The universe's most massive star may be smaller than we thought

The universe's most massive known star just got its best ever close-up, and it reveals the star might be smaller than astronomers previously thought.

Astronomers using the Gemini South telescope in Chile photographed the star R136a1, which is located about 160,000 light-years from Earth in the center of the Tarantula Nebula in the Large Magellanic Cloud — a dwarf companion galaxy of the Milky Way. Their observations show that the giant star (and others like it) may not be as massive as previously thought. 

"Astronomers have yet to fully understand how the most massive stars — those more than 100 times the mass of the sun — are formed," according to a statement from the National Science Foundation's (NSF) NOIRLab, which operates the Gemini South telescope. "One particularly challenging piece of this puzzle is obtaining observations of these giants, which typically dwell in the densely populated hearts of dust-shrouded star clusters."

Related: What Is the Most Massive Star?

Gemini South's Zorro instrument uses a technique known as speckle imaging, which combines thousands of short-exposure images of stars deep in the universe to cancel out the blurring effect of Earth's atmosphere. This technique allowed astronomers to more accurately separate the brightness of R136a1 from its nearby stellar companions, resulting in the sharpest image yet of the giant star ever acquired. 

While previous observations suggested that R136a1 was between 250 to 320 times more massive than the sun, the new Zorro observations show that the mass of the giant star may be closer to 170 to 230 times that of the sun — which still qualifies it as the most massive known star. 

"Our results show us that the most massive star we currently know is not as massive as we had previously thought," Venu M. Kalari, lead author of the study and astronomer at the NSF's NOIRLab, said in the statement. "This suggests that the upper limit on stellar masses may also be smaller than previously thought."

A star's brightness and temperature is based on its mass. In other words, more massive stars appear brighter and hotter. Astronomers estimated R136a1's mass by comparing its observed brightness and temperature with theoretical predictions. Since the new Zorro images more accurately separated the brightness of R136a1 from its nearby stellar companions, astronomers were able to estimate that the star has a lower brightness and, in turn, lower mass than previous measurements showed, according to the statement. 

Massive stars like R136a1 grow rapidly, burning through their fuel reserves in only a few million years before dying fiery deaths in supernova explosions, which seed galaxies with heavy elements responsible for the formation of new stars and planets. This is the fate of most stars that are more than 150 times the mass of the sun. However, if stellar masses are smaller than previously thought, then supernovae may also be more rare than expected, the researchers noted. 

The study has been accepted for publication in The Astrophysical Journal.

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