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TDE

What happens when a star is ripped apart within the accretion disk of an active black hole? New research probes the infrared echo from one of the first known events of this kind to learn more.

Feeding Habits of Black Holes

The centers of galaxies are hubs of activity, and in many galaxies, it’s mealtime. A galaxy’s central supermassive black hole can feast on a disk of dust and gas that persists for tens to hundreds of thousands of years, creating a luminous and variable active galactic nucleus.

On shorter timescales — more of a galactic snack — there are tidal disruption events, which occur when a star ventures too close to a supermassive black hole and is summarily torn apart by the black hole’s titanic tidal forces.

In the past decade, it’s become clear that these events aren’t mutually exclusive, and a black hole might snack on a star while dining on a disk. What does it look like when a tidal disruption event happens in the accretion disk of an active galactic nucleus?

Dusty Disk and Shredded Star

multi-wavelength light curve of PS16dtm

Multi-wavelength light curves of PS16dtm. The infrared behavior (triangles) is significantly different from the optical and ultraviolet behavior (squares and circles). Click to enlarge. [Jiang et al. 2025]

To probe this question, Ning Jiang (University of Science and Technology of China) and collaborators studied PS16dtm, one of the first tidal disruption events thought to occur in an active galactic nucleus disk. The team amassed archival data from the Neil Gehrels Swift Obsevatory, the Asteroid Terrestrial Last Alert System, the All Sky Automated Survey for SuperNovae, and the Wide-field Infrared Survey Explorer, piecing together a multi-wavelength record of the system’s behavior.

These data show that the ultraviolet and optical emission from the event rocketed to peak brightness in a matter of weeks before declining more gradually over the course of multiple years. The infrared emission behaved differently, rising more slowly and remaining high rather than falling. This persistently bright infrared emission is not a universal feature of tidal disruption events and instead appears to be a unique feature of tidal disruption events in active galactic nucleus disks.

Torus Insights

Why does the infrared emission behave so differently from other wavelengths? The answer lies beyond the accretion disk of the active galactic nucleus, in a vast dusty torus that surrounds the system. As light from the disrupted star reaches the torus, the dusty gas absorbs the light and re-emits it in the infrared, creating an infrared “echo” of the tidal disruption event.

schematic of the accretion disk and torus system

A schematic of the black hole, accretion disk, and dusty torus. The gray area shows the portion of the torus that may have been evaporated due to the tidal disruption event. The black hole and accretion disk are not to scale; if they were to scale, they would be too small to be visible in this diagram. [Jiang et al. 2025]

Using a dust echo model, Jiang’s team showed that in order for the infrared emission to remain bright for so long, the tidal disruption event powering the emission must be extremely luminous — so luminous that the radiation from the star’s destruction may have evaporated some of the torus, pushing its inner radius outward from 3.6 to 5.2 light-years. The fact that so much of the system’s energy emerges in the form of infrared light may offer a solution to the “missing energy” problem, in which tidal disruption events observed only at optical wavelengths appear to emit less energy than expected. In general, tidal disruption events that happen within active galactic nucleus accretion disks may also be more energetic than typical tidal disruption events due to interactions between the disrupted star and the gas of the disk.

For now, observing this dusty torus directly is out of reach, as it is too faint and too small to be captured by existing facilities. Jiang and coauthors speculated that GRAVITY+, an upgrade to the existing GRAVITY instrument on the Very Large Telescope, may be able to observe the torus directly. In the meantime, upcoming surveys like the Vera C. Rubin Observatory Legacy Survey of Space and Time will discover hundreds to thousands more tidal disruption events, giving researchers a glimpse into the intricacies of stellar disruption.

Citation

“The Extraordinary Long-Lasting Infrared Echo of PS16dtm Reveals an Extremely Energetic Nuclear Outburst,” Ning Jiang et al 2025 ApJL 980 L17. doi:10.3847/2041-8213/adaeb9

bubble nebula

As the solar system travels through the Milky Way, our planet and the life it harbors are exposed to a variety of environments. A recent study suggests that a nearby supernova may have played a role in the evolution of life on Earth.

local bubbble

Map of the Local Bubble showing the locations of surrounding stellar associations. The solar system lies near the center of the Local Bubble, and the surrounding stellar associations hosted supernovae that blew up the bubble. Click to enlarge. [Modified from Nojiri et al 2025]

The Local Bubble

About 6 million years ago, the solar system wandered into a 1,000 light-year-wide void known as the Local Bubble. Winds from massive stars and ionizing radiation from an estimated 15 supernovae carved out this bubble over the last 15 million years. As the solar system traveled from the bubble’s edge to its current position in the center, at least nine of the bubble-sculpting supernovae exploded, showering the Earth with supernova byproducts and intense radiation.

Evidence of these past explosions is embedded within the Earth’s crust — radioactive isotopes formed only in supernovae have decayed over time in deep-sea sediments. Less conspicuous are the ways cosmic rays from supernovae may have impacted the Earth; high-energy charged particles rained into the solar system, exposing the planet and the life on it to powerful radiation. Studying the Local Bubble and tracing the history of supernova explosions within it will allow researchers to gauge how the local environment has impacted our planet and the life it harbors.

