Most Massive Star in Class By Itself
Hubble Space Telescope images of the young star cluster NGC 3603 showing the position of the newly-weighed most massive star "A1" and its companion "C".
Credit: HST/Universite de Montreal

Astronomers have confirmed the weight of the most massive star in the galaxy.

This behemoth, estimated to be roughly 116 times the mass of the sun, dwarfs most other stars in the galaxy. In fact, the next most massive star is about 89 solar masses, and it is a gravitationally bound sister to the record setter.

The next most massive ever weighed is 83 solar masses. Theory holds that stars can be up to about 150 solar masses.

Discovery of the record-setting stars were first announced last year. The new measurements are rough, and the stars might turn out to be considerably heavier or lighter.

"A star having a little over a hundred times more material in it than the sun is rare," said researcher Anthony F. J. Moffat of the Universit? de Montr?al. "For every star like it you get tens of thousands of stars like the sun formed. So finding them is hard. That?s why very few have been known and measured."

Moffat, with Olivier Schnurr, Jules Casoli and Andr?-Nicolas Chen? and Nicole St-Louis of the Universit? de Montr?al used measurements from the European Southern Observatory's Very Large Telescope in Chile, and infrared images from the Hubble Space Telescope to refine estimates of the two giant stars' masses.

By measuring how the stars orbit around each other, the researchers were able to calculate each one's mass, though the level of uncertainty is still quite high (the larger weighs 116 plus-or-minus 30 solar masses, while the smaller weighs 89 plus-or-minus 15 solar masses). The astronomers hope to refine these calculations even further with future observations.

The heavyweight binary, called A1, is in the star cluster NGC 3603, which lies in the Carina spiral arm of the Milky Way, around 20,000 light-years away from our solar system.

The two A1 stars are thought be "Wolf-Rayet" stars, which are very hot, heavy and evolved stars that appear to be losing a large amount of mass in a stellar wind, similar to our sun's solar wind, but stronger. These stars are so large and luminous that the pressure of their outward-pouring radiation outweighs the inward pull of their gravity, so material is constantly being blown away into space.

These stars can lose a significant portion of their mass ? ten of percent of their total bulk ? to this process over their lifetimes. The more massive a star is, the shorter its lifetime, since it tends to burn through itself quicker. These very huge stars only live for 2 to 3 million years before dying in supernovae explosions.

Now that the researchers have found that the massive stars in A1 are Wolf-Rayet stars, it may help them locate similar finds.

"We're on to something new, that the most massive stars are probably like this," Moffat told "This is a new revelation, I think. We should look for Wolf-Rayet stars like this."

Confirming the mass of these gigantors is important because theories of star formation predict the existence of extremely massive stars weighing up to 150 times the mass of our sun. Besides these two, though, no other stars yet discovered have come close to that upper range.

The same research team previously announced the discovery of this system in 2007, but the research had not been published yet in a peer-reviewed journal. In the meantime, they have further refined the measurements, and have recently detailed their findings in the September 2008 issue of the journal Monthly Notices of the Royal Astronomical Society.