Final answer:
Sgr A* is an extremely dense object with a mass of around 4 million solar masses, situated within an area no larger than 0.3 AU in diameter. Observations in radio and X-ray wavelengths have provided insights into its size and the nature of its surrounding accretion disk and hot gas.
Step-by-step explanation:
From the motions of nearby stars, the mass of the radio source Sgr A* is found to be concentrated within a very small volume. Astronomical observations indicate that Sagittarius A* (Sgr A*), which is the bright radio source located at the nucleus of the Milky Way galaxy, has a mass of approximately 4 million solar masses. The fact that this mass is confined within a radius of about 17 light-hours correlates to a size no larger than about 0.3 AU, roughly the size of Mercury's orbit, indicating an extremely dense central object.
Further evidence of Sgr A*'s nature comes from the study of its accretion disk through radio astronomy and Very Long Baseline Array (VLBA) measurements, providing high spatial resolution that suggests the radio source is confined to a diameter of no more than 2.5 light-minutes. This data aligns with additional observations in the X-ray spectrum from the Chandra X-ray satellite, which reveals a glowing region due to the presence of various X-ray sources such as neutron stars and stellar black holes with accretion disks, as well as gas at a temperature of 10 million K around Sgr A*.