One of the great ironies in astronomy is that while technology allows deeper probes of the universe and clearer pictures of its contents, basic information about stars remains hard to come by. Measuring the distance to a star is a complex undertaking. Determining diameters is incredibly difficult and has been accomplished accurately in just a handful of cases.

And weighing a star? You might as well wish upon it.

"For single stars, the only way to estimate a star's mass is from its luminosity," explains Philip Massey of the Lowell Observatory in Flagstaff, Arizona.

Astronomers combine measurements of a star's brightness with assumptions either of how the star evolved or about the composition of its atmosphere. The two methods generate different weight estimates, however.

"Evolutionary models predicted one mass, while stellar atmosphere models predicted masses that were two times smaller," Massey says.

So he and some colleagues employed a well-known trick to accurately weigh a star -- not just any star, but one suspected of being among the heaviest known. They chose a binary star system, in which two stars orbit around one another.

Massey's team used the Hubble Space Telescope to monitor the orbital interplay of the two stars as they behaved under known laws of gravity. The mass of each star was then calculated with high precision.

The effort yielded the discovery of the heaviest binary star known. The star, called R136-038, is a whopping 57 times more massive than our Sun. Its companion is no lightweight, at 24 solar masses.

"The advantage of course of using a binary system to measure the mass of a star is that it is absolutely fundamental, and not dependent upon any stellar models," Massey said. "The masses come just from Newton's laws."

And what would Newton say about this discrepancy between evolutionary and atmospheric models?

"We found excellent agreement between the stellar evolutionary model masses and the direct measurement of the mass [which] gives us some assurances that we can deduce the masses of high-mass stars from evolutionary models with some reliability."

The result should help other astronomers weigh the true behemoths of the universe, lone objects thought to be 150 to 180 times more massive than the Sun.

"We don't really know what the upper limit is to how large a star can form," Massey notes.

The finding will be detailed in the Feb. 1 issue of the Astrophysical Journal. Laura Penny of the College of Charleston and Lowell's Julia Vukovich also worked on the study.

The binary stars they examined are roughly 175,000 light-years from Earth, in the Tarantula Nebula, within a nearby galaxy called the Large Magellanic Cloud.

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