Erika 

Moon Europa. Image taken by #NASAJuno and processed by myself. #Europa #Jupiter #News #Astronomy #Science #NASA #Photography 🐘 mstdn.social/@GirlInSpace

When the @ehtelescope Horizon Telescope released its first image of M87* in 2019, the most striking feature was a bright ring surrounding a dark centre. The glow comes from hot plasma close to the black hole, about 55 million light-years away. Its gravity bends the light on its way to us and captures some of it, leaving a shadow against the surrounding glow. An independent team has now returned to the telescope measurements to ask how much of that familiar shape the data can actually pin down. The question arose from the way the image was made. Radio telescopes on different continents observed M87* at the same time. Each pair recorded information about structure on a particular scale and in a particular direction. The gaps between those measurements leave room for more than one image to fit the data. In one earlier reanalysis, changing the size of the patch of sky being reconstructed was enough to produce an elongated bright patch where the ring had been. Samuel Lai and colleagues at Australia’s @CSIRO took a different approach. They combined the signals in a way that cancels out calibration errors introduced at individual telescopes while preserving information about the shape of the source. Their algorithm had no ring built into it. Yet when they applied it to the original observations, a ring emerged around a dark centre, measuring 42.2 ± 1.5 microarcseconds across. At M87’s distance, that diameter is consistent with a black hole about 6.5 billion times as massive as the Sun. The finer details, including exactly where the plasma appears brightest, are still harder to recover. What now stands on firmer ground is what most of us remember from that first image. A different team, working from the same measurements with a different method, recovered both the ring and its size. 🍁 nature.com/articles/s41586-0…

On 18 November 2026, Voyager 1 will become the first human-made object to reach a distance of one light-day from Earth. @NASA predicts the milestone at approximately 10:16 UTC, when the spacecraft will be about 25.9 billion kilometres away. Nothing will change around the probe at that moment; the significance is in the distance itself. A light-day measures distance: how far light travels through a vacuum in 24 hours. That is approximately 173 times the mean distance between Earth and the Sun. Voyager 1 has spent more than 49 years getting that far, travelling at around 61,000 kilometres per hour. Light covers the same separation in a single day. That delay governs every conversation with the spacecraft. Radio waves travel at the speed of light, so a command takes approximately 24 hours to arrive. Even if Voyager replies immediately, its response needs another 24 hours to reach Earth. Engineers must allow roughly two days just for the signals to make the round trip, before adding any time needed to carry out an instruction or analyse the result. Voyager 1 launched on 5 September 1977, visited Jupiter in 1979 and Saturn in 1980, and continued outward. On 25 August 2012, it crossed the heliopause, the boundary of the Sun’s solar-wind bubble, and entered interstellar space. The Solar System extends much farther, through the distant Oort cloud, whose outer regions remain far beyond Voyager’s position. Its work has changed with the journey. The cameras were switched off in 1990, but its magnetometer and plasma-wave instrument remain active. Its generators convert heat from radioactive decay into electricity, and their output keeps declining. Engineers conserve power by switching off equipment and finding ways to keep the remaining systems warm and operating. It also carries the Golden Record, a gold-plated copper disc containing 115 images, greetings in 55 languages, music and natural sounds from Earth. Those images are encoded in analogue form. The record will remain aboard after the spacecraft can no longer communicate. For now, a machine launched in 1977 is still sending scientific measurements home from interstellar space.

