By Dr David Whitehouse
BBC News Online science editor
Astronomers may have found a star that has a similar "heartbeat" of activity to that seen on our own Sun.
A montage of solar X-ray images taken over 11-years
On the Sun, there is an 11-year cycle to the rise and fall in the numbers of sunspots and in the amount of intense radiation released into space.
It is this huge fluctuation in X-rays that has now been recorded coming from a star given the designation HD 81809.
The observations were made in orbit with the XMM-Newton telescope which is operated by the European Space Agency.
Esa's Fabio Favata and colleagues recorded changes in the X-ray brightness of HD 81809, a star that is located 90 light-years away in the constellation Hydra.
The brightness has varied by more than 10 times over the past two and a half years, reaching a well defined peak in mid 2002.
However, since the observations only stretch back a short time - XMM was only launched in 1999 - the astronomers cannot say much about the star's long-term pattern of activity; but they are hopeful something significant has been detected.
"We may have caught HD 81809 at the beginning of an X-ray activity cycle," says Favata.
Understanding our Sun's 11-year activity cycle is important because of the effects it can have on the Earth's global climate.
When sunspots disappeared from our star in the 17th Century, the Earth chilled a little, showing that the Sun has a profound influence on climate, both directly and indirectly.
Scientists would like to understand when this might happen again.
Down and out
Spots on our Sun have been known since antiquity, but it was English astronomer Sir Thomas Harriot who was the first person to record them using a telescope in 1610.
Sunspots were subsequently investigated by Galileo in Italy, Fabricus in Holland and Scheiner in Italy.
But only a few years after these pioneering observations, the frequency of spots seen on the Sun declined, only to pick up again around 1715.
Although the disappearance of sunspots was remarked upon at the time, astronomers were not then aware of any cycles in the numbers.
That was recognised by German astronomer Heinrich Schwabe in 1843. His sunspot counts clearly displayed a rising and falling of sunspot numbers every 11 years or so.
It was a periodicity that was soon found in the disturbances seen in the Earth's magnetic field, later attributed to the Sun's influence.
Since then, astronomers have looked for a similar cycle of activity in other stars. But it was only in the past few decades that they have been able to detect them.
Observations made by Sally Baliunas and colleagues showed that some Sun-like stars show variations in certain emissions in the same way that the Sun does in its activity cycle.
Starspots were detected on some stars and were seen to come and go over a few years.
Observations at Jodrell Bank showed that some stars underwent a large variation in the number of explosions on their surfaces and took this as a sign of a stellar activity cycle.