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Omega Centauri is the largest of the Milky Way Galaxy’s globular clusters, huge collections of hundreds of thousands or even millions of stars packed together into a roughly spherical ball generally a hundred light years across or so. Omega Cen (as its friends call it) is close to twice that size, and so massive it may not actually be a globular cluster at all; instead it’s likely the core of a small galaxy that was stripped of its stars by the Milky Way long ago.

Either way, it’s so big and relatively close by as these things go (about 17,000 light-years) that it’s been an object of intense scrutiny for decades. In fact, it’s that long baseline of observations that has finally revealed a treasure long sought-after: a stellar mass black hole amongst the stellar crowd.

Black holes like this form when a massive star explodes, and its core collapses. Globular clusters probably had lots of these stars when they first formed billions of years ago, but stars like that are short-lived and all exploded long ago. This means there should be thousands of black holes in clusters like Omega Cen, but until now none has been found (with the exception of an intermediate mass black hole in its very center, but that’s a different species of black hole and not what we’re talking about here).

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It’s possible the black holes all got flung away. Due to a process called mass segregation, heavy objects like black holes fall to the center of the cluster as they interact gravitationally with all the stars around them, and lighter stars propagate outward. If two black holes happen to get too close together during the process they could give each other a gravity assist (the so-called “slingshot”) and throw each other out of the cluster. Others might get close enough to a binary star that one of the two stars gets ejected and the black hole winds up in orbit around the other. It’s complex, and there are lots of possibilities.

Finding any potential black holes isn’t easy. If they’re gobbling down matter, say ripping it away from a nearby star, then they’ll emit X-rays. While observations of Omega Cen show lots of point-source X-ray emitters, they’re all neutron stars (also the remnants of collapsed cores of massive stars, but have an upper mass limit of around 2.8 times the mass of the sun). But there’s another way to find them…

If a black hole and a star are in a binary system, then they orbit their mutual center of mass, called the barycenter. Normally this motion is too small to see, but we do have some pretty good eyes.

That includes Hubble and JWST. Hubble has been observing Omega Cen for over 20 years! JWST also has keen vision and has observed it in 2025, adding to the baseline. By very carefully measuring the positions of stars in the cluster, a team of astronomers found one moving on a curved path, indicating it was orbiting a companion object. By measuring the shape of the curve, the astronomers determined the object has a mass of about 4.46 times that of the sun (± about 1 solar mass). A star that mass would be very bright (and also long dead in the ancient cluster), so, since nothing is seen, the object must be a black hole! [link to journal paper]

The normal star in the system is about 0.8 times the sun’s mass, and the pair has a decently elliptical orbit around each other with a roughly 95-year period. Along its long axis the orbit is about 9 billion kilometers across, around the same size as Neptune’s orbit around the sun.

The figure above shows the measurements made of the position of the visible star, with its actual motion through space subtracted away to show the orbit better. The dashed line is a best mathematical fit of the orbit. Note that while it looks elliptical in the figure, we’re looking down the long axis of the ellipse, so it’s distorted heavily by perspective! The long axis is actually from about the 8 o’clock to 2 o’clock positions, nearly horizontal.

I’ll note that these measurements are phenomenally precise. The axes scales are in milliarcseconds. An arcsecond is a tiny measurement on the sky; the moon is 1,800 arcsec across. A milliarcsec is a thousandth of that! That’s much less than the size of a pixel on the Hubble and JWST cameras, and getting measurements on that scale is difficult, painstaking work.

To be clear: what they’re seeing is the physical motion of that star as it moves around the black hole! Luckily, the observations were made when the two were closest together in their mutual orbit (called periastron), so the motion was rapid enough to measure over two decades.

The uncertainties in the measurements are still substantial, though the mass is determined well enough to be pretty certain it’s a black hole. More observations with JWST (already planned) should help nail that down, though.

That’s important. A binary system like this has a finite lifetime; as it passes by other stars the two components can be pulled apart by the gravity of those stars. The astronomers calculate the mean lifetime of this system is about 800 million years, which is in itself an interesting number: it means this system isn’t primordial; that is, these stars weren’t born together. If they had the system would be about 12 billion years old, and would long ago have been torn apart. So the black hole and the star must have become bound together relatively recently.

More observations getting better measurements of the system will allow astronomers to better understand how it formed, how it’s evolved, and what its eventual fate will be. It will also help them determine how many more systems are like it in the cluster. There are almost certainly many black holes that remain undetected. Maybe they’re in longer orbits that make it harder to see the visible star’s motion. Maybe they’ve all been ejected from the cluster entirely. That seems unlikely, but only much better observations will be able to say.

Hopefully, this is the first of many such black holes found, and, even more hopefully, over time more will be found in other clusters as well (a handful have already, but only a handful). There’s still much we don’t understand about globulars — or stripped dwarf galaxies, as the case may be — and observations like this will go a long way in helping.

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