Since they're basically invisible, it can be hard to pin down just how big a black hole is. They can range anywhere from a few times the mass of the Sun up to millions or billions times that mass, but there's a potential class that are even bigger than that. A new study of data gathered by NASA's Chandra X-ray telescope has found that these so-called "ultramassive" black holes may be larger and more common than we thought.

The smallest class is the stellar mass black holes, which can be from about five to 30 times the mass of the Sun. In the middle there sits a proposed group known as intermediate-mass black holes, between 100 and 10,000 solar masses. And finally there's the heavyweights that lurk at the center of galaxies, supermassive black holes with masses of millions or even a few billion Suns.

But there's quite a gap between a million and billion, leading some astronomers to claim that there should be another class at the top for the very biggest. These ultramassive black holes would include objects of tens of billions of solar masses, such as S5 0014+813, which contains one of the largest known black holes at about 40 billion solar masses.

To learn more about these strange celestial objects, researchers in Canada and Spain examined Chandra data of 72 galaxies up to 3.5 billion light-years away, in one of the brightest and most massive galaxy clusters in the universe. The bigger a black hole, the bigger the jets of gas and matter they throw off, and astronomers can use the brightness of these jets to calculate the black hole's mass.

By analyzing the radio wave and X-ray emissions given off by these black holes, the team determined that the objects are on average about 10 times more massive than previously thought. In fact, about 40 percent of those studied were calculated to have masses of more than 10 billion Suns, which could comfortably class them as ultramassive.

The sheer size of these objects could turn our understanding of the formation of black holes and galaxies on its head. Previously it was believed that supermassive black holes and the galaxies around them form and grow in tandem, but these ultramassive black holes appear to be growing faster than their host galaxies. They may even be threatening to tear them apart.

"We have discovered black holes that are far larger and way more massive than anticipated," says Mar Mezcua, co-lead author of the study. "Are they so big because they had a head start or because certain ideal conditions allowed them to grow more rapidly over billions of years? For the moment, there is no way for us to know."

The research was published in the Monthly Notices of the Royal Astronomical Society.