Everybody knows that whales in the ocean are big, but do they ever stop growing? Given the opportunity and the food supply, would whales keep growing without limit, or would some intrinsic quality check their size at some leviathan amount?

Beth Dykstra
The core of R136a in the Large Magellanic Cloud. Sally Oey, assistant professor in the astronomy department, used star clusters like R136a to find the upper limit of stellar masses. (Courtesy of NASA)

This was the analogy that Sally Oey, an assistant professor in the University’s Department of Astronomy, wrestled with in the course of determining the size limits of the most massive stars in the universe.

Working with C.J. Clarke, a researcher at Oxford University, the two may have a figure for the maximum size of stars, if not whales: between 120 to 200 times the mass of our own sun.

In the pecking order of stars in the universe, our sun is only medium-sized. Not too big and not too small, it fits in the middle of charts that rank stars according to their masses. According to these charts, smaller stars are much more numerous, and bigger stars are much rarer.

However, “it was an open question as to whether stars could be arbitrarily large, or max out at some point,” Oey said.

Previous studies had analyzed the sizes of stars in the cluster R136a, the brightest stellar group with individual stars discernible when viewed from Earth. In the Feb. 10 issue of the Astrophysical Journal, Oey and Clarke present their analysis of many more clusters, including R136a.

The results, Oey said, “strengthened the statistics quite strongly” for a stellar mass limit of around 120 solar masses, or multiples of mass of the sun.

Their analysis combed through the published data sets of star sizes in many different clusters and found that the Initial Mass Function, or the curve that describes the probabilities of finding stars of different sizes, didn’t hold for very massive stars.

Instead of just becoming very rarely seen, very massive stars weren’t found in their observations. The IMF “breaks down at 150 solar masses,” Oey said.

Previously, stars of up to 1000 solar masses were thought to exist. Subsequent observations by the Hubble Space Telescope were able to distinguish that these “stars” were actually combinations of many smaller stars in the same patch of sky.

These findings could hold implications for the way mass calculations are done for remote areas of the universe. Because small stars greatly outnumber their larger counterparts, small stars make up most of the mass of a stellar region.

“If we overestimate the number of big stars, we may overestimate the overall mass,” Oey said.

Many questions remain, though. How stars of any size form from swirling nebulae, or the birthplaces of stars, is still a mystery. Another of Oey’s interests is in the dynamics of star formation triggered by multiple supernova explosions in relatively close confines. The reason for the mass limit is still unknown, though.

“Is it due to intrinsic physical limitations … or is it fundamental to the size of the cloud?” Oey wondered. “How stars are formed is still fairly poorly understood.”


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