Wolf–Rayet Star R136a1 Gets Sharpest Ever Image

By James Anderson •  Updated: 08/20/22 •  6 min read

R136a1 is the highest mass star known in the Universe – new images from the Gemini Observatory suggest it is less massive than previously thought. Researchers led by NOIRLab astronomer Venu M. Kalari utilized the abilities of the 8.1-meter Gemini South telescope in Chile for the work.


Sharpest Image Ever of R136a1, Largest Known Star.
Credit: International Gemini Observatory/NOIRLab/NSF/AURA.

The formation of the most massive stars, more than 100 times the mass of the Sun, is not yet fully understood by astronomers. Studying them is difficult because these giants typically dwell in the densely populated hearts of dust-shrouded star clusters, making observations challenging.

Giant stars also burn their fuel reserves in only a few million years. To put that in perspective, our Sun is less than halfway through its ten billion-year lifespan.

Add together densely packed stars, relatively short lifetimes, vast astronomical distances, and distinguishing individual massive stars in clusters is a daunting technical challenge.

What Is r136a1’s Mass?

R136a1 lies at the center of the R136 star cluster, located about 160,000 light-years from Earth. Cluster R136 belongs to the large NGC 2070 open cluster in the Tarantula Nebula in the Large Magellanic Cloud.

Astronomers had to push the limits of the Zorro instrument on the Gemini South telescope, but they obtained the sharpest-ever image of R136a1.

Earlier observations indicated that R136a1’s mass was somewhere between 250 to 320 times the mass of the Sun. These newer Zorro observations suggest that this giant star may be only 170 to 230 times the mass of the Sun. Even with this lower estimate, R136a1 still weighs in as the most massive known star.

Astronomers estimate a star’s mass by comparing its observed brightness and temperature with theoretical predictions. The sharper Zorro image allowed Kalari and his colleagues to more accurately separate the brightness of R136a1 from its nearby stellar companions.

That led to a lower estimate of its brightness and thus its mass.

Pair Instability Supernovae

R136a1 - Zorro and Hubble

Comparison Observation of R136a1, Zorro and Hubble. Credit: International Gemini Observatory/NOIRLab/NSF/AURA

The results also have implications for the origin of elements in the Universe heavier than helium. Such elements form during the cataclysmic explosion deaths of stars greater than 150 times the mass of the Sun.

These events are referred to as pair-instability supernovae by astronomers. If R136a1 is less massive than previously thought, the same could be true of other massive stars, and therefore pair-instability supernovae may be rarer than expected.

Pair-instability supernovae can only happen in stars with a mass between 130 to 250 solar masses and a low to a moderate abundance of elements other than hydrogen and helium. These supernovae destroy the parent star, leaving no neutron star or black hole behind.

Speckle Imaging

R136a1 illustration

Illustration of Largest Known Star in the Universe. This is an illustration of R136a1, the largest known star in the Universe, which resides inside the Tarantula Nebula in the Large Magellanic Cloud.
Credit: NOIRLab/NSF/AURA/J. da Silva/Spaceengine

Astronomers did previous observations of the star cluster containing R136a1 with the Hubble Space Telescope and various ground-based telescopes. None of these telescopes could capture images of high enough resolution to pick out all the individual stellar members of the nearby cluster.

Gemini South’s Zorro instrument exceeded the sharpness of previous observations by using speckle imaging, which enables ground-based telescopes to overcome much of the blurring effect of Earth’s atmosphere. This atmospheric blurring effect makes stars look like they are twinkling in the night sky.

Astronomers and engineers have devised various approaches to dealing with this atmospheric turbulence. Methods include locating observatories at high, dry sites and equipping telescopes with adaptive optics systems – assemblies of computer-controlled deformable mirrors and laser guides that correct atmospheric distortion.

In speckle imaging, by taking many thousands of short-exposure images of a bright object and carefully processing the data, it is possible to cancel out almost all of this blurring. Zorro captured individual observations with exposure times of just 60 milliseconds.

Astronomers captured forty thousand of these individual observations of the R136 cluster over 40 minutes. Each of these snapshots is so short that the atmosphere didn’t have time to blur any individual exposure, and by carefully combining all 40,000 exposures, the team could build up a sharp image of the cluster.

“This result shows that given the right conditions an 8.1-meter telescope pushed to its limits can rival not only the Hubble Space Telescope when it comes to angular resolution but also the James Webb Space Telescope,. This observation pushes the boundary of what is considered possible using speckle imaging,”

said co-author Ricardo Salinas, the instrument scientist for Zorro.

When observing in the red part of the visible electromagnetic spectrum, Zorro has an image resolution of about 30 milliarcseconds. This is a slightly better resolution than the James Webb Space Telescope and about three times sharper resolution than the Hubble Space Telescope at the same wavelength.

r136a1 Star Type

R136a1 is a high-luminosity WN5h star, placing it on the extreme top left corner of the Hertzsprung–Russell diagram. A Wolf–Rayet star is distinguished by the strong, broad emission lines in its spectrum. This includes ionized nitrogen, helium, carbon, oxygen and occasionally silicon, but with hydrogen lines usually weak or absent.

A WN5 star is classified on the basis of ionized helium emission being considerably stronger than the neutral helium lines, and having roughly equal emission strength from NIII, NIV, and NV. WNh stars as a class are massive luminous stars still burning hydrogen at their cores.

The emission spectrum of a WN5 star is produced in a powerful dense stellar wind, and the enhanced levels of helium and nitrogen arise from the convectional mixing of CNO cycle products to the surface.

Original Study: Venu M. Kalari, Elliott P. Horch, Ricardo Salinas, Jorick S. Vink, Morten Andersen, Joachim M. Bestenlehner, Monica Rubio. Resolving the core of R136 in the optical. arXiv:2207.13078v2 astro-ph.SR

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