two separate images from the South African Astronomical Observatory's 1 meter telescope, the nebula is probably a light echo from shells of formerly unseen material lost by the star during its previous evolution. Light-years in diameter, the shells progressively reflect the light from V838 Mon's outbursts, providing an opportunity to look back at the history of this remarkable star's behaviour. Credit: Lisa Crause (Univ. Cape Town), Warrick Lawson (Australian Defence Force Academy)
Cape Town - Dec 06, 2002
A highly unusual new variable star has attracted a great deal of attention amongst the astronomical community in recent months: the star has significantly changed its appearance during this time.

Variable stars, as their name suggests, are stars that vary in brightness. With some stars, this dimming and brightening can occur periodically on timescales from a few seconds to many years; others vary in brightness on an irregular or once-off basis.

This eccentric new star, however, has changed completely in appearance over the last few months. "Normally, similar astronomical events happen on an enormously long timescale so one does not often get to witness significant changes in individual objects during the course of a human lifetime; that is what makes this so exciting", says University of Cape Town PhD student Lisa Crause, working at the South African Astronomical Observatory (SAAO).

The highly unusual star, now known as V838 Mon -- one of the several hundred variables in the constellation Monocerotis (the mythological unicorn) -- has attracted enormous attention since its appearance earlier this year.

The object was first noticed by an amateur astronomer in Australia. Professional astronomers cannot spend valuable telescope-time scanning the sky for such objects. But once alerted, astronomers working at large observatories around the world can carefully monitor and study these newly identified objects.

V838 Mon has evolved dramatically since its discovery nine months ago. First, it brightened about a hundredfold, then it faded slowly before erupting to around ten thousand times its original brightness. As V838 Mon started fading after this last outburst, a light-echo appeared and continues to expand around the star.

This echo is due to light from the eruption bouncing off dust surrounding the star and getting scattered towards us. If we assume that these are dust shells centred on the star, we can estimate V838 Mon's distance from the rate at which the echo has grown. In this way, we calculate that the star is about 7500 light years away, which means that the outbursts that we have been following, actually took place 7 500 years ago and the news has only just reached us! The light-echo provides more clues about the nature of this unusual star. Images obtained with the Hubble Space Telescope in April show several rings and arcs of material surrounding the star, maybe indicating that these sorts of outbursts have been happening roughly every century for the past thousand years.

Unlike most other sciences, astronomy can only gather data by analysing collected light: one cannot get a spoonful of star! "We have to make the most of the information that the light of different energies provides," says Crause. "Everything we know about the Universe comes from analysing these light signals".

When the star experienced a less violent eruption in March this year, data obtained with the SAAO's 1.9-m telescope in Sutherland showed that another shell of material was ejected during this time. Lower energy light gathered with another SAAO telescope indicates that a new dust cocoon condensed around V838 Mon in April and this caused the star to suddenly fade from view. Another consequence of this dust obscuration is that the star now looks much redder than it used to, in the same way that sunsets are redder when the air is polluted.

As the star remains faint, it is quite difficult to observe with the telescopes currently available.

Fortunately, we have the services of the Southern African Large Telescope (SALT) to look forward to in years to come. This giant 11-m light bucket will give astronomers the advantage of more than thirty times the light gathering area of the 1.9-m, the largest telescope in Sutherland at this stage.

"While we do not yet understand all the mechanisms responsible for V838 Mon's complex behaviour, nor can we predict what it may do in the future, we can be sure that it will continue to challenge and entertain us!" says Crause.

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 Satellite To Study The Space In Space Nears Launch
 Wallops Island - Oct 18, 2002
A satellite that will examine the "empty" space between stars has reached a milestone towards a December launch with its arrival this week at the Vandenberg Air Force Base, Calif., for integration with the launch vehicle.



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