Then and Now: The Galactic Center

Featuring the Work of Nobel Laureate Andrea Ghez

Galaxy center with and without adaptive optics
Sky and Telescope Magazine (UCLA Galactic Center Group / W. M. Keck Observatory Laser Team)
As scientists have attempted to prove the existence of black holes in the universe, it has led to a series of discoveries that ultimately landed Dr. Andrea Ghez her 2020 Nobel Prize in Physics. When Albert Einstein completed his reformulation of the law of gravity in 1915 – his General Theory of Relativity – it was implied that extremely dense objects from which not even light can escape were theoretically possible. But Einstein, who died in 1955, never believed that these "black holes" could form in the real world.
In the 1960s, discoveries of exotic types of stars, including pulsars (first seen by Jocelyn Bell Burnell, whose male superiors received the Nobel Prize for it), gave the first observational indications that black holes might actually exist. But since we cannot actually "see" a black hole, if we were going to find one in the galactic center, where theories predicted one should exist, we would have to do it by observing the motions of stars in the region. The challenge was how to look at it edge-on from our position in the galactic suburbs, where our view is obscured by interstellar dust, as well as the turbulence in our atmosphere.
Adaptive optics reveals a binary star system
An early AO system at Palomar Observatory shows the amorphous image on the left to actually be that of a binary star system. — Sky and Telescope Magazine (Palomar Observatory / NASA-JPL)
Enter Dr. Andrea Ghez, who earned a bachelor’s degree in physics from MIT in 1987 and a doctorate from Caltech in 1992, then joined the UCLA faculty in 1994. She wanted to prove that there was a massive black hole at the center of the Milky Way, and she had an idea about how to do it.
Uranus with and without adaptive optics
Adaptive optics dramatically sharpens features in Uranus's atmosphere, such as storms and banding. — Sky and Telescope Magazine (Heidi B. Hammel and Imke de Pater)
As related in a Scientific American article by the director of the Keck Observatory in Hawaii, Hilton Lewis, after arriving at the observatory, Ghez requested modifications to software that would leverage a new technique. Adaptive optics (AO), which corrects for the earth’s turbulent atmosphere to create sharper images, allowed us to peer through the dust and make detailed observations of the orbits of stars in the galactic center. This data would imply the size of the object they were orbiting – the suspected massive black hole.
Adaptive optics using laser
Though adaptive optics can be done with natural guide stars (i.e., by utilizing a nearby bright star in the sky), it's not always feasible, especially over a wide field of view. When bright stars are not available, observatories implement laser guide stars, a bright artificial star created by shining a laser beam into the sky. The image below shows the improvement from a natural guide star image of the galactic center (right) with a laser guide star image of the same field (left). — Sky and Telescope Magazine (UCLA Galactic Center Group - W.M. Keck Observatory Laser Team)
“Andrea did her work at the W.M. Keck Observatory’s twin telescopes on Maunakea, Hawai’i, in the calm and clear air almost 14,000 feet above the Pacific Ocean,” Lewis wrote. “She started using the very first instrument commissioned on Keck Observatory’s Near Infrared Camera (NIRC), [but it] was never designed to do what Andrea needed—an ultrafast readout of images and then a restacking of the result to remove the effects of the atmosphere’s turbulence. But she was not to be denied—and we made the changes. And it worked! It was supremely hard and time-consuming to make sense of the data, but Andrea persisted.”
A quarter century of determined effort has proven that there is a 4.2 million solar mass black hole at the heart of our galaxy, further confirming Einstein’s theory (and further disproving his intuition). “For the discovery of a supermassive compact object at the center of our galaxy” Ghez was awarded the Nobel Prize, half of which she shares with Reinhard Genzel of UC Berkeley and the Max Planck Institute for Extraterrestrial Physics. (The other half of the prize was awarded to Roger Penrose of the University of Oxford “for the discovery that black hole formation is a robust prediction of the general theory of relativity.”)
The animation below, from the UCLA Galactic Center Group, led by Ghez, shows the fruits of 25 years of observation, innovation and calculation. Click the play button to see the motions of stars as they orbit the supermassive black hole in the center; timestamp is shown in the upper right.
As Ghez has indicated in interviews and talks since the announcement of the Nobel Prize, the accolades are wonderful, but it's the work that matters the most. And the work of the Galactic Center Group goes on; the video below shows the kind of resolution future systems will provide.
Animation showing the improvement in resolution in the central 0.5” of the Galaxy from seeing-limited to Keck + Adaptive Optics to future Extremely Large Telescopes + Adaptive Optics. Actual orbits are shown in the seeing-limited and Keck cases, while additional simulated orbits are included for the ELT case. See a side-by-side comparison in this 3-panel. — UCLA Galactic Center Group

For more on Dr. Andrea Ghez and the work that earned her the Nobel prize, please see this UCLA Newsroom article or the UCLA Galactic Center Group’s website, which features more images and animations.
Access Ghez’s Nobel lecture here.