 Another way to detect a black hole is to find stars orbiting an invisible point. The black hole at the center of our galaxy provides an excellent example. The central object in the Milky Way is known as Sagittarius A star, or SAG A star for short. It is surrounded by so many stars and gas and dust that it is almost impossible to see. After decades of careful observations, the speeds and orbits of around 45 stars around SAG A star have been calculated. This enabled measuring the precise location of the point they are all orbiting around. The measured orbits also identified the gravitational pull from this point, which in turn gave us its mass at four million times the mass of our sun. But when we look at this point, we don't see anything. This was strong evidence that SAG A star was a black hole, because stars are known to be unstable at much smaller masses. The star S2 is of particular interest, because it passes closer to SAG A star than any other. It's a single main sequence star with 15 times the mass of our sun. The measurements of the star showed that its orbit took it to within 20 light hours of Sagittarius A star in 2002, without bumping into anything. That puts SAG A star's 4 million sun mass into a very small place. For many astrophysicists, this constituted proof that it was indeed a supermassive black hole. But others pointed out that an extremely dense dim star cluster could produce these results. But if SAG A star were a cluster, S2's orbit would wobble. It did not wobble. This was the final proof point. 500 years after Copernicus put the sun at the center of our solar system, this team identified Sagittarius A star as a supermassive black hole at the center of our galaxy. But we weren't done with S2. This orbital period is 16 years. Following the 2002 passing, a major effort was mounted to upgrade ESO's very large telescope array to enable the precision needed to reveal the true geometry of space and time near this object and test Einstein's theory of general relativity. These new instruments followed S2 very closely. At the start of 2018, it was accelerating towards SAG A star, reaching relativistic speeds. On May 19th, it reached the closest approach, Perry Center. At that point, it was traveling at 7,650 km per second, or 4,753 miles per second. That's almost 3% of the speed of light. This distance from the black hole was just 18 billion kilometers, or 11 billion miles. That's only 120 times our distance from the sun. The separation on the sky between the two points was just 15 milli arc seconds. It was also reddening in color as the black hole's gravitational field stretched its light to longer wavelengths. The color change in this illustration is exaggerated for effect. The actual reddening is quite small and would not be visible to the naked eye. S2's velocity changes close to the black hole were an excellent agreement with the predictions of general relativity. In addition, the change in the light wavelength agreed precisely with what Einstein's theory predicted. Here's a full dome illustration that shows how SAG A star might look to viewers on a planet orbiting S2 as it orbits the black hole.