Huge star just can't stop eating

An image of the Omega Nebula and a cluster of hot, young stars taken by the European Southern Observatory's Very Large Telescope. The silhouetted disk is below and to the right of the cluster, indicated by a square.
An image of the Omega Nebula and a cluster of hot, young stars taken by the European Southern Observatory's Very Large Telescope. The silhouetted disk is below and to the right of the cluster, indicated by a square.ESO/VLT
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When humans feel full, something generally tells them to stop eating. With stars, that may not be the case. A new study of a big eater could answer an old question about how the heaviest stars gain their girth.

Going against every diet scheme ever conceived, a star already 20 times heavier than our sun feasts on a diet rich in gas and sprinkled with dust and, eschewing the idea of three meals a day, eats all the time

The prognosis: The star will continue to put on the pounds, going against current theory.

Self-imposed diet?
Scientists understand the development of lower mass stars like our sun, which are born when a giant cloud of material collapses under its own weight. A rotating core "protostar" develops. Remaining stuff, also rotating, flattens into a disk around the protostar's equator. Think of it as a dinner plate loaded with gas and dust not consumed during the main meal.The leftovers are drawn toward the star and, by a common process called accretion, consumption continues.

At some point, the protostar contracts enough that its density triggers thermonuclear fusion. A star is born. This is where the problem with current theory comes in.

Once a star is burning for real, it sends out a wind of charged particles as well as radiation in many wavelengths. All this puts pressure on the material trying to fall onto the star. Scientists observe this head-butting interaction even today with leftover dust in our solar system.

"The radiation pressure and the stellar wind from a star of eight solar masses, however, is so strong that it stops further accretion of gas," explains study leader Rolf Chini of the Ruhr-Universitat Bochum in Germany. "At least this is what theoreticians have told us for 20 years. Therefore, the big question is: 'How do stars above about eight solar masses form at all if this accretion scenario fails?'"

Stars more than eight times the bulk of the sun are fairly common.

Something's gotta give
One long-held possible solution is to have low-mass stars collide and merge. This would be most likely to occur in dense star clusters, where most stars are born and where the odds of collision are higher. In fact a study last month found this to be a great way, in theory, to fuel the creation of black holes.

But perhaps there's a way for stars to simply keep munching, Chini says. He studied one in the Omega Nebula, more than 7,000 light-years away. While accretion disks are routinely observed being up to 200 astronomical units (AU) wide, the star in the Omega Nebula has a disk that's 20,000 AU. One AU is the distance from Earth to the sun.

The star and its disk are still partly shrouded by a natal cloud of material, so they were observed in infrared, which allowed the scientists to peer through the cloud to detect heat from the star and a silhouette of the disk.

The huge star -- whose mass is more than twice what should signal satiation -- never stops eating, Chini explained in an e-mail interview. It pulls gas and dust in at its equator, swallowing some and spitting the rest out in a pair of jets that flow along the axis of rotation, perpendicular to the dinner plate.

Such setups are seen around many low-mass stars, but this is the first time one with such a huge dinner plate has been detected around a massive star. The plate holds more than a hundred Suns worth of meals.

"This suggests that all stars are formed in the same way -- that is by accretion through a disk -- and that the collision scenario is not required," Chini said.

Key criterion
The dinner plate is seen edge-on from our earthly point of view. Its odd shape is the key to making all this work.

"The trick to overcome the radiation pressure is a flared disk, which is very thin at the stellar equator so that the interaction between the star and the disk is minimized," Chini explained. "Calculations show that with such a configuration about 30 percent of the disk mass can be accumulated while the rest is blown off." Most of that blowing off is done via the two polar jets.

The observations were made with several telescopes, including two run by the European Southern Observatory. The results are detailed in the May 13 issue of the journal Nature.

The finding supports recent computer models suggesting stars up to 40 times the mass of the sun might form in this manner.