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Star is confirmed single and ready to test Einstein’s theory

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Star is confirmed single and ready to test Einstein’s theory
Astronomers determined the star S0-2, which will test Einstein’s Theory of General Relatively when it swings by our galaxy’s supermassive black hole later this year, does not have a significant binarycompanion.
By Jake Parks  |  Published: Wednesday, February 28, 2018
http://www.astronomy.com/news/2018/02/star-is-confirmed-single-and-ready-to-test-einsteins-theory


S0-2 is one of only a handful of stars that orbit very close to the Milky Way’s supermassive black hole. Later this year, astronomers will eagerly observe S0-2 (light blue orbit) as it makes its closest pass by the black hole, which will provide a stringent test of Einstein’s Theory of General Relativity.
S. Sakai/A. Ghez/W. M. Keck Observatory/UCLA Galactic Center Group

At the center of the Milky Way galaxy, some 26,000 light-years away, lies a supermassive black hole known as Sagittarius A*. With a mass of roughly 4 million Suns, this colossal cosmic object is a gravitational heavyweight. When something, such as a star, passes by the black hole's outer rim, its enormous gravitational pull accelerates the star to speeds of up to 18.5 million miles (30 million kilometers) per hour – or about three percent the speed of light. The short list of high-velocity stars that make such close approaches to Sagittarius A* are known as S-stars.

One such S-star is named Source 2 (also known as S0-2 or S2). After nearly 16 years of anticipation, astronomers are making their final preparations to observe S0-2 as its orbit brings it exceptionally close to Sagittarius A* in a few short months. Astronomers are so eager for this close approach because it will serve as yet another stringent test of Einstein’s Theory of General Relativity. But, until last week, there was a slight sense of unease in the air as astronomers suspected S0-2 was a binary star system like many other S-stars. This would have complicated the upcoming test.

Fortunately, in a study published February 6 in The Astrophysical Journal, a team of astronomers found that S0-2 does not have a significant binary companion that will obscure the precise measurements required to test Einstein’s theory. Based on their results, the team found that even if S0-2 does have a companion, its mass is too small to affect any tests planned for the star's closest approach to the black hole.

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S0-2 is an important star for testing gravitational redshift because it makes the closest known approach to Sagittarius A*, reaching a distance of just 17 light-hours, or three times the distance between the Sun and Pluto. This means that S0-2 should feel a noticeable nudge in its orbit due the extreme gravitational forces it will experience during its closest pass by Sagittarius A* since its last (16 years ago). Although astronomers knew about S0-2 when it made its last close pass, at the time, they did not have instruments precise enough to definitively observe a gravitational redshift in its starlight. Now they do. 

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