Sagittarius A* The Milky Way’s Supermassive Black Hole; Location, Facts, Size, Mass

Sagittarius A*, the black hole, embedded in the larger Sagittarius A complex, is a strong source of astronomical radio waves. It is the central supermassive black hole of the Milky Way, located in the Sagittarius Constellation. It is located near the border of two constellations Scorpius and Sagittarius, close to the butterfly cluster and Shaula. The Milky Way’s Supermassive Black Hole, also known as Sgr A*, is a compact, extremely bright point source. Using infrared, X-ray, spectroscopic, and radio interferometric investigations, scientists have found out about the very small dimensions of this region and the huge mass compacted there. The discovery of conclusive evidence that Sagittarius A* is a black hole was announced in a paper in 2018. Sagittarius A* does not act like some other large black holes that rotate so fast that they bend space. However, scientists say this may no longer hold.

What is Sagittarius A? Where is located?

The Milky Way’s Supermassive Black Hole, also known as Sgr A, is a complex radio source, located in the constellation Sagittarius in the center of the Milky Way, containing a supermassive black hole. Sagittarius A consists of three components. Sagittarius A East, the supernova remnant, Sagittarius A west, the spiral structure, and the main topic of this article, Sagittarius A*, which is a huge compact radio source located at the center of the spiral. Sagittarius A*, the supermassive black hole, is located at the center of the Milky Way Galaxy, in the Sagittarius constellation. It has a distance of 26,000 light-years from earth. Sagittarius A* is one of the three components of The Milky Way’s Supermassive Black Hole

The history of the Sagittarius A*

It has been a long time since astronomers are aware of the existence of the Sagittarius A*. but it took them a long way to determine whether it existed and the way it grows, and how big it is. In August 1931, the father of radio astronomy, Karl Jansky, discovered a radio signal coming from a location in the direction of the Sagittarius constellation, to the center of the Milky Way. They named the discovered radio source this black hole.

Later, astronomers found that black hole consists of several overlapping sub-components. In the 1970s, Bruce Balick and Robert Brown discovered a bright and very compact point-like radio source, which was what we know as Sagittarius A* (pronounced “Sagittarius A-Star) today. At that time, scientists couldn’t determine enough characteristics of the compact object to precisely describe it. By the 1980s, astronomers knew that the central component of Sgr A* is likely a black hole. In the early 1990s, when scientists had new technics for measuring the orbits of the fast-moving stars orbiting this area, they found out that the mass of Sgr A* is tightly concentrated.

In 2002, researchers said they have observed the motion of a star named S2 near Sgr A* for ten years. The analysis this team had done showed new facts about the region. They said that Sgr A* can’t contain a cluster of dark stellar objects or a mass of degenerate fermions. This strengthens the evidence for Sagittarius A* to be a black hole. These observations revealed the fact that Sagittarius A* is the central supermassive black hole. In 2004, a team of researchers reported that they have discovered a potential intermediate-mass black hole, which is orbiting 3 light-years from Sagittarius A*. This new observation added support to the idea that supermassive black holes absorb smaller black holes and stars, and this is the way they grow.

By 2008, scientists were able to track complete orbits. This year, a group of researchers who had monitored stellar orbits around Sagittarius A* for 16 years, said that the object at the center of those orbits has a mass of 4,310,000 times the mass of the sun, while its orbit is not bigger than the Pluto’s orbit. Their research was the best empirical evidence to that day, that supermassive black holes do exist. They also gained information needed to determine the mass of and distances to the stars. In 2014, a gas cloud passed through the Sgr A* region and didn’t disappear beyond the event horizon which was what theorists expected to happen.

In 2015, an X-ray flare 400 times brighter than usual was observed from Sagittarius A*. Nasa said that the reason behind this phenomenon might be an asteroid breaking apart while falling into a black hole. In 2019, astronomers saw a sudden brightening of Sagittarius A*, 75 times bigger than usual, which might be caused by the supermassive black hole having encountered another object. It was in the same year that scientists announced that this supermassive black hole is consuming nearby matter at a much faster rate compared to what it did over the previous year. In 2020, the Nobel Prize in Physics was awarded to Reinhard Genzel and Andrea Ghez for discovering that Sagittarius A* is a supermassive compact object; being a black hole is the only explanation known today for such a phenomenon. These stars are still being tracked by researchers to test fundamental physics.

How did the scientists estimate the mass of Sagittarius A*?

Researchers have estimated the mass of Sgr A* in 2 different ways:

First, two groups in two different countries (the United States and Germany) were monitoring the orbits of the stars that were close to the black hole. They used Kepler’s laws to figure out the enclosed mass. The group working in Germany estimated the mass of 4.31±0.38 million sun masses, and the American group estimated the mass of 4.1±0.6 million sun masses. This estimate is ten times higher than previous estimates as the mass is limited inside a sphere having a 44-million-kilometer-diameter.

Then, more recently, scientists have used the measurement of the proper motions of a sample of several thousand stars and a statistical technique to estimate the black hole’s mass and the distributed mass in the central parsec which is composed of stars and stellar remains.

The results of these observations reveal an implication of the Milky Way. This supermassive black hole having a comparatively small mass, and low radio and infrared emission lines having low luminosity brings the idea of the Milky Way not being a Seyfert galaxy into mind.

As said before, the black hole itself cannot be seen, so a black hole is only observable if it is near Sagittarius A*. In such black holes, the radio and infrared energy that is generated comes from gas and dust that is heated to millions of degrees when it falls into the black hole.

Last words

In this article, we briefly talked about the complex radio source Sagittarius A and discussed Sagittarius A* in more detail including its discovery and main characteristics. Now you have information about the history of Sagittarius A* discovery, and then being recognized to be a black hole and then a supermassive black hole as technology gave scientists better tools to observe space.

References :

NASA

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Britannica

Wikipedia

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