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illustration of a spider pulsar

In the vast menagerie of cosmic critters, perhaps none is as impressively deadly as the spider pulsar. Spider pulsars are a subset of millisecond pulsars: tiny, extraordinarily fast-spinning remnants of dead massive stars. Like other types of pulsars, spider pulsars emit narrow beams of radio emission that can sweep across our field of view as the pulsar spins, generating the characteristic radio pulses from which pulsars get their name. Locked in a close embrace with a low-mass stellar companion, a spider pulsar roasts its companion with high-energy radiation and strips away the companion’s atmosphere with powerful particle winds. Eventually, like the ill-fated mates of certain spiders, the companion star may be entirely destroyed.

Just as there are many species of spiders, so too does it appear that there are different “species” of spider pulsars. So far, two categories are well established, and a third has started to emerge. These categories are based on the mass of the pulsar’s companion star and the orbital period of the system. Black widows have the lowest-mass companions, weighing in at just 5% of the mass of the Sun, and orbital periods less than one day. Redbacks, named after black-and-red spiders also known as Australian black widows, are similarly closely orbiting but have companions with masses ranging from 0.1 to 1.0 solar mass. Finally, the emerging class of huntsman pulsars, named for a large, long-legged type of spider, have low-mass red giant companions and relatively long orbits of 5–10 days.

Today, the Monthly Roundup will introduce three recent investigations of spider pulsars.

Discovery of a Second Huntsman

In 2016, researchers reported the discovery of a ~2-solar-mass millisecond pulsar in a 5.4-day orbit with a 0.33-solar-mass red giant companion stripped of its outer atmosphere. This pulsar — with its evolved stellar companion and long orbital period — was unlike any other spider pulsar known at the time and is now the prototype of a new class of spider pulsars nicknamed “huntsman” pulsars. A year later, a second system with similar properties was found, though the pulsar was not directly detected and remains a candidate.

Now, a team led by Jay Strader (Michigan State University) has announced the discovery of the second confirmed huntsman: the pulsar PSR J1947–1120. This source initially came to light in gamma-ray observations by the Fermi Gamma-ray Space Telescope. Millisecond pulsars are known to emit gamma rays, and many pulsars are first discovered in gamma rays. Using new and archival observations from the Neil Gehrels Swift Observatory (X-rays), XMM-Newton (X-rays), Gaia (optical), the Zwicky Transient Facility (optical), the Southern Astrophysical Research telescope (optical), and the Green Bank Telescope (radio), Strader’s team followed up on the gamma-ray detection and identified both the pulsar and its companion.

stellar spectrum

A spectrum of the PSR J1947–1120 system. The spectrum is consistent with a cool K-type star. Click to enlarge. [Strader et al. 2025]

While more observations are needed to pin down the precise properties of the system, current data point to a pulsar with a 2.24-millisecond rotation period in a 10.3-day orbit with a 0.25–0.40-solar-mass red giant star. Modeling suggests that the red giant is highly stripped, retaining only 0.06–0.2 solar mass of envelope material.

Strader’s team outlined a potential formation mechanism for huntsman pulsars that involves a red giant companion in the “red bump” phase. During this phase of stellar evolution, red giants temporarily become smaller and less luminous, resulting in a pileup or “bump” in the Hertzsprung–Russell diagram. Accretion onto the pulsar also ceases during this period, matching what is seen in huntsman systems. Through stellar evolution modeling, the team showed that the properties of the two known huntsman pulsars can be attained by systems containing red giant stars in the red bump phase.

Why, then, have so few huntsman pulsars been found compared to other types of spider pulsars? The red bump phase of red giant evolution is brief, cosmically speaking, lasting on the order of tens of millions of years. Other types of spider pulsars arise in setups that are longer lived, on the order of billions of years, making it simply more likely to find a black widow or redback in the field than a huntsman.

Searching for Spiders That Will Consume Their Companions

Spider pulsars often exhibit eclipses: periods during which the pulsar’s radio signal passes through the ablated material of the companion star, causing the signal to drop out. While radio eclipses are a common feature of spider pulsars, not all spider pulsars experience this phenomenon. The inclination of the system and physical size of the companion star may both influence whether a spider pulsar exhibits eclipses.

The occurrence of radio eclipses may also be related to the mass-loss rate of the companion star, with higher mass-loss rates being associated with eclipses. The rate at which spider pulsars ablate their companions is a crucial piece of information for establishing spider pulsars as the missing link in the creation of solo millisecond pulsars.

Typically, millisecond pulsars like spider pulsars are thought to arise in binary systems. Accretion onto the pulsar from the binary companion spins up the pulsar, helping it achieve its extreme speed. While this mechanism explains the origins of millisecond pulsars in binary systems, it can’t explain solo millisecond pulsars — unless these singletons are spider pulsars that have entirely ablated their companions.

To explore this possibility, astronomers need to study the eclipses and mass-loss rates of a large sample of spider pulsars; so far, none of the known spider pulsars are blowing away their companions quickly enough for their companion stars to disappear within the universe’s current age.

Example of the detected radio eclipses. [Kumari et al. 2025]

In a recent research article, Sangita Kumari (National Centre for Radio Astrophysics, Tata Institute of Fundamental Research) and collaborators used the Giant Metrewave Radio Telescope (GMRT) to search for eclipses in 10 pulsars systems. In all, Kumari’s team observed eclipses for the first time in three systems and constrained the cutoff frequency — crucial to understanding the origin of the eclipses — in a further four systems for which eclipses had previously been observed. Three systems showed no eclipses.

