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.