A hidden giant resides beyond the stars of the Sagittarius constellation, near the Scorpius border: a supermassive black hole known as Sagittarius A* (pronounced “Sagittarius A-star” and abbreviated as Sgr A*). This enigmatic entity emits vast amounts of energy in the form of X-rays and radio waves, far exceeding what one would expect from a typical star-sized object.
After decades of studying the movements of stars close to Sagittarius A*, astronomers have estimated that Sgr A* has a mass equivalent to 4.3 million solar masses, although its size is comparable to that of a red supergiant star like Antares. Remarkably, it remains invisible to direct observation. The only theoretical object that could match this description is a supermassive black hole.
In 2000, Reinhard Genzel and Andrea Ghez were awarded half of the Nobel Prize in Physics for their pivotal discovery confirming that Sgr A* is indeed a supermassive black hole, while the other half recognized Sir Roger Penrose for his contributions to the understanding of black holes and general relativity.
The journey to this groundbreaking revelation began in 1931 when Karl Jansky from Bell Telephone Laboratories was testing radio antennas for communication purposes. He detected radio waves coming from an unidentified source in the sky, which was later confirmed to be of extraterrestrial origin and traced back to the Milky Way in the Sagittarius direction. This source was designated Sagittarius A (Sgr A).
Following World War II, advancements in radio antenna technology allowed for astronomical observations at radio frequencies. Various radio telescopes were established globally, including in the United States, the United Kingdom, and Australia. In 1974, astronomers Bruce Balick and Robert L. Brown utilized the National Radio Astronomy Observatory”s baseline interferometer in Virginia to identify Sgr A* as the source of the strongest radio emissions from the galactic center. They found that this compact radio source was embedded within a larger and brighter radio source, Sgr A.
The observation of stars orbiting Sgr A*, especially a notable star named S2, has enabled scientists to calculate the mass and size constraints of the object. These measurements have led to the conclusion that Sgr A* serves as the supermassive black hole at the center of the Milky Way, located approximately 26,000 light years from Earth.
Ordinary black holes form when a massive star, typically over eight solar masses, exhausts its nuclear fuel, resulting in a dramatic core collapse followed by an explosive supernova. If the remaining core exceeds around three solar masses, its intense gravitational pull distorts space-time to the extent that light cannot escape, creating a “normal” stellar-mass black hole. Prominent supergiant stars such as Antares, Betelgeuse, Deneb, and Rigel have masses exceeding fifteen solar masses and are destined to undergo supernova events, ultimately yielding neutron stars or black holes.
In contrast, supermassive black holes can possess masses in the millions or even billions of solar masses. It is believed that these massive entities formed in the early stages of the universe, at the centers of large spiral and elliptical galaxies over 12 billion years ago. The giant elliptical galaxy known as Messier 87 (M87), located in the Virgo constellation, boasts a supermassive black hole at its core, estimated to have a mass 6.5 billion solar masses, making it more than a thousand times more massive than Sgr A*.
The first-ever image of a black hole was captured in 2019, showcasing the one in M87. The second black hole to be imaged was Sgr A*, with astronomers releasing the first image of its accretion disk in May 2022, utilizing the Event Horizon Telescope, a collaborative network of radio observatories worldwide. While the black hole itself is not visible, astronomers can observe nearby objects influenced by its gravitational pull. The energy detected in radio and infrared wavelengths emanates from gas and dust clouds ignited to millions of degrees as they spiral into the black hole.
So, the next time you gaze at the stunning constellation Sagittarius on a clear summer or fall evening, remember that beyond those twinkling stars and shrouded in thick interstellar dust lies the heart of the Milky Way, home to an unseen powerhouse: a supermassive black hole.
Augensen serves as the director of the Widener University Observatory and is an emeritus professor of physics and astronomy at Widener University.
