UCLA astronomers at Keck Observatory discover pivotal star to test Einstein’s theory

UCLA astronomers at Keck Observatory discover pivotal star to test Einstein’s theory

Credit: Dan Birchall

The two W. M. Keck Telescopes on Mauna Kea, Hawaii, observing the galactic center. The lasers are used to create an artificial star in Earth's upper atmosphere, which is then employed to measure the blurring effects of the lower atmosphere (the unfortunate effect that makes the stars twinkle in the night sky). The blurring gets corrected in real time with the help of a deformable mirror. This is the adaptive optics technique.

UCLA astronomers at Keck Observatory discover pivotal star to test Einstein’s theory

Credit: Ethan Tweedie

Today, UCLA astronomers using the W. M. Keck Observatory reported the discovery of a remarkable star that orbits the enormous black hole at the center of our Milky Way galaxy in a blistering 11-and-a-half years, the shortest known orbit of any star near this black hole.

The star, known as S0-102, may help astronomers discover whether Albert Einstein was right in his fundamental prediction of how black holes warp space and time, said Andrea Ghez, leader of the discovery team and professor of physics and astronomy, who holds UCLA’s Lauren B. Leichtman and Arthur E. Levine Chair in Astrophysics, and is a co-author. The research is published Oct. 5 in the journal Science.

Before this discovery, astronomers knew of only one star near the black hole with a very short orbit: S0-2, which Ghez used to call her “favorite star” and whose orbit is 16 years. (The “S” is for Sagittarius, the constellation containing the galactic center; its name is Latin for the archer.)

“I’m extremely pleased to find two stars that orbit our galaxy’s supermassive black hole in much less than a human lifetime,” said Ghez, who studies 3,000 stars that orbit the black hole, and has been studying S0-2 since 1995. Most of the stars have orbits of 60 years or longer, she said.

“It is the tango of S0-102 and S0-2 that will reveal the true geometry of space and time near a black hole for the first time,” Ghez said. “This measurement cannot be done with one star alone.”

Taft Armandroff, Director of the W. M. Keck Observatory, noted that “The pivotal research by Ghez’s UCLA group using the Keck Observatory has evolved from proving that a supermassive black hole exists in the center of our Galaxy, to testing the very fundamentals of physics. This is truly an exciting time in astronomy.”

Black holes form out of the collapse of matter to such high density that nothing can escape their gravitational pull, not even light. They cannot be seen directly, but their influence on nearby stars is visible and provides a signature, said Ghez, a 2008 MacArthur Fellow.

“Today, Einstein is in every iPhone, because the GPS system would not work without his theory,” said Leo Meyer, a researcher in Ghez’s team and lead author of the study. “What we want to find out is, would your phone also work so close to a black hole? The newly discovered star puts us in a position to answer that question in the future.”

“The fact that we can find stars that are so close to the black hole is phenomenal,” said Ghez, director of the UCLA Galactic Center Group. “Now it’s a whole new ballgame in terms of the kinds of experiments we can do to understand how black holes grow over time, the role supermassive black holes play in the center of galaxies, and whether Einstein’s theory of general relativity is valid near a black hole, where this theory has never been tested before. It’s exciting to now have a means to open up this window.

“This should not be a neighborhood where stars feel particularly welcome,” she added. “Surprisingly, it seems that black holes are not as hostile to stars as was previously speculated.”

Over the past 17 years, Ghez and colleagues have used the W. M. Keck Observatory, which sits atop Hawaii’s dormant Mauna Kea volcano, to image the galactic center at the highest angular resolution possible. They use a powerful technology Ghez has helped to pioneer called adaptive optics to correct the distorting effects of the Earth’s atmosphere in real time. With adaptive optics at the Keck Observatory, Ghez and her colleagues have revealed many surprises about the environment surrounding supermassive black holes, discovering, for example, young stars where none were expected and seeing a lack of old stars where many were anticipated.

“The Keck Observatory has been the leader in adaptive optics for more than a decade, and has enabled us to achieve tremendous progress in correcting the distorting effects of the Earth’s atmosphere with high–angular resolution imaging,” Ghez said. “It’s really exciting to have access to the world’s largest and best telescope. It is why I came to UCLA and why I stay at UCLA.”

In the same way that planets orbit around the sun, S0-102 and S0-2 are each in an elliptical orbit around the galaxy’s central black hole. The planetary motion in our solar system was the ultimate test for Newton’s gravitational theory 300 years ago; the motion of S0-102 and S0-2, Ghez said, will be the ultimate test for Einstein’s theory of general relativity, which describes gravity as a consequence of the curvature of space and time.

“The exciting thing about seeing stars go through their complete orbit is not only that you can prove that a black hole exists, but you have the first opportunity to test fundamental physics using the motions of these stars,” Ghez said. “Showing that it goes around in an ellipse provides the mass of the supermassive black hole, but if we can improve the precision of the measurements, we can see deviations from a perfect ellipse — which is the signature of general relativity.”

As the stars come to their closet approach, their motion will be affected by the curvature of space-time, and the light travelling from the stars to us will be distorted, Ghez said.

S0-2, which is 15 times brighter than S0-102, will go through its closet approach to the black hole in 2018.

The deviation from a perfect ellipse is very small and requires extremely precise measurements. Over the last 15 years, Ghez and her colleagues have dramatically improved their ability to make these measurements.

Over the years, Keck Observatory has been the primary tool in Ghez’s research. As the two biggest, fully-steerable telescopes, Keck Observatory collects more light than any other system in the world. Additionally, Keck has been continually developing and installing leading-edge instruments to allow astronomers like Ghez to make critical discoveries. 

In 1998, Ghez answered one of astronomy’s most important questions, showing that a monstrous black hole resides at the center of our Milky Way galaxy, some 26,000 light-years away from Earth, with a mass approximately four million times that of the sun. The question had been a subject of raging debate among astronomers for more than a quarter of a century.

In 2000, she and colleagues reported that for the first time, astronomers had seen stars accelerate around a supermassive black hole. Their research demonstrated that three stars had accelerated to more than 250,000 mph as they orbited the supermassive black hole. The speed of S0-102 and S0-2 should also accelerate to more than 250,000 miles per hour at their closest approach, Ghez said.

In 2003, Ghez reported that the case for the Milky Way’s black hole had been strengthened substantially and that all of the proposed alternatives could be excluded. In 2005, she and her colleagues took the first clear picture of the center of the Milky Way, including the area surrounding the black hole, using laser guide star adaptive optics technology at the Keck Observatory.

Ghez is the first woman to receive the prestigious Crafoord Prize by the Royal Swedish Academy of Sciences, which she was awarded this May.

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Big Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.


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