August 19, 2026
By Eurasia Review
Key Takeaways:
Precise measurements of a reaction powering Type-I X-ray bursts reduce uncertainty by more than tenfold and confirm that the nickel-copper cycle traps some nuclear material, affecting the bursts’ light curves.
Together the studies replace theory and estimates with experimental evidence, helping scientists better understand how stellar explosions forge and distribute chemical elements.
New clues about how stars explode and forge the chemical elements that make up everything from planets to people have come to light, thanks to research led by the University of Surrey.
In two separate papers published in Physical Review Letters, researchers investigated the nuclear reactions that take place inside supernovae and X-ray bursts – two of the Universe’s most powerful explosions. Together, the findings will help scientists build more accurate models of how stars explode and how newly formed elements are produced.
The first study focused on supernovae – bright, powerful explosions that mark the death of massive stars. Despite centuries of observations, scientists still do not fully understand how these explosions unfold. One of the best clues comes from radioactive titanium-44, which is produced during a supernova and can still be detected by space telescopes long after the explosion.
At Argonne National Laboratory in the United States, Surrey researchers obtained the first experimental data needed to determine the rate of a nuclear reaction that controls how much titanium-44 is produced during a supernova. They found the reaction happens much more slowly than previously thought, increasing predicted titanium-44 production by up to 35 per cent.
This will allow astronomers to compare computer models more closely with real observations, bringing them a step closer to understanding how supernovae occur.
The study’s lead, Dr Christopher Cousins, a postdoctoral researcher in the Nuclear Physics Group, said:
“It’s exciting to see just how far the field has come. A measurement like this would have been considered out of reach only a couple of decades ago, but it now gives us new insight into one of the biggest unanswered questions in astrophysics.”
The second study investigated X-ray bursts (Type-I) – the most frequent stellar explosions in the Universe – which occur when a dense neutron star pulls material from a nearby companion star, triggering repeated thermonuclear reactions that build heavier elements and release enormous amounts of energy.
Working at the new Facility for Rare Isotope Beams (FRIB) in Michigan, USA, the team measured a nuclear reaction that powers X-ray bursts with far greater precision than previously possible, reducing uncertainty about how the reaction behaves by more than tenfold. The results settle a long-running question over the role of the nickel-copper cycle – a process that can temporarily trap nuclear material during an explosion – showing it influences X-ray burst light curves.
Lead author, Connor O’Shea, who is a postdoctoral researcher within the University of Surrey’s Nuclear Physics Group, said:
“One of the biggest unknowns was whether material becomes trapped in the nickel-copper cycle during an X-ray burst. We’ve shown that it does, but likely only a small proportion, giving us a much more realistic picture of these explosions.”
Professor Gavin Lotay, Professor of Nuclear Physics at the University of Surrey and principal investigator on both studies, said:
“Despite decades of research, we still don’t fully understand the nuclear reactions that power some of the Universe’s most spectacular stellar explosions. These two studies answer important questions about what happens inside both X-ray bursts and supernovae, providing experimental evidence where scientists previously had to rely on theory and estimates.
“Together, they give us a much clearer picture of how these explosions happen, allowing us to compare our models more closely with astronomical observations and bringing us closer to understanding how the chemical elements are created and spread throughout the Universe.”
Dark stars may have left gravitational-wave echos across the Universe
New study shows that pulsar timing arrays could probe how the Universe’s first supermassive black holes were born
image:
Predicted nanohertz gravitational-wave backgrounds from descendants of early supermassive black-hole seeds. Models in which black holes originate from collapsed supermassive Dark Stars can reach the gravitational-wave background measured by Pulsar Timing Arrays, whereas the much rarer direct-collapse-black-hole population considered in the study produces a substantially weaker signal.
view moreCredit: Ghodla and Ilie, Physical Review D (2026).
Hamilton, NY — August 19, 2026 — A mysterious background of extremely low-frequency gravitational waves detected by networks of pulsars may carry information about events that began more than 13 billion years ago — including the formation of some of the first supermassive black holes in the Universe.
In a new study published as a Letter in Physical Review D, Sohan Ghodla and Cosmin Ilie of Colgate University investigate whether supermassive black holes formed in the early Universe could ultimately produce a substantial fraction of the gravitational-wave background now observed by Pulsar Timing Arrays, or PTAs.
Their results establish a direct connection between two seemingly very different observational frontiers: observations of unexpectedly massive black holes in the young Universe and gravitational waves produced by supermassive black-hole binaries billions of years later.
