Stars Reveal the Mass of Sag A*

Dec 15, 2021 | Daily Space, Milky Way, Stars, Supermassive Black Holes

IMAGE: These annotated images, obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) between March and July 2021, show stars orbiting very close to Sgr A*, the supermassive black hole at the heart of the Milky Way. One of these stars, named S29, was observed as it was making its closest approach to the black hole at 13 billion kilometres, just 90 times the distance between the Sun and Earth. Another star, named S300, was detected for the first time in the new VLTI observations. CREDIT: ESO/GRAVITY collaboration

One of the most amazing breakthroughs of modern astronomy has been the ability to image stars and gas blobs orbiting the center of our galaxy. The early work was done by Andrea Ghez at UCLA using the Keck telescope to peer into the heart of the galaxy and see what infrared light could be seen making its way out through the dust of the inner galaxy.

Seeing what’s going on in the center of our galaxy is a doubly difficult problem. Not only are astronomers trying to make out the details of objects located 27,000 light-years away, but they are also trying to make those details out through the fog of intervening matter. If you’ve ever gone out on a summer’s night and looked at the glowing swath of the Milky Way with binoculars, you may have noticed it is rich in not just stars but also bright and dark nebulae that block light from behind them.

Exactly what colors are blocked depends on how much of what size dust and gas is out there. In general, dust will first scatter blue light, making things look more and more red, just like sunlight looks redder and redder the closer the Sun gets to the horizon. When a cloud of material gets thick enough, it will block all visible light, and the only way to see what is behind the dust is to look in infrared light. So Andrea Ghez developed techniques to look toward the center of the galaxy in the infrared and to do so in such a way that the blurring caused by our own atmosphere could be taken out of the images. 

Working in the 1990s, she started by taking high-speed images and then aligning them using brighter objects, and with the build-up of image after image, she was able to tease out the light of stars orbiting our galaxy’s central supermassive black hole, which is named Sagittarius A*. Over the decades, she and others have pushed the world’s largest telescopes to the limits of their abilities and worked to understand both the mass of Sag A* and the small volume it occupies. This work demonstrated that our galaxy doesn’t have a swarm of dense objects or any other “not a black hole” possibilities for what’s in its core, and it earned Ghez, along with competing observer Reinhard Genzel, the Nobel Prize.

VIDEO: This animation shows the orbits of the stars S29 and S55 as they move close to Sgr A* (centre), the supermassive black hole at the heart of the Milky Way. As we follow the stars along in their orbits, we see real images of the region obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) in March, May, June and July 2021. In addition to S29 and S55, the images also show two fainter stars, S62 and S300, the latter having only been discovered following these new VLTI observations. CREDIT: ESO/GRAVITY collaboration/L. Calçada

While this is all amazing, astronomers are never satisfied with their results, and there is always someone looking for some way to see more and understand more. And while Ghez may have used the biggest telescopes in the world to look at Sag A*, she hadn’t combined them to work together as a single combined telescope.

Using the Very Large Telescope in Chile, which is actually a system of four 8.2-meter telescopes and four 1.8-meter ancillary telescopes, researchers, including Genzel, were able to make out details closer to the center of the galaxy than ever before, and along the way, they caught the motions of stars. The observations were taken between March and July 2021, and during this time, in May, the star S29 made its closest approach to Sag A*, getting just 90 astronomical units (AU) from the black hole. For comparison, Voyager 1 is 155 AU from the Sun, and New Horizons is more than 50 AU out. S29 would be within our solar system if Sag A* were placed where our Sun is. This team is seeing a solar system sized volume 27,000 light-years away! And these observed motions put the mass of Sag A* at 4.3 million solar masses.

Honestly, I don’t care that much just how much mass that black hole has. Folks, we are watching stars fly around in the center of our galaxy, and we’re watching some intervening stars sit unmoving in our field of view. Those moving stars – that is motion seen over months not years, and this is amazing.

More Information

ESO press release

“Mass distribution in the Galactic Center based on interferometric astrometry of multiple stellar orbits,” R. Abuter et al., to be published in Astronomy & Astrophysics


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