Sagittarius A*: Our Galactic Neighbor

By: Josiah Houck

Ever heard of Sagittarius A*? It’s kind of a big deal. It weighs as much as 4.5 million of our suns put together, and it has a diameter of 44 million kilometers. It is, as some would say, supermassive. And it’s relatively close to earth. So how come you’ve never heard of it? (or, if you have heard of it, how come you don’t think about it more often?)

Sgr A* (as it is lovingly known by astronomers and astrophysicists) is the name of the Milky Way Galaxy’s hub. It is the gravitational center around which the spiral arms of our galaxy spin, and without it our solar system would be either flying through space, confused and directionless, without a place to call home, or, we’d be stuck in some other galaxy knowing there was something better out there.

Our relationship with Sgr A* is a complex one. Albert Einstein was one of the first to discover just how intertwined our lives are with this gravitationally immense neighbor.  He showed in his theory of relativity that gravity actually impacts the “flow” of time, and that areas of the universe like Sgr A* actually warp time with their attractive forces.

Theoretical physicists (like Einstein, Stephen Hawking, and Sheldon from Big Bang Theory) have surmised that in order to satisfy their extensive calculations, areas such as this must exist. The term that has become popular to describe them is “black hole.” Essentially, a black hole is an area through which material can only flow one way. “Material” in this case refers to “spacetime,” which is a convenient way of grouping time and space together in equations, making them a singular unit composed of four dimensions (the fourth being time).

General relativity predicts that black holes exist because gravity arises from the curvature of space. Without getting into the intricacies of the mathematics involved in the claim, suffice it to say these black holes are the cause of these spatial curvatures, bending spacetime with their gravitational pulls.

There are two main groupings of black holes: stellar-mass, and supermassive. The first is created from the remnants of a massive star that has collapsed. A star more than eight times as large as our sun (or much larger) collapses in on itself and creates a supernova explosion or gamma ray burst.

It is not known exactly how or when supermassive black holes form, though there are several theories, but it is known that they are massive in the literal sense, ranging from hundreds of thousands to billions of solar masses large. Interestingly though, all black holes are hyper-condensed, so even one that had a solar mass of one billion would fit within the orbit of Neptune (Broderick and Abraham, Scientific American, 2009).

Sgr A* is nowhere near that big, but it still qualifies as supermassive, as well as super relevant. It is only 24,000 light years away, which seems like an insane distance to preface with “only.” BUT, when you consider that the entire Milky Way Galaxy is 100,000 light years across, that means that we are relatively close compared to the rest of our galaxy. To further put it into perspective, the next-closest supermassive black hole that astrophysicists have found suitable for study is 55 million light years away. So, I maintain that it is a close neighbor.

If you think of the galaxy as a community, the Milky Way Manor, or something along those lines, Sagittarius A* is the rich, powerful resident with a lot of sway about town. All the stars revolve around it, and are subject to its daily whims. Luckily, it is consistent and non-volaitaile. For now, it seems content to keep the arms of the Milky Way spinning, while occasionally devouring an unlucky photon or gaseous particle that goes to close to its event horizon. Let’s hope it stays that way, lest it become an annoying (or deadly) neighbor. For now, this is the only neighborhood we can afford to live in.


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