Modeling Nearby Supernovae and Cosmic Radiation

Starting with the decay rate of the deep-sea isotopes, scientists estimate peaks in radioactive iron deposits approximately 2–3 and 5–6 million years ago. Using this information and the recent mapping of stellar associations in the Local Bubble, Caitlyn Nojiri (University of California, Santa Cruz) and collaborators modeled the necessary supernova input to produce the level of radioactive material present on Earth. From their modeling, they estimate that the iron peak ~2.5 million years ago can be attributed to a single supernova explosion from either the Upper Centaurus Lupus or Tucana Horologium stellar associations. The iron peak 5–6 million years ago, the authors suggest, arises from the solar system passing through the enriched outer shell of the Local Bubble.

cosmic ray spectra

Cosmic-ray spectra (top panel) and the amount of cosmic radiation received at various depths on Earth (bottom panel) for the modeled supernova in the Upper Centaurus Lupus stellar association. Click to enlarge. [Modified from Nojiri et al 2025]

Given the amount of radioactive iron deposited on Earth, the authors predict a powerful supernova progenitor capable of releasing some of the highest-energy cosmic rays in the universe. Through knowing the approximate location of the supernova and modeling its energy output, the authors estimate the amount of cosmic radiation Earth was exposed to from the time the supernova exploded to now. In their model, cosmic-ray radiation varies over time as the supernova evolves, meaning the Earth received a much higher volume of cosmic rays for the first 100,000 years after the explosion.

Impacts on Life

What does this cosmic-ray exposure mean for life on Earth? Though the exact effects of this radiation are not certain, biological studies have shown that radiation exposure can cause DNA to break, which can accelerate the rate at which genetic mutations and evolutionary changes occur. The authors note a prior study that showed the rate of virus diversification in Lake Tanganyika in Africa accelerated 2–3 million years ago. Though this cannot be definitively attributed to the supernova, the overlapping timeframes are suggestive of cosmic radiation playing a role in the evolution of life on our planet.

This study underscores the importance of considering cosmic radiation when it comes to understanding the environmental factors that drove biological evolution on Earth. Further studies must be performed in order to constrain the threshold at which this radiation goes from driving species diversification to becoming detrimental to life and its evolution. The Local Bubble has left imprints on the solar system and on Earth in ways that astronomers and biologists will continue to uncover. 

Citation

“Life in the Bubble: How a Nearby Supernova Left Ephemeral Footprints on the Cosmic-Ray Spectrum and Indelible Imprints on Life,” Caitlyn Nojiri et al 2025 ApJL 979 L18. doi: 10.3847/2041-8213/ada27a

image of the Sun with clouds crossing the Sun's disk

Even in the sunny spots where solar telescopes are stationed, clouds are a fact of life. Rejecting cloud-obscured solar images means losing a lot of data, but new machine-learning methods show that we don’t have to cast off cloudy images after all.

Contending with Clouds

Dunn Solar Telescope

The Richard B. Dunn Solar Telescope in Sunspot, New Mexico. Even this sunny site experiences clouds. [NSF/NSO/AURA; CC BY 4.0]

Space-based solar telescopes have the luxury of perfect “weather” every day, but ground-based solar observatories have to contend with clouds. A study undertaken at Udaipur Solar Observatory in India, for example, found that the Sun’s disk was unimpeded by clouds only 63% of the time. If clouds can’t be removed reliably from images taken the remaining 37% of the time, that means losing a ton of information about the Sun. Is there a way to salvage these images?

So far, existing methods of handling cloud-contaminated images fall short, either failing to remove the clouds fully or failing to reconstruct the obscured solar features underneath. Now, a study led by Zhenhong Shang (Kunming University of Science and Technology) has shown how a new machine learning method rescues clouded-out images of the Sun’s disk.

Machine Learning Method

The model developed by Shang’s team handles the two critical aspects of cloud removal: 1) identifying and characterizing cloudy images and 2) removing the clouds and recovering the features underneath.

examples of clear and cloudy solar images

Examples of (a) clear, (b) mildly cloudy, (c) moderately cloudy, and (d) severely cloudy images from various telescopes in the GONG network. [Shang et al. 2025]

First, the team’s method reports not only whether an image is cloudy but also how cloudy it is. This is an improvement over previous methods that returned only a yes or no to the question of cloud cover. Previous methods that searched only for variations in brightness across the solar disk could also be fooled by widespread cloud cover, whereas the new method also takes in information about the median brightness of the disk, identifying images that are nearly completely cloud covered.

For the second step of the procedure, Shang’s team used data from the Global Oscillation Network Group (GONG), six solar telescopes that maintain eyes on the Sun every hour of every day. The team selected mildly, moderately, and severely cloud-covered images from this dataset. To train the machine learning model to remove clouds, they used 760 pairs of clear and cloudy images from the GONG telescopes. When cloud cover is intermittent, these clear–cloudy pairs are constructed from images taken by the same telescope. When cloud cover is persistent, a cloudy image from one observatory is paired with a clear image from another observatory, taken just seconds later. This dataset was also augmented by resizing and rotating some of the images, making sure it can handle even more cloud-cover scenarios.

examples of cloud removal model performance

Examples of the model’s cloud-removal and feature-retrieval performance. This particular test used data from solar telescopes not in the GONG network. [Shang et al. 2025]

Clear Skies Ahead

After testing and validating their method against a separate set of GONG images, Shang’s team compared the model’s performance to that of existing methods. On nearly every quantitative and qualitative metric, the new machine learning method outperformed existing methods. The new method showed particular improvements in recovering details of the solar surface under dense cloud cover.

The team also demonstrated the method’s performance on data from other ground-based solar observatories, showing its generalizability and readiness to tackle the challenge of cloud removal.

Citation

“Cloud Removal in Full-Disk Solar Images Using Deep Learning,” Zhenhong Shang et al 2025 ApJS 276 56. doi:10.3847/1538-4365/ad93ca

Dimorphos

final full image from DART

The final full image from DART, taken 2 seconds before the spacecraft crashed into the asteroid moonlet Dimorphos. [NASA/Johns Hopkins APL]

In 2022, humanity performed its first test of its ability to defend itself against an oncoming asteroid impact. When NASA’s Dual Asteroid Redirection Test (DART) spacecraft slammed into Dimorphos, the 160-meter-wide moonlet of the asteroid Didymos, a spray of material escaped the moonlet’s surface, and Dimorphos’s path around its parent asteroid was altered — showing how a 610-kilogram spacecraft can change the course of a 4.8-billion-kilogram asteroid. Both the spray of ejecta and the change in orbit reflect the transfer of momentum from the spacecraft, which came in at a screaming 22,530 kilometers per hour.