In addition to characterizing the eclipses they detected, the authors calculated the mass-loss rates for two of the pulsar’s companions. The mass-loss rates were too small to fully ablate the companion stars within the age of the universe, as has been the case for other spider pulsars under study. Additionally, the team found no correlation between the pulsar spin-down rates — thought to be related to the mass-loss rate — and the presence of eclipses, suggesting that other factors must play an important role in the creation of eclipses.

A Spider on the Dividing Line

J1908+2105 is a spider pulsar that was discovered in a search for counterparts to unidentified gamma-ray sources. The pulsar rotates every 2.56 milliseconds, and it’s in a 3.51-hour orbit with a companion star with a minimum mass of 0.055 solar mass, placing the pulsar between black widows and redbacks. As one of only a few known pulsars that may sit upon the dividing line between black widows and redbacks, studying J1908+2105 may help researchers understand whether these pulsars are evolving from one class to the other.

average radio pulses

Average radio pulses at three different frequencies, showing the pulse behavior when not eclipsed. [Ghosh et al. 2025]

Ankita Ghosh (National Centre For Radio Astrophysics, Tata Institute of Fundamental Research) and collaborators used the Giant Metrewave Radio Telescope (GMRT) and the Parkes radio telescope to study J1908+2105’s radio eclipses. The observations covered several frequency bands, allowing the team to search for frequency-dependent changes in the eclipse profile and duration, as well as to determine the magnetic-field properties of the system and the mass-loss rate of the companion.

The team detected J1908+2105’s eclipses at frequencies up to 4 gigahertz, making this one of only a few pulsars with observable eclipses at such high frequency. They explored several possible sources for the eclipses, such as reaching the plasma frequency cutoff — the frequency at which radio waves are unable to travel through a plasma — and determined that synchrotron absorption is the likeliest cause.

Ghosh’s team measured the mass-loss rate from observations made at several frequencies, finding in the highest frequency band a mass-loss rate large enough for the companion to be ablated within 3 billion years. The authors caution that changes in the orbital properties of the system, spurred by the companion’s mass loss, likely mean that the rate will slow over time, and it may not be possible for the companion to be fully evaporated.

Citation

“PSR J1947−1120: A New Huntsman Millisecond Pulsar Binary,” Jay Strader et al 2025 ApJ 980 124. doi:10.3847/1538-4357/ada897

“Unveiling Low-Frequency Eclipses in Spider Millisecond Pulsars Using Wideband GMRT Observations,” Sangita Kumari et al 2025 ApJ 979 143. doi:10.3847/1538-4357/ad93ba

“Exploring Unusual High-Frequency Eclipses in MSP J1908+2105,” Ankita Ghosh et al 2025 ApJ 982 168. doi:10.3847/1538-4357/adb8e0

Tycho crater's peak

While humans have not set foot on the Moon in over 50 years, multiple spacecraft and rovers have surveyed our closest companion in great detail for decades. Since 2009, the Lunar Reconnaissance Orbiter has orbited the Moon, collecting data in order to map the Moon’s surface and identify potential landing sites and resources for future lunar missions. A recent study focuses on the Moon’s north pole, using data from the orbiter to create new highly detailed maps.

LRO

Artist’s impression of the Lunar Reconnaissance Orbiter orbiting above the surface of the Moon. Click to enlarge. [NASA/GSFC]

Navigating the Moon’s Surface

When making the 238,900-mile journey to the surface of the Moon, it’s important to be able to stick the landing. After being pelted with rogue solar system rocks for billions of years, the Moon’s surface is home to thousands of craters, making peaks and valleys that vary significantly depending on where you land. Thus, having a detailed understanding of the Moon’s surface is imperative to choosing where to aim and knowing what can be accomplished once we get there. 

In recent years, researchers have created detailed maps of the Moon’s south pole to identify regions with rough terrain, boulders, steep edges of craters, and flatter areas safer for landings. In addition to mapping geological features, these studies have found candidate water-ice sources that will be targeted in future missions. With continued observations, higher-resolution studies can reveal more detailed information about other regions on the Moon as well.

Mapping the North Pole

Recently, Michael K. Barker (NASA Goddard Space Flight Center) and collaborators used data from the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter to produce high-quality topographical maps of the Moon’s north pole. LOLA uses a laser to measure variations in the Moon’s surface, and from this information, the authors computed new maps of the surface height, slope, and roughness — how bumpy or uneven the terrain is — in the north polar region. The new surface height and slope maps provide information regarding regions of high and low elevation as well as how rapidly those changes in elevation occur. Additionally, the authors created a new high-resolution map of permanently shadowed regions — areas never hit with direct sunlight, keeping any possible traces of water ice cool and intact.

maps

Lunar surface maps of slope (left column) and roughness (right column) across smaller (top) and larger (bottom) spatial scales. Click to enlarge. [Barker et al 2025]

How do these maps of the north pole compare to previously computed maps of the south pole? The authors found that both poles exhibit diverse terrains that have been shaped by impact cratering and smoothed by landslides and slumps. Showing fewer large-scale linear roughness features than the south pole, the north polar region has likely been less affected by the ejecta from recent large impacts. Furthermore, when looking at the relationship between the roughness and the temperature of the Moon’s dusty surface, the authors found that rougher areas tend to exhibit higher temperatures on both poles. The exact reason for this trend is not yet known, but it could be linked to the presence of subsurface icy materials, including water ice. 

