Bounced? The Becklin-Neugebauer object, the brightest star in this infrared photograph, may have been catapulted from its home cluster by gravity.

Slingshot Star?

A massive star streaking through space at the breakneck speed of about 40 kilometers per second was recently booted from its birthplace in the center of the Orion nebula, according to a paper to appear in the 20 May issue of The Astrophysical Journal Letters. If that interpretation is correct, it suggests that violent gravitational slingshots in such clusters may occur more often than many astronomers had assumed.

The star is known as the Becklin-Neugebauer object (BN), after the two astronomers who discovered it in 1967. It lies enshrouded in the Orion nebula, a huge star-forming region 1500 light-years away. In 1995, radio observations hinted at the star's high velocity. Now Jonathan Tan of Princeton University Observatory claims he knows the cause for its remarkable speed: The star was ejected by the Trapezium cluster, a group of young stars, just 4000 years ago.

Tracing its observed motion back in time, Tan infers that BN was flung from the Trapezium cluster as a result of a close encounter with the massive star Theta-1 Orionis C. Since then, Tan says, BN has left a trail through space. As it flew by a protostar known as “source I,” roughly 500 years ago, BN triggered an eruption of gas, still visible today, Tan asserts. And finally, he claims that hot shock waves produced as the star plows through the interstellar gas can been seen in images from NASA's Chandra X-ray Observatory.BN wouldn't be the first runaway star. In fact, at least three stars are known to have escaped the Trapezium cluster some 2.5 million years ago. Astronomers believe a star can be ejected when an orbiting companion star explodes. But there was no such supernova blast in the Orion nebula 4000 years ago. So if Tan is right, BN must have been ejected by a close stellar encounter instead. The fact that we happen to see such a gravitational ejection more or less in progress might suggest that they're more frequent than astronomers thought."It's plausible, but it's hard to prove that this is the only possible explanation" of the observations, says Tim de Zeeuw of Leiden Observatory in the Netherlands. But other astronomers are more skeptical. "It is pretty unlikely" that Tan's ejection scenario is right, says Robert O'Dell of Vanderbilt University in Nashville, Tennessee. For instance, it doesn't tie in with popular models of the three-dimensional layout of the Orion nebula, he says. O'Dell also questions whether the reference stars Tan used to measure the velocity of BN are truly stationary, as Tan assumed.

Related sites
Abstract of Tan's paper, with link to full text
A flight through the Orion Nebula