Comets from sister stars


Upating Comets

Did our comets come from sister stars?

How did the Solar System acquire its never-ending supply of comets to keep startling us? An explanation comes in today’s Science magazine, from Harold Levison and David Kaufmann of the Southwest Research Institute in Boulder, Colorado, working with Canadian and French colleagues.

The presumed source of supply is a very distant cloud of 100 billion or more comets, loosely bound to the Sun, called the Oort Cloud. The new report suggests that, in the tight cluster of stars in which the Sun was born, comets were scattered hither and yon in close encounters between stars, and many of our comets were captured from the Sun’s sisters.

In this extract from Comets I am at pains to stress that Jan Oort wasn’t the inventor of the distant comet cloud.

Ernst Öpik is an Estonian astronomer and musician who has recently been running the Armagh Observatory in Northern Ireland. For most of his long life he has adopted the role of cosmic garbage-sorter, concerning himself with the stray material of the Solar System. In 1932 he calculated that an invisible cloud of comets and meteors, surrounding the Sun at enormous distances, could survive throughout the long lifetime of the Solar System. In 1950 the doyen of Dutch astronomers, Jan Oort of Leiden, who is better known for classic work on the nature of galaxies, reworked Öpik’s idea. He emphasised a different aspect of it, namely that passing stars would cause a few of the objects to fall out of the cloud and into the heart of the Solar System, to become observable as ‘new’ comets.

Thus was the fabulous Öpik-Oort Cloud conceived, as the source of the comets. I abridge the name to the Öoo Cloud and defend this coinage on grounds of sight and sound. It looks like an untidy collection of roughly round objects of various sizes, and it is pronounced ‘Er, oh!’ – just what a neophyte comet lover is liable to utter when he is first told that there are many billions of the things out there.

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Stars rush about


Pick of the pics

Young stars rush about like naughty children

Flicking between images of a star cluster taken by the WFPC2 camera in the Hubble Space Telescope in 1997 and 2007 reveals individual stars moving, like those seen in the boxes. In two years of close examination, German astronomers have gauged the motions of more than 700 stars and found them to be faster than expected. Most of the cluster stars move by less than 1/10 of a pixel over the ten-year period, which is not discernible by eye. The object is the massive compact Young Cluster, NGC 3603, lying 20,000 light-years away in the Carina spiral arm of the Galaxy. Credit: NASA, ESA and Wolfgang Brandner (MPIA), Boyke Rochau (MPIA) and Andrea Stolte (University of Cologne)

Extracts from the Hubble press release

[If the flicking doesn’t work, go to this Hubble url to see it offered, on the right. NC]

With a mass of more than 10,000 suns packed into a volume with a diameter of a mere three light-years, the massive young star cluster in the nebula NGC 3603 is one of the most compact stellar clusters in the Milky Way and an ideal place to test theories for their formation.

A team of astronomers from the Max-Planck Institute for Astronomy in Heidelberg and the University of Cologne led by Wolfgang Brandner (MPIA) wanted to track the movement of the cluster’s many stars. Such a study could reveal whether the stars were in the process of drifting apart, or about to settle down.

The results for the motion of these cluster stars were surprising: this very massive star cluster has not yet settled down. Instead, the stars’ velocities were independent of their mass and thus still reflect conditions from the time the cluster was formed, approximately one million years ago.

In the long term such massive compact star clusters may lead to the development of the huge balls of stars known as globular clusters, whose tightly packed stars remain held together by gravity for billions of years.

Wolfgang Brandner (MPIA): This is the first time we have been able to measure precise stellar motions in such a compact young star cluster.

Andrea Stolte (Cologne): This is key information for astronomers trying to understand how such clusters are formed, and how they evolve.

Boyke Rochau (MPIA) Our measurements have a precision of 27 millionths of an arcsecond per year. This tiny angle corresponds to the apparent thickness of a human hair seen from a distance of 800 km.