Where do these results take us? The differences between the north and south poles motivate further exploration of both locations. Future human and robotic missions will allow for more robust studies of the Moon’s surface, which will test for the relationship between surface features and subsurface ices, explore the geography on much smaller scales, and set the grounds for sustained science on the Moon. 

Citation

“Large-scale Roughness Properties of the Lunar North and South Polar Regions as Measured by the Lunar Orbiter Laser Altimeter (LOLA),” Michael K. Barker et al 2025 Planet. Sci. J. 6 83. doi:10.3847/PSJ/adbc9d

double-faced white dwarf

A recent study brings the number of known double-faced white dwarfs to seven. These rare objects, which feature compositional changes across their surfaces, may arise under the influence of magnetic fields.

The Rise of Two-Faced Stars

Hubble image of the Ring Nebula

When a super-hot white dwarf illuminates the diffuse shells of gas that surround it, we see a glowing planetary nebula. The central white dwarf is visible in this image of the Ring Nebula. [NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]

At the end of their hydrogen-burning lifetimes, low- to intermediate-mass stars shed their outer layers and bare their cores, evolving into Earth-sized spheres of degenerate electron matter called white dwarfs.

In 2023, researchers reported the discovery of a bizarre white dwarf: one with a surface composed of hydrogen on one side and helium on the other. A handful of other white dwarfs with split or patchy surface compositions have since been discovered, giving rise to the growing class of double-faced white dwarfs.

Many aspects of these strange objects are still unknown, including exactly how they form, how common they are, and what kinds of surface composition distributions are possible. To answer these questions, researchers must find more double-faced white dwarfs — and the hunt is on.

A Closer Look at “Binary” White Dwarfs

Because the composition of a double-faced white dwarf varies across its surface, these objects can present different compositional “faces” as they spin, sometimes resembling one kind of white dwarf and sometimes another. To find white dwarfs with this behavior, Adam Moss (University of Oklahoma) and collaborators investigated six systems earmarked as being possible unresolved binary systems containing one hydrogen-rich white dwarf and one helium-rich white dwarf.

The team suspected that these “binary” systems might instead be double-faced white dwarfs exhibiting features of two types of white dwarfs. One last white dwarf, previously identified as having spectral features from both hydrogen and helium, rounded out the sample.

spectra of a double-faced white dwarf

Spectra of white dwarf J0847+4842 taken at the beginning, middle, and end of the night from Apache Point Observatory. The spectra have been offset vertically for clarity. Vertical blue lines mark the locations of hydrogen lines, and vertical red lines mark the locations of helium lines. [Adapted from Moss et al. 2025]

Analyzing spectra from the Gemini North telescope, Apache Point Observatory, and the MMT Observatory, Moss’s team found spectral variations in two of the seven targets. The five targets with no spectral variations are likely bona fide binary systems, while the two time-varying targets showed clear signs of the surface composition variations that characterize a double-faced white dwarf. The spectra of these two objects were well fit with a model featuring polar caps rich in hydrogen and an equatorial belt rich in helium.

Potential Magnetic Origins

These two newly discovered double-faced white dwarfs bring the number of known objects in this class to seven. What does the current sample tell us about the origins of double-faced white dwarfs?

Four of the seven sample members are known to have magnetic fields, which is a far larger proportion than the general population of white dwarfs. This suggests that magnetic fields play an important role in creating these objects’ patchwork surfaces.

Moss and collaborators favor a scenario in which magnetic fields affect the process of convection, which can alter the elemental makeup of a white dwarf’s surface as it cools. In this scenario, stronger magnetic fields at the magnetic poles suppress convection, while weaker fields near the magnetic equator allow convection to proceed. Convection brings helium-rich material to the star’s surface, creating a helium-rich belt sandwiched between hydrogen-rich polar caps. Future modeling will explore how complex magnetic field configurations affect convection and the composition of white-dwarf surfaces, advancing our understanding of this rare class of objects.

Citation

“The Emerging Class of Double-Faced White Dwarfs,” Adam Moss et al 2025 ApJ 983 14. doi:10.3847/1538-4357/adbd3a

artist's impression of a pulsar

Researchers recently discovered an object with regular radio pulses less frequent than those of a pulsar but more frequent than those of a long-period radio transient. What do current observations tell us about what this object might be?

Between Two Extremes

The sky at radio wavelengths is inhabited by many objects that brighten, fade, and emit pulses. The most rapidly and regularly varying radio emission comes from pulsars: rotating remnant cores of massive stars that emit beams of radio waves along their magnetic poles. As the beamed emission from a pulsar sweeps across our field of view, it creates the characteristic pulses that give pulsars their name.

Pulsars have pulse periods from milliseconds to a few seconds. Over the past few years, astronomers have discovered a small but growing number of objects with radio pulses with periods of minutes to hours. Though these long-period radio transients (LPRTs) share some properties with pulsars, it’s not yet clear what powers them. One path to uncovering their identity is discovering objects in the period gap between them and pulsars — a region typically missed in searches for pulsars or LPRTs.

Serendipitous Discovery

In a recent publication, Yuanming Wang (王远明; Swinburne University of Technology and ARC Centre of Excellence for Gravitational Wave Discovery) and collaborators reported their discovery of an object that lies within this gap, PSR J0311+1402.