Could an impact from a spacecraft like DART actually nudge a hazardous asteroid onto a safer course? The timeliness of this question recently ratcheted up a notch: the probability of the asteroid 2024 YR4 striking Earth on 22 December 2032 has risen above 2%.

Discovery images of asteroid 2024 YR4

Discovery images of 2024 YR4. [ATLAS]

Answering this question isn’t straightforward. Among the factors that influence the outcome of a mission like DART — or a future mission to reshape the trajectory of a hazardous asteroid — is the target asteroid’s structure and material strength. Today, we’re taking a look at three research articles that tackle simulations of an impactor striking an asteroid to understand the role of asteroid structure and strength, providing results relevant to both planetary defense and the study of asteroid properties.

Subsurface Structure and Impact Outcomes

First up, a team led by Kaiyi Dai (Macau University of Science and Technology; Imperial College London) and Xi-Zi Luo (Macau University of Science and Technology) simulated the outcome of an impactor striking an asteroid at an angle, just as DART did, rather than head on. Their goal was to understand how the presence of boulders — like those expected to make up “rubble-pile” asteroids like Didymos and Dimorphos — affects the transfer of momentum from the impactor to the target asteroid. Specifically, the team used a three-dimensional shock physics code to explore the effect of different subsurface arrangements of boulders.

Plots of simulated momentum transfer efficiency

Momentum transfer efficiency as a function of the impact angle for different boulder configurations. Click to enlarge. [Adapted from Dai et al. 2024]

In simulations that contained no boulders, Dai and Luo found that head-on collisions excavated the largest amount of ejecta and transferred momentum most efficiently, but oblique impacts launched the fastest-moving ejecta. The addition of boulders into the simulations altered the momentum of the ejected material: surface boulders provided an “armoring” effect, reducing the transfer of momentum and resulting in a smaller crater, whereas buried boulders had an “anti-armoring” effect, increasing the efficiency of momentum transfer and carving out a larger crater. The more deeply buried the boulders, the less they affected the outcome.

An impactor striking a rubble-pile asteroid doesn’t just dislodge material: it also melts some of it. The simulations showed that the amount of melted material, as well as whether the melted material remained on the surface or was ejected into space, varied substantially with the angle of the impact and the configuration of subsurface boulders. This suggests that the volume of melted material remaining on an asteroid’s surface could be used to discern the structure of the material beneath the surface.

Dai and Luo’s team noted that applying these simulation results to the outcome of the DART mission will require simulating a more realistic impactor, as the DART spacecraft’s shape and composition is more complicated than that of the spherical projectile used in these simulations.

Impact of Asteroid Strength and Porosity

Mallory DeCoster (Johns Hopkins University Applied Physics Laboratory) and collaborators also investigated the outcomes of high-velocity impacts on rubble-pile asteroids. The team focused on the effects of not only the boulders at the impact site, but also the “matrix material” of finer particles between the boulders. Because this matrix material effectively holds a rubble-pile asteroid together, the strength of the material plays a large role in determining the outcome of an impact.

Examples of matrix, large boulder, and small boulder configurations

Examples of matrix, large boulder, and small boulder configurations. Matrix material is shown as gray, the impactor is the small gray rectangle, and boulders are shown in different colors (the color of the boulder does not have any meaning). [Adapted from DeCoster et al. 2024]

The team carried out 52 two-dimensional shock physics simulations of impacts onto an asteroid that is 163 meters across — roughly the size of Dimorphos. The properties of the impactor were tuned to be similar to the DART spacecraft: a porous aluminum cylinder 2 meters long and 0.5 meter wide with a mass of 501 kilograms and an impact velocity of 6.6 kilometers per second. The porosity of the simulated cylinder reflects the fact that spacecraft aren’t solid, and this porosity changes the way the impactor interacts with the target.

DeCoster and collaborators placed boulders with diameters of 1–10 meters randomly within their simulated asteroids, filling 62% of each asteroid with boulders before topping off the rest of the volume with matrix material. The simulated asteroids were categorized as “matrix,” “large boulders,” and “small boulders” depending on the primary makeup of the impact site. The strength and porosity of the matrix material were varied between simulations.

The simulations showed that more porous matrix material resulted in larger impact craters, but this outcome was moderated by the distribution of boulders on and under the surface; for matrix-rich asteroids, the presence of weaker, more porous material boosted the crater width, while asteroids with many large or small boulders showed smaller differences in crater width with matrix strength and porosity. None of these factors particularly influenced the depth of the crater.

Matrix-rich asteroids tended to experience larger momentum transfer than boulder-rich asteroids, but this finding was complicated by certain boulder-rich simulations in which entire boulders were thrown into space, resulting in a large momentum transfer for those simulation runs. These results highlight the complex factors that determine how an asteroid responds to an impact.

Estimating Cratering on Dimorphos

The final of today’s three research articles focuses on simulations of the DART spacecraft impacting Dimorphos. After DART slammed into the asteroid, the Italian Space Agency’s Light Italian CubeSat for Imaging of Asteroids (LICIACube) surveyed the aftermath from a safe distance. LICIACube imaging showed fantastic sprays of ejecta and boulders tumbling through space, but the precise size of the crater excavated by DART’s impact is still unknown. That will change with the arrival of the European Space Agency’s Hera mission, which launched in October 2024. Upon reaching Dimorphos and Didymos in late 2026, Hera will assess the impact site and measure the size of the impact crater.

