Previous estimates of a black hole called M82 X-1 have ranged wildly from 20 times the mass of our sun to 10,000 solar masses. Now astronomers applying a rate-of-pulsation technique say it’s around 400 times the mass of our sun. Not too big and not too small, this new estimate, reported in Nature this week, is the most accurate measurement of a medium-mass black hole to date. “I wouldn’t bet my house on it,” Richard Mushotzky from University of Maryland, College Park, tells Time, “but I might bet my car.”

Nearly all black holes fall into one of two classes: big and colossal. They range from 10 to 100 times the mass of our sun on the small end, with supermassive black holes (those residing at the center of most galaxies) measuring more than a million times the mass of our sun. And then there are these intermediate-mass black holes that are so hard to measure, even their existence is sometimes disputed. “For reasons that are very hard to understand, these objects have resisted standard measurement techniques,” Mushotzky says in a news release.

A bright x-ray source in a nearby galaxy called Messier 82 (M82) about 12 million light years from Earth has been previously interpreted as an intermediate-mass black hole about 100 to 10,000 solar masses; some models have suggested that it’s only 20 times that of the sun. For a less ambiguous method of mass determination, Mushotzky, UMD’s Dheeraj Pasham, and Tod Strohmayer of the NASA Goddard Space Flight Center turned to stable oscillations in x-ray emissions. 

Between 2004 and 2010, NASA’s Rossi X-Ray Timing Explorer satellite telescope observed M82 X-1 about 800 times and recorded the individual x-ray particles it emitted. The researchers mapped the intensity and wavelength of the x-rays in each sequence, and when they stitched the sequences together, they found two repeating flares of light. These displayed a rhythmic pattern of light pulses: One occurred 5.1 times per second, the other occurred 3.3 times per second. The two light oscillations were like two dust motes stuck in the grooves of a vinyl record spinning on a turntable, Mushotzky explains. If the oscillations were musical beats, they would produce a specific syncopated rhythm with a 3:2 beat, like “Mean Mister Mustard sleeps in the park, shaves in the dark, try’na save paper.”

These 3:2 oscillations have correlated well with black hole mass for smaller black holes in the past, so the team applied the technique to their candidate intermediate-mass black hole. They detected the oscillations in the x-rays from M82 and calculated that the source is a black hole with a mass of around 428 solar masses, plus or minus 105 solar masses. An alternative model estimated 415 solar masses, plus or minus 63 solar masses. 

The black hole in question, Mushotzky says, is a “just-right-sized” version of this class of astral objects.