However, you must keep in mind that this object is found in the central ~100 AU of a galaxy that is something like 30 kiloparsecs or more in radius, so in every image you have seen so far of the Milky Way Galaxy, Sgr A* would be much smaller than the single pixel in the center of the image. In the context of the Milky Way as a whole, Sgr A* is considered to be the very center of the Galaxy. Sgr A* is one example of a class of objects called Super-Massive Black Holes, or SMBHs. Clearly the supernova explosion of one star could never produce a single black hole with a mass so large, so this object must have formed in a different manner. The mass that results from the study of this star and other nearby stars is 4 million solar masses! The only type of object that astronomers believe can have a mass of approximately 4 million stars, but a radius of about 100 AU, is a black hole. Using Kepler's laws, if we measure the period and semi-major axis of this star's orbit around Sgr A*, we can calculate the mass of this object. They have measured the orbit of a star that comes within 17 light-hours of the object in the core of our Galaxy, which is a distance that is only a few times larger than the orbit of Pluto around the Sun. Using the highest resolution IR cameras available, astronomers have repeatedly observed the stars orbiting around Sgr A*. As described in our press release, astronomers have used NASA’s Chandra X-ray Observatory to take a major step in understanding why material around Sgr A is extraordinarily faint in X-rays. See UCLA's Galactic Center Group Animation of the Stars Orbiting Sgr A*. The center of the Milky Way galaxy, with the supermassive black hole Sagittarius A (Sgr A), located in the middle, is revealed in these images. Recently, observations of stars also found to be orbiting Sgr A* have given us significant new insight into the nature of this object. Astronomers suspect that hot gas circles both black holes at the same velocity, close to the speed of light. However, it took far more time and effort to bring Sgr A into focus, due to its smaller size and its location within our own galaxy. It appears to be motionless, but we see gas apparently orbiting the source. The images of both black holes are based on data taken by the EHT of the respective sources in 2017. (Note, the equation for T is based on circumference/velocity where circ2 and v where replaces what would normally be r). This object emits a large amount of radiation in IR, X-rays, and gamma-rays. In both cases, the spin parameter is calculated to be a/M0.65 in order to match the orbit periods (T), as per fig. ![]() Now let us consider the nature of Sgr A* in particular.
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