In my last Cosmic Game Connection, I talked about red nugget galaxies – an ancient type of galaxy found in data from the early universe – and that a few found in the modern universe had black holes at their centers that were larger than expected.
But did you know that nearly all large galaxies (maybe all) have supermassive black holes at their centers? In fact, the closest one to home lies at the center of our very own Milky Way galaxy!
The very center of our galaxy, where we think that black hole lies, is called Sagittarius A*. A radio signal was first seen from near where we thought the center of the Milky Way was in the 1930s. Later, specific sources were identified in that region, including Sagittarius A*. Evidence began mounting for a black hole at that location.
The material around the Sagittarius A* makes it hard to see that region in visible light – light our eyes are sensitive to; however, if we look in infrared, X-ray, and gamma-ray wavelengths, we can see through the dust and gas to the stars and stellar remnants that live there.
Here are some cool things about Sagittarius A*:
- Black holes don’t emit light, so we can only “see” them by the effects they have on their environments. One of the best pieces of evidence for Sagittarius A* being a black hole come from tracking the stars that orbit it. A team out of UCLA has been tracking them for over 20 years.
- In the above animation, the “star” marker at the center shows where Sagittarius A* is. The dots are stars they have tracked orbiting the black hole, and the lines indicate the orbits. Note that they’ve tracked a couple of the stars through a full orbit or more.
- Using the orbits and Kepler’s Law, it’s fairly easy to calculate the mass of the black hole, which tips the scales at a bit more than 4 million times that of our sun!
- NASA’s Swift mission monitored Sagittarius A* in X-rays, taking a 17-minute image every few days over the course of about seven years from 2006 to 2013. They saw flares from the region very close the the black hole a number of times. Flares like that happen when the black hole pulls in some gas or dust. As the matter is pulled in, it moves faster and faster. That acceleration causing the matter to emit X-ray light. Using that data, the researchers estimate the black hole flares about every five to ten days. In other words, it’s getting a snack every week or two!
- A recent report from the Chandra X-ray Observatory found evidence for a swarm of “ordinary” black holes near the supermassive one in our galaxy. These stellar-mass black holes form when a massive star dies, leaving behind a black hole with the mass of 5-30 times that of our sun. They found a dozen or so of these smaller black holes within 3 light-years of Sagittarius A* – that’s closer to the center of our galaxy than our nearest neighbor star is to us.
Even more remarkable, those dozen or so that they spotted are just the tip of the black hole iceberg, so to speak. Since black holes by themselves can’t be detected, the ones they’re seeing are those that were in some way eating matter. But there are likely to be hundreds more than we can’t see! All within that 3 light-year distance! A veritable black hole mine field.
On the personal side, I first saw that animation of the stars orbiting the center of the galaxy over a decade ago. At the time, none of the stars had made a full orbit, but it was clear they were orbiting something. I sat and watched a 10-second animated gif of those partial orbits for several minutes. And I would go back to it from time to time. I found it simply mesmerizing. The fact that we can track those stars and clearly see evidence for a black hole is so exciting.
And, in case you missed my red nugget galaxy video, be sure to check it out!
- Galactic Center: NASA’s Great Observatories Examine the Galactic Center Region
- UCLA Galaxy Center Group – Animations
- Sagittarius A*: NASA’S Chandra Catches Our Galaxy’s Giant Black Hole Rejecting Food
- NASA’s Swift Catches X-ray Action at Milky Way’s Center
- Sagittarius A* Swarm: Black Hole Bounty Captured in the Milky Way Center