The discovery was serendipitous, with three of the object’s pulses appearing in two minutes of test observations made with the Australian Square Kilometre Array Pathfinder (ASKAP)’s newly installed Commensal Realtime ASKAP Fast Transient Coherent (CRACO) system. CRACO is sensitive to pulse periods within the gap between pulsars and LPRTs.

Light curve of PSR J0311+1402

Light curve of PSR J0311+1402 from MeerKAT observations. Click to enlarge. [Wang et al. 2025]

The team followed up on this detection with observations from ASKAP, Murriyang, MeerKAT, and the Green Bank Telescope. These observations paint a coherent picture of an object with 0.5-second-long pulses every 41 seconds, placing it squarely in the gap between pulsars and LPRTs.

Pulsar or Long-Period Radio Transient?

Could PSR J0311+1402 be a pulsar with an unusually long spin period? The shape of PSR J0311+1402’s pulses and the spectral index (related to how the pulses vary as a function of frequency) resemble those of typical pulsars. However, the spin-down rate of this object, measured from the minuscule change in pulse period over time, potentially places PSR J0311+1402 below the “death line.” Objects below the death line are theorized to be unable to produce radio emission through electron–positron pair production.

plot of change in spin period over time versus spin period for various pulsars and LPRTs

Change in spin period over time versus spin period for various pulsars and LPRTs. PSR J0311+1402 is shown as a star outlined in red. The dotted, dashed, and solid black lines indicate the death lines for various magnetic field models. PSR J0311+1402 falls below all death lines except for the extreme case of a twisted multipole configuration. Click to enlarge. [Wang et al. 2025]

Is PSR J0311+1402 more likely to be an LPRT with a surprisingly short period? Isolated LPRTs have strongly polarized radio emission and are located at low galactic latitudes, while PSR J0311+1402 has weakly polarized radio emission and is located at a moderate galactic latitude. While LPRTs in binary systems have also been found, and have different properties, no companion has been detected for PSR J0311+1402.

Together, these factors suggest that PSR J0311+1402 is more likely to be a slowly rotating pulsar than a short-period LPRT, though further observations to refine the calculation of its spin-down rate are needed. And where there’s one object, there are usually more, suggesting that future searches attuned to periods within the gap between pulsars and LPRTs may find more of these in-between objects and help to illuminate their properties.

Citation

“The Discovery of a 41 s Radio Pulsar PSR J0311+1402 with ASKAP,” Yuanming Wang et al 2025 ApJL 982 L53. doi:10.3847/2041-8213/adbe61

plumes of Enceladus

Enceladus and its tiger stripes

This enhanced-color image from the Cassini orbiter shows the southern hemisphere of Enceladus and its four prominent “tiger stripes.” [NASA/JPL/Space Science Institute]

In 2005, Enceladus became the first moon known to spout plumes of water. Now, researchers are reanalyzing six years of data from the Cassini orbiter to learn more about the origin of these plumes.

Plumes on an Icy Moon

Enceladus, the sixth-largest moon of Saturn, is encased in a shell of reflective ice thought to be several miles thick. This icy crust is marred by craters, chaotically crisscrossed terrain, and — most notably — four long, nearly parallel fissures nicknamed tiger stripes.

The Cassini orbiter spied geysers of water ice and vapor bursting through these fissures, powered by the release of tidal stresses from interactions with another of Saturn’s moons, Dione. The plumes provided the first evidence for an ocean of liquid water beneath Enceladus’s surface. There is much still to learn about the plumes, including whether they spray directly from Enceladus’s subsurface ocean or from a network of shallow pools within the icy crust.

Drawing Back the Curtain

To learn more about the behavior of Enceladus’s plumes and the association between the plumes and the fissures, Joseph Spitale (SETI Institute) and collaborators analyzed 15 epochs of plume activity spread across the years 2009–2015. Their goal was to determine which fissure each plume originated from.

demonstration of the curtain-based plume-mapping method

A demonstration of the advantage of the curtain-based method over a method that attempts to identify the origin of individual jets. Click to enlarge. [Spitale et al. 2025]

Spitale’s team modeled the plumes as “curtains” that emerge from the fissures. This is an improvement upon previous methods that focused on individual narrow plumes or “jets,” requiring the use of multiple images to triangulate the position of each jet.

The team had previously used the curtain method to identify the source fissures of plumes during a few time periods. In this work, they refined their method and expanded their analysis to a broader set of observations.

Fissure Findings

map of fissure activity on Enceladus

Map of plume activity along the fissures of the tiger stripes. This map omits small fissures that were never active. (The full map of fissures used in this study can be seen here.) The majority of locations along each fissure were active in all 15 epochs. [Adapted from Spitale et al. 2025]

The analysis linked curtains of erupting plumes to fissures that had been digitized by eye. Most locations along the fissures were active in every epoch, and those that were never or infrequently active were located on the outskirts of the fracture system.

Previously, researchers have noted that the plumes on Enceladus vary in intensity by approximately a factor of three. With the finding that only the ends of the fissures ever truly become inactive, it’s unlikely that the change in intensity is due to fissures turning on and off as Enceladus encounters varying levels of tidal stress. Instead, it’s likely that most locations along the fissures are always active, regardless of tidal stress, with tidal stresses controlling the overall intensity of the eruption and the on-off behavior of just the tips of the fissures.

The distribution of fissure activity may hint that the plumes emerge directly from the subsurface ocean. In order for pools within the ice shell to be the source of the plumes, the locations of the pools must vary in a particular way with the thickness of the ice shell, or there must be a single plume-supplying chamber spanning the entire area of the tiger stripes — a configuration seemingly unable to withstand the immense weight of the ice crust.

