 The world's great space observatories, the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory have collaborated to produce this unprecedented look at the central region of our galaxy. Hubble documented vast arcs of gas heated by stellar winds from very large stars. Spitzer's infrared picked up the pervasive heat signals of all these stars. And Chandra detected X-ray sources from ultra-dense neutron stars and small black holes. Together they produced this spectacular image. 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 in gas and dust that it is almost impossible to see. Teams of astronomers and astrophysicists have been working on understanding Sagittarius A-star for over 25 years. The UCLA Galactic Center Group, along with the Keck Observatory on top of the Monarchy and Volcano in Hawaii, and the European Southern Observatory and its array of very large telescopes in Chile, and the Max Planck Institute for Extra-Terrestrial Physics in Germany and many others have made dramatic progress in advancing our understanding of this critically important part of our galaxy. After decades of careful observations, the speeds and orbits of around 45 stars around Sagittarius 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 10 to 15 times the mass of our sun. Observations 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. Its 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 kilometers per second, or 4,753 miles per second. 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 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. But understanding what is happening this far away is always prone to errors. I remember when we thought there was a gas cloud G2 that would be entering the black hole in 2014. This never materialized. In our current case, some astronomers point out that massive non-luminous objects such as stellar mass black holes might be present and could affect the orbital dynamics of S2. More research is needed to rule out this possibility. Here's a full dome illustration that shows how SAGA star might look to viewers on a planet orbiting S2 as it orbits the black hole. We'll cover black holes and why our supermassive black hole might look like this. But first we'll cover how the ESO, very large telescope, actually measured the minute distance is associated with S2 and SAGA star 26,000 light years away.