Remarkably, the researchers find that one possible population of early black-hole seeds — black holes left behind by supermassive Dark Stars — could potentially account for a dominant contribution to the observed PTA signal. “Pulsar timing arrays are usually thought of as probes of supermassive black-hole binaries in the relatively recent Universe,” said Ilie. “What our work shows is that the signal may also contain information about how the ancestors of those black holes formed at cosmic dawn. In that sense, gravitational waves observed today could provide a new window onto the birth of the first supermassive black holes.”
Using cosmic clocks to detect enormous black holes
Pulsar Timing Arrays use rapidly rotating neutron stars called pulsars as extraordinarily precise cosmic clocks. Passing gravitational waves subtly alter the arrival times of radio pulses reaching Earth. By monitoring many pulsars over years, collaborations around the world have detected evidence for a stochastic gravitational-wave background at nanohertz frequencies.
The leading astrophysical explanation is a cosmic population of inspiraling supermassive black-hole binaries. Black holes with combined masses greater than about a billion times the mass of the Sun are particularly important contributors at PTA frequencies. But building such enormous black holes raises another question: Where did their original seeds come from? Observations with facilities including the James Webb Space Telescope and Chandra have revealed massive black holes surprisingly early in cosmic history, intensifying interest in mechanisms capable of producing massive black-hole seeds rapidly.
Ghodla and Ilie asked whether descendants of such early seeds could survive, grow with their host galaxies, eventually form binaries, and generate the gravitational-wave background measured billions of years later.
A possible gravitational-wave signature of Dark Stars
The researchers considered two early black-hole formation channels: direct-collapse black holes and the collapse of supermassive Dark Stars.
Dark Stars are a proposed type of primordial star whose principal energy source is heating associated with dark matter rather than ordinary nuclear fusion. In the WIMP dark-matter scenario considered in the study, Dark Stars can remain comparatively cool and extended while continuing to accrete matter, potentially reaching masses of a million Suns or more before collapsing into massive black holes.
In this work Ghodla and Ilie followed the cosmological evolution of black holes produced by such seeds, modeled the halos in which they reside, calculated their merger rates and predicted the resulting gravitational-wave background. They find that if supermassive-Dark-Star remnants have a number density of order (10^{-3}\ {\rm Mpc}^{-3}), their descendants can make a major — and potentially dominant — contribution to the PTA gravitational-wave signal.
The competing direct-collapse-black-hole population considered in the study is expected to be far rarer, with characteristic densities around (10^{-6}\ {\rm Mpc}^{-3}), and consequently produces a much smaller contribution.
Turning gravitational waves into a census of the early Universe
The key insight of this work is the fact that existing PTA measurements can be used to place an upper limit on how abundant the early seeds of Supermassive Black holes could have been. “Produce too many of these massive seeds and you end up over-producing the PTA-detected signal. Produce too few, and you need other sources to efficiently assemble these supermassive black holes later in the life of the universe to match PTA observations,” said Ghodla.
For the models explored in the study, the researchers find that seed densities approximately in the (10^{-2})–(10^{-1}\ {\rm Mpc}^{-3}) range would begin to overproduce the measured gravitational-wave background, with the precise constraint depending strongly on the masses of the dark-matter halos in which the seeds formed. This means PTA observations can potentially do something unexpected: constrain populations of objects that existed at redshifts greater than 10, even though the gravitational-wave-producing mergers of their descendants occur much later. The calculation also confirms a previous result that binaries with total black-hole masses roughly above (10^9) solar masses dominate the predicted PTA signal. Lower-mass binaries contribute considerably less.
The result provides a new observational connection among dark matter physics, the formation of the first luminous objects, the origin of supermassive black holes and gravitational-wave astronomy.
“Dark Stars were originally proposed as objects that might be seen directly at cosmic dawn,” Ilie said. “This work points to a completely different way of testing their possible role in cosmic history. Their descendants could leave a gravitational-wave imprint that persists all the way to the present-day Universe.”
Future improvements in PTA measurements, combined with better constraints on high-redshift black-hole populations and their host galaxies, could therefore help distinguish among different scenarios for the origin of the Universe’s first supermassive black holes.
Journal
Physical Review D
Article Title
Reconstructing PTA measurements via early seeding of supermassive black holes
Article Publication Date
17-Aug-2026
Milky Way's fastest star orbits our supermassive black hole so closely it feels its spin
image:
This sequence of images, taken with the GRAVITY instrument at ESO’s Very Large Telescope Interferometer (VLTI), show several stars orbiting Sagittarius A*, the supermassive black hole at the centre of our galaxy. One of these stars, known as S301, was recently found to pass much closer to the black hole than any other known star.
S301 orbits Sagittarius A* every 8.7 years. At its closest approach — similar to the distance between the Sun and Saturn — the star moves at more than 8% the speed of light. The size of Neptune’s orbit is shown as a reference.