Citation

“Curtain-Based Maps of Eruptive Activity in Enceladus’s South-Polar Terrain at 15 Cassini Epochs,” Joseph N. Spitale et al 2025 Planet. Sci. J. 6 67. doi:10.3847/PSJ/adb7d7

black hole

Gravitational waves from merging black holes encode the mass and spin of the original black holes in the system. These properties can be heavily influenced by the interactions the black hole binary experiences prior to merging, especially when these systems are within dense star clusters. A recent study explores how black hole binaries are impacted by stellar collisions in cluster environments and what that may mean for the gravitational waves we detect here on Earth.

Gravitational Waves from Black Hole Binaries

black hole merger

Animation of a black hole binary merger. As the black holes merge, they create gravitational waves, warping the fabric of spacetime. Click to enlarge. [LIGO/T. Pyle]

Since the first gravitational-wave detection in 2015, the LIGO, Virgo, and KAGRA (LVK) detectors have continued to record rumbles from across the universe. A number of these gravitational waves were born from the merging of two black holes in a binary system, and the LVK observations allow researchers to decipher the properties of the black holes involved. These measured masses and spins of the binaries’ black holes could point us toward an explanation for the actual origin of the binary systems themselves — currently an open question that makes predicting black hole mergers and their subsequent gravitational waves difficult.

Multiple formation mechanisms have been proposed that are sensitive to the characteristics of the black holes in the binary. Importantly, the spin (rotation) of the black holes can be used to distinguish how the system likely formed. From theory, stellar-mass black holes likely form with little spin but will spin up as they interact with stars and other black holes throughout their lifetimes. Interactions causing spin-up are more likely to happen in crowded areas like star clusters, so understanding how binary black holes form and evolve within these dense environments will enable comparison to LVK observations.

Simulations of Clusters

To explore how collisions and close encounters with stars influence black hole properties within dense star clusters, Fulya Kıroǧlu (CIERA; Northwestern University) and collaborators performed eight simulations across a range of cluster models. Varying characteristics such as metallicity, cluster radius, and binary fraction for massive stars, the authors explore how the black holes in each cluster evolve over 12 billion years. 

Through these simulations, the authors track the number of black hole–star collisions and find that, depending on the cluster properties, black holes and stars undergo collisions at early and late times in the cluster’s life. For stellar clusters with high-density cores and more massive star binaries, black holes are more likely to collide with high-mass stars, increasing the rate at which spinning black holes are formed. Additionally, the metallicity of the star cluster can significantly impact how many black hole–star interactions occur. For merging black hole binaries in star clusters, especially those with high metallicity, over 50% had at least one black hole that had spun up after a previous collision with a star. These results build an expected distribution of binary black hole merger spins that can be compared to gravitational wave detections.

figure

Effective spin versus primary black hole mass distribution. The results from the simulations in this study are shown in purple and cyan. The background points show the predicted distribution based on LVK observations. Click to enlarge. [Kıroǧlu et al 2025]

Comparing to Observations

What do the simulation results mean when it comes to LVK observations? The authors compare the distribution of binary black hole spin and mass for all mergers in their models to the predicted distribution based on LVK observations. Their modeling reproduces the predicted trends from gravitational-wave data, and recent observations suggest a population of binary black hole mergers with high spins consistent with this work.

As LVK continues to detect gravitational waves, further simulations with more detailed hydrodynamic models will be necessary in order to uncover the full range of possible outcomes of binary black hole–star interactions. This work serves as a critical step in revealing the origins of gravitational waves.  

Citation

“Black Hole Accretion and Spin-up through Stellar Collisions in Dense Star Clusters,” Fulya Kıroǧlu et al 2025 ApJ 979 237. doi:10.3847/1538-4357/ada26b

asteroid 2024 yr4

The chances of the asteroid 2024 YR4 striking Earth in 2032 have tumbled from a peak of 3.1% in February 2025 to nearly zero today, though a collision with the Moon is still possible. Brand new JWST observations give a fresh perspective on this near-Earth asteroid.

From Discovery to Today

Discovery images of asteroid 2024 YR4

Discovery images of 2024 YR4. [ATLAS]

On 27 December 2024, the Asteroid Terrestrial-impact Last Alert System (ATLAS) spotted an asteroid, cataloged as 2024 YR4, with a slim chance of a collision with Earth on 22 December 2032. As observations rolled in, the probability of 2024 YR4 slamming into Earth crept upward, reaching a peak of 3.1% on 18 February 2025. Just a few days later, revised estimates of the asteroid’s orbit reduced the odds of an Earth impact to just 0.004%. Though 2024 YR4 no longer poses a threat to Earth, there is still a small chance it will hit the Moon in 2032.

A New Look at 2024 YR4

As reported today in Research Notes of the AAS, a team led by Andrew Rivkin (The Johns Hopkins University Applied Physics Laboratory) used JWST to observe 2024 YR4 in March 2025. Using the Mid-Infrared Instrument (MIRI) and the Near-Infrared Camera (NIRCam), the team determined the size, albedo, and spectral energy distribution of the asteroid.

By modeling the asteroid’s thermal emission, Rivkin’s team estimated 2024 YR4’s diameter to be 60 ± 7 meters (197 ± 23 feet). This is consistent with previous size estimates that used measurements of the asteroid’s apparent magnitude and estimates of its albedo, or the fraction of light that is reflected from its surface. As a comparison, the asteroid that caused the Tunguska event — an air burst that toppled millions of trees, shattered windows, and sparked forest fires — is thought to have been in the range of 40–100 meters (130–328 feet).

