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galaxy cluster MACS J0416.1-2403

JWST images of galaxy clusters are rich with gravitationally lensed galaxies. Recent work examines a source in a lensed galaxy that may offer a glimpse into the lives of massive stars billions of years ago.

Cosmic Gems arc

JWST image of the gravitationally lensed Cosmic Gems arc, in which multiple individual star clusters are visible. [ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration; CC BY 4.0]

Glimpses of Single Stars

Galaxy clusters, the largest gravitationally bound structures in the universe, create the conditions necessary for astronomers to perform an extraordinary feat: examine individual massive stars and star clusters at far greater distances than our telescopes can typically achieve. This is possible thanks to gravitational lensing, the bending of spacetime by an immense mass, which warps and magnifies the light from more distant objects.

These glimpses of single stars and star clusters offer a rare chance to study massive stars in our universe’s distant past directly. In particular, these observations allow us to probe whether factors like the multiplicity fraction — how many massive stars are in binary or multiple systems — have changed over cosmic time.

Peering Through a Gravitational Lens

In a recent research article, a team led by Hayley Williams (University of Minnesota) reported on their examination of an intriguing source in a gravitationally lensed galaxy called the “Warhol arc.” This galaxy, located at a redshift of z = 0.94 (when the universe was roughly 6 billion years old), is gravitationally lensed by the massive galaxy cluster MACS J0416.1−2403. The cluster is located at a redshift of z = 0.396, corresponding to when the universe was about 9.4 billion years old.

Using data from the JWST Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) program and the Canadian NIRISS Unbiased Cluster Survey (CANUCS), Williams’s team analyzed a source in the Warhol arc called W2, which previous work suggests is either a binary star system or a small star cluster.

Multiplicity and Microlensing

Warhol arc

Top row: The Warhol arc during four epochs of JWST observations. Bottom row: On the left, a magnified image of W2 during the first epoch. The remaining images show the difference in brightness between subsequent epochs and the first epoch. Click to enlarge. [Williams et al. 2026]

Across four epochs spanning 126 days, the JWST observations show the source W2 within the Warhol arc. Williams and collaborators performed spectral fitting of the JWST light curves to investigate the multiplicity of the source. They found that the data are best matched by a binary system containing stars with temperatures of 3500K and 12600K.

W2 varies between observations in both brightness and color, a fact that the authors suggested is due to microlensing by a star within the lensing galaxy cluster, rather than variability within the binary system itself. Under this hypothesis, the orbital motions of the binary bring the stars across the microlensing caustic — a region in which the magnification is exceptionally high — and the brightness and color of W2 vary as the components of the binary approach and recede from the caustic.

Williams and collaborators also performed stellar population modeling to explore the binary configurations that could match the observations. They found that the stars likely have masses of 21–24 solar masses, with one being a cool red supergiant and the other a hot, main-sequence companion. Depending on the precise evolutionary stage of the binary, it’s possible that one of the stars is nearing a supernova explosion. Lending more support to the binary system hypothesis, the microlensing measurements constrain W2 to be no larger than 90 au — too small for even a compact star cluster.

The team closed by proposing further observations of W2’s position to rule out the possibility that the microlensing rate there is unusually high, an outcome that may suggest that microlensing of two unrelated stars, rather than a binary system, is responsible for these observations.

Citation

“JWST’s PEARLS: A Candidate Massive Binary Star System in a Lensed Galaxy at Redshift 0.94,” Hayley Williams et al 2026 ApJ 997 292. doi:10.3847/1538-4357/ae2003

'Oumuamua

Traversing the galaxy from places yet known, a few interstellar objects have taken a quick dip into our solar system. These objects have inspired a frenzy of questions regarding their origins and paths that led them here. A recent study took to nearby planet-forming stellar systems as possible launching posts.

Interstellar Object Origins

As 3I/ATLAS was first spotted entering the solar system in July 2025, all eyes turned to the curious interstellar visitor. Just the third interstellar object astronomers have observed, 3I/ATLAS joins 1I/ʻOumuamua (2017) and 2I/Borisov (2019) in capturing our attention and igniting many questions regarding their origins and journeys to our corner of the Milky Way. While astronomers have characterized these objects’ compositions, sizes, and paths through the solar system, the mechanisms by which these objects leave their origin systems and travel the galaxy before arriving here are not well constrained. 

These three large bodies are just part of the interstellar material visiting the solar system — spacecraft have detected an influx of small interstellar dust particles, and while not yet directly confirmed, interstellar meteoroids burning up in Earth’s atmosphere are possible. This loose material likely originates in dusty, planet-forming disks (debris disks) around stars — rubble kicked out into interstellar space, sending chemicals, organic compounds, and perhaps even the precursors of life to other nearby systems. Understanding the origins and trajectories of such material will allow us to investigate planet formation and how developing systems may impact their environments.