From a distance, Saturn’s rings look like a continuous surface. Up close, we would find countless individual particles following their own orbits: dust grains, pieces of ice and larger blocks that collide, gather into temporary clumps and separate again. We have been trying to understand this structure for four centuries. Galileo observed it in 1610 without being able to resolve its shape. Huygens recognised a ring surrounding Saturn in 1655, and Maxwell showed in 1859 that a stable ring system had to consist of independently orbiting particles. We now know that all four giant planets, Jupiter, Saturn, Uranus and Neptune, have rings. We have also found them around much smaller bodies, including Chariklo, Haumea, Quaoar and Chiron. Their materials, dimensions and histories vary considerably. Each particle stays in orbit through the combined effect of its motion and the gravity of the central body. Particles closer to the planet complete their orbits faster than those farther away. Collisions dissipate energy and reduce vertical motion. Around a planet flattened by its rotation, the material tends to settle close to the equatorial plane, producing a disc that can be extraordinarily thin compared with its width. Tidal forces help determine what happens to that material. Gravity pulls more strongly on the near side of a moon than on its far side. If this difference overcomes the forces holding a weak satellite together, the satellite can break apart. The Roche limit estimates where this becomes possible, depending on the densities and physical properties involved. Inside that region, loose fragments also have greater difficulty assembling into a lasting moon. Applying the idea requires care: internal strength, collisions and orbital disturbances all affect the outcome. Rings can therefore form in several ways. Colliding moons can scatter debris. A satellite moving too close to its planet can lose material or disintegrate. Micrometeoroid impacts can knock dust off small moons. Some models also consider material left from the early formation of a planetary system. Reconstructing which process occurred requires an explanation that fits the present composition, the amount of material and its orbital arrangement. Creating a ring in a simulation is only the beginning. Saturn has the most extensive and brightest dense ring system among the planets. Its main regions, the A, B and C rings, are dominated by water ice, with abundant particles ranging from millimetres to metres in size. The outer edge of the A ring puts the diameter of the main system above 270,000 kilometres. Many regions are only a few tens of metres thick, although waves and other structures can rise much higher. The bands visible in photographs reflect differences in particle density, size and the amount of light passing through the material. Moons continually influence this structure. Their gravitational disturbances can maintain sharp boundaries, open gaps and generate waves. Some act as shepherd satellites, restricting the spread of nearby material. Cassini also observed small embedded moons and the disturbances they create. Other waves originate in Saturn itself: oscillations inside the planet produce small changes in its gravitational field that affect the rings. Measuring these waves has allowed researchers to investigate Saturn’s interior, including an extensive transition region between its core and its surrounding envelope. The age of the rings remains disputed. Cassini measured their mass and composition, along with the incoming flux of interplanetary dust. Because that dust should gradually contaminate the ice, the relatively small amount of dark material was interpreted as evidence of youth. A study published in 2023 calculated an exposure age of roughly 100 to 400 million years under particular assumptions. That range was widely reported as the formation age, although the calculation depends on how much material arrives, how much remains and how the rings have evolved. Later work has challenged some of those assumptions. Simulations published online in 2024 suggest that much of the incoming contaminating material could be vaporised and removed after impacts. A reassessment published in Icarus in 2026 found that changes to the treatment of incoming dust, together with possible cleaning processes, can permit ages of billions of years. Ancient rings therefore remain a possibility. Contamination, mass loss and orbital evolution still need to fit into a consistent explanation before their age can be established with confidence. Saturn’s individual rings also have different sources. The E ring is continually supplied with ice particles expelled by the plumes of Enceladus, connecting a subsurface ocean to the planet’s surroundings. The narrow F ring changes through interactions with small moons. Much farther out lies the faint, dusty Phoebe ring, tilted relative to the main ring plane. Asking for “the age of Saturn’s rings” therefore brings together components that may have formed and been replenished through different processes. Jupiter provides a different example. Its rings, discovered by Voyager 1 in 1979, are faint and dominated by dark dust. The system includes a main ring, a thicker inner halo and two very diffuse outer components associated with Amalthea and Thebe. Impacts on these moons, as well as on Metis and Adrastea, release particles that supply the rings. Small grains respond to sunlight and Jupiter’s intense electromagnetic environment, changing orbit or being removed. Maintaining the system requires fresh material. The particles visible today can be much younger than the system producing them. Uranus has thirteen known rings. Many are narrow, dark and difficult to observe; others contain much more widely dispersed dust. Their discovery in 1977 came from small drops in a star’s brightness before and after Uranus passed in front of it. Those interruptions revealed material that telescopes could barely distinguish directly. The rings lie approximately around Uranus’s equator and share its extreme tilt relative to the plane of its orbit around the Sun. The disruption of earlier moons is one proposed origin, although the system’s detailed history remains uncertain. Even Uranus’s rings differ substantially from one another. A 2026 study combined Keck, Hubble and Webb observations to examine its two outer rings, μ and ν. The μ ring’s spectrum is consistent with water-ice particles, probably released by impacts on the small moon Mab. The ν ring has properties consistent with rocky material mixed with organic substances and may receive dust from larger bodies that have yet to be identified. Two rings around the same planet can be supplied by objects with different compositions. Neptune has at least five main rings: Galle, Le Verrier, Lassell, Arago and Adams. They are faint, dark and dusty. Within Adams, some material is concentrated into arcs, denser sections occupying only part of the ring’s circumference. Their persistence is difficult to explain because differences in orbital speed tend to spread the particles out. Resonances involving nearby moons, and possibly small bodies sharing similar orbits, are among the mechanisms being investigated. Observations from Earth and space allow researchers to follow their changes. Their composition offers evidence