From Orion’s Sword to the Pleiades
Dr Pavel Kroupa
University of Kiel, Germany
(+49) (0)431 880 4101
pavel@astrophysik.uni-kiel.de
What will the Orion Nebula look like in 100 million years time? Astronomers Dr Pavel Kroupa, Dr Sverre Aarseth and Dr Jarrod Hurley believe they have had a glimpse of the future enabling them to reveal the answer. They have calculated how the young star cluster at the heart of the Orion Nebula, presently about 1 million years old, will change over time. Their amazing result is that it will ultimately bear a remarkable resemblance to the familiar Pleiades or Seven Sisters. Looking at it another way, 100 million years ago, the Pleiades cluster was like the Orion Nebula cluster (ONC).
Over several decades, the Norwegian astrophysicist Sverre Aarseth who works at the Institute of Astronomy in Cambridge, UK, has developed a powerful and complex computer program that makes it possible to calculate the way in which a star cluster alters over time taking into account how the gravity of each star influences all of the other stars. Thanks to the computing techniques he has pioneered, researchers can calculate realistic predictions involving thousands of stars, including binary star systems. Building binary stars into the calculations is important because most stars form in pairs and some are born in systems with three or more members. But close encounters between binary stars as they move around in a cluster are particularly difficult to compute.
Working with Aarseth and his former Australian student at Cambridge, Jarrod Hurley, Pavel Kroupa (an Australian presently working at the University of Kiel, Germany) tailored the computer code to deal effectively with the significant mass of gas expelled from a large interstellar gas cloud when a star cluster forms within it. For the Orion Nebula cluster calculation, Kroupa assumed that about 400,000 years ago the radiation from new-born massive hot stars drove out two thirds of the gas initially present. He was also able to draw on his own earlier work which identified the three-dimensional distribution of stars in the ONC.
The computation carried out by Kroupa and his colleagues is the most realistic achieved for a star cluster. After the gas has been blown out, a rich star cluster forms, almost identical to the Pleiades, even down to the detailed properties of the binary stars. Two thirds of the stars that form quickly disperse into space, leaving behind a ‘nucleus’, which survives as an identifiable cluster. In principle, it will be possible to pick out any stars ejected early on from the Pleiades by their present speeds and direction of travel through space. If they can be located by the upcoming space missions that will map the positions and motions of millions of stars in the Milky Way galaxy, this will confirm that the predictions of the calculations are essentially correct and that a large proportion of all stars in the Galaxy, probably including our own Sun, have indeed formed originally in clusters.
Notes for Editors
1. Additional contact: Dr Sverre Aaseth, Institute of Astronomy, University of Cambridge, phone (+44) (0)1223 337508, e-mail sverre@ast.cam.ac.uk
2. Formation of star clusters
Between the stars, gas clouds assemble through the action of gravity and are dispersed by the heating effect of stars. Throughout the Milky Way galaxy, gas clouds form and dissolve, lasting perhaps 5 or 10 million years. On rare occasions, so much gas piles up in one place it becomes unstable and starts to contract under its own gravity. Such a cloud may be 30 to 100 light years across and contain as much mass as 100,000 Suns. As the cloud contracts, clumps within it collapse ever faster. Within the clumps, smaller knots form individual protostars. General contraction proceeds, and protostars continue to form until they heat and disperse so much of the surrounding gas that star formation ceases. At this stage, the heat input from the very young stars overwhelms the gas cloud, which is destroyed as all remaining gas is expelled into interstellar space.
3. The Orion Nebula
The Orion Nebula is visible to the naked eye as a misty patch in Orion’s ‘sword’, just below the three bright stars forming the ‘belt’. In effect, it is like a bubble or blister that has broken through at the edge of a huge dark interstellar cloud lying behind it. It is about 1,300 light years away. The bubble has been created by the intense radiation from the cluster of young stars at its centre. The four brightest stars are popularly known as ‘The Trapezium’. Their radiation causes the inside surface of the cavity to glow.
4. The Pleiades (The Seven Sisters)
Easily visible to the naked eye in the constellation Taurus, the Pleiades is a cluster of about 3000 stars in all. Most people can pick out the six brightest without the help of binoculars or a telescope. It is about 400 light years away and 30 light years across. The faint bluish nebulosity seen in photographs of the Pleiades (though not readily seen by visual observers) is not a remnant of the cloud from which the cluster formed, but a different interstellar cloud through which the cluster happens to be drifting at the present time.