Deliveries from Debris Disks

Debris disks offer ideal testbeds for tracing small bodies and dusty material through the galaxy. Interactions with large planets in debris disks can launch loose material like planetesimals and dust into interstellar space. To explore how debris disks may drive interstellar visitors to our solar system, Cole R. Gregg and Paul A. Wiegert (The University of Western Ontario) simulated how much material the 20 nearest debris disks would contribute to the interstellar object population of our solar system.

sky projection for particle arrival directions

Arrival direction on the sky for particles that reach the solar system from the 20 debris disks studied here. The arrival locations for 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS are shown for comparison. Click to enlarge. [Gregg and Wiegert 2025]

The authors modeled the trajectories of ejected material from each debris disk within a simulated Milky Way, running the simulations backward and forward in time to trace the ejecta over 100 million years. These simulations show that material from each of the 20 systems is currently expected to be within our solar system. For interstellar objects (≥100 meters), the simulations predict that about two objects from these debris disks are currently within the inner solar system.

Trickier to spot, smaller meteoroids (≥200 microns) that would appear as meteors in Earth’s atmosphere are also expected. The only way to observationally confirm interstellar origins for meteors is based on how fast they are moving relative to the material in our solar system, or their excess velocities. Based on the simulations, many of these smaller particles have low excess velocities, making them observationally difficult to distinguish from bound solar system objects.

Observational Connections and Expectations

'oumuamua zoom in plot

Zoomed-in view focusing on the direction where 1I/ʻOumuamua entered the solar system. The interstellar object falls near the arrival directions of particles from HD 38858 and HD 166. Click to enlarge. [Gregg and Wiegert 2025]

How do these simulations compare to observations? From their analysis, the authors identified where the interstellar material from each debris disk would first enter the solar system. Comparing these to the entry locations of 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS, ʻOumuamua appears near the entry locations for debris from HD 166 and HD 38858. (2I/Borisov and 3I/ATLAS had no such match up in this sample of debris disks.) However, tracing ʻOumuamua’s travel backward, its closest approaches to these systems are about 32 and 18 light-years away, respectively — likely too far to have originated from either system but still a clear proof of concept that we can place interstellar objects near potential systems of origin. 

Where are the expected interstellar meteoroids? Current meteor detectors including the Canadian Meteor Orbit Radar (CMOR) and the Global Meteor Network (GMN) search the sky for incoming material. While the simulations predict hundreds of meteoroid-sized particles from each debris disk currently in the inner solar system, the small collecting area of CMOR and the limiting size of grains detectable by GMN statistically require decades of observations to detect even a single interstellar meteoroid. Thus, the lack of detections thus far is not surprising, but with advanced instrumentation, discoveries of more interstellar particles and objects are imminent.

Citation

“A Catalogue of Interstellar Material Delivery from Nearby Debris Disks,” Cole R. Gregg and Paul A. Wiegert 2025 PSJ 6 309. doi:10.3847/PSJ/ae284f

Vera Rubin Observatory

The detection of gravitational waves and light from a single source was one of the most important discoveries of the gravitational wave era. Can upcoming data from Vera C. Rubin Observatory help astronomers repeat that feat?

kilonova as seen by Hubble

These images from the Hubble Space Telescope show the fading light of the kilonova associated with the gravitational wave event GW170817. [NASA and ESA Acknowledgment: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)]

Not Yet Repeated

When two neutron stars merge, the collision sends ripples through spacetime and generates an electromagnetic signal called a kilonova. In 2017, researchers detected gravitational waves and kilonova emission from colliding neutron stars — the first and, to date, only electromagnetic signal definitively associated with a gravitational wave event.

In a research article published this week, a team led by Simon Stevenson (Swinburne University of Technology; OzGrav) considered how the much-anticipated Legacy Survey of Space and Time (LSST), carried out by Rubin Observatory, can enhance our ability to detect kilonovae and pair them with their gravitational wave counterparts.

Survey Simulations

When a new gravitational wave signal is detected, researchers rush to search for an electromagnetic counterpart to the signal. While Rubin Observatory can aid this type of reactive search, Stevenson’s team focused on a different strategy: spotting kilonovae during routine survey operations and using these detections to trigger a targeted hunt through gravitational wave data, searching for signals missed by automated detection algorithms.

Over the 10-year course of LSST, Rubin will scan the visible southern sky every few days, uncovering a wide variety of transient sources such as supernovae, novae, and tidal disruption events. Because Rubin will amass a huge amount of data each night, astronomers worldwide will use data brokers to pass along the most promising signals. Stevenson’s team used for their analysis a bespoke and modular data broker called Fink. Fink currently handles 200,000 alerts per night from the Zwicky Transient Facility and will be scaled up to handle 10 million alerts per night as Rubin requires.

simulated kilonova light curves

Examples of simulated kilonova light curves for different redshifts and models (Kasen and Bulla). Click to enlarge. [Adapted from Stevenson et al. 2026]

Searching for Signals

Though Rubin is a transient-tracking powerhouse, it’s not ideally suited to discovering kilonovae specifically; these events evolve so rapidly that Rubin may only be able to collect a few data points before the event fades from its view. To gauge Rubin’s efficacy as a kilonova detector, Stevenson and collaborators simulated kilonova light curves and identified detections with a signal-to-noise ratio greater than 5; these detections will be passed to the data broker. They found that Rubin will detect about 42 kilonovae per year, but only a few will have a strong enough signal to be passed to the broker.