of a potentially violent past. A study published in 2026 identified signatures of minerals in the rings and some inner moons that form through interactions between liquid water and rock. The authors propose that this material came from the interiors of larger bodies that were destroyed: earlier moons destabilised after Triton’s capture, or a large object torn apart by Neptune’s tides. Some of the debris could then have formed the present rings and small satellites. The observed composition supports this reconstruction, while its detailed dynamics still need to be tested. In October 2026, researchers announced a new arc, Sincérité. Comparing Keck images from 2021 with Webb observations from 2022 revealed a concentration of material trailing Fraternité. Meanwhile, Courage and Liberté, seen by Voyager 2, were no longer detected, and Égalité had weakened. The arc system is reorganising over decades. One possibility is that material is moving from fading leading arcs into other concentrations farther behind, although that interpretation requires further observations. Small bodies have considerably broadened the problem. In 2014, astronomers announced two narrow rings around Chariklo, discovered through a stellar occultation observed in 2013. Chariklo is a centaur roughly 250 kilometres across, orbiting in the region of the giant planets. Its rings demonstrated that a relatively small body can retain material in an organised ring system. Proposed origins include collisions, material loss and episodes of gravitational interaction. Small moons might help confine the rings, but that possibility requires establishing which satellites actually exist. An analysis of Webb observations published in September 2026 found that Chariklo’s inner ring was more opaque than expected, while the outer ring produced a much weaker signal. These results are consistent with changes in the amount or distribution of material, but comparisons also depend on the wavelength used. A ring can block visible light differently from infrared light. Further occultations are needed to distinguish physical changes from these wavelength effects. Haumea, a rapidly rotating and highly elongated dwarf planet, has a ring discovered in 2017 through another stellar occultation. It lies approximately in Haumea’s equatorial plane, about 2,300 kilometres from the centre, and has an estimated width of around 70 kilometres. Its orbital motion is close to a relationship in which Haumea completes three rotations for each revolution of the ring particles. Such relationships, called spin–orbit resonances, can influence where material survives. The ring’s origin and lifetime remain uncertain. Quaoar presents a particularly demanding case for models. This trans-Neptunian object, roughly 1,100 kilometres across, has two rings approximately 2,500 and 4,100 kilometres from its centre. Both lie beyond the Roche limit calculated using commonly assumed material properties. Under those conditions, particles might be expected to assemble into small satellites. Their persistence requires closer examination of how particles rebound during collisions, together with gravitational disturbances and resonances. The Roche limit describes a real physical effect, but it cannot by itself explain every location where a ring can survive. A study published in 2026 also reported an additional occultation event near Quaoar, consistent with an arc or small satellites. The detection expands the possible structure of the system, although the object responsible still needs to be established. A brief decrease in starlight can yield a precise measurement while leaving its physical interpretation unresolved. Chiron, another centaur, has a particularly variable environment. A study published in 2025 analysed a 2023 occultation and found signals best explained by three confined rings, accompanied by a broader diffuse structure and another faint outer feature. Comparisons with earlier observations indicate substantial changes in the surrounding material. The authors suggest that we could be observing a ring system undergoing formation and evolution. The source of its particles remains under investigation, including a possible connection with Chiron’s activity. It offers an opportunity to study how a debris environment changes while we watch. The future of these systems depends on exchanges of energy and angular momentum. Collisions and gravitational interactions redistribute material: some can move towards the central body while some spreads outwards. Moons can restrict this spreading or remove particles. Where conditions allow fragments to remain together, new satellites can form. Where impacts and tides disrupt them, material becomes available for rings again. These processes allow rings and moons to develop from one another during a system’s evolution. At Saturn, measurements show ring material entering the atmosphere. Some small particles acquire an electric charge, allowing the magnetic field to influence their motion in the process known as “ring rain”. If the estimated loss rates were representative over long periods, a substantial part of the system could disappear within a few hundred million years. That condition matters. Extending measurements from a spacecraft mission across such long intervals requires understanding how the rates vary and whether material is replenished. The loss is real; the timescale remains uncertain. This evolution is separate from changes in our viewing angle. Roughly every fifteen years, Saturn’s rings appear almost edge-on from Earth. Their enormous apparent surface narrows to a fine line, making them difficult to see through small telescopes. This periodic effect comes from viewing geometry. The material continues to orbit Saturn. For Jupiter, dust production by its moons should remain essential as long as processes continue removing small grains. Around Uranus and Neptune, satellite interactions and impacts can alter both the rings and their sources. Small bodies face additional consequences from possible close encounters with planets and episodes of activity. Their masses, particle properties and loss rates remain too poorly constrained to assign reliable lifetimes. Observing change provides evidence of evolution; predicting its outcome requires more information. Rings could also appear around planets that currently lack them. Phobos is slowly moving closer to Mars through tidal interactions with the planet. Some models suggest that, within a few tens of millions of years, its weakest material could break apart and form a ring. The timing and outcome depend on the moon’s internal structure and strength. Some of that debris would eventually fall onto Mars. Future observations will need to measure this evolution more precisely: repeating occultations, comparing images obtained under equivalent conditions, analysing spectra and searching for small satellites that might control the material’s distribution. Researchers also need to distinguish the age of individual particles from the age of the system maintaining them. In some cases, we have watched a ring for decades while its components are replenished. In others, almost everything we know comes from a handful of occultations. Establishing how long rings survive starts with measuring what changes between one observation and the next.

