UCLA: Star Found Racing Around Galactic Center's Black Hole

[bystander]

Astronomers discover star racing around black hole at center of our galaxy: pivotal star to test Einstein’s theory
University of California, Los Angeles | Keck Observatory | 2012 Oct 04

Discovery crucial to revealing fabric of space and time around black hole

[i]Keck telescopes observe the center of our galaxy (Credit: Ethan Tweedie)[/i]

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[i]Reconstruction of the orbits of two stars—S0-2 and S0-102—near the black hole at the Milky Way’s center. (Credit: Andrea Ghez et al./UCLA/Keck)[/i]

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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.

The Shortest-Known–Period Star Orbiting Our Galaxy’s Supermassive Black Hole - L Meyer et al

• Science 338(6103) 84 (05 Oct 2012) DOI: 10.1126/science.1225506

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[neufer]

Star seen whizzing around supermassive black hole
by Colin Stuart, Oct 4, 2012

<<Astronomers using the Keck telescope have found a new star orbiting very near to the supermassive black hole believed to be at the centre of the Milky Way. This is only the second star that researchers have observed completing an entire orbit – and its discovery confirms the black hole's presence beyond reasonable doubt. Future observations of both orbiting stars could provide a unique test of general relativity.

The Keck telescope atop Mauna Kea in Hawaii has been used since the mid-1990s to systematically probe the area surrounding the centre of the Milky Way. In doing so, astronomers revealed several stars that appear to be orbiting a central object dubbed Sgr A* ("Sagittarius A Star"). From measurements of the stars' orbital characteristics, it was calculated that Sgr A* must weigh in at around four million times the mass of the Sun. The only known astrophysical object that could be so massive, yet exist in such a small space, is a black hole.

However, only the orbit of one star – S0-2 – had data covering its entire 16.5 year journey around the centre. Data on the rest of the stars cover less than 40% of their orbits – the remainder has been projected using modelling. In order to characterize an orbit, astronomers believe that 50% of a star's orbit needs to be observed. With only S0-2 breaking this threshold, some sceptics questioned whether a central black hole existed at all.

Now, astronomers, including Andrea Ghez at the University of California, Los Angeles, have revealed the discovery of a new star named S0-102. "The orbital period of this star is just 11.5 years – the shortest of any star known to orbit the black hole," Ghez told physicsworld.com. "Improvements in adaptive optics have allowed us to find fainter stars and measure them more acurately," she says. With adaptive optics, the telescope's mirror is not a single surface, rather a tiled surface made up of smaller mirrors. A laser guide is fired into the sky above the telescope and the distortion of the laser due to atmospheric turbulence is measured. The shape of the mirror can then be adapted by moving individual tiles in order to compensate for the distortion.

This technique will also allow the future observation of S0-102 at apoapsis – its furthest distance from the black hole. "This will reduce our uncertainties in parameters like the black hole's mass," says Ghez. Having a second star to observe will also allow astronomers to improve their understanding of S0-2's orbit. In particular, it will help provide a more precise measurement of S0-2's periapsis – its closest approach to the black hole – in 2018. During periapsis, the star experiences a stronger gravitational force, causing an additional redshift in its light. The precise amount of redshift is predicted by Einstein's general theory of relativity. The experiment can be repeated when S0-102 reaches its own periapsis in 2021.

General relativity also predicts the precession of a star's periapsis. "The fact that space is warped by the gravity of the black hole means that orbits overshoot each time. The point of periapsis moves on in the direction that the star is already orbiting," explains Ghez. This is similar to the precession of Mercury's orbit within our own solar system – a puzzle that, when explained by Einstein in 1915, provided an early endorsement of his ideas. However, this particular test of relativity is not possible with a single star. "The situation isn't as simple as two stars orbiting a single black hole," says Ghez. "There are likely to be other things orbiting in there too, such as stellar-mass black holes and neutron stars," she adds. This means that the orbiting stars do not see a symmetrical distribution of mass as they pass through this crowded region. If general relativity is to be tested, it has to be treated as an unknown parameter. If the mass distribution is also unknown, you need two stars to solve the equations. "With future advances in adaptive optics, and the next generation of telescopes, we will now be able to see whether Einstein's relativity stands up in this extreme gravitational environment," Ghez hopes.

"It is pretty spectacular that they've observed the whole orbit of a second star," Nils Andersson, head of the General Relativity Group, at the University of Southampton, UK, says. "It shows there has to be a black hole in the centre, and it helps pinpoint how massive it is," he adds. However, he believes there are stronger tests of general relativity. "I think the best test beyond the solar system is still two pulsars orbiting around each other. That sort of system puts more constraints on Einstein's theory," he explains.>>