A supermassive black hole, known as Sagittarius A* (pronounced “Sagittarius A-star”), lies at the core of our galaxy, the Milky Way. This invisible entity, located approximately 26,000 light years away in the constellation Sagittarius, has a mass estimated at 4.3 million times that of our Sun. Through decades of research and advanced astronomical technology, scientists have confirmed that this black hole emits significant energy in the form of X-rays and radio waves, far exceeding what would be expected from a typical stellar object.
The journey to uncover the nature of Sgr A* began in 1931 when astronomer Karl Jansky from Bell Telephone Laboratories detected unusual radio signals coming from the direction of Sagittarius. This discovery marked the first identification of extraterrestrial radio waves, leading to the designation of the source as Sagittarius A (Sgr A). Following World War II, advancements in radio telescope technology allowed astronomers to further investigate this enigmatic region.
In 1974, astronomers Bruce Balick and Robert L. Brown utilized the baseline interferometer at the National Radio Astronomy Observatory in Virginia. Their observations revealed that the brightest radio emissions from the galactic center originated from a compact object embedded within the larger Sgr A source. This finding was pivotal, as Sgr A is recognized as the brightest radio source in the sky.
Over the years, astronomers closely studied the movement of stars orbiting around Sgr A*, particularly a star named S2. By analyzing the motion of these stars, researchers determined the mass and other characteristics of the central black hole. The precise measurements indicated that Sgr A* was indeed a supermassive black hole, confirming its central role in the Milky Way.
On May 12, 2022, a significant milestone in astronomy was achieved when scientists unveiled the first image of the accretion disk surrounding Sgr A* using the Event Horizon Telescope. Although the black hole itself remains invisible, the image captured the surrounding gas and dust, which are heated to millions of degrees as they spiral into the black hole. This groundbreaking achievement followed a similar success in imaging the supermassive black hole at the center of the giant elliptical galaxy Messier 87 (M87) in 2019.
Understanding black holes requires distinguishing between different types. Ordinary black holes form when massive stars exceeding about eight solar masses exhaust their nuclear fuel, triggering a core collapse. If the remnant core exceeds approximately three solar masses, it creates a gravitational field so strong that nothing, not even light, can escape, resulting in a stellar-mass black hole. In contrast, supermassive black holes, like Sgr A*, are millions to billions of solar masses and are believed to have formed in the early universe, possibly at the centers of developing galaxies over 12 billion years ago.
The study of black holes continues to evolve, with astronomers exploring their formation and impact on galaxy development. The presence of supermassive black holes at the centers of most large galaxies raises questions about their role in cosmic evolution. As scientists delve deeper into these mysteries, they remain committed to expanding our understanding of the universe.
As you gaze at the constellation Sagittarius on a clear evening, remember that beyond the visible stars lies a powerful force—an invisible supermassive black hole at the heart of our galaxy. The ongoing exploration of this phenomenon not only enhances our comprehension of black holes but also deepens our understanding of the cosmos itself.