Further Findings and Future Directions

The asteroid’s albedo is between 0.08 and 0.18, with a best-fit value of 0.13. Many classes of asteroids have albedos in this range, so this estimate doesn’t place 2024 YR4 in any particular class. However, Rivkin’s team noted that these values are compatible with previous spectra of the asteroid that suggest an S-type (stony) classification.

The team also estimated the potential impact of a 2024 YR4-like asteroid, finding that a collision with Earth would release the energy equivalent of 2–30 megatons of TNT, with a blast damage radius of up to 80 kilometers (50 miles).

Future modeling using sophisticated thermophysical models will help to define the asteroid’s properties further. And in early 2026, researchers may have another chance to observe 2024 YR4 with JWST and refine their understanding of a potential lunar impact in 2032.

Citation

“JWST Observations of Potentially Hazardous Asteroid 2024 YR4,” A. S. Rivkin et al 2025 Res. Notes AAS 9 70. doi:10.3847/2515-5172/adc6f0

30 Doradus

This Monthly Roundup covers three investigations of the high-energy universe, from a hunt for a cosmic particle accelerator in the Milky Way to an examination of a fading quasar in the distant past.

Investigating the Most Energetic Neutrino Ever Detected

In February 2023, the Cubic Kilometre Neutrino Telescope (KM3NeT) — a neutrino telescope at the bottom of the Mediterranean Sea — detected a particle called a muon with an energy of roughly 100 petaelectronvolts (a hundred quadrillion electronvolts). The muon was likely produced by an incoming neutrino with an energy of 220 petaelectronvolts — the highest-energy neutrino ever observed.

The orientation of the event suggests an astrophysical origin, but the source of this neutrino is unknown. One possibility is that the neutrino arose in a transient event that produced extremely high-energy cosmic rays: relativistic charged particles like protons, electrons, and atomic nuclei. Cosmic rays could produce neutrinos and gamma rays through interactions with photons of the cosmic microwave background. The neutrinos zip off into space, unhindered by intervening gas or magnetic fields, while the cosmic rays can be waylaid for thousands of years, caught up in the magnetic fields that lace the space between galaxies. Gamma rays fall in between the two extremes, slowed slightly by interactions with the photons of the extragalactic background. Repeated interactions between the gamma rays and background photons create a cascade of gamma rays across a range of energies.

plot of gamma-ray flux as a function of time since the neutrino's arrival

Gamma-ray flux as a function of time since the neutrino’s arrival for different intergalactic magnetic field strengths. Stronger magnetic fields lead to lower flux and later arrival times. The gray lines show the five-sigma detection limits of different instruments. Click to enlarge. [Fang et al. 2025]

Detecting this gamma-ray cascade would provide a valuable clue in the search for the origin of the ultra-high-energy neutrino detected in 2023. In a recent research article, Ke Fang (Wisconsin IceCube Particle Astrophysics Center) and collaborators estimated the flux of gamma rays that would be associated with this high-energy neutrino. The team’s estimates accounted for varying distances to the source as well as different strengths of the intergalactic magnetic field. The stronger the magnetic field, the weaker the gamma-ray flux when it arrives at Earth, and the later the arrival time at Earth.

For weak magnetic fields, the gamma-ray cascade should have arrived at Earth hours or days after the neutrino was detected in 2023. These gamma rays are potentially detectable as long as the magnetic field is weaker than 3 × 10-13 Gauss. For magnetic field strengths greater than 3 × 10-13 Gauss, the gamma rays wouldn’t arrive until more than a decade later, and they would likely be too faint to detect. If no gamma rays are detected, the non-detection could be used to place a lower limit on the strength of the intergalactic magnetic field.

The Hunt for a Galactic PeVatron

Across the universe, charged particles are being accelerated to near the speed of light, achieving energies in the petaelectronvolt, or PeV, range. The sources of these particles are called PeVatrons, and observations have revealed that these cosmic particle accelerators exist in the Milky Way. Supernovae, massive stars, pulsars, and pulsar wind nebulae are all candidate PeVatrons. To find out more, astronomers look to gamma rays, which can be produced when cosmic rays interact with dense matter.

Recently, the Large High Altitude Air Shower Observatory (LHAASO) collaboration investigated a possible galactic PeVatron called G35.6−0.4. G35.6−0.4 is a radio source that is thought to be associated with the gamma-ray source HESS J1858+020. Observations of this region show a supernova remnant and an H II region containing multiple X-ray point sources.

gamma-ray map of the region of interest in this study

LHAASO’s Water Cherenkov Detector Array (left) and Kilometer Square Array (right) observations of the region around the gamma-ray source HESS J1858+020. Black solid circles and black crosses represent extended and pointlike sources, respectively, resolved in this work. Dashed circles show sources resolved by LHAASO in previous work. The cyan symbols show the locations of gamma-ray sources identified by other facilities. Click to enlarge. [LHAASO Collaboration 2025]

To learn more about the origins of the gamma rays from this complex region, the collaboration used data from LHAASO, a ground-based gamma-ray and cosmic-ray observatory. Data from two of LHAASO’s detectors show five gamma-ray sources throughout the region, one of which may be associated with the previously detected gamma-ray source HESS J1858+020. The team also amassed data from other sources, pulling together a picture of the molecular and atomic gas and massive stars present in the region.