A new theoretical study suggests that gravitational fields could undergo “reconnection”, a process with similarities to the magnetic reconnection that powers solar flares. Physicists developed the idea within Einstein’s general relativity. On the Sun, magnetic reconnection rearranges the connections between magnetic field lines, transferring energy to the surrounding plasma. The researchers investigated whether an analogous rearrangement could occur in gravity, using a mathematically equivalent formulation of Einstein’s equations. Their earlier work identified conditions under which connections between gravitational field structures remain preserved as spacetime evolves. These structures can be represented as two-dimensional “sheets” within four-dimensional spacetime. They provide a mathematical description of the field’s organisation. The new calculations explore what happens when those preserving conditions fail: the sheets can change their connections. The authors derived measures of how quickly this reconnection proceeds and how it alters exchanges of energy and momentum between gravity and matter. Spacetime itself remains continuous throughout this proposed process. Its possible relevance lies in understanding extreme events such as black hole mergers, where gravitational fields evolve dramatically. Reconnection has yet to be identified in nature; simulations are needed to establish where it occurs and whether it leaves a measurable signature in gravitational waves. 🍁 share.google/RGPyO3J62pGU57X…

Polaris appears almost stationary in the northern sky while other stars seem to circle around it. Its position makes it useful for finding north, but that alignment is temporary. Earth’s rotation axis, the imaginary line around which our planet spins, slowly changes direction in space. This movement is called AXIAL PRECESSION. Over approximately 25,800 years, the axis sweeps out a roughly conical surface, and its projection onto the sky traces an approximately circular path. The cause lies in Earth’s shape and its gravitational interactions. Rotation makes our planet slightly flattened at the poles and wider around the equator. The gravitational pull of the Moon and the Sun on this equatorial bulge produces a torque, a turning effect that gradually redirects Earth’s angular momentum. As a result, the orientation of the spin axis slowly changes. The Moon provides the larger contribution to this motion. There are two different aspects of the axis to consider: its inclination and the direction in which it points. Earth’s current axial tilt is about 23.4°, measured from a line perpendicular to its orbital plane. Precession changes the direction of that tilted axis. The tilt angle itself, called obliquity, also varies, following a separate cycle of approximately 41,000 years. A simplified precession diagram therefore shows an axis maintaining the same tilt as it turns around the perpendicular to the orbital plane. These changes gradually move the celestial poles across the background of stars. Around the time the Egyptian pyramids were built, the north celestial pole lay near Thuban, in Draco. Today it lies close to Polaris. In roughly 12,000 years, Vega will serve as a bright northern reference, although it will remain several degrees from the pole and provide a less precise marker than Polaris does today. Precession also shifts the equinox points, where the celestial equator intersects the Sun’s apparent annual path across the sky. This is the origin of the term “precession of the equinoxes”. Hipparchus identified the effect in the second century BCE by comparing observations made at different times. Modern astronomers account for it when calculating celestial positions and pointing telescopes accurately. There is a climate connection, too. Earth follows a slightly elliptical orbit, reaching its closest point to the Sun in early January. Precession changes where the seasons occur along that orbit, altering the seasonal distribution of incoming sunlight. The orbital ellipse also slowly changes orientation; together, these motions produce climatic precession rhythms of roughly 19,000–23,000 years. Alongside variations in obliquity and orbital shape, they form the Milankovitch cycles that help regulate the growth and retreat of ice sheets over geological timescales. The actual motion includes smaller oscillations called nutation, superimposed on the slow precession. Its strongest component has a period of about 18.6 years, associated with the changing orientation of the Moon’s orbital plane. Precise models of Earth’s orientation include both motions, allowing observations made from our moving planet to be placed accurately on a celestial map.

