A new study led by PhD student Ethan Winch explores how stellar spin and mass loss shape the Universe’s heaviest black holes — connecting gravitational wave discoveries to the lives of the earliest stars.
When the first gravitational waves rippled through Earth’s detectors in 2015, it was a historic moment. A century after Einstein predicted them, we finally had direct proof that gravitational waves are real — and not just that. The source of those waves surprised everyone: two enormous black holes, each around 30 times the mass of the Sun, smashing together over a billion light-years away.
The detection of the first gravitational waves from 30-40 solar mass Black Holes. Image Credit: LIGO, Caltech
Before then, most astronomers thought stellar-mass black holes — those formed when stars die (as opposed to the supermassive black holes lurking in galactic centres) — would usually be much lighter, typically between 5 and 15 solar masses. So finding black holes in the 30–40 solar mass range raised an immediate question: how did such heavy black holes form?
One important clue comes from how massive stars lose material during their lives. Massive stars shed their outer layers through powerful stellar winds, and crucially, the strength of these winds depends on how “metal-rich” the star is. The work of the Armagh massive star group, led by Prof. Jorick Vink, showed that the winds of Wolf-Rayet stars — hot, massive stars close to the end of their lives — weaken dramatically at low metallicity. This means stars in primitive, metal-poor environments can retain much more of their mass until they die.
This suggested that the massive black holes detected by LIGO might have formed from low-metallicity stars — stars born in early, metal-poor galaxies very different from the Milky Way.
But the story doesn’t end there.
Another critical piece of the puzzle involves a phenomenon called pair instability. Very massive stars can become so hot and energetic that the light inside their cores — photons — transforms into matter and antimatter, draining pressure which is vital for keeping the star stable, and triggering violent instabilities. If a star’s core becomes too massive, this process can completely disrupt the star or prevent it from forming a black hole of certain sizes — creating what astronomers call the second mass gap.
A new study, recently accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS) and led by PhD student Ethan Winch at the Armagh Observatory and Planetarium (AOP), takes this story a step further. The research team — Ethan Winch, Gautham Sabhahit, Jorick Vink, and Erin Higgins — asked: how do stellar rotation and mass loss together shape the maximum mass of black holes?
Using detailed simulations with the stellar evolution code MESA, they explored how massive stars behave when they spin rapidly. They found that fast rotation drastically alters a star’s internal structure. Once a star spins at about 60% of its critical speed (the point at which it would start to break apart), it becomes chemically homogeneous — meaning the star mixes its interior thoroughly and evolves like a stripped-down, compact star.
This chemical mixing leads to smaller final core masses, setting a hard limit on how massive a black hole can grow before encountering pair instability. The study confirms that this threshold remains at a carbon-oxygen core mass of about 36 solar masses, even for these fast-spinning stars.
Excitingly, this matches the “bump” in the black hole mass distribution observed by LIGO and Virgo — providing strong evidence that rotation, metallicity, and mass loss are key players in shaping the heavy black holes we observe today.
In short, the gravitational waves that first proved Einstein right have also opened a new window into the lives and deaths of the Universe’s most massive stars — revealing that spin, stellar winds, and cosmic chemistry all weave together to create the heavy black holes we are now finally beginning to understand.
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