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.
An SAAO image of V838 Mon was last chosen as NASA's astronomy image of the day: http://antwrp.gsfc.nasa.gov/apod/ap021003.html
IMAGE CAPTION: [http://www.saao.ac.za/news/v838mon.jpg (28KB)] The images show the expansion and evolution of the light-echo surrounding V838 Mon between May and September this year. The reddening of the star is due to it cooling and being obscured by a dusty cocoon which formed during April.