DART observations of Dimorphos and a simulated target constructed to match the observations

DART observations of Dimorphos before impact (panels a and b). Panels c and d show the rubble-pile target constructed to match the DART observations. Click to enlarge. [Stickle et al. 2025]

While they wait for Hera to finish its journey, scientists are fine-tuning their simulations. Angela Stickle (Johns Hopkins University Applied Physics Laboratory) and collaborators used information gleaned from the DART mission — namely, the change in Dimorphos’s orbit, the characteristics of the ejecta, and the appearance of the asteroid’s surface — to predict the size of the resulting crater.

Stickle’s team modeled Dimorphos as a collection of boulders held together by matrix material, with the sizes and positions of the surface boulders determined from pre-impact photographs of the asteroid. The sizes of the subsurface boulders were drawn from the same size–frequency distribution as observed for the surface boulders, and these boulders were given random non-spherical shapes. The impactor was a DART-like conglomeration of three spheres, with the large central sphere representing the body of the spacecraft and two smaller, lower-mass side spheres representing the spacecraft’s solar panels.

LICIACube observations of ejecta compared to simulated ejecta

Comparison of LICIACube observations of ejecta rays (panel a) to simulated ejecta (panels b through d). Click to enlarge. [Stickle et al. 2025]

Using three shock physics codes, Stickle’s team examined the transfer of momentum, the creation of a crater, and the production of filamentary ejecta rays like those seen by LICIACube. A rubble-rich structure seems to be necessary to create dramatic ejecta rays, supporting the classification of Dimorphos as a rubble-pile asteroid. The transfer of momentum — a marker of the ability of an impactor to deflect the asteroid from its initial trajectory — depended strongly on the material properties of the asteroid: less-cohesive asteroid material with a larger resistance to crushing correlated with larger momentum transfer.

Estimated crater diameters ranged from 15 to 60 meters, with the likeliest size for Dimorphos being in the range of 40–60 meters. The team notes that this calculation is complicated, as crater sizes continue to evolve beyond the end of the simulations in a way that depends on the strength of the material and the gravity of the asteroid.

The upcoming measurements from the Hera mission will provide an important test of these model results, providing further information on the outcome of the DART mission and helping to advance our understanding of humanity’s ability to deflect hazardous asteroids.

Citation

“Impact Momentum Transfer — Insights from Numerical Simulation of Impacts on Large Boulders of Asteroids,” Kaiyi Dai et al 2024 Planet. Sci. J. 5 214. doi:10.3847/PSJ/ad72eb

“Statistical Analysis of Near-Surface Structure and Material Properties on Momentum Transfer in Rubble Pile Targets Impacted by Kinetic Impactors,” Mallory E. DeCoster et al 2024 Planet. Sci. J. 5 244. doi:10.3847/PSJ/ad7cff

“Dimorphos’s Material Properties and Estimates of Crater Size from the DART Impact,” Angela M. Stickle et al 2025 Planet. Sci. J. 6 38. doi:10.3847/PSJ/ad944d

Trapezium Cluster

Silhouetted against the background glow of the Orion Nebula, the protoplanetary disk 114–426 provides an excellent opportunity to study a site of planet formation. The discovery of water ice in this disk suggests that ice can survive in disks in the dense star cluster environments where most stars form.

Clues to Our Planetary Past

artist's impression of a protoplanetary disk

Artist’s impression of a protoplanetary disk around a young star. [A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO); CC BY 4.0]

To understand how our solar system came to be, researchers can search for clues close to home, piecing together the compositional and dynamical hints that persist today — or they can look farther afield, taking cues from distant young planetary systems in the throes of formation.

Protoplanetary disks — disks of gas and dust that encircle young stars and set the stage for planet formation — provide an opportunity to understand how our solar system may have formed. In today’s article, JWST provides a new perspective on an unusual protoplanetary disk.

A Disk’s Silhouette

The Orion Nebula is a prolific star-forming region that’s home to hundreds of young stars. At the nebula’s center is the Trapezium Cluster, a tight-knit grouping of massive stars that illuminates the surrounding nebula. Located near the Trapezium Cluster, the protoplanetary disk 114–426 (named according to its coordinates) is notable for both its size and its location. At more than 1,000 au across — about 25 times the distance from the Sun to Pluto — it’s one of the largest disks in the region. It’s also oriented edge-on from our perspective, showing off a narrow disk of gas and dust that is illuminated from behind by the bright glow of the Orion Nebula.

protoplanetary disk 114–426

JWST image of the protoplanetary disk 114–426 in Orion. In this image, 1.15- and 1.40-micron light is blue, 1.62-micron light is green, 1.82-micron light is yellow-green, and 2.77-micron light is red. [Adapted from Ballering et al. 2025]

Previous observations of 114–426 with the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) allowed for detailed studies of the dust grains in the disk. Now, Nicholas Ballering (Space Science Institute and University of Virginia) and collaborators have used JWST’s Near Infrared Camera to scan the disk in 12 wavelength bands from 1 to 5 microns (1 micron = 10-6 meter).

The JWST observations showcase 114–426’s edge-on, dusty disk and two bright lobes of scattered light from the hidden star at the disk’s center. The two lobes are asymmetrical, which previous research suggests means that the inner part of the disk is tilted. A tilted disk could mean that the hidden central star is actually a binary system, or that the star is orbited by a massive planet.

Icy Interpretation

spectra of the disk 114–426

Spectra from several locations in the disk. Each spectrum shows a dip at 3 microns, indicative of water ice. Click to enlarge. [Ballering et al. 2025]

Looking at the portions of the disk that extend beyond the lobes of scattered light, Ballering’s team identified several wavelengths where the spectrum dips, indicating absorption of the background light. One such feature around 3 microns is due to water ice, which has been identified in other protoplanetary disks.

