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* or Sgr A* for short. (It lies approximately 26,000 light-years away). It is surrounded by so many stars and gas and dust that it is extremely difficult to see. After decades of carful observations, the speeds and orbits of around 45 stars around Sgr A* 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 4 million times the mass of our Sun. But, when we look at this point, we don’t see anything. This was strong evidence that Sgr A* was a black hole because stars are known to be unstable at much smaller masses.
These new instruments followed S2 very closely. At the start of 2018 it was accelerating towards Sgr A* reaching relativistic speeds. On May 19th, it reached its closest approach. At that point, it was traveling at 7650 km/s (or 4753 mi/s). That’s almost 3% of the speed of light. Its 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 mas. 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 reddening is quite small and would not be visible to the naked eye. S2’s velocity changes close to the black hole were in excellent agreement with the predictions of general relativity. In addition, the change in the light wavelength agreed precisely with what Einstein’s theory predicted.