At the heart of the Milky Way galaxy lies a supermassive black hole known as Sagittarius A* (pronounced “Sagittarius A-star”), which has captivated astronomers for decades. This invisible giant, situated near the constellation Sagittarius, has a mass estimated at 4.3 million solar masses and emits vast amounts of energy, primarily in the form of X-rays and radio waves. Its presence challenges our understanding of galactic formation and evolution.
The Journey to Discovery
The story of Sagittarius A* began in 1931 when Karl Jansky, a radio engineer with Bell Telephone Laboratories, detected mysterious radio interference coming from the Milky Way. This discovery, which marked the first detection of extraterrestrial radio waves, led to further investigations in the direction of Sagittarius, ultimately identifying the source as Sagittarius A.
After World War II, advancements in radio astronomy facilitated the construction of radio telescopes worldwide, including in the United States, the United Kingdom, and Australia. In 1974, astronomers Bruce Balick and Robert L. Brown utilized these advancements to identify Sagittarius A* using the baseline interferometer at the National Radio Astronomy Observatory in Virginia. Their research revealed that the strongest radio emissions were produced by a compact object within a much larger radio source, confirming the existence of the supermassive black hole.
Further studies of stars orbiting Sagittarius A*, particularly a star designated as S2, enabled astronomers to determine the mass and size limits of the black hole. These observations solidified the understanding that Sagittarius A* is indeed the central supermassive black hole of the Milky Way.
Understanding Black Holes
Ordinary black holes form when massive stars, typically greater than eight solar masses, exhaust their nuclear fuel. This results in a core collapse, leading to a supernova explosion that ejects a significant portion of the star’s mass. If the remaining core exceeds about three solar masses, the gravitational pull becomes so intense that not even light can escape, leading to the formation of a stellar-mass black hole.
In contrast, supermassive black holes, such as Sagittarius A*, can possess millions or even billions of solar masses. It is widely believed that these black holes formed early in the history of the universe, around 12 billion years ago, at the centers of large galaxies.
For example, the giant elliptical galaxy Messier 87 (M87), located in the constellation Virgo, contains a supermassive black hole estimated to be 6.5 billion solar masses. This black hole was the first to be imaged, with results published in 2019. Sagittarius A*, on the other hand, was imaged later, with the first image of its accretion disk released on May 12, 2022, by a global network of radio observatories known as the Event Horizon Telescope.
The black hole itself remains invisible, but its influence can be observed through the behavior of nearby objects. The emissions detected originate from gas and dust heated to millions of degrees as they spiral into the black hole, creating a dynamic environment around this cosmic powerhouse.
As observers gaze upon the constellation Sagittarius on clear evenings, it serves as a reminder of the incredible phenomena occurring beyond visible stars. Enshrouded by vast clouds of interstellar dust lies the core of the Milky Way, where an invisible giant, Sagittarius A*, continues to shape our understanding of the universe.
Augensen, the director of the Widener University Observatory and an emeritus professor of physics and astronomy at Widener University, encapsulates the significance of this discovery: “The study of supermassive black holes not only enhances our understanding of galactic centers but also reveals the intricate workings of the universe as a whole.”
