Sagittarius A*

Sagittarius A*, often abbreviated as Sgr A*, is the supermassive black hole located at the Galactic Center of the Milky Way.

It is a very compact and bright astronomical radio source. Because a black hole has immense gravitational force, light cannot escape from it. This means the black hole itself is invisible to us. Instead, astronomers observe nearby objects that are influenced by its gravity. We see the energy from gas and dust that has been heated to millions of degrees.

This material falls toward the black hole, creating a visible signature in radio and infrared energy.

The mechanism of observation relies on the behavior of matter near the event horizon. As gas and dust fall toward the center, they form an accretion disk.

In 2019, observations using the HAWC+ instrument on the SOFIA aircraft showed that magnetic fields play a role. These fields cause the surrounding ring of gas and dust to flow into an orbit around the black hole. This orbiting material stays at temperatures ranging from 10,000 to 100,000 Kelvin. This process helps keep the black hole's emissions relatively low. Sometimes, magnetic interactions in this hot gas cause bright flares. These flares may also occur when an asteroid breaks apart while falling into the black hole.

Astronomers study several distinct components and signals to understand this region. The larger radio source is known as Sagittarius A, or Sgr A. Within that larger source, there is a bright, compact component called Sagittarius A*.

Scientists also look for specific types of energy, such as X-ray flares. In 2015, NASA reported an X-ray flare that was 400 times brighter than usual. There are also radio filaments associated with the area. Another interesting feature is the potential for gravitational lensing. This effect can create a ring-like structure around the black hole. This ring would have a diameter about 5.2 times the Schwarzschild radius of the black hole.

The history of discovering Sgr A* spans many decades. In April 1933, Karl Jansky discovered radio signals coming from the direction of the Sagittarius constellation. Later, in 1974, Bruce Balick and Robert L. Brown discovered the specific compact source Sgr A*. Robert Brown assigned the asterisk to the name in 1982. He did this because the radio source was "exciting," and in science, excited states are often marked with asterisks.

In 2022, the Event Horizon Telescope Collaboration released the first actual image of the accretion disk. This image was based on radio interferometer data collected in 2017. It took five years of calculations to process the data from eight observatories.

The significance of Sgr A* is tied to its massive scale. The current best estimate for its mass is 4.297 million solar masses.

This means it is over four million times heavier than our Sun. Astronomers determined this mass by watching the orbits of stars like S2. The star S2 moves very quickly near the center. In 2018, it was recorded moving at 2.55% of the speed of light. By analyzing these Keplerian orbits, scientists can calculate the mass and the volume of the object. They found the mass is confined within a radius of no more than 17 light-hours.

There are many surprising facts about the environment surrounding the black hole. For example, the diameter of Sgr A* is smaller than the orbit of Mercury. Astronomers have also found evidence of other objects nearby. In 2004, a team reported a potential intermediate-mass black hole called GCIRS 13E. This object orbits about 3 light-years away from Sgr A*. This discovery supports the idea that supermassive black holes might grow by absorbing smaller black holes and stars. Additionally, the radio emissions do not always appear centered on the black hole itself. They often arise from a bright spot near the event horizon.

Studying Sagittarius A* allows scientists to connect many different fields of physics. The observations provide a rigorous test for Albert Einstein's general theory of relativity. When the star S2 passed close to the black hole, it showed a gravitational redshift. This effect matched the predictions of general relativity within a 10 percent precision.

This work led to major recognition in the scientific community. Reinhard Genzel and Andrea Ghez were awarded the 2020 Nobel Prize in Physics. They proved that Sgr A* is a supermassive compact object. Sir Roger Penrose also received a share of the prize for his work on black hole formation.