Super Stars Require Right Environment to Rise

Super Stars Require Right Environment to Rise
Simulation of the collapse of an interstellar gas cloud into a massive star. The left side shows the whole cloud, and the right side shows a zoom-in around the massive star in the center.
(Image: © Mark Krumholz)

Hollywood megastars need just right the rightconditions to explode onto the scene, a phenomenon shared by rare supermassivecelestial stars.

Twoscientists think they have decoded the gassy recipe to create stars as much as100 times bigger than the sun, perhaps solving the mysteryof their formation. Mark Krumholz Princeton University in New Jersey and hiscolleague Christopher McKee of the University of California Berkeley usedmathematical models to show how small stars can primesuperstar formation.

"Gravitytends to break interstellar gas clouds into small pieces, preventing massivestar formation," Krumholz said. "But little stars heating up a gascloud can smooth it out, forcing gravity to create a hugestar."

Krumholzand McKee detail their findings in the Feb. 28 issue of the journal Nature.

Starpower

Althoughmassive stars are about a million times rarer than the most common stars —those about 80 percent smaller than the sun — they are the movers and shakersof the universe.

"They'revery rare, but massive stars are the dominant players in galaxies,"Krumholz said. "They're the things that can push around and heat upinterstellar gas, which is essentially where all stars come from."

He alsoexplained that big stars seed the cosmos with elements that are required for life.

"Theyenrich the universe with metals from theirsupernovae," he said, noting that only enormous stars are powerfulenough to fuse together small atoms and create the heavy materials.

Hotinfluence

To form agalactic superpower, Krumholz said an interstellar gas cloud needs to bethousands of times more dense than average. Problem is, gravity tends to breakdense gas clouds into pieces and thwart massive star formation.

"Thechallenge isn't getting enough gas, it's getting the cloud into a small enoughregion and preventing its breakup," he said.

If a fewsmall stars form within the cloud, Krumholz explained, they can heat up thecloud and increase its "column density," or pressure. The heatingprocess prevents gravity from taking control of the cloud, breaking it up andforming only small stars.

"Heatingup the gas helps pressure win over gravity's influence, ultimately forcing thegas cloud to collapse in a massive star," Krumholz said.

The newview of star formation highlights the rarity of massive stars — the only kindastronomers on Earth can seein distant galaxies — but leads to the possibility that more stars form ingalaxies than previously thought.

"Theremay be significant parts of galaxies where massive stars can't form, butlower-mass stars like the sun can," Krumholz said. "We estimate thenumber of stars in a galaxy on the amount of light we see, and if massive starsare missing, then it's possible that we've dramatically underestimated the rateof star formation in distant regions of the universe."

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