![]() Modeling a Massive Black HoleĪfter constructing the images, the team used models to explore conditions close to Sgr A*, determine the black hole’s properties, and test theories of gravity. The diameter of the ring - 51.8 microarcseconds, encircling a shadow with a diameter of 48.7 microarcseconds - precisely matches the predictions of the general theory of relativity for a black hole four million times as massive as the Sun, providing yet another validation of this leading theory of gravity. These stunning images provide the first visual confirmation that the massive object at the center of the Milky Way is indeed a black hole. At the center of the ring is a dark region called the shadow, which contains the black hole’s event horizon - the surface beyond which nothing, not even light, can escape the black hole’s grasp. An image created from these data reveals a bright, asymmetrical ring of emission from the superheated gas that orbits the black hole. The articles published today focus on the best set of observations, which were taken on April 7, 2017. Building on the programs used to create the image of M87*, the team devised new algorithms to process their data, accounting for Sgr A*’s rapid variation and removing the effects of scattering by interstellar gas and dust. Unlike M87*, which looked similar each day it was observed, Sgr A*’s brightness varied during a single day’s observations. These images do not show the time variability of Sgr A*. ![]() The bottom row shows average images derived using different data-processing techniques and the prevalence of each morphology (bar graphs in lower-left corners). Image of Sgr A* constructed from observations made on April 7, 2017. How did the EHT collaboration handle these challenges, and what do the resulting images tell us? Inspecting Images On top of that variability, there’s the difficulty of viewing angle: to observe Sgr A*, the researchers had to peer through 27,000 light-years of gas and dust between Earth and the galactic center. Because Sgr A* is much smaller than M87*, our black hole’s appearance can change on much faster timescales, leading to hourly brightness variations that made processing the EHT data challenging. Sgr A*’s size and location create some unique observational challenges. M87* likely feeds on massive reservoirs of gas while Sgr A* sips from the stellar winds of a few dozen stars in its neighborhood. M87* is 2,000 times farther away than Sgr A*, but it’s also 1,500 times more massive and far more luminous. M87* is known to be active, shooting out a jet of particles that stretches 5,000 light-years from the galaxy’s center, while our own supermassive black hole is relatively subdued. ![]() Although M87* and Sgr A* appear around the same size from Earth - Sgr A* is roughly the scale of a donut placed on the surface of the Moon, as viewed from Earth - the two supermassive black holes are vastly different. ![]() M87, located 53 million light-years away, emits a powerful jet of electrons and other particles in this visible-light image from the Hubble Space Telescope. The images of M87* released in 2019 helped us to determine this black hole’s mass, spin, and orientation, and they provided a valuable test of how massive objects bend spacetime. In April 2017, eight telescopes in six locations around the world collected the faint radio emission from Sgr A*, M87*, and a handful of other objects. The EHT is a global network of radio dishes stretching from Antarctica to Spain that can be combined to make a planet-size telescope. Now, our knowledge of the black hole at the center of our galaxy - Sgr A*, pronounced “sadge-ay-star” - is getting a huge boost thanks to years of work by the Event Horizon Telescope (EHT) collaboration. Over the past decade, gravitational-wave observatories, telescope arrays, and detailed computer models have advanced our understanding of black holes throughout the universe. The close-up X-ray view of the innermost half light-year zooms in on Sgr A*. This composite (blue) and infrared (red and yellow) view shows the crowded environment of the galactic center.
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