The team estimated that about half an Earth mass of ice and dust is present in the areas of the disk that are backlit by the Orion Nebula. Given that this disk is subjected to intense ultraviolet radiation from the nearby Trapezium Cluster, it may seem surprising that water ice can survive there. However, the team’s calculations showed that the disk is far too cold for ice to sublimate, and the ultraviolet radiation from nearby stars isn’t sufficient to remove the ice from the surface of dust grains.

This finding suggests that water ice is likely to survive in disks around stars in cluster environments like this one. The fact that ice persists is important for planet formation, as ice helps dust grains clump together into pebbles and eventually planets. Ice may also play a role in the transport of water from the cold outer regions of disks to the temperate inner regions where habitable planets may reside.

Citation

“Water Ice in the Edge-On Orion Silhouette Disk 114–426 from JWST NIRCam Images,” Nicholas P. Ballering et al 2025 ApJ 979 110. doi:10.3847/1538-4357/ad9b7a

galaxy merger

Over the course of its lifetime, the Milky Way has encountered many other galaxies — from quick flybys to full on collisions, each interaction is important to our galaxy’s evolution. A series of simulations has predicted how galaxy interactions have contributed to star formation across the lifetimes of Milky Way–mass galaxies. 

Merging Galaxies

As galaxies move through the universe, they interact and merge with other galaxies. Sometimes these interactions are quick flybys, leaving behind little evidence of the encounter. Other times, galaxies smash into each other, creating extended tails and messy morphologies. From observations and simulations, scientists have suggested that the merging of galaxies can also trigger increased star formation. However, it is unclear if this behavior is consistent for galaxies across cosmic time. 

In the local universe, observations have shown increased star formation rates for galaxies with nearby neighbors and lower star formation rates for galaxies that are more isolated, likely experiencing very few interactions and mergers. Earlier in the universe, mergers were more common as galaxies were closer together; however, directly observing evidence of galaxy mergers from the distant past is difficult, leading to an incomplete understanding of how ancient mergers impacted star formation rates. Fortunately, simulations can explore where our telescopes can’t quite reach, allowing scientists to track galaxy interactions over time. 

Galaxy Interaction Simulations

Simulation snapshot showing the gas (left) and stellar (right) surface density maps of a major merger event. Click to enlarge. [Li et al 2025]

In order to explore the impact galaxy mergers and interactions have on the star formation activity in Milky Way–mass galaxies, Fei Li (University of Toronto) and collaborators performed simulations using the Feedback in Realistic Environments (FIRE) cosmological simulation suite. In these simulations, the authors track major, minor, and mini interactions that a Milky Way–mass galaxy experiences over ~12 billion years. Over this time, the galaxy’s interaction history and star formation rate is monitored, allowing the authors to explore how interactions may play a role in the galaxy’s star formation. 

torque star formation rate comparison

Torque (blue) and star formation rate (orange) measurements during major merger interaction snapshots from the simulations ran in this study. Not all peaks in star formation rate correspond to peaks in torque, and not all peaks in torque cause a peak in star formation rate. Click to enlarge. [Li et al 2025]

The simulations show that in major interactions, where the interacting galaxies are closer in mass, there is a positive correlation between the torque exerted by the companion and the star formation rate — increased torque often causes a burst of star formation. However, the star formation history in the galaxy shows multiple starbursts, and most often, these starbursts are unrelated to any sort of merger or interaction with another galaxy. In the minor and mini interactions, there is no significant relationship between the torque and star formation rate for the galaxy. While major mergers can trigger some starburst activity, the overall pattern in star formation for Milky Way–mass galaxies is independent of interaction history — less stellar mass created early on can be directly attributed to mergers than previously assumed.

Observation and Simulation Comparison

How do the authors’ results compare to other simulations and observations that explore interaction impacts? Observations that compare the star formation activity of merging and non-merging galaxies find no significant difference between the star formation activity in these two groups of galaxies across the last ~12 billion years, which is consistent with the results of this simulation. Other simulations predict that massive galaxies like the Milky Way go from bursty to steady star formation across cosmic time, and that mergers and interactions do not leave a significant mark on this overall trend of star formation activity. 

This study finds that Milky Way-mass galaxies experience most major interactions during their early years, experience only a few major mergers, and their star formation activity across cosmic time is predominantly driven by internal dynamics within the galaxy itself. These findings provide predictions for what we may observe as advanced telescopes continue to peer into the distant past.

Citation

“The Effect of Galaxy Interactions on Starbursts in Milky Way–Mass Galaxies in FIRE Simulations,” Fei Li et al 2025 ApJ 979 7. doi:10.3847/1538-4357/ad94ef

Hubble Space Telescope image of stars in the Milky Way's galactic bulge

Regulus, the brightest star in the constellation Leo, is known to float through space with three companion stars. New research shows that it may have yet another member in its stellar entourage.

The Growing Regulus Family

constellation Leo

The constellation Leo, showing the location of Regulus. Click to enlarge. [IAU/Sky & Telescope; CC BY 4.0]

Looking at a field of stars, it’s impossible to know at a glance which stars are associated with one another. Astronomers discern the connections between stars through careful measurements of stars’ distances and motions, as well as similarities in age or metal content.

Using these techniques, astronomers have discovered that Regulus, the nearest B-type star to Earth, has three companions. First to be identified was Regulus B, a 0.8-solar-mass K-type star. Regulus B itself was discovered to have a companion, a 0.32-solar-mass M-type star named Regulus C, in 1873.

In 2008, technological advances allowed for the discovery of an even smaller companion: a 0.31-solar-mass stellar core that sweeps around Regulus every 40 days. Although this companion has yet to be imaged directly, it was detected spectroscopically only a few years ago.

