Black holes are among the most mysterious and fascinating objects in the universe. They are regions of space where gravity is so strong that nothing, not even light, can escape. Scientists have been studying black holes for decades, but they are very hard to observe directly. Most of the black holes we know of are either very small, with masses of a few times that of our sun, or very large, with masses of millions or billions of suns. These black holes are usually detected by their effects on the surrounding matter, such as gas, dust, stars and planets.
But what about the black holes that are in between? The ones that have masses of hundreds or thousands of suns? These are called intermediate-mass black holes, and they are very rare and elusive. Astronomers think they may form when massive stars collapse at the end of their lives, or when smaller black holes merge together. They may also be the seeds of the supermassive black holes that lurk at the centers of galaxies. However, finding them is not easy, as they are often hidden in dense clusters of stars or far away from us.
That’s why a recent discovery by a team of astronomers is so remarkable. They have accidentally found an intermediate-mass black hole that is so close and so bright that you can spot it with a backyard telescope! The black hole is located in a globular cluster called Messier 4 (or M4), which is a spherical collection of several hundred thousand stars that orbits around our Milky Way galaxy. M4 is about 6,000 light-years away from us, which means it is one of the closest globular clusters to Earth. It is also visible to the nɑƙeɗ eye in the constellation Scorpius.
The astronomers were not looking for a black hole when they stumbled upon it. They were using data from the Hubble Space Telescope and the Gaia spacecraft to study the motions and distances of the stars in M4. They noticed that some of the stars near the center of the cluster were moving very fast and erratically, as if they were being pulled by a powerful invisible force. They calculated that this force could only be explained by a single object with a mass of about 800 times that of our sun. The only plausible candidate for such an object is a black hole.
“This is not your garden-variety black hole,” said Eduardo Vitral, an astrophysicist at the Space Telescope Science Institute in Maryland and lead author of the study. “It’s too tiny for us to be able to explain other than it being a single black hole. Alternatively, there might be a stellar mechanism we simply don’t know about, at least within current physics.”
The black hole is not only massive, but also very bright. It emits a lot of radiation as it devours gas and dust from its surroundings. In fact, it is so bright that it outshines all the other stars in M4 combined. It has a visual magnitude of 14.5, which means that it can be seen with a decent telescope and a very dark night. This makes it one of the few black holes that can be observed directly by amateur astronomers.
“This black hole is such an outlier that while you should never say never, I don’t believe we will find another one like this,” said Christian Wolf, an associate professor at the Australian National University and co-author of the study. “Now we want to know why this one is different – did something catastrophic happen? Perhaps two big galaxies crashed into each other, funnelling a whole lot of material onto the black hole to feed it.”
The discovery of this black hole is not only exciting for astronomers, but also for anyone who is curious about the wonders of the cosmos. It shows that there are still surprises waiting for us in our cosmic backyard, and that we can explore them with our own eyes.
