Astronomers Unveil Secrets of Milky Way’s Supermassive Black Hole
The core of the Milky Way galaxy houses an enigmatic force known as a supermassive black hole, designated as Sagittarius A* (pronounced “Sagittarius A-star”), or Sgr A*. Located near the constellation Sagittarius, this invisible entity has a mass estimated at 4.3 million solar masses and emits vast amounts of energy in the form of x-rays and radio waves, surpassing what one would expect from an ordinary star-sized object.
Through decades of astronomical observations, researchers have meticulously mapped the movements of stars in the vicinity of Sgr A*. Their findings culminated in the groundbreaking conclusion that this invisible powerhouse is indeed a supermassive black hole. In recognition of their pivotal work, Reinhard Genzel and Andrea Ghez were awarded half of the Nobel Prize in Physics in 2000, with the other half going to Sir Roger Penrose for his contributions to the understanding of black holes.
Tracing the Journey to Discovery
The history of Sgr A* traces back to 1931, when scientist Karl Jansky of Bell Telephone Laboratories stumbled upon radio waves that originated from an unknown source in the Milky Way. This discovery marked the beginning of a series of advancements in radio astronomy, which allowed astronomers to explore the cosmos in a new light. Following World War II, the development of radio antennas transformed them into powerful telescopes capable of mapping the universe at radio frequencies.
Significant progress was made in 1974, when astronomers Bruce Balick and Robert L. Brown utilized the National Radio Astronomy Observatory’s baseline interferometer in Virginia. Their research identified Sgr A* as a compact radio source embedded within the larger radio source known as Sagittarius A. This discovery was vital, as Sgr A is recognized as the brightest radio source in the sky.
The observation of stars orbiting Sgr A*, particularly a notable star named S2, provided critical insights into the mass and radius limits of this black hole. These measurements ultimately confirmed that Sgr A* serves as the central supermassive black hole of the Milky Way.
The Nature of Supermassive Black Holes
Supermassive black holes, such as Sgr A*, differ significantly from smaller black holes, which form when massive stars exhaust their nuclear fuel. This process leads to a core collapse followed by a supernova explosion, resulting in a stellar-mass black hole if the remnant core exceeds approximately 3 solar masses. In contrast, supermassive black holes can reach millions or even billions of solar masses, with origins believed to date back to the early universe, around 12 billion years ago.
An example of a supermassive black hole is found in the giant elliptical galaxy Messier 87 (M87), which is estimated to contain a black hole of approximately 6.5 billion solar masses. This black hole was famously the first to be imaged in 2019, showcasing the advances in astronomical technology and research.
In May 2022, astronomers achieved another milestone by releasing the first image of the accretion disk surrounding Sgr A* using the Event Horizon Telescope. This international collaboration of radio observatories provided the first visual evidence of the region surrounding the black hole, although the black hole itself remains invisible. Researchers observe the behavior of nearby objects and the intense radio and infrared energy emitted as gas and dust spiral into the black hole, reaching temperatures of millions of degrees.
As stargazers look upon the constellation Sagittarius on clear evenings, they are reminded of the hidden marvels of our galaxy. Beyond the visible stars, shrouded by interstellar dust, resides the core of the Milky Way, where the forces of nature are at their most extreme, embodied by the extraordinary presence of a supermassive black hole.
