This video may not look like much, but it's the most definitive evidence we have that our Milky Way galaxy is orbiting around a seriously massive black hole. Created by a group at UCLA, it shows the orbits of stars around that black hole — and it shows us a little something else, too.
For decades, astrophysicists had puzzled over what lurks at the center of our galaxy. The Milky Way is a spiral galaxy, shaped something like a whirlpool of stars, and each "arm" of the whirlpool appears to have been flung out from a central area. The problem? We can't see the center of the galaxy at all — it's hidden behind a haze of dust, stars, and gas. So astronomers started to look at the galactic center at the far red end of the light spectrum, looking for low-energy light waves that could pass through all that dust. They saw a lot of stars orbiting around . . . something. But that something emitted no light in any part of the spectrum. It was as if the center of our galaxy was just a blank.
Image via NASA
Which sounds a little bit like a black hole, doesn't it? Most astronomers thought so. But it wasn't until we plotted the orbits of stars at the center of the galaxy that we had clear evidence. Below, you can see another video of these orbits, created using imagery from the Very Large Telescope in Chile.
Look closely at these orbits, and what do you see? One of them is a very narrow ellipse, which shoots its star quickly around that blank at the center of the galaxy. Knowing the shape of that ellipse helped astronomers determine how large the black hole might be — it has to be smaller than the narrowest part of the ellipse.
But they also wanted to know how massive the object is, or how much mass it has crammed into that relatively small space. Only the mass would really reveal whether it was a black hole or not. Luckily, there is a way to discover mass using Kepler's 3 laws of planetary motion. The 17th century astronomer Johannes Kepler discovered that there's a predictable mathematical relationship between how long it takes objects like planets to revolve around a star (this is called the orbital period) and the distance of those planets from their sun. Building on Kepler's laws, Newton and Leibniz discovered that the mass of that star is also in a mathematical relationship with the period and distance.
Here's the equation: Mp^2 = a^3
Translation: mass times period squared equals distance cubed (actually orbital axis cubed, which is where the "a" comes from).
The upshot: If you want to know how massive the object is at the center of our galaxy, all you need to know are the orbital period of a star revolving around it, as well as the star's distance from the object. Then you can solve for the mass.
Luckily, our infrared telescopes allowed astronomers to observe the orbits of those stars around the mystery object over a long period of time, which is what you see in both those videos. So we were able to figure out A) the stars' orbital periods and B) how far away from the object their orbits are. Which is how we discovered that the object is likely 4.1 million solar masses, and 6.2 light hours in diameter (roughly Uranus' orbit around the Sun). That means this object has crammed the masses of 4.1 million Suns into a space that's as small as Uranus' orbit. Holy freakin' moly. Only a black hole is capable of doing that.
And that is how math, along with careful observation through infrared telescopes, allowed us to "see" the invisible object around which our entire galaxy is revolving — and determine that it was a black hole.
For those who want to know more about the math, you can learn more in this paper.
Thanks to Martin Sirk and everybody attending San Francisco State's amazing Observational Astronomy class last week for helping me understand this idea!