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Movie reveals details of massive star formation
Posted: November 17, 2009

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A new high resolution movie of star formation based on radio images of the vast stellar nursery within Orion's Great Nebula shows that massive stars form just like their smaller siblings.

Artist impression of the hot disc of ionized gas around Orion Source I. A cool wind of gas is driven from the upper and lower surfaces of the disc (as indicated by the colored arrows) and is sculpted into an hourglass shape by tangled magnetic field lines (shown as thin blue lines). The entire disc and winds are rotating: red colours represent material with a component of motion away from the observer while blue represents material moving towards the observer. Image: Bill Saxton, NRAO/AUI/NSF.

The glowing gas of the nebula is powered by a group of young stars, but behind it lies a cluster of even younger stars and gas. This youthful hotbed of star formation is so obscured by dust and gas from which the newborn stars are formed that radio telescopes are needed to reveal the hidden details. In this study one particular protostar, Source I, was probed using the National Science Foundation’s Very Long Baseline Array (VLBA). The team observed Source I at monthly intervals over two years and then assembled the individual images into a time-lapse movie.

“In astronomy, it’s rare to see changes over the course of a human lifetime. With this new movie, we can see changes over just a few months as gas clumps swarm around this young protostar,” says Smithsonian astronomer Ciriaco Goddi.

The VLBA detected thousands of silicon monoxide gas clouds called masers – naturally occurring laser-like beacons often associated with star formation – many of which existed long enough for their motions to be tracked across the sky and along our line of sight, yielding their 3-D motions through space. “Source I is the richest source of masers in the Galaxy, that we know of,” says Lynn Matthews, lead author of the research. “Without the masers, we couldn’t track the gas motions in such detail so close to this massive star, and would be relatively blind to its formation.” Some of the masers were as close to the protostar as Jupiter is to our Sun.

This movie shows data from the VLBA, which imaged clumps of gas flowing away from the young protostar over the course of two years. Gas moving toward us is coloured green or blue, while gas moving away from us is coloured yellow, orange and red. The protostar is not visible to the VLBA; its location is marked by a red dot and crosshairs. Image: L. Matthews (MIT).

Capturing the birth of massive stars is challenging, since not only are they rare, but they tend to spend their early years enveloped in dust and gas, preventing them from being seen in visible light, but the new movie reveals that massive stars form much like their smaller siblings, with a rotating accretion disc and magnetic fields playing crucial roles. The movie also shows material flowing outward perpendicular to the disc in two large V-shapes – the edges of cone-shaped streams of gas – and appear to curve as they leave the disc. Such outflows foster star formation by carrying angular momentum away from the system.

“The bending path of these masers provides key evidence that magnetic fields may be influencing gas motions very close to the protostar,” says Claire Chandler of the National Radio Astronomy Observatory. The magnetic field lines of massive stars may extend outwards in a helix shape (rather than in loops as occurs for iron filings sprinkled around a bar magnet), with gas streaming along those field lines.

“Magnetic fields are supposed to be weak and unimportant to the birth process for massive stars,” says Matthews. “But masers would not travel along gentle arcs unless they experience some sort of force, probably a magnetic force.”

The current data does not show whether the magnetic field arises in the star or in the accretion disc, but future observations planned with the Expanded Very Large Array (E-VLA) and the Atacama Large Millimeter Array (ALMA) may be able to shed light on the matter. The team also plans to look for other fingerprints of magnetic fields around Source I.

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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
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