Sagittarius A*

A giant black hole lives in our galaxy.

It sits in the middle of our home in space. This black hole is very heavy. It is as heavy as four million suns.

It pulls on things nearby. We can see gas and dust falling in. This gas gets very hot and glows. We use special tools to see this light.

It is a very exciting part of space.

At the center of our Milky Way galaxy lives a giant black hole. Its name is Sagittarius A*.

We cannot see the black hole itself. This is because light cannot escape its pull. Instead, we see things near it. Gas and dust fall toward the black hole. This material gets very hot and glows. This creates a bright ring called an accretion disk.

In 2022, scientists shared the first image of this disk. They used a global network called the Event Horizon Telescope. This tool uses radio waves to see the object.

Sagittarius A* is supermassive, which means it has a huge mass. It is about 4.297 million times heavier than our Sun. Scientists found this by watching stars orbit the center. One star, named S2, moves very fast in a circle. These orbits help us measure the black hole. This work earned several Nobel Prizes in Physics. The study of these stars proves the black hole is there.

At the very center of our Milky Way galaxy sits a giant object called Sagittarius A*.

This is a supermassive black hole, which means it has a huge amount of mass. Because of its extreme gravity, even light cannot escape from it. We cannot see the black hole itself with our eyes. Instead, we observe the things moving very close to it. We see gas and dust that get heated to millions of degrees. This material glows and creates a bright ring around the center.

This bright ring is known as an accretion disk. It forms as gas and dust fall toward the black hole. Magnetic fields help guide this material into an orbit. As the gas swirls, it gets incredibly hot and releases energy. This energy shows up as radio and infrared light. Scientists can study these signals to learn about the black hole. In 2022, the Event Horizon Telescope released the first real image of this disk.

This image was made using data from eight radio observatories. It took five years of hard work to process the data.

People have been studying this area for a long time. In 1933, Karl Jansky found radio signals coming from the Sagittarius area. Later, in 1974, Bruce Balick and Robert L. Brown discovered a tiny, bright radio source. They called this specific part Sagittarius A*. In 1982, Robert Brown added the asterisk to the name. He chose it because the source was very "exciting." This name helps tell it apart from the larger radio source nearby.

Measuring such a distant object is a very hard job. Astronomers watched stars like S2 move in circles near the center. By tracking these orbits, they calculated the black hole's mass. The best estimate is 4.297 million solar masses. This means it is 4.297 million times heavier than our Sun. This discovery was so important that it won Nobel Prizes. Reinhard Genzel and Andrea Ghez won in 2020 for their work. Sir Roger Penrose also won for his work on how black holes form.

Studying Sagittarius A* helps us understand how the whole galaxy works. It even helps us test famous ideas like Einstein's theory of relativity. When the star S2 passed close to the center, it followed Einstein's rules perfectly. We can also see bright flares from the black hole. These flares might happen when magnetic fields interact with hot gas. Sometimes, the black hole might even swallow an asteroid.

Watching these events helps us see the invisible forces of space.

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.