The Quest for the Galaxy’s Core
Every star seen in the sky is a part of the same massive Galaxy. The Milky Way contains our solar system and between 100 billion and 400 billion other stars, many of which have their planets.
The Milky Way earned its name from how it appears in the sky: like a streak of spilled milk. Stars, dust, and gas make up the hazy white band. We see it as a flat stripe because we’re looking at it from within its disc; if we could travel above the Milky Way, we’d see it as a massive spiral stretching 100,000 light-years across.
There’s also a bulge in the center, which is primarily made up of old stars. When you look at a spiral galaxy from the side, you can see lovely spiral arms where stars are formed. Our solar system is located on Orion’s arm, approximately 25,000 light-years (2.5 X 1017 miles) from the Galaxy’s core.
If you lived in the Milky Way’s center, you’d see a sky filled with stars, up to a million times denser than what we’re used to seeing. Our Sun is around four light-years away, and stars in the center of the Galaxy are about 0.4–0.04 light-years apart. The Milky Way’s spiral arm structure has broken down and changed into a “bulge” of stars in the inner 10,000 light-year area. Sagittarius A*, a black hole one million times the mass of the Sun, is in its core and the primary force in that region of the Galaxy (pronounced “Sagittarius A-star”). The Milky Way is one of over 100 billion galaxies in the cosmos, but it is the one where we live.
What exactly is Sagittarius A*, the star at the heart of our Galaxy?
To prove the existence of a black hole there, we must demonstrate that a massive amount of mass is jammed into a minimal volume. Because a black hole emits no radiation, confirming its existence is difficult, as we learned in Black Holes and Curved Spacetime. Astronomers must demonstrate that a black hole is the only logical explanation for our findings—that a small region has substantially more mass than a dense cluster of stars or whatever else made of ordinary matter could account for.
To put some figures into context, a galactic black hole with a mass of roughly 4 million Suns would have an event horizon radius of only about 17 times the size of the Sun. This region of space would have a significantly higher density than any star cluster or other conventional astronomical object. As a result, we must determine both the diameter and mass of Sagittarius A*. To provide us with the requisite evidence, we will need both radio and infrared data.
Why is it called Sagittarius A*?
The core of our galaxy is called Sagittarius A* (pronounced A-star), or Sgr A*. This name comes from a nineteenth century astronomer who cataloged thousands of stars and star clusters. The first * indicates that it’s a star, as opposed to other types of objects (such as nebulae or globular clusters). The second * means that it was classified by a man named John Herschel. To make things even more confusing, there are actually two different Sagittarius As in our galaxy. In addition to Sgr A*, there is also a bright radio source known as Sagittarius A West. These two sources are separated by about 0.5 degrees in right ascension—the distance measured along the celestial equator—and they appear to be gravitationally bound together. That makes them look like one object, but they aren’t related to each other. Sgr A* is an active supermassive black hole at the center of our galaxy; Sagittarius A West is a stellar-mass black hole (meaning it formed when a massive star collapsed) located in one of its companion galaxies.
