In the early 1960s astronomers discovered a monster.<\/p>\n
Something in the constellation of Virgo was pouring out radio waves, but no counterpart in visible light was initially seen. That changed when observers used some clever techniques<\/a> to glimpse a faint blue \u201cstar\u201d sitting at the radio source\u2019s exact position. Eventually they were able to determine that this object, called 3C 273, was not a star at all but rather something much stranger located a staggering two billion light-years from Earth.<\/p>\n To be visible at all across such vast stretches of space, the \u201cquasi-stellar object\u201d (quasar for short) 3C 273 had to be overwhelmingly bright. Scientists ultimately settled on a feeding black hole at the heart of a far-distant galaxy as the most likely engine for 3C 273\u2019s ridiculous luminosity. And this wasn\u2019t just any black hole but a positively Brobdingnagian one, likely containing 900 million times the mass of our sun.<\/p>\n [Sign up for Today in Science, a free daily newsletter]<\/a><\/p>\n Since that time we\u2019ve found many more such supermassive black holes. In fact, by the 1980s astronomers were starting to suspect that every big galaxy had a supermassive black hole in its center. Thanks to observations from the Hubble Space Telescope and other facilities, we now know that to be true\u2014which means there could be as many as a trillion such giants in the observable universe.<\/p>\n And they can get massive\u2014very, very massive. Many have been found with a billion times the sun\u2019s mass, and the beefiest can be even higher than that.<\/p>\n This naturally raises the question: Just how big can one get?<\/p>\n Answering it, however, gets a bit tricky. A notional upper limit could pop out of mass measurements for many black holes, but such observations are difficult and often rely on indirect evidence and incomplete accounting of all the physics involved. With that in mind, though, this approach suggests that the biggest black holes top out around a few tens of billions of solar masses\u2014that\u2019s as hefty as a smallish galaxy! Only a handful of these ultraheavyweights are known<\/a>, and the uncertainties in their masses can be quite large.<\/p>\n Still, is it possible that some could be even bigger? After all, in principle, a black hole could grow without end because these objects gain mass by eating anything and everything that gets too close; if you could somehow offer up the entire universe as a meal, a black hole would happily consume it.<\/p>\n But piling the whole cosmos on a black hole\u2019s dinner plate isn\u2019t very realistic, of course. According to research published in the Monthly Notices of the Royal Astronomical Society: Letters in 2015<\/a>, under physically possible (but implausibly ideal) conditions, the theoretical upper limit for a feeding, growing black hole should be a whopping 270 billion solar masses! More likely, though, the largest we\u2019ll ever find will be closer to a mere 50 billion or so.<\/p>\n The discrepancy boils down to just how close an object must get to a black hole to be pulled in. Even the largest black holes are only a few tens of billions of kilometers across\u2014on a similar scale to the size of our solar system\u2014which is tiny on the cosmic stage. From a distance, you\u2019re perfectly safe from their gravity. If a solar-mass black hole suddenly replaced our sun, we\u2019d have fatal problems\u2014such as freezing to death\u2014but our falling in wouldn\u2019t be one of them; Earth and the other planets would continue in their orbits as if nothing changed. Similarly, our Milky Way galaxy has a central supermassive black hole called Sgr A*<\/a> (pronounced \u201cSagittarius A star\u201d) that\u2019s about four million solar masses. It\u2019s some 26,000 light-years away from us, but it causes us no distress at all.<\/p>\n This really means that it\u2019s rather rare for anything to fall into a black hole\u2014and even when it happens, the mechanics aren\u2019t straightforward. Most material won\u2019t plunge headlong into the cosmic dumpster\u2019s maw. Instead its orbital speed increases as it falls toward the black hole so that it whirls madly around the compact object. This captive matter will form a flattened disk called an accretion disk.<\/p>\n Within the disk, material closer in will orbit faster than matter farther out. This generates incredible friction, heating the disk to millions of degrees. Matter that hot glows fiercely, which is one way we can detect black holes in the first place: although they\u2019re invisible, the effect they have on nearby material can be seen, even clear across the universe, as with 3C 273.<\/p>\n The disk can be so hot that material within it can actually be blown away by the intense radiation. Disks can have powerful magnetic fields that can also draw matter away<\/a>. Together these effects limit how rapidly a black hole can feed: a glut of infalling material can cause the disk to get so big and hot that it repels any additional approaching matter. This is called the Eddington limit; think of it as how rapidly a black hole can eat without\u2014and pardon the indelicacy, but an analogy is an analogy\u2014vomiting it back out.<\/p>\n So it takes time for a black hole to grow. And time is limited: the universe had a finite beginning. At best a black hole has had 13.8 billion years\u2014the age of the cosmos\u2014to stuff itself\u2014and the earliest evidence we\u2019ve found for black holes dates to a few hundred million years after that time<\/a>, further limiting their cosmic feeding frenzy.<\/p>\n Factoring in these temporal limitations, the biggest black hole today should be no larger than 270 billion times the mass of the sun. And that\u2019s only if all its feedstock is revolving in the same direction as the black hole\u2019s spin<\/a>, which acts as a digestive aid, allowing material to fall in more rapidly. If the black hole doesn\u2019t spin, or the material falls in the opposite direction to that spin, the upper limit falls to the 50 billion solar mass figure.<\/p>\n That smaller number is indeed in the ballpark of the highest-mass black holes we\u2019ve detected<\/a>. Some, like one called TON 618, appear to be a bit bigger, but there is a lot of uncertainty in that number<\/a>, and the lower limit is probably a little fungible as well.<\/p>\n