For anyone who sees a white trail behind a plane and immediately thinks “they’re spraying us”: let’s talk physics. 😁 Burning jet fuel produces water vapour. At around 10 kilometres above the ground, air temperatures can fall below −40 °C. As the hot exhaust mixes with that cold air, it cools, allowing water vapour to condense into tiny droplets that freeze. Those ice crystals form the visible contrail. Water vapour itself is invisible. How long the trail lasts depends on atmospheric conditions. In dry air, the ice turns directly back into water vapour and the trail quickly disappears. In sufficiently humid air, the crystals can grow by taking up water vapour from their surroundings. The trail can then last for hours and spread into broader clouds. Temperature and humidity vary across different regions and altitudes. Two planes that look close together from the ground can therefore leave very different trails. A single aircraft can also leave a broken trail as it crosses areas with different conditions. Crossing flight paths and winds help explain the patterns we see overhead. Aviation releases pollutants, and persistent contrails also affect the climate. These are real effects that deserve research and action. A photograph of a white line in the sky, however, is not evidence of a secret spraying operation. Before reporting a conspiracy at 10,000 metres, it helps to understand what happens when water cools down. 😉

El Gordo is an enormous galaxy cluster made up of two smaller clusters colliding at thousands of kilometres per second. Its light has travelled for roughly seven billion years to reach us. The distribution of its galaxies, hot gas and dark matter offers a way to investigate how these components behave during a collision. In the standard cosmological model, dark matter particles have negligible interactions with one another beyond gravity. A new study explores whether allowing them to collide could explain some unusual features of El Gordo. These include the brightest X-ray emission from one subcluster lying ahead of the centre of its dark matter distribution. The simulations introduce dark matter particles with two internal energy states. Some collisions redistribute kinetic energy, while others use part of that energy to raise particles into an excited state, reducing the energy associated with their motion. Including this additional interaction allows the model to reproduce properties of the merger while satisfying constraints from weak gravitational lensing, which measures how the cluster’s mass distorts the appearance of background galaxies. The interpretation remains uncertain. It depends strongly on estimates of the main cluster’s mass and the projected separation between the two dark matter concentrations. More precise measurements could make this particular explanation difficult to sustain. Testing other massive merging clusters will be essential to assess whether the proposed interactions occur in nature. ☁️ share.google/m6zOiT9IdgNozf0…

Researchers at the University of Ottawa have been testing an alternative cosmological model that describes a universe approximately 26.7 billion years old and seeks to explain observations without invoking dark matter or dark energy. The framework is called CCC+TL. Its proposed age remains a model-dependent prediction; the standard cosmological estimate is about 13.8 billion years. CCC+TL stands for “covarying coupling constants plus tired light.” It allows certain physical constants to evolve together over cosmic time and attributes part of the redshift of distant light to photons losing energy during their journey. Cosmic expansion also contributes to the redshift. Combining these effects changes how observed redshifts translate into distances and cosmic ages, giving early galaxies considerably more time to develop within this framework. The study highlighted in the announcement, published in Universe, examines Big Bang nucleosynthesis: the formation of helium, deuterium and other light nuclei in the early universe. The authors find that CCC+TL can preserve the standard model’s predicted abundances if nuclear reaction rates and neutron decay rates scale together with the altered expansion rate. Their calculations then produce nearly identical abundances. This agreement depends on those coordinated changes. Adopting a lower baryon density can reduce the longstanding lithium discrepancy, but simultaneously produces too much deuterium. A separate paper examines galaxy rotation using the SPARC database, which contains 175 disc galaxies. A modified version of the CCC prescription reproduces rotation curves with performance comparable to MOND fitted individually to each galaxy. This extends the framework’s application to galactic dynamics, although fitting rotation curves alone cannot establish which explanation of the gravitational discrepancy is correct. The researchers have also explored gravitational lensing in the Bullet Cluster. Their preliminary model produces additional gravitational effects concentrated around the galaxies, resembling the observed separation between the lensing signal and the hot gas. The paper acknowledges that this resemblance partly follows from constructing the extra-gravity profiles around member galaxies. A full comparison using the observed positions of multiple lensed images remains a further step. Other tests remain at different stages of development. A supernova time-dilation analysis is still under review, while the cosmic microwave background analysis announced by the team has not yet been published. The cautious interpretation is that these studies provide conditional consistency checks and reasons for further investigation. They have yet to establish a revised age for the universe or a comprehensive replacement for ΛCDM. 🍁 share.google/dopYSTDmhYx2XBP…

