Discovery Files

Sharpest image to date of universe's most massive known star

This star and possibly other colossal stars may be less massive than 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. 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.

Giant stars also live fast and die young, burning through their fuel reserves in only a few million years. In comparison, the sun is less than halfway through its 10-billion-year lifespan. The combination of densely packed stars, relatively short lifetimes and vast astronomical distances makes distinguishing individual massive stars in clusters a daunting technical challenge.

By pushing the capabilities of the Zorro instrument on the Gemini South Telescope of the International Gemini Observatory, operated by the U.S. National Science Foundation's NOIRLab, astronomers have obtained the sharpest image to date of R136a1 — the most massive known star. The findings are reported in The Astrophysical Journal.

The colossal star is a member of the R136 star cluster, which lies 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.

Previous observations suggested that R136a1 had a mass somewhere between 250 to 320 times the mass of the sun. The new Zorro observations, however, indicate that this giant star may be only 170 to 230 times the mass of the sun. Even with this lower estimate, R136a1 still qualifies as the most massive known star.

Astronomers can estimate a star's mass by comparing its observed brightness and temperature with theoretical predictions. The sharper Zorro image allowed NSF's NOIRLab astronomer Venu Kalari and colleagues to more accurately separate the brightness of R136a1 from its nearby stellar companions, which led to a lower estimate of its brightness and therefore its mass.

"Our results show us that the most massive star we currently know is not as massive as we had previously thought," said Kalari. "This suggests that the upper limit on stellar masses may also be smaller than previously thought."