Black holes are by far the most massive objects in the universe. They are the only objects whose gravitational pull is so strong that not even light can escape them. Relative to their mass, most black holes are not overly large. Some are hundreds of times the sun's mass, yet they are not much bigger than the sun. The black hole at the center of our galaxy is over one million times the sun's mass, yet it is only about the size of our solar system. The reason why black holes can be so massive yet also relatively small is that their extreme mass is compressing them into a small area of space. However, some black holes are truly gigantic. What is the biggest black hole in the universe?
The Phoenix Cluster
The largest black hole ever discovered is located within the Phoenix galaxy cluster around 8.5 billion light years away. The Phoenix Cluster is one of the most intensely studied galaxy clusters in our universe. It contains as many as 1,000 individual galaxies, and its galaxies also have an abnormally high rate of star formation. Perhaps the most interesting galaxy in the cluster is the central galaxy, called Phoenix A. Star formation in Phoenix A is over 700 times higher than in the Milky Way. In our galaxy, one star forms every year on average. In Phoenix A, 740 stars form every year. Phoenix A is currently undergoing a phase known as a starburst phase, wherein star formation is occurring at an accelerated rate. In fact, Phoenix A is one of the most active galaxies ever discovered.
Star formation alone is not what makes Phoenix A an interesting target for research. At the center of the galaxy there exists the largest black hole ever discovered. Estimates of its mass and size make this black hole a true behemoth, and it is unlike anything in our galaxy or in any of the galaxies in our galactic neighborhood. The mass of the central black hole is estimated to be 100 billion times larger than the sun. The supermassive black hole is even more massive than some galaxies, and is about 10% the mass of the entire Milky Way. The event horizon of the black hole has a diameter of 590 billion kilometres, or about 100 times the distance between the sun and Pluto. Travelling at the speed of light, it would take you 71 days to traverse the entire circumference of the black hole. The mass of this black hole is so large that it would have been impossible for it to form from a star. Rather, the supermassive black hole of Phoenix A likely formed from the collision of multiple supermassive black holes that formed shortly after the Big Bang, which means that it may be one of the oldest black holes in the universe. Furthermore, the black hole itself is actually growing in size and mass. It is currently absorbing vast amounts of material and is increasing its mass by 60 suns every year.