For decades, astronomers have been trying to understand why some massive stars contain significantly more helium at their surfaces than stellar evolution models predict. A new study led by researchers at the Instituto de Astrofísica de Canarias (@IAC_Astrofisica) provides compelling observational evidence that the explanation lies in the evolutionary history of binary star systems. Massive stars, particularly those of spectral type O, are among the hottest, most luminous and shortest-lived stars in the universe. Their intense nuclear fusion processes gradually alter their internal chemical composition, converting hydrogen into helium and, at later evolutionary stages, producing heavier elements. However, the chemical composition we measure through spectroscopy comes primarily from their outer layers. Explaining how material processed deep inside a star reaches its surface has remained a significant challenge, especially when the observed helium abundance exceeds the predictions of conventional single-star models. An international team examined 45 spectroscopic binary systems containing O-type stars, using observations from the IACOB, MONOS and OWN projects. These are systems in which the orbital motion reveals the presence of a companion, even though its light cannot be directly distinguished in the observed spectrum. The researchers compared surface helium abundances with orbital properties, including periods and eccentricities, and identified a striking relationship. Seven of the 45 systems showed enhanced helium abundances, and all seven had orbital periods shorter than approximately 15 days. They were also classified as runaway stars, meaning their motion differs significantly from that of the surrounding stellar population. Four displayed relatively high orbital eccentricities, while the remaining three showed ellipsoidal variations associated with tidal distortion. The statistical relationship between helium enrichment and these orbital characteristics was particularly strong, suggesting that the stars share a common evolutionary mechanism. The most plausible explanation involves mass transfer between stellar companions. In a massive binary system, the initially more massive star evolves faster and eventually expands enough for material to flow towards its companion. As the interaction progresses, helium-enriched material from deeper layers of the donor can be transferred onto the receiving star, changing the chemical composition of its surface. The receiving star may therefore appear unusually helium-rich even though that enrichment did not originate entirely from its own internal evolution. The donor star can subsequently lose much of its outer envelope, becoming a stripped star, and may eventually collapse into a neutron star or black hole. If a supernova occurs, the associated mass loss and possible asymmetric kick can modify the binary's orbit and give the surviving system an unusual velocity. This provides a possible explanation for the relationship between helium enrichment, orbital eccentricity and runaway motion. One of the systems included in the analysis is Cygnus X-1, a well-known binary containing a stellar-mass black hole. The researchers emphasise that these observations strongly support binary interaction as the dominant explanation for helium enrichment in the studied O-type stars, although the evolutionary histories of individual systems have not been directly reconstructed. The precise nature of several unseen companions also remains uncertain, and further observations will be necessary to distinguish between stripped stars and compact remnants. The implications extend beyond the chemical composition of individual stars. Binary interactions influence stellar masses, rotation rates, supernova progenitors and the formation of compact objects. Improving these evolutionary models is therefore essential for understanding the populations of neutron stars and black holes, the production of gravitational-wave sources and the chemical evolution of galaxies. Perhaps one of the most useful consequences of this research is that surface helium enrichment could become an observational signature of previous binary interactions. A star may retain chemical evidence of a companion's influence long after that companion has become difficult to detect or the original system has been disrupted. This gives astronomers an additional way to reconstruct the evolutionary histories of massive stars from the properties we can still observe today. 🍁 arxiv.org/pdf/2608.05421

Imagine rearranging Earth into a broad, flat disk while keeping its present mass and the laws of physics we know. To make the experiment reasonably clear, suppose the disk has a roughly uniform distribution of rock, dimensions comparable to those of a terrestrial planet, and slow rotation. We would also need something extraordinary to hold it in that shape temporarily. Under those assumptions, we can work out how gravity would affect people, oceans and air, and why the disk would deform once its support disappeared. Gravity acts between every piece of matter and every other piece. The gravitational pull you feel comes from the combined attraction of the entire planet. Each part contributes a force with its own strength and direction, and those contributions add together. For a perfectly spherical, spherically symmetric planet, the external gravitational field behaves exactly as though all its mass were concentrated at its centre. Earth is close enough to that arrangement for gravity near its surface to point approximately towards its centre. That is what defines “down” wherever you stand. A disk has a different geometry. Directly above the centre of one face, the sideways contributions cancel by symmetry, leaving an attraction perpendicular to the surface. Move away from the centre, however, and that cancellation becomes incomplete. For the roughly uniform disk imagined here, gravity develops a component directed inward across the surface. Towards the outer regions, its direction becomes increasingly tilted relative to the ground. This follows from adding the attractions of the disk’s individual parts. Close to a disk, we cannot simply replace all that matter with a point at its centre and expect an exact answer. That would change ordinary experiences. A hanging plumb line would point partly towards the disk’s interior. A dropped object would accelerate both towards the surface and inward. Walking towards the edge would require working against that inward attraction, so a geometrically flat road could demand the effort of an uphill walk. To stand comfortably, you would need to align your body against the local direction of gravity, which would no longer be perpendicular to the ground everywhere. The strength of your weight would also depend on the disk’s dimensions and mass distribution; preserving Earth’s total mass would not preserve present-day surface gravity at every location. The oceans would respond to the same altered gravitational field. Water at rest settles onto an equipotential surface: a surface along which moving a small amount of water would not change its gravitational potential energy. Its surface is locally perpendicular to effective gravity. This is why “level” water can follow a curved planetary surface. On our hypothetical disk, water spread across a flat face would experience an