Now, researchers may have tracked down Regulus’s smallest companion yet.

A Potential Companion

SDSS J100711.74 +193056.2, or SDSS J1007+1930 for short, is a brown dwarf in Regulus’s vicinity. Brown dwarfs lie in between stars and planets, massive enough to temporarily experience fusion of deuterium and sometimes lithium, but not massive enough to initiate hydrogen fusion. SDSS J1007+1930’s mass is estimated to be just 0.06 solar mass. The brown dwarf sits 12.6 light-years from Regulus, and its proper motion across the sky is similar to that of Regulus, suggesting that the two objects might be associated.

near-infrared spectrum of SDSS J1007+1930

A near-infrared spectrum of SDSS J1007+1930 (gray is the high-resolution spectrum, black is the smoothed spectrum) along with the spectrum of an L9-type brown dwarf standard (purple). Click to enlarge. [Adapted from Mamajek & Burgasser 2025]

To explore this possibility, Eric Mamajek (NASA’s Jet Propulsion Laboratory) and Adam Burgasser (University of California San Diego) collected spectra of SDSS J1007+1930 using the 10-meter Keck II telescope. Their aim was to determine if SDSS J1007+1930’s properties were similar to those of Regulus or any of the stars in its retinue.

Assembling the Clues

Mamajek and Burgasser found that SDSS J1007+1930 bears a close resemblance to members of the Regulus system in several ways. The brown dwarf is less metal-rich than the Sun, with a metallicity similar to that of Regulus B. Its radial velocity is also similar to that of Regulus B and Regulus itself. Finally, spectral analysis suggests that SDSS J1007+1930 is unlikely to be young, and its age may instead be similar to the 1–2 billion year age of the system.

These clues leave open the possibility that SDSS J1007+1930 is associated with Regulus — but is it? So far, the available evidence isn’t conclusive, but it does suggest that SDSS J1007+1930 formed either within the Regulus system or in the same natal star cluster. Even if SDSS J1007+1930 is currently linked to the Regulus system, the association might be short lived. Given the brown dwarf’s distance from Regulus, it’s possible that the object is no longer gravitationally bound to the system, and a future interaction with a passing star or even a massive gas cloud could steal the brown dwarf away.

Citation

“SDSS J100711.74+193056.2: A Candidate Common Motion Substellar Companion to the Nearest B-Type Star Regulus,” Eric E. Mamajek and Adam J. Burgasser 2025 AJ 169 77. doi:10.3847/1538-3881/ad991b

Artist's impression of a pulsar

Neutron star crust is the strongest material in the universe, but it’s not infinitely strong. New research explores how the cracking of a neutron star’s crust might determine how fast these extreme stellar remnants can spin.

Stellar Speed Limit

Neutron stars are the remnant cores of massive stars that have expired as core-collapse supernovae. These stars are extraordinarily compact, typically packing more than the mass of the Sun into a sphere with a volume about a billion times smaller than Earth’s volume.

Neutron stars also rotate extremely quickly, with the fastest-spinning neutron stars zipping around hundreds of times each second. That’s fast — but it turns out to be only about half as fast as a neutron star could hypothetically spin before being ripped apart by its rotation, a speed known as the breakup rate. What’s stopping neutron stars from spinning even faster?

Crust-Covered Pasta

Jorge Morales and Charles Horowitz (Indiana University) have explored one possibility: that the cracking of a neutron star’s crust occurs at roughly half the star’s breakup rate, and that once the crust cracks, the star can spin no faster.

Neutron stars are composed of a strange substance called nuclear pasta, which is enclosed by a shell of atomic nuclei and electrons. Both of these layers are reportedly billions of times stronger than steel.

plot of strain versus rotation rate

The strain experienced by a neutron star’s crust as a function of the rotation rate divided by the breakup rate. [Morales & Horowitz 2025]

Morales and Horowitz modeled the strain that a neutron star’s crust encounters under different spin rates and neutron star masses. They showed that as a neutron star spins, the material around its equator bulges outward while the material near its poles draws inward. When the strain of this deformation surpasses the crust’s strength, the crust splits along the star’s equator, at the base of the crust where the crust meets the interior nuclear pasta. For reasonable estimates of the crust’s breaking strength, this happens at 58% of the star’s breakup rate — roughly the maximum spin speed drawn from observations.

Continuous Gravitational Waves

Why does the breaking of a neutron star’s crust inhibit faster rotation? This is especially relevant for neutron stars that are expected to “spin up” to faster speeds over time as they accrete matter from their companions; even these appear to obey the spin speed limit.

After the crust breaks, the pieces of the crust can shift around. This may transform the neutron star from one that is symmetric around its spin axis to one that is asymmetric. These objects might produce a continuous gravitational wave signal, and Morales and Horowitz proposed that any angular momentum added to the star — through accretion, for example — is carried away in the form of gravitational waves, preventing the star from spinning faster.

Morales and Horowitz noted that there are more aspects of this scenario to be explored, such as the impact of crustal magnetic fields, the importance of relativistic effects, and the prospects of detecting gravitational waves from these sources.

Citation

“Limiting Rotation Rate of Neutron Stars from Crust Breaking and Gravitational Waves,” J. A. Morales and C. J. Horowitz 2025 ApJL 978 L8. doi:10.3847/2041-8213/ad9ea7

cartoon of Thorne–Żytkow object formation

A Thorne–Żytkow object is a star within a star — a star with a neutron star at its core. These objects are theorized to form in close binary systems, but new research reveals complications in this proposed formation pathway.

A Star Within a Star

The term “star” encompasses a wide variety of objects, from our familiar Sun to roiling supergiants dozens of times as massive and hundreds of times as wide. Certain types of stars are only theorized, like those containing huge amounts of dark matter or with cores composed of strange quarks. One such theorized star — and the subject of today’s article — is a Thorne–Żytkow object, also known as a hybrid star.

illustration of an X-ray binary system

An artist’s impression of an X-ray binary, in which a compact object accretes material from a companion star and emits X-rays during intermittent outbursts. [ESO/L. Calçada; CC BY 4.0]

Thorne–Żytkow objects might form in close binary systems when a star engulfs its neutron-star companion. Because of their potential binary origin, it’s possible that Thorne–Żytkow objects arise in X-ray binary systems containing a neutron star that collects gas from its companion in a super-heated accretion disk.

After being engulfed by its companion, the neutron star is thought to sink to the star’s core. There, it is hypothesized to energize the surrounding star through accretion and nuclear fusion, creating a curious mix of elements that distinguishes a Thorne–Żytkow object from an ordinary star.

Diverging Paths

At least, that’s the theory. But as a recent research article led by Tenley Hutchinson-Smith (University of California, Santa Cruz; University of Copenhagen) shows, more work is needed to understand whether X-ray binary systems could truly evolve into Thorne–Żytkow objects. At the core of this question is how the inspiraling neutron star affects the companion that has engulfed it. Does the engulfing star hold on to its extended gaseous envelope, or does it lose its atmosphere and diverge from the Thorne–Żytkow object path? And how long would the Thorne–Żytkow phase last — could the neutron star remain at the center of its companion indefinitely, or does the neutron star eventually gain mass and collapse into a black hole?

The team used as the basis of their exploration the X-ray binary system LMC X-4, which contains a 1.57-solar-mass neutron star and an 18-solar-mass primary star. The stars are locked in a tight gravitational embrace, separated by only 14 solar radii and orbiting one another every 1.4 days.

simulation snapshots showing gas density

Simulation screenshots showing the density of gas as the neutron star (white circle) spirals in toward the core of its companion star. Click to enlarge. [Adapted from Hutchinson-Smith et al. 2024]

Total Collapse of the Heart

Using a three-dimensional fluid dynamics simulation, Hutchinson-Smith and collaborators followed the evolution of LMC X-4 as the primary star engulfed the neutron star. As the neutron star spiraled inward, the energy released ejected only a small amount of gas, and the neutron star accreted only a small amount of matter from the companion star. At that point, the formation of a Thorne–Żytkow object seemed inevitable, but the merger of the neutron star with the companion star’s core set the system on a different course.

plot of luminosity and duration of gamma-ray emission

Comparison of the luminosity and duration of the gamma-ray burst produced by the collapse of the neutron star in LMC X-4 with the properties of long gamma-ray bursts (LGRBs) and ultra-long gamma-ray bursts (ULGRBs). Click to enlarge. [Hutchinson-Smith et al. 2024]

As the neutron star melded with the companion’s core, it imparted angular momentum to the core. This created an accretion disk that fed the neutron star until it collapsed into a black hole. The collapse launched a relativistic jet and powered gamma-ray emission that was about as bright and as long-lasting as an ultra-long gamma-ray burst. Feedback from accretion onto the black hole ejected nearly all of the gaseous envelope, definitively halting the short-lived Thorne–Żytkow phase.

Thus, Hutchinson-Smith’s team has demonstrated that a Thorne–Żytkow object is unlikely to result from the evolution of an X-ray binary system like LMC X-4 — though this evolution may provide a path to powering ultra-long gamma-ray bursts. This suggests that accretion and feedback leading to the collapse of the neutron star and the ejection of the stellar envelope must be taken into consideration when exploring the formation of Thorne–Żytkow objects.

Citation

“Rethinking Thorne–Żytkow Object Formation: The Fate of X-Ray Binary LMC X-4 and Implications for Ultra-Long Gamma-Ray Bursts,” Tenley Hutchinson-Smith et al 2024 ApJ 977 196. doi:10.3847/1538-4357/ad88f3

Illustration of a Neptune-like exoplanet

With nearly 6,000 exoplanets discovered to date, it’s clear that not all of the planets in our galaxy resemble the planets in our solar system. Today’s Monthly Roundup explores three types of planets with no analogs in our solar system: a warm Neptune orbiting its tiny host star in less than 4 days; a polar Neptune orbiting perpendicular to its host star; and a famous sub-Neptune whose structure is a matter of debate.

Close-In Neptune, Breaker of Chains

LHS 3154b is a typical Neptune-mass exoplanet in an atypical place. This planet has a mass of at least 13.2 Earth masses, but it orbits an M-dwarf star just 11% the mass of the Sun, swinging around its tiny host star every 3.7 days. It’s not clear how such a setup came to be, as conventional theories of planet formation provide few avenues for such massive planets to form around low-mass stars.

Donald Liveoak and Sarah Millholland (Massachusetts Institute of Technology) have proposed an explanation for LHS 3154b: it’s several planets in a trench coat. In other words, rather than having formed as the single planet observed today, LHS 3154b is the product of a series of collisions between multiple planets that once existed in the system.