For the handful of detections that are passed to the broker, what are the prospects for tracking down the associated gravitational wave signals? Due to the survey cadence, most kilonova detections occurred 1–2 days after the gravitational waves from the neutron star collision would have reached Earth, but delays of up to 5 days were possible. This means that searches for gravitational wave signals must sift through multiple days of data — a computationally intensive prospect that might require the development of new search techniques. The computational requirements of this search are increased by the possibility of contaminants (signals misclassified as kilonovae and passed on by the data broker), which will require additional followup.

Despite these challenges, it’s clear that Rubin’s upcoming detections of kilonovae will advance our study of these rare events. With LSST expected to get underway early this year, the next multi-messenger signal may be just around the corner.

Citation

“Strategy for Identifying Vera C. Rubin Observatory Kilonova Candidates for Targeted Gravitational-Wave Searches,” Simon Stevenson et al 2026 ApJ 998 8. doi:10.3847/1538-4357/ae244e

illustration of the Milky Way's dark matter halo

New research suggests that the mysterious “little red dots” spotted in the early universe could be supermassive black holes birthed in the collapse of dark matter halos.

Another Theory for Little Red Dots

little red dots

JWST images of six very distant galaxies dubbed “little red dots.” [NASA, ESA, CSA, STScI, Dale Kocevski (Colby College)]

Many theories exist for the origins of little red dots: compact, reddish objects spotted mainly in the first billion years after the Big Bang. The leading theory states that that little red dots are growing black holes encased in dense gas. The black holes at the centers of these little red dots appear to be tens or even hundreds of millions of times the mass of the Sun, raising questions about how they grew so massive so quickly. One possibility is that they started out quite massive, arising from black holes born from the collapse of massive gas clouds in the early universe.

A recent research article explores that theory, with a twist: that the black holes at the heart of little red dots were born not out of regular, baryonic matter, but out of dark matter.

A Dark (Matter) Past

In the early days of the universe, tiny fluctuations in the density of dark matter began to grow, eventually collapsing into vast spheroidal structures called dark matter halos. In the leading theory of cosmology, ΛCDM, dark matter interacts with itself and normal matter only through gravity. This leads to the creation of stable dark matter halos that act as the invisible scaffolding within which the first stars and galaxies grow.

In today’s article, Fangzhou Jiang (Peking University) and collaborators investigated the evolution of halos containing self-interacting dark matter. Particles of self-interacting dark matter, as the name suggests, can interact with one another through collisions and exchange heat. This small adjustment, sometimes invoked in alternatives to ΛCDM to explain the properties of dwarf galaxies, can lead to the collapse of dark matter halos into black holes.

Seeding Black Holes

Jiang and coauthors examined whether this process could explain the observed population of little red dots. First, the team examined whether the timescales are feasible: is it possible for dark matter to assemble itself into massive halos and collapse into black holes, all within the first billion years of the universe?

The answer appears to be yes — in this framework, dark matter halos with masses between 106.5 and 108.5 solar masses were able to form black holes between 104.5 and 106.5 solar masses by a redshift of z = 8.5, when the universe was just 600 million years old.

plot comparing observed and modeled black hole populations

Comparison of the inferred black hole population in little red dots (red diamonds) to models of black holes formed in dark matter halo collapse (shaded areas) at two redshifts. The red area shows the fiducial model, while the blue and green areas show the effects of changing the dark matter self-interaction cross-section. The dashed red lines show an extreme case in which the presence of baryons accelerates halo collapse. Click to enlarge. [Jiang et al. 2026]

Jiang’s team then performed semianalytic modeling to explore how these “seed” black holes grow and evolve through accretion and mergers. This analysis generated a distribution of black hole masses consistent with what has been observed in little red dots. The team cautioned that their results are sensitive to the interaction cross-section of the dark matter particles, as well as various other parameters like the black hole accretion duty cycle. Their modeling also assumes that the halo collapse takes place before any baryonic matter collects within it. Intriguingly, the authors found that the presence of baryons accelerates the collapse of the dark matter halo, speeding the seeding process.

To close, Jiang and collaborators noted that this process is not mutually exclusive with other black hole seeding mechanisms, and as studies of self-interacting dark matter continue, its ability to create black holes in the early universe should be examined further.

Citation

“Formation of the Little Red Dots from the Core Collapse of Self-Interacting Dark Matter Halos,” Fangzhou Jiang et al 2026 ApJL 996 L19. doi:10.3847/2041-8213/ae247a

Sagittarius B2

New research uses distant sources to investigate a question close to home: how the Milky Way replenishes its supply of star-forming gas.

Dynamic Galaxies

UGC 12158

This Hubble Space Telescope image shows UGC 12158, a barred spiral galaxy that is likely very similar to the Milky Way. This image also features an asteroid streