Situated approximately 27,000 light-years from Earth, at the core of our Milky Way, exists a black hole with a mass exceeding 4 million solar masses. This black hole, intriguingly named Sagittarius A*, has a unique history dating back to 1954 when researchers at Ohio University catalogued celestial radio sources. The most luminous radio wave source in each constellation was tagged with the letter A, later an asterisk was added to the source in the Milky Way center to highlight its uniqueness.
In 2020, the Nobel Prize in Physics was awarded to Reinhard Genzel and Andrea Ghez for their ground-breaking evidence proving Sagittarius A* as a compact supermassive entity. Fast forward to May 2022, the EHT collaboration unveiled an image of the accretion disk surrounding Sagittarius A*, confirming its true nature as a black hole.
The image, displayed in the top right, does not depict the black hole itself - as it is impossible to photograph due to its non-radiating nature. Instead, it portrays the orange-colored material in the vicinity being influenced by the black hole's gravitation, while the dark area at the donut's center is the shadow cast by the black hole.
On the other hand, the larger image reveals the stars nestled in the Milky Way's core, captured in infrared by Hubble and in X-rays by Chandra. The stars in this area, particularly one called S2, have been heavily researched to understand the physical properties of the black hole, especially through the calculation of their orbits, providing a highly accurate measurement of Sagittarius A*'s mass.
The X-ray and infrared image spans an area of 7 light-years, while the radio wave image's diameter is a mere 10 light-minutes. Observing the black hole in visible light, however, proves impossible due to the immense amount of dust between us and the Milky Way's center, which virtually absorbs all radiation in those wavelengths.
Credit: X-ray - NASA/CXC/SAO, IR - NASA/HST/STScI; Inset: Radio - Event Horizon Telescope Collaboration.
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