Black holes are among 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 a galaxy cluster known as the Phoenix Cluster, located 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.
Black Hole of Phoenix A
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. The event horizon of the black hole has a diameter of 366-billion miles (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.
The supermassive black hole of Phoenix A is believed to be a primordial black hole, meaning that it is likely among the first black holes to have formed after the Big Bang. The exact origin of supermassive black holes remains a mystery, yet their origin likely predates the formation of the first large galaxies. Since nearly every large galaxy contains a supermassive black hole at its core, astronomers believe that supermassive black holes play an important role in the formation of galaxies. However, this is somewhat of a chicken or the egg problem. Did supermassive black holes come first and cause the formation of galaxies, or did galaxies cause the formation of supermassive black holes? What came first, black hole or galaxy? While this question remains unanswered, there are two models that seek to explain how supermassive black holes came to be.
First, supermassive black holes may have formed directly from the gravitational collapse of gas and dust after the Big Bang, either prior or during the formation of the first stars. Before stars formed in vast numbers, the universe contained a vast amount of star-forming material. If enough material clumped together and collapsed under its own gravity, it could have resulted in the direct formation of a supermassive black hole. The second explanation is that supermassive black holes formed from the merging of smaller black holes. The first stars to form after the Big Bang would have been far more massive than stars that form today, and again, this would have been due to the sheer abundance of star-forming material in the early universe. The more massive a star is, the faster it burns through its fuel supply and the shorter its lifespan. When the most massive stars reach the end of their life, they gravitationally collapse to become stellar black holes. In the early universe, these stellar black holes would have had masses of around 100,000 suns, and if enough of them merged together, they would have resulted in the formation of supermassive black holes. It could very well be the case that both explanations are true, or that there is another yet to be discovered explanation for the origin of black holes like the one in Phoenix A.