Final answer:
Evidence for a supermassive black hole at our Galaxy's center includes the gravitational effects on nearby stars, orbital data, and observations of accretion disks and energetic emissions in galactic centers, all pointing to a massive, dense entity that can only be explained by the presence of a black hole with a mass of millions of suns.
Step-by-step explanation:
The evidence of a supermassive black hole at the center of our Galaxy is garnered from the observation that 4.6 million solar masses are confined within a volume with a diameter no larger than Mercury's orbit. This much mass in such a tight space means that no collection of normal stellar objects could explain this concentration, strongly pointing to the existence of a supermassive black hole.
Various observations, including the orbits of nearby stars as analyzed by the UCLA Galactic Group, using powerful telescopes like the W.M. Keck Observatory, reveal these stars are orbiting a massive, invisible region consistent with a supermassive black hole. Additionally, Hubble Space Telescope observations have shown accretion disks and energetic emissions in other galactic centers, indicative of the presence of black holes with incredible masses.
The notion that such objects would have collapsed into a single black hole over a relatively short period suggests their likelihood of being anything other than a supermassive black hole is very slim, considering the age of the Galaxy. The unique behavior of quasars, which are believed to be young galaxies with a supermassive black hole at their cores, also supports this conclusion.
Therefore, based on these findings, we can confidently assert the presence of a supermassive black hole at the heart of the Milky Way Galaxy.