Sagittarius A

Sagittarius A* (Sgr A*) is the supermassive black hole at the center of the Milky Way, with a mass of about 4 million times that of the Sun.

Location and Observational History

Sagittarius A* is located at the Galactic Center, near the border of the constellations Sagittarius and Scorpius, approximately 26,000 light-years from Earth. It was first discovered in 1974 as a compact radio source by Bruce Balick and Robert L. Brown, and the asterisk () was added in 1982 to denote its particularly strong radio emission within the larger Sagittarius A complex. Observations of stars orbiting Sgr A, especially the star S2, have been crucial in determining its mass and confirming its black hole nature.

Mass and Size

Sagittarius A* has an estimated mass of around 4.15–4.3 million solar masses, equivalent to roughly 8.21 × 10^36 kg. Its Schwarzschild radius, which defines the event horizon, is approximately 7.3 million miles (11.7 million kilometers), or about 0.041 light-years. The diameter of the black hole is tiny compared to the Milky Way itself, which spans about 100,000 light-years.

Observations and Imaging

In May 2022, the Event Horizon Telescope (EHT) released the first image of the hot gas and accretion disk surrounding Sgr A*’s event horizon, providing direct visual evidence of the black hole’s presence. The black hole itself cannot be seen because light cannot escape its gravitational pull; instead, astronomers observe the behavior of nearby gas, dust, and stars influenced by its gravity. X-ray and infrared observations, including those from NASA’s Chandra X-ray Observatory and the Hubble Space Telescope, reveal hot gas swirling around Sgr A*, with less than 1% of the material within its gravitational influence actually reaching the event horizon.

Significance

Sagittarius A* is a dormant supermassive black hole, occasionally accreting gas or dust. Its study provides insights into galactic dynamics, black hole physics, and the evolution of galaxies. The precise tracking of stars orbiting Sgr A* has earned astronomers Andrea Ghez and Reinhard Genzel the 2020 Nobel Prize in Physics for their work confirming the black hole’s mass and properties. Its behavior is similar to active galactic nuclei in other galaxies but on a smaller scale.

Summary

Sagittarius A* is a key astronomical object that shapes the dynamics of the Milky Way’s center. With a mass millions of times that of the Sun, a compact event horizon, and observable effects on nearby stars and gas, it remains one of the most studied and fascinating black holes in the universe.