Because of the crowded nature of this area, this investigation wasn’t able to clearly point to the source of the gamma rays. The authors outlined three possible sources for the gamma rays: 1) winds from hidden massive stars or outflows from protostars within the H II region, 2) particles escaping from the supernova remnant and interacting with nearby molecular clouds, and 3) an as-yet-undetected pulsar wind nebula. While none of these scenarios is a clear front-runner, neither could any of them be ruled out (though the supernova remnant scenario faces the greatest feasibility challenges). Future searches for massive stars or pulsar wind nebulae in this region may provide further clues.

Fading Light from a Quasar at Cosmic Dawn

For the third and final article, we’re looking back into the distant past at one of the most powerful objects in the universe: a quasar. Quasars are extraordinarily luminous galactic centers in the early universe, powered by accretion of gas onto a growing black hole. Because of their extreme brightness, quasars are visible from billions of light-years away, giving researchers a glimpse into the early evolution of supermassive black holes.

Jianwei Lyu (吕建伟; University of Arizona) and collaborators investigated HSC J2239+0207, a quasar located at a redshift of z = 6.2498, when the universe was roughly 900 million years old. This redshift places the quasar near the end of the epoch of reionization, when the formation of the first stars and galaxies ionized the universe’s abundant neutral hydrogen gas. This quasar is an intriguing target because previous observations have shown that the black hole that powers it is roughly 15 times more massive than expected for the stellar mass of its host galaxy. The quasar’s accretion rate is low, indicating that the black hole may be nearing the end of its growth spurt.

JWST spectrum of the quasar HSC J2239+0207

JWST spectrum of the quasar HSC J2239+0207 (blue line). Click to enlarge. [Lyu et al. 2025]

Lyu’s team analyzed JWST spectra of this quasar, estimating the black hole’s mass to be roughly 300 million solar masses (about 75 times more massive than the Milky Way’s supermassive black hole) and its accretion rate to be just 40% of the theoretical limit. This is unusual, since quasars at this point in the universe’s history typically have accretion rates at or above the theoretical limit. The unexpectedly low accretion rate for HSC J2239+0207 could mean that the black hole’s growth is slowing down. However, the authors caution that it could be a temporary slowdown caused by a lack of fuel rather than a permanent shutdown.

The team also investigated a gas cloud located one arcsecond from the quasar. This object could be several things: an isolated high-redshift galaxy, a galaxy falling toward the quasar host galaxy, tidally disrupted material stripped from a galaxy passing nearby, or material blown out of the quasar host galaxy by the quasar itself. The authors favor this final scenario, which is indicative of black hole feedback at work.

Feedback may be the reason that this black hole is so massive compared to the stellar mass of its host galaxy. Powerful radiation and winds from the black hole could have suppressed the rate of star formation as the black hole grew. With the black hole’s activity winding down, star formation should have a chance to ramp up, bringing the galaxy into alignment with the expected stellar mass–black hole mass relation.

Citation

“Cascaded Gamma-Ray Emission Associated with the KM3NeT Ultrahigh-Energy Event KM3-230213A,” Ke Fang et al 2025 ApJL 982 L16. doi:10.3847/2041-8213/adbbec

“An Enigmatic PeVatron in an Area Around H II Region G35.6−0.5,” Zhen Cao et al 2025 ApJ 979 70. doi:10.3847/1538-4357/ad991d

“Fading Light, Fierce Winds: JWST Snapshot of a Sub-Eddington Quasar at Cosmic Dawn,” Jianwei Lyu et al 2025 ApJL 981 L20. doi:10.3847/2041-8213/adb613

Milky Way center

Could there have been two massive black holes in our galaxy’s center at one time? New research suggests that this scenario could explain oddities in the population of hypervelocity stars in the Milky Way’s halo.

From the Center to the Halo

stars at the center of the Milky Way

A near-infrared image from the Very Large Telescope of stars at the center of the Milky Way. [ESO/S. Gillessen et al.; CC BY 4.0]

At the heart of the Milky Way, in the neighborhood of Sagittarius A* (Sgr A*) — our galaxy’s supermassive black hole — there is a population of massive young stars. Living in such an extreme environment has its dangers, and Sgr A*’s tidal forces can tear apart stellar binaries, capturing one star into a snug orbit around the black hole and flinging the other away at high speeds.

Surveys of the Milky Way halo have indeed found massive hypervelocity stars that hastily departed the galactic center some 50–250 million years ago, cementing the idea that interactions between stellar binaries and Sgr A* can send stars careening through the galaxy.

There are two odd things about the observed population of hypervelocity stars, though: 1) their velocities top out around 700 km/s, though theory suggests roughly half of these stars should move faster than this cutoff; and 2) none of the stars remaining in orbit around Sgr A* appear to be the left-behind binary companions of these hypervelocity stars.

A Long Time Ago in a Galaxy Very, Very Close By

A second black hole in the Milky Way’s center, present long ago, may explain these oddities. In a research article published this week, Chunyang Cao (Peking University) and collaborators outlined how the existing population of hypervelocity stars could be produced with the help of an intermediate-mass black hole that entered our galaxy billions of years ago and has since merged with Sgr A*.

