By making detailed measurements of the black hole that lies in nearby galaxy M87, astronomers have found that its mass is equivalent to 6.6 billion Suns.

M87 is an elliptical galaxy that resides 50 million light years away, and the derivation of its mass is the largest black hole ever "weighed" via a direct technique. The result was achieved by combining detailed studies of both the core of the galaxy and of its outermost regions.

The Near-Infrared Field Spectrograph (NIFS) on the 8-metre Gemini North telescope was used by Karl Gebhardt and Joshua Adams of The University of Texas to track the motions of stars careening around the black hole with ten times greater resolution than previous studies have allowed. Meanwhile, University of Texas graduate student Jeremy Murphy used the VIRUS-P instrument on the Harlan J. Smith Telescope at the university's McDonald Observatory to probe the outer reaches of the galaxy known as the dark halo due to the presence of dark matter.

"That has been an enormous struggle for a long time, trying to get what the dark halo is doing at the edge of the galaxy, simply because, when you look there, the stellar light is faint," says Gebhardt.

The VIRUS-P instrument can observe a large chunk of sky at once, adding together the faint light from dim stars to create a detailed picture. Combining the two datasets allowed the team to determine the black hole's mass as 6.6 billion Suns, and is a key step towards understanding how stars assembled themselves in a galaxy over time.

"How do you make a galaxy?" asks Gebhardt. "These two datasets probe such an enormous range, in terms of what the mass is in the galaxy. That's the first step to answering this question. It's very hard to understand how the mass accumulates unless you know exactly what's the distribution of mass: how much is in the black hole, how much is in the stars, how much is in the dark halo."

Tantalizingly, Gebhardt says that the study will eventually lead to astronomers being able to "see" a black hole. "There's no direct evidence yet that black holes exist," Gebhardt says, "... zero, absolutely zero observational evidence. To infer a black hole currently, we choose the 'none of above' option. This is basically because alternative explanations are increasingly being ruled out."

But given the size of M87's monster black hole, Gebhardt says that in the future astronomers might be able to detect its event horizon – the cliff-edge of a black hole beyond which nothing can escape. M87's event horizon spans a diameter three times larger than the orbit of Pluto in our own Solar System, but this could be revealed using a world-wide network of telescopes to search for the shadow of the event horizon on a disc of gas that surrounds M87's black hole.

In the meantime, obtaining such as robust mass for a black hole is only just the beginning for hunting down and weighing even larger galaxies residing at greater distances from our Milky Way.