Recent research has illuminated the enigmatic heart of our galaxy, the Milky Way, revealing the presence of a supermassive black hole known as Sagittarius A* (pronounced “Sagittarius A-star”). Located near the border of the constellations Sagittarius and Scorpius, this invisible entity possesses a staggering mass of 4.3 million solar masses and emits immense energy in the form of X-rays and radio waves.
The journey to uncover the mysteries of Sagittarius A* has spanned decades, with astronomers meticulously mapping the movements of stars in its vicinity. Their studies suggest that this black hole, while having a mass comparable to that of a red supergiant star like Antares, remains unseen. The only feasible explanation for its properties is that it is indeed a supermassive black hole.
Historical Milestones in Astronomy
The history of Sagittarius A* dates back to 1931, when Karl Jansky of Bell Telephone Laboratories detected radio interference from an unknown source in the sky. His findings led to the identification of this source as Sagittarius A, which is positioned within the Milky Way’s core. Following World War II, advancements in radio technology allowed astronomers to use radio telescopes to explore the cosmos more effectively.
In 1974, astronomers Bruce Balick and Robert L. Brown utilized the National Radio Astronomy Observatory’s baseline interferometer in Virginia to pinpoint Sagittarius A*. Their research revealed that the strongest radio emissions at the galaxy’s center originated from a compact radio object embedded in the larger radio source known as Sgr A, the brightest radio source in the sky. Observations of stars orbiting this black hole, particularly a star named S2, have been crucial in determining the mass and size of Sagittarius A*.
Understanding Black Holes
Ordinary black holes form when massive stars, typically over eight solar masses, exhaust their nuclear fuel. This process leads to a core collapse and a subsequent supernova explosion, which ejects a significant portion of the star’s mass into space. If the remnant core exceeds three solar masses, it creates a gravitational field so strong that not even light can escape, resulting in a stellar-mass black hole.
In contrast, supermassive black holes, such as Sagittarius A*, can reach millions or even billions of solar masses. They are thought to have formed during the early stages of galaxy evolution over 12 billion years ago. For example, the giant elliptical galaxy Messier 87 (M87) houses a supermassive black hole estimated at 6.5 billion solar masses, significantly larger than Sagittarius A*.
A major breakthrough occurred in May 2022 when astronomers unveiled the first image of the accretion disk surrounding Sagittarius A* using the Event Horizon Telescope, a global network of radio observatories. While the black hole itself remains hidden, the behavior of nearby objects and the energy emitted from heated gas and dust provide critical insight into its properties.
As you gaze at the constellation Sagittarius on a clear evening, consider that beyond its stars, shrouded by clouds of interstellar dust, lies the core of the Milky Way, where an invisible powerhouse—Sagittarius A*—dominates the galactic center. The ongoing exploration of this supermassive black hole continues to reveal the complexities of our universe.
Augensen, the director of the Widener University Observatory and an emeritus professor of physics and astronomy, emphasizes the significance of these discoveries as they deepen our understanding of black holes and their roles in galaxy formation and evolution.