To test this theory, Liveoak and Millholland simulated the evolution of a system containing 11 small planets with an average mass of 2 Earth masses. The planets in this system are arrayed in what’s known as a resonant chain, in which the orbital periods of the planets are integer multiples of one another. This configuration is thought to arise naturally when young planets migrate within a protoplanetary disk. Liveoak and Millholland nudged the planets out of this stable configuration by having the planets begin to lose their atmospheres. With their orbital stability disrupted, the planets abandoned their carefully balanced orbits and collided.

masses and orbital periods of planets in systems with a close-in Neptune-mass planet

Masses and orbital periods of the planets remaining in the six simulated systems that generated a close-in Neptune-mass planet. The planetary system architectures are remarkably similar. [Liveoak & Millholland 2024]

Planets akin to LHS 3154b — defined here as planets with masses of 12–20 Earth masses and orbital periods less than seven days — arise in just 1.2% of simulations. This result shows that while it’s possible to create a planet like LHS 3154b this way, it’s not surprising that more planets like LHS 3154b have yet to turn up. The simulations did provide a clue to find evidence of this process in other systems, though: the systems yielding close-in Neptunes always had one or two companions. The Neptune-mass planet is always the closest planet to the star, and the remaining planets have orbital periods around 30 days. By searching for systems containing a short-period Neptune with an outer companion, researchers can study the chain-breaking process that may be responsible for LHS 3154b’s existence.

Stability of Polar Neptunes

The planets in our solar system orbit in nearly the same plane as the one defined by the Sun’s spin, but it’s clear that not all planetary systems in our galaxy are so orderly. A sizeable chunk of the exoplanet population orbits nearly perpendicular to their stars’ spins. Curiously, roughly half of this population are planets with masses similar to that of Neptune. These highly inclined exo-Neptunes share several characteristics, including slightly elongated orbits, puffy atmospheres with the possibility of past mass loss, and, in some cases, a massive planet in the same system, orbiting farther from the host star.

Emma Louden (Yale University) and Sarah Millholland (Massachusetts Institute of Technology) investigated whether disk-driven resonance could torque exo-Neptunes into their perpendicular orbits while also explaining the other properties of this exoplanet population. In this framework, a young exo-Neptune’s orbit would evolve under the gravitational influence of an outer giant planet and a dissipating planetary disk. Over time, the dissipation of the disk combines with a nodal resonance between the inner Neptune and the outer giant planet, eventually launching the inner Neptune into a highly inclined orbit. This mechanism clearly links polar-orbiting Neptunes with outer giant planets.

diagram showing the process of disk-driven resonance

A diagram that describes the disk-driven resonance for creating polar-orbiting planets. [Louden & Millholland 2024]

This process is thought to take place in the first 10 million years of a planetary system, and it’s not yet clear whether this setup is stable over billions of years. Louden and Millholland simulated the evolution of this setup under the influence of tides. Remarkably, the authors found that exo-Neptunes in polar orbits are stable over long stretches of time, though some planets with smaller orbital inclinations, in the 45–80-degree range, are not stable.

Louden and Millholland then considered two known polar Neptunes, WASP-107b and HAT-P-11b. Both of these planets have an outer giant planet in their system, and both planets show signs of mass loss due to tidal heating. Louden and Millholland showed that these planets’ orbital configurations are incredibly stable, on par with the orbital stability of Uranus and Neptune in our own solar system. While this study doesn’t provide proof of the disk-driven resonance hypothesis, it does demonstrate the feasibility of the concept, and further observational evidence can strengthen the hypothesis.

Is K2-18b Covered in a Supercritical Ocean?

Of the many sub-Neptune exoplanets — those with masses between the mass of Earth and the mass of Neptune — K2-18b is perhaps the most famous. Discovered in 2015 in data from the Kepler Space Telescope, K2-18b has a mass of 8.63 Earth masses and a radius of 2.6 Earth radii. The structure of planets of this size is a matter of debate: do sub-Neptunes have solid surfaces, like Earth, or are they primarily gaseous, like Neptune? Or are they somewhere in between, unlike any of the planets in our solar system?

Possible structures for K2-18b

Possible structures for K2-18b that have been explored in previous work and will be tested in this work (far right). Click to enlarge. [Luu et al. 2024]

K2-18b’s structure has been difficult to pin down, even with the help of JWST. Atmospheric spectroscopy enabled the detection of methane (CH4) and carbon dioxide (CO2) in K2-18b’s atmosphere. Water, carbon monoxide, and ammonia were not detected. Through modeling, some researchers have found these data to be consistent with the planet having a thin atmosphere with a habitable — even speculated to be inhabited — surface, while others have concluded that K2-18b is gas rich, with no solid surface.

Now, Cindy Luu (University of Texas at San Antonio) and collaborators have explored yet another possibility: that K2-18b is covered with an ocean of hot, supercritical water. A supercritical fluid is hot enough to become gaseous but is under enough pressure that it is left in a strange in-between state that shares properties with both its liquid and gaseous forms. Pure water goes supercritical at 647.15K (705℉/374℃).

Luu’s team used geochemical modeling to test the hypothesis that a global supercritical water ocean could explain the observed chemical abundances of K2-18b’s atmosphere. In this scenario, the boundary between the ocean and the atmosphere would be fuzzy, with a greater degree of mixing than would be found at a typical ocean–atmosphere boundary. The team found that an ocean with a temperature between 710K and 1050K could reproduce the observed chemical abundance ratios in K2-18b’s atmosphere. What’s more, this scenario naturally reproduces the non-detection of carbon monoxide.

While the current investigation shows that a supercritical water ocean is consistent with the existing atmospheric abundances measured with JWST, more work is needed to explore this exotic possibility further. More data on carbon monoxide and ammonia, in particular, will be critical to future research.

Citation

“Formation of Close-In Neptunes around Low-Mass Stars through Breaking Resonant Chains,” Donald Liveoak and Sarah C. Millholland 2024 ApJ 974 207. doi:10.3847/1538-4357/ad7383

“Polar Neptunes Are Stable to Tides,” Emma M. Louden and Sarah C. Millholland 2024 ApJ 974 304. doi:10.3847/1538-4357/ad74ff

“Volatile-Rich Sub-Neptunes as Hydrothermal Worlds: The Case of K2-18b,” Cindy N. Luu et al 2024 ApJL 977 L51. doi:10.3847/2041-8213/ad9eb1

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