inward pull and begin redistributing towards the central region. Its eventual surface would be curved, with its depth and extent determined by the amount of water, the underlying terrain and the disk’s gravitational field. A large central accumulation is a reasonable consequence of this model, although its exact appearance would require calculation. The atmosphere would redistribute as well. Air remains around Earth because gravity binds it, while pressure differences help support it against that attraction. In the disk experiment, the direction and strength of gravity would vary across the surface, changing the pressure structure needed for balance. At comparable temperatures, air would tend to concentrate more strongly in the deeper gravitational potential towards the central region. The adjustment would produce air flows and change conditions across the disk. Predicting completely airless edges, however, would require additional assumptions about its dimensions, temperature and atmospheric mass. Flatness alone does not establish that outcome. All of this assumes that something continues to hold the rocky disk in place. Without that support, the planet’s own material would respond to gravity too. Rock can preserve irregular shapes when the gravitational stresses are modest. Small asteroids can therefore remain elongated or angular. As a body becomes larger and more massive, its own gravity produces stresses that can overcome the strength of its material. Rock can fracture, deform and, under sustained pressure and suitable temperatures, flow. This is why a small stone can keep its corners while a planet cannot maintain an enormous, unsupported rocky extension indefinitely. For an object with Earth’s mass and broadly planetary dimensions, a thin, slowly rotating disk would be far from mechanical equilibrium. Its material would rearrange towards a more compact, rounded configuration. Internal pressure would resist further compression, while gravity would continue acting throughout the body. The resulting balance is called hydrostatic equilibrium when pressure supports the material against gravity. A sufficiently relaxed, slowly rotating planet approaches a spherical shape because that geometry allows this balance to hold in every direction. Earth’s solid structure preserves mountains and other departures from equilibrium, but those are small compared with the planet itself. Rotation modifies that balance. In a rotating frame, the centrifugal effect is strongest farthest from the rotation axis, favouring a wider equator. Earth is therefore slightly flattened at the poles and wider around its equator. Faster rotation can produce greater flattening, but it also changes the stability of the body. We could not arbitrarily increase the spin of an Earth-mass rocky planet and assume it would become a stable, extremely thin disk. Its angular momentum, internal structure and ability to remain gravitationally bound would all matter. The deformation of our unsupported disk would release gravitational potential energy as matter moved into a more tightly bound arrangement. Some of that energy would become motion; collisions, friction and deformation would convert part of it into heat. Depending on the starting configuration, the process could involve enormous fractures, violent movements of rock and water, and substantial heating. An exact countdown or a particular explosion sequence requires a defined physical model. Radius, thickness, density, temperature, material strength and rotation all affect the evolution. A characteristic gravitational timescale can give an estimate, but a realistic account of the disruption and eventual settling would require a dynamical calculation. The magnetic-field claim also needs care. Earth’s main magnetic field is generated by the motion of electrically conducting fluid in its outer core. Rearranging the planet would profoundly disturb that system, but the surface’s flatness alone does not tell us whether a magnetic dynamo could operate in the resulting interior. Nor does losing an internally generated magnetic field automatically remove an entire atmosphere. Venus provides a useful example: it has a very dense atmosphere despite lacking an Earth-like intrinsic magnetic field. Atmospheric escape depends on several interacting processes, including gravity, heating and interactions with the solar wind. Earth’s nearly spherical shape is a physical consequence of its mass, its materials and its rotation. The same gravity that determines which way an object falls also governs how oceans settle and how the planet’s interior supports its own weight. Changing Earth into a disk would change all those relationships together. With ordinary rock, Earth’s mass and slow rotation, maintaining that flat shape would require a supporting mechanism capable of resisting forces that naturally drive the planet towards a rounded form.

Supermassive black holes existed when the universe was less than a billion years old, raising a difficult question: how did they acquire so much mass so quickly? One possibility is that some began as unusually massive black holes formed inside dense star clusters. A larger initial mass would reduce the growth required to reach the enormous masses inferred for early quasars. Webb has revealed environments where this process could have operated. In the Cosmic Gems arc, astronomers identified five compact star clusters seen when the universe was around 460 million years old. Firefly Sparkle contains ten clusters observed roughly 600 million years after the Big Bang. Both galaxies are magnified by gravitational lensing: foreground galaxy clusters bend and amplify their light, allowing Webb to distinguish structures that would otherwise be too small and faint to study individually. Within a sufficiently dense cluster, stars can collide and merge repeatedly. Successive mergers may produce a very massive star that eventually collapses into a black hole. Whether this happens depends on the cluster’s mass, size and internal dynamics: stars must interact frequently enough for substantial growth to occur within their short lifetimes. Compact clusters therefore provide particularly favorable conditions for this formation route. Low abundances of elements heavier than helium also help stars retain mass by reducing losses through stellar winds. Astronomers modeled stellar collisions and wind losses in these systems. They predict black holes of approximately 1,600–2,700 solar masses in Cosmic Gems and 180–810 solar masses in Firefly Sparkle. Comparisons with existing simulations support their approach, although unknown cluster structures, rotation and binary populations introduce uncertainties. Webb has observed the clusters; the proposed black holes remain unconfirmed. Such objects would occupy the intermediate mass range between ordinary stellar remnants and supermassive black holes. They would still need to grow substantially through gas accretion and potentially mergers. Forming relatively massive seeds early would make that subsequent growth easier, while the availability of surrounding material would help determine how far it could proceed. 🍁 share.google/bZ8E8T79h1EQbEa…

