Published online 25 August 2004 | Nature | doi:10.1038/news040823-10


Jets expelled by star shed light on explosion

Astronomers dissect the anatomy of a supernova.

<mediar rid='m1'/>Superb images from an orbiting observatory have revealed the enormous jets of material that sprayed outwards when a recent supernova erupted. They suggest the star's explosion was more complicated than scientists had previously suspected.

The supernova remnant is a neutron star called Cassiopeia A, and it is believed to be the youngest such object discovered in our galaxy. The explosion occurred around 10,000 years ago.

Astronomers believe that light from the explosion first reached the Earth in about 1667, and was possibly observed in 1680 by John Flamsteed, founder of the Greenwich Observatory and the first 'astronomer royal' of England.

Martin Laming of the Naval Research Laboratory in Washington DC and his colleagues used the Chandra X-ray Observatory to capture detailed images of the exploded star.

Chandra was delivered into orbit around the Earth by the space shuttle Columbia in 1999, and clocked up 1 million seconds of observations of Cassiopeia A over nine different occasions this year. That is equivalent to eleven and a half days' observation in total.

Peaceful heart

When the researchers analysed the pictures they were surprised to see two jets of material, extending roughly 10 light years from the centre of the cloud. Jets are sometimes produced in the most violent star explosions, which can leave behind neutron stars that are incredibly dense and rotate rapidly.

But the relatively peaceful cinder at the heart of Cassiopeia A shows none of the other features normally associated with spinning neutron stars, such as whizzing clouds of charged particles or rotating beams of radiation. <mediar rid='m2'/>

The observation suggests that jets could be more common in relatively normal supernova explosions than astronomers thought, says Una Hwang, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who led the research, due to be published in Astrophysical Journal Letters1.

The jets are rich in silicon but relatively poor in iron, the researchers found. Since iron is usually found deep in a star's core, this means that the jets must have formed immediately after the star exploded, but before its core was blasted apart.

When the core disintegrated, the remaining iron was thrown out sideways, almost perpendicular to the initial jets. The blast has left a quiet neutron star lying at the centre of the cloud of hot atoms, says Hwang. 

Naval Research Laboratory, Washington DC

  • References

    1. Hwang U., et al. Astrophys. J. Lett., in the press (2004).