By Dr David Whitehouse
BBC News Online science editor
Astronomers have seen a comet vaporise into a cloud of gas as it plunged on to the fiery surface of a hot, young star.
An artist's impression of the cometary collision
The comet - a 100km-wide body made of rock and ice - was ripped apart and destroyed by the heat of the star.
Its demise was witnessed by observers using the giant Hobby-Eberly Telescope in Texas to observe the star Lk H-alpha 234, which is 3,200 light-years away.
"This is a quite extraordinary event," says Professor Eric Feigelson, of Penn State University, US.
"This discovery is significant because this is the youngest star ever found with this kind of infall of a comet-like body," says Jian Ge, assistant professor of astronomy at Penn State.
The star has a mass of about six times the mass of our Sun and an estimated very young age of about 100,000 years.
"This detection indicates that solid bodies of 100km in size can form this early around a star," Ge says.
Five sets of observations taken during October and November 2003 indicated that the star's light was absorbed by clouds of hydrogen and helium surrounding it.
Scientists believe that the gas clouds were the wreckage of a comet that got too close.
"The spectacular appearances and disappearances suggest a comet-like body," says researcher Abhijit Chakraborty.
Because astronomers know how hot the star is and how close to the star a comet can survive, they have been able to deduce the motion of the comet during its infall on to the star.
It would have disintegrated at about one-tenth of the distance between the Sun and the Earth, they think.
Understanding planetary formation
According to Eric Feigelson, evidence for cometary infall has been seen in the spectrum of the nearby star beta Pictoris, which is older and less massive than Lk H-alpha 234, but not with the dramatic spectral variations seen here.
The infall provides new data for understanding planetary formation and the timescale involved in the evolution of a massive star system.
"The main reason we see comets in our Solar System is that large snowballs in the outer parts of the Solar System are disturbed by Jupiter's gravity," says Ge.
"Eventually, some of the snowballs fall towards the inner Solar System and we see then as comets."
The team is now monitoring a number of similar stars as well to understand how common and how often this type of comet-like body occurs around these massive, young stars.