S301 passes so close to Sagittarius A* that its orbit could be affected by the rotation of the black hole, which twists space-time around it. The effect is small, but could be potentially measured in the next few years with the VLTI and ESO’s Extremely Large Telescope (ELT), currently under construction.
The orbit of S301 is displayed here with an elliptical curve. In each frame, the solid curve shows the path that the star has already completed up to that point, whereas the dashed one marks the path it will follow afterwards.
view moreCredit: ESO/GRAVITY collaboration
Astronomers have discovered the fastest known star in our galaxy, the Milky Way, orbiting the black hole at its centre. The star, named S301, was detected with the European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI) and reaches speeds of 25 000 km/s as it travels around the four-million-Solar-mass black hole. It comes closer to it than any other observed before, so close that it feels the effects of the black hole’s rotation.
“Decades carefully tracking stars orbiting our galaxy’s central black hole, Sagittarius A*, have led to this breakthrough discovery of a very promising star. Because it orbits so close to Sagittarius A*, S301 opens a new window to the fundamental properties of spacetime in this extreme black-hole environment,” says Nobel Prize winner Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and founding member of the collaboration that made the new observations.
“What is special about this star is that it’s orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the Sun. That is unprecedented,” says Felix Mang, PhD student at MPE and author of the study published today in Nature.
During its closest passage to the black hole, the star travels at around 25 000 kilometres per second — 100 000 times faster than a commercial plane, or over 8% of the speed of light — making it the record holder for the fastest star in the Milky Way. S301 also comes closer to Sagittarius A* than any other star observed so far, approaching the black hole at around the distance of Saturn to the Sun. [1] Because S301 comes so close to Sagittarius A*, it is the first star known that could be used to directly measure the rotation of a black hole.
Like most things in our Universe, astronomers predict that Sagittarius A* spins. According to Einstein’s general theory of relativity, a spinning black hole drags spacetime along with it and twists it, which impacts the orbits of surrounding stars. The effect is felt more strongly for objects orbiting fast-rotating black holes at close range.
“With this star we hope to measure, within the next 10 years, the spin of the black hole," says Mang. MPE researcher Stefan Gillessen, who also had a leading role in the new study, adds: “For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein’s theory.” Juan Osorno, an astronomer at LIRA Observatoire de Paris–PSL, France, who also had a key role in the study adds: “Without this star, we would need to measure the motion of other stars for several more decades to get anywhere close to measuring the spin of the black hole.”
Finding S301, which appears two billion times fainter than Betelgeuse (the orange star in the constellation Orion) in the sky, was no easy feat. The team used the VLTI, a facility at ESO’s Paranal Observatory in Chile and its GRAVITY instrument, now known as GRAVITY+ following an infrastructure upgrade. [2] The VLTI’s superpower lies in its ability to combine the light from four 8-metre telescopes to create a ‘virtual’ telescope with 15 times the spatial resolution of a single 8-metre telescope.
“Worldwide, Paranal is the only place where you can do this type of observations because no other observatory in the world has four 8-metre telescopes that can act together as an interferometer,” says co-author Frank Eisenhauer, GRAVITY+ Principal Investigator and Director at MPE.
With GRAVITY, and later with GRAVITY+, the team managed to catch a first glimpse of the new star in spring 2023 and have followed it since to constrain its orbit. They could also trace S301’s orbital history back to 2017, finding that it last made its closest approach to the central black hole in early 2023. S301’s orbital properties, and the fact that stars cannot form so close to a massive black hole, indicate that the star was likely part of a binary pair that was torn apart by the tidal forces of Sagittarius A*. In the process, S301 became trapped by the black hole’s gravity while its companion star was kicked out with high velocity, most likely enough to leave the galaxy altogether.
Follow-up observations with GRAVITY+, and with the MICADO instrument on ESO’s upcoming Extremely Large Telescope (ELT), will be crucial for tracing S301’s path over the next decade, as it makes its next closest passage in 2031. Observing at least two complete orbits of S301 allows its trajectory to be constrained with high enough precision to enable the team to directly determine the spin of Sagittarius A* for the first time. “That would be a dream come true,” says Mang.
Notes
[1] At its closest approach, the star passes just 1.78 billion km from the black hole, around 12 times the Sun-Earth distance or just 20% larger than the Sun-Saturn distance.
[2] Following decades mapping stars orbiting the Milky Way’s centre using different ESO facilities, the team has been using the GRAVITY instrument on the VLTI for this purpose since 2017. A series of upgrades to both the instrument and the interferometer, called GRAVITY+, has been implemented gradually over the last few years, and has allowed them to find increasingly fainter objects.
