Observations by the W. M Keck Telescopes provide clear evidence for the existence of a supermassive black hole at the centre of our Milky Way galaxy. Because of dust, the galactic centre cannot be seen at visible, ultraviolet or soft x-rays wavelengths, but the W. M. Keck Telescopes have obtained incredible images of the stars and gas clouds at the centre of our Milky Way galaxy using infrared optics.
This image from the Keck telescopes shows the stars closest to the black hole hole and their orbits within the central 1.0 x 1.0 arcseconds of our Galaxy [http://www.galacticcenter.astro.ucla.edu/blackhole.html]. The superimposed orbits show the positions of the stars at earlier times in the period 1995-2014. The movements of these stars provide very convincing evidence for an invisible supermassive black hole at the centre of our Milky Way galaxy.
The star with the shortest period is S0-102, which orbits the Galaxy’s black hole with a period of just 11.5 years. [Meyer L et al., Science, Volume 338, Issue 6103, 84, 2012]. This orbit period is similar to the 11.9 year orbit period of Jupiter.
Boehle et al. [The Astrophysical Journal, Volume 830, Issue 1, 23, 2016] used the orbits of two stars (S0-2 and S0-38) to determine the mass of the black hole. Star S0-2 has a period of 16 years and S0-38 a period of 19 year. Using these orbits, Boehle et al. determined the mass of the black hole mass to be (4.02 ± 0.16 ± 0.04) × 106 (4 million) solar masses.
The technology and seeing capability of the Keck Telescopes enables the telescope to see through the clouds of gas obscuring the galactic centre and a high enough resolution to observe stars completing orbits around the black hole.
Although massive, the black hole in the centre of our galaxy is very modest compared to black holes in some galaxies which exceed billions of solar masses.