2/ The black holes we’ve seen merging tend to be roughly the same mass as each other, and that’s likely due to the way they formed (details in the article). But this time, the two were very different masses! One was 32 times the Sun’s mass, the other 8.
3/ As they orbited each other, the heavier one made a little circle while the lighter one made a big circle. As they did, they made ripples in the fabric of spacetime — gravitational waves — with different frequencies.
4/ The ratio was 3:2, which in music is a perfect fifth. So the “singing in tune” is a fanciful way of thinking about it. The important part is that the difference in masses meant other properties could be determined, ones that are nearly impossible with equal-mass black holes.
5/ That includes the distance to the pair, the spin of the more massive black hole, and more. It also hints at the formation mechanism of the black hole binary itself.

As usual, it’s all in the article.

But I have a broader point.
6/ When a new field of astronomy opens up it’s usually due to some tech breakthrough, and the objects we find at first are the biggest, the brightest, the easiest to find. In this case, roughly equal mass black holes are common and easy to spot, so that’s what we find. At first.
7/ But as we find more, the oddballs start turning up, and those can really lead to better understanding of what we’re seeing. So just getting that one breakthrough? Yeah, it’s a big deal, but it’s the followup work that may be less sexy but is critically important.
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