More information
This research was presented in a GRAVITY+ Collaboration paper titled “Discovery of a star sensitive to the spin of Sgr A*” to appear in Nature (doi: 10.1038/s41586-026-10894-w).
The team is composed of: K. Abd El Dayem (LIRA, Observatoire de Paris, Universitê PSL, CNRS, Sorbonne Université, Université de Paris, France), R. Abuter (European Southern Observatory, Garching, Germany [ESO Germany]), N. Aimar (Faculdade de Engenharia, Universidade do Porto, Portugal [FEUP], and Centro de AstrofÃsica e Gravitação, IST, Universidade de Lisboa, Portugal [CENTRA]), P. Amaro-Seoane (Universitat Politècnica de València, Spain and Max Planck Institute for Extraterrestrial Physics, Garching, Germany [MPE]), A. Berdeu (ESO and LIRA), J. P. Berger (Univ. Grenoble Alpes, CNRS, Grenoble, France [IPAG]), G. Bourdarot (MPE), W. Brandner (Max Planck Institute for Astronomy, Heidelberg, Germany [MPIA]), A. Burkert (University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität, Munich, Germany [LMU] and MPE), D. Calderon (Max Planck Institute for Astrophysics [MPA], Garching, Germany), C. Correia (FEUP and CENTRA), J. Cuadra (Universidad Adolfo Ibañez, Viña del Mar, Chile and Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes [TITANS], Chile), R. Davies (MPE), D. Defrère (Institute of Astronomy, KU Leuven, Belgium [KU Leuven]), L. Delit (LIRA), A. Drescher (IPAG and MPE), F. Eisenhauer (MPE and Department of Physics, Technical University of Munich, Germany [TUM]), L. Esteras Otal (ESO Germany), M. Fabricius (MPE), H. Feuchtgruber (MPE), N. M. Förster Schreiber (MPE), A. Foschi (LIRA), P. Garcia (FEUP and CENTRA), R. Garcia Lopez (School of Physics, University College Dublin, Ireland), A. Generozov Astronomy Dept. and Oden Institute, University of Texas at Austin, USA), R. Genzel (MPE and Departments of Physics & Astronomy, Le Conte Hall, University of California, Berkeley, USA) S. Gillessen (MPE), F. Gonté (ESO Germany), X. Haubois (European Southern Observatory, Santiago, Chile [ESO Chile]), S. F. Hönig (School of Physics & Astronomy, University of Southampton, United Kingdom [Southampton]), M. Houllé (IPAG), S. Joharle (MPE), A. Kaufer (ESO Chile), J. Kammerer (ESO Germany), P. Kervella (LIRA), J. Kolb (ESO Germany), L. Kreidberg (MPIA), R. Laugier (KU Leuven), S. Lacour (LIRA), O. Lai (Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange [Lagrange]), J.-B. Le Bouquin (IPAG), J. Leftley (Southampton) B. Lopez (Lagrange), D. Lutz (MPE), F. Mang (MPE and TUM), A. Mérand (ESO Germany), F. Millour (Lagrange), M. Montargès (LIRA), N. Morujão (FEUP and CENTRA), H. Nowacki (Lagrange), M. Nowak (LIRA), S. Oberti (ESO Germany), J. Osorno (LIRA), T. Ott (MPE), T. Paumard (LIRA), C. Paladini (ESO Chile), H. B. Perets (Physics department, Technion - Israel Institute of Technology, Haifa, Israel), K. Perraut (IPAG), G. Perrin (LIRA), R. Petrov (Lagrange) P. O. Petrucci (IPAG), T. Piran (Racah Institute of Physics, The Hebrew University of Jerusalem, Israel [Racah]), N. Pourré (IPAG), S. Rabien (MPE), D. C. Ribeiro (MPE), S. Robbe-Dubois (Lagrange), M. Sadun Bordoni (MPE), J. Sánchez Bermúdez (Instituto de AstronomÃa, National Autonomous University of Mexico, Mexico), D. Santos (MPE), R. Sari (Racah) J. Sauter (MPIA), S. Scheithauer (MPIA), J. Scigliuto (Lagrange) J. Shangguan (MPE), T. T. Shimizu (MPE), F. Soulez (Univ. Lyon, Univ. Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon, France), J. Stadler (LMU), C. Straubmeier (1st Institute of Physics, University of Cologne, Germany), E. Sturm (MPE), M. Subroweit (Cologne), C. Sykes (Southampton), L. J. Tacconi (MPE), P. Thévenet (LIRA), I. Urso (LIRA), F. Vincent (LIRA), J. Woillez (ESO Germany), G. Zins (ESO Chile).
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