Gravitational Doppler effect in the orbit of the star S2 around the massive black hole Sgr A * at the center of the Milky Way Galaxy

Abstract In 2018 it was announced that the star S2, which had been monitored since 1992, orbiting the massive black hole Sgr A * in the center of the Milky Way galaxy, obeyed as programmed by Einstein's Theory of General Relativity. The orbital movements of this star and dozens more in orbit of the black hole Sgr A * reveal that the mass of the black hole is greater than four million solar masses. The intense gravity of the black hole Sgr A * causes a gravitational Doppler effect predicted in the Theory of General Relativity and which was observed in the light emitted by the star S2. We didactically discuss the phenomena involved in these observations made for the star S2.