Cao’s team outlined how Sgr A* could gain an intermediate-mass black hole companion when our galaxy captures and incorporates a dwarf galaxy — something that is thought to have happened multiple times in the Milky Way’s history. Orbiting with Sgr A* in a binary system, the intermediate-mass black hole acts as a goalkeeper, gravitationally kicking away stars that approach the galactic center. The presence of the intermediate-mass black hole prevents most binary systems from being disrupted very close to Sgr A*, where they would score the largest gravitational boost and achieve the highest velocities.

predicted and observed radius and velocity distributions for hypervelocity stars in the Milky Way

Predicted (green and blue shaded areas) and observed (gray and black lines) velocity and galactocentric radius distributions of hypervelocity stars in the Milky Way. The two observation lines show the results from the Multiple Mirror Telescope (MMT) hypervelocity star survey and the MMT survey plus proper-motion corrections from the Gaia spacecraft. The pink and purple lines show the predictions of models that do not incorporate an intermediate-mass black hole. Click to enlarge. [Cao et al. 2025]

A Model Population

Cao’s team modeled the population of hypervelocity stars that would result from this scenario and found that the velocity and radius distributions of their model population closely matched what is observed. Crucially, the model predicts that just 2.5% of hypervelocity stars should have velocities greater than 700 km/s and can accurately reproduce the population of stars that remain in the galactic center. The model also predicts that 10% of ejected stars should be intact binary systems rather than lone binary members, which is consistent with observations.

The authors posited that the most likely source of an intermediate-mass black hole in the right time frame for this scenario to play out would be the merger with the Gaia–Sausage–Enceladus dwarf galaxy about 10 billion years ago. Modeling suggests that this dwarf galaxy’s black hole was roughly 300 times less massive than the Milky Way’s central black hole, and the two black holes likely merged 10 million years ago.

Though the intermediate-mass black hole may be long gone, its influence may still be felt: the merger of the two black holes would have caused Sgr A* to recoil, potentially accounting for the motion of Sgr A* seen today.

Citation

“A Recent Supermassive Black Hole Binary in the Galactic Center Unveiled by Hypervelocity Stars,” Chunyang Cao et al 2025 ApJL 982 L37. doi:10.3847/2041-8213/adbbf2

NGC 3603

JWST observations of distant galaxies have revealed incredibly dense and bright clusters of stars in the early universe. New simulations show how these clusters may have formed through galactic disk fragmentation.

From the Early Universe to the Present Day

While JWST observations were expected to shed light on the earliest stages of galaxy evolution, certain discoveries raised more questions than answers. How structures like galactic disks, supermassive black holes, and massive star clusters developed so rapidly is an open question.

simulation results showing gas and stellar density

The gas density (left) and stellar density (right) for the main galaxy, labeled “0,” and its two galactic companions. Click to enlarge. [Adapted from Mayer et al. 2025]

In a recent research article, a team led by Lucio Mayer (University of Zurich) tackled the question of how dense star clusters like those spotted by JWST formed in the early universe. Mayer and collaborators used a hydrodynamical simulation with a resolution of just 6.5 light-years to study the aftermath of a merger of two massive gas-rich galaxies at a redshift of z = 7.6, when the universe was about 700 million years old. The resulting galaxy has a stellar mass of 80 billion solar masses, placing it on the extreme high-mass end of galaxies during this time period. The galaxy also has two galactic companions that are 8 and 400 times less massive.

In the wake of the galaxy collision, the resulting galaxy forms a dense, gas-rich disk. Because of the high density of this gas disk, fragmentation of the disk into massive star clusters occurs quickly, on timescales of a million years. In less extreme environments, the rate of star formation is moderated by feedback — for example, from supernova explosions that inject energy and momentum into star-forming clouds, heating and disrupting the clouds and slowing the rate of star formation. Here, however, fragmentation happens too fast for supernova feedback to stop it, resulting in massive, dense clumps of gas that collapse further to form star clusters.

Cosmic Gems arc

The gravitationally lensed Cosmic Gems arc, imaged by JWST at a redshift of z = 10.2 (about 500 million years after the Big Bang) bears considerable similarity to the smallest of the three galaxies in this simulation. [ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration; CC BY 4.0]

Cluster Assemblage

The central galaxy and its two smaller companions all formed star clusters through this method. The mass of the clusters appears to scale with the mass of the galaxy, with the largest galaxy forming the largest number of clusters and the most massive clusters. In total, roughly 20–30% of the total stellar mass of each galaxy forms through fragmentation of the disk, creating clusters with masses of 105–108 solar masses. The large fraction of galactic stellar mass belonging to massive star clusters, as well as the rapid time frame of their formation — the simulation spans just 6 million years — are both consistent with JWST’s observations of star clusters in the early universe.

These star clusters may provide a clue to another early universe mystery: the development of supermassive black holes. The extreme density of the simulated star clusters provides a natural avenue for the formation of intermediate-mass black holes with masses up to 105 solar masses. These black holes could sink to the center of the galaxy and coalesce to form a single supermassive black hole with a mass of 10 million solar masses — comparable in mass to many of the black holes seen with JWST.

Intriguingly, in the smallest galaxy explored in the team’s simulation, the black hole’s mass would be only a factor of a few less massive than the total mass of stars in the galaxy, potentially also explaining why some galaxies appear to have over-massive black holes.

Citation

“In Situ Formation of Star Clusters at z > 7 via Galactic Disk Fragmentation: Shedding Light on Ultracompact Clusters and Overmassive Black Holes Seen by JWST,” Lucio Mayer et al 2025 ApJL 981 L28. doi:10.3847/2041-8213/adadfe

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