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Space

The Milky Way Monster: the supermassive black hole at the center

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In the depths of most galaxies “blacken” supermassive black hole a million to a billion times more massive than the sun. Today we have strong evidence that our Galaxy, the Milky Way, contains such a supermassive “monster.”

One of the most important achievements in astronomy in the last twenty years has been the discovery that most (if not all) galaxies have supermassive black holes deep in their centers. Galaxies usually have one supermassive black hole, but it is rare for some galaxies to have two or three supermassive black holes at a time.

The first suggestion of a supermassive black hole in the central part of the Milky Way (Figure 1) was obtained by observing a strong and compact source of radio waves in the southern constellation Sagittarius, called Sagittarius A *, or abbreviated S * 2 (Abbreviated Sgr). ). Theoretical studies have shown that gas circulating around a high-speed black hole can emit such radio waves, so the existence of a supermassive black hole in this region is taken as the best explanation for the mysterious radio source.

supermassive black hole

Because black holes cannot be observed directly, evidence of their existence comes from, for example, measuring the velocity of a material (stars, gas) in the immediate vicinity of an alleged black hole, and by measuring the light coming from the area where it is assumed. that the black hole is located. Roughly speaking, the speed of the material is a measure of the concentrated mass around which the material moves (orbits), and light gives us information about whether that mass is contained in stars.

There are a number of moving stars in the center of the Milky Way. However, in practice, the observations of these stars are problematic, because the many clouds of dust that can be found between us and the central part of the Milky Way interfere with our view of the center. A mitigating circumstance is the possibility of using infrared light (instead of visible light), and we manage to bypass this problem. The infrared rays from the stars in the center of the Galaxy are easily pierced through the clouds of dust and provide us with a clear picture of the central part of our Galaxy.

Over the years, astronomers have patiently tracked the motion of such a star, called S2, around the supposed supermassive black hole at the center of the Milky Way (Figure 3). In 2002, a study1 was published in which the continuous measurements of the S2 position for 10 years were presented. As you can see from the picture, this time S2 has completed two-thirds of its elliptical orbit around SgrA *, which means that S2 has a period of about 15 years.

From this and other features of the orbit, it is measured that S2 moves around a mass four million times that of the Sun. Because a relatively small amount of light has been measured compared to a massive mass, a convincing conclusion has been drawn that the entire mass is not in the form of stars, but is concentrated in a compact object whose gravitational influence is responsible for S2’s motion. The supermassive black hole best explains such features, so the movement of the S2 (and also that of several other stars) is the strongest evidence for the existence of a supermassive black hole in the Milky Way.

 

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