Ground-based telescope discovers the most massive star is smaller than we thought

Deep in the heart of the Tarantula Nebula – approximately 157,000 light years away from us – there lies a massive star.

Originally thought to be between 250 and 320 times larger than the mass of our own Sun, the massive star R136a1 was pushing the limit for how big stars could actually be.

But the sharpest ever images of the star have just been released, and it shows that R136a1 isn’t pushing the limit as hard as we thought – it’s a modest 196 times the size of our Sun.

Luckily for the now shrunken star, even with this lower estimate, it’s still the most massive star we know of.

“Our results show us that the most massive star we currently know is not as massive as we had previously thought,” explained NOIRLab astronomer Venu Kalari, lead author of the new study.

“This suggests that the upper limit on stellar masses may also be smaller than previously thought.”

The star is part of a large collection of stars inside the NGC 2070 star cluster, which you can see in this magical Hubble picture below.

752px grand star forming region r136 in ngc 2070 captured by the hubble space telescope
Hubble Space Telescope image of the R136 super star cluster. Credit: NASA, ESA, F. Paresce (INAF-IASF, Bologna, Italy), R. O’Connell, and the WFC 3 Science Oversight Committee

What’s potentially most exciting about this research for ground-based astronomers is just how sharp the new images of the star cluster are. Although they might look blurry, it’s a huge jump on past optical telescopes – both ground and space.

NGC 2070 has previously been observed using Hubble (shown below) and a variety of ground-based telescopes but none of these telescopes could pick out all the individual stellar members of the cluster. This’s why R136a1 was thought to be bigger than it actually was.

Read more: The James Webb Space Telescope has captured the Cartwheel Galaxy in vivid pinks

Most of the time optical space telescopes are better than those on the ground because of the atmosphere on Earth warping the ability to see far flung objects. The atmosphere is unfortunately full turbulence which distorts light.

But Gemini South Telescope’s Zorro instrument – which is based in Chile – has a better resolution than previous observations by using a technique known as “speckle imaging.” This enables ground-based telescopes to overcome much of the blurring effect of Earth’s atmosphere by taking thousands of short-exposure images of a bright object and processing the data. This approach, as well as the use of adaptive optics, can dramatically increase the resolution of ground-based telescopes, as shown by the team’s sharp new Zorro observations of R136a1.

This comparison image shows the sharpness of the Zorro imager (left) when compared to an image from Hubble (right). Credit: International Gemini Observatory/NOIRLab/NSF/AURA

“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,” says Ricardo Salinas, a co-author of this paper and the instrument scientist for Zorro.

“This observation pushes the boundary of what is considered possible using speckle imaging.”

Of course, because of the way this speckling image technique works we’ll need to take these results with a pinch of salt. The speckled images are reconstructed of thousands of images, as opposed to one long exposure.

But if this speckled image technique is able to work as well as it seems, it brings a whole new life into ground-based optical telescopes, and we’re excited to see what comes next. The research will appear in The Astrophysical Journal and can be read in full on preprint server arXiv.

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