The #JWST has revealed massive black holes already present during the universe’s first billion years, raising questions about how they formed and grew so quickly. It has also discovered “little red dots,” compact, reddish sources whose physical nature remains under discussion. Some appear consistent with growing black holes embedded in dense gas, although their unusual properties leave room for several interpretations. Understanding these objects could help astronomers identify the conditions in which the earliest massive black holes developed. A study investigates the role of stellar collisions in these environments. Stars rarely collide in our region of the Milky Way, but the exceptionally dense clusters present in the young universe could have experienced frequent encounters. At sufficiently low relative speeds, collisions can merge stars into larger objects. Repeated mergers could eventually assemble an extremely massive star whose collapse would produce a substantial black hole, giving its subsequent growth a much larger starting mass. The researchers combined an analytical model with conditions drawn from cosmological simulations and tested selected predictions using Monte Carlo calculations. For some cluster configurations, they estimated that stars could accumulate tens of thousands to around a hundred thousand solar masses. They also examined clusters containing an existing black hole. Close to it, higher stellar velocities can make collisions destructive, releasing material that builds up a dense gaseous environment. This offers a possible source of the gas invoked in explanations of little red dots. The results identify plausible physical conditions for these processes, while their connection to particular observed objects remains unconfirmed. The model simplifies stellar encounters and cluster evolution, and its gas estimates omit cooling and feedback. These uncertainties affect both the predicted stellar masses and the amount of gas retained. More detailed calculations are needed to establish whether collisions can reproduce the observed properties of little red dots and explain a significant fraction of early massive black holes. 🍁 share.google/xw3y65fX9LYwFs0…

The stars and gas in a galaxy are constantly moving. In galaxies such as the Milky Way, much of that motion is organised into rotation around the galactic centre. Understanding where this rotation comes from requires looking back to the formation of galaxies, when gravity began gathering gas and dark matter into growing structures. The early universe contained small differences in density, and the regions that would become galaxies had irregular shapes and uneven surroundings. Nearby concentrations of matter pulled more strongly on some parts than others, exerting gravitational torques that gave these regions angular momentum. Gas arriving along cosmic filaments and encounters between growing structures supplied additional angular momentum. When two systems merged, some of their orbital motion could become internal rotation in the resulting galaxy. Angular momentum describes motion around a point or axis and depends on mass, velocity and how matter is distributed. Without an external torque, a system’s total angular momentum remains constant. If its matter contracts towards the rotation axis, its rotation can therefore become faster. Conservation explains how existing rotation changes during contraction; the initial angular momentum comes from the system’s formation and interactions. Gas also behaves differently from stars and dark matter. It can collide, heat up and radiate energy, allowing it to settle into a thinner configuration. Its angular momentum limits how far it can collapse towards the centre and helps determine the size of the resulting disc. Stars born within that disc inherit much of the gas’s orbital motion, producing a population that largely travels around the galaxy in the same direction. Simulations show how cooling and the alignment of incoming gas contribute to the formation of thin stellar discs. Subsequent evolution can change this organisation. Mergers redistribute angular momentum between stars, gas, dark matter and material expelled into the surroundings. Their effects depend on the masses, gas content and orbital orientations of the galaxies involved. Some elliptical galaxies retain substantial ordered rotation, while others have very little overall rotation and contain stars moving on orbits with many different orientations. The fact that their stars are moving does not necessarily mean that the galaxy has a strong collective spin. Observations distinguish these different populations through detailed maps of stellar velocities. Spiral arms have their own dynamics. In density-wave models, stars and gas can move through regions of enhanced density rather than remaining permanently inside the same arm. Gas becomes compressed as it encounters these regions, encouraging the formation of young stars that make the arms particularly conspicuous. The visible spiral pattern and the orbital motion of individual stars therefore need to be distinguished. The central supermassive black hole contributes to the gravitational field, but its influence dominates mainly in its immediate neighbourhood. Across most of a galaxy, stellar and gas orbits respond to the combined distribution of stars, gas and dark matter. The black hole contains only a small fraction of the galaxy’s total mass. Measuring orbital speeds also reveals information about that mass distribution. In the outer regions of many spiral galaxies, speeds remain higher than the observed stars and gas can explain under standard gravity. Extended dark matter haloes provide the standard explanation for this discrepancy. However, approximately constant orbital speed does not mean constant angular speed: a star farther out travels around a larger orbit and takes longer to complete it. Our solar system, for example, takes roughly 230 million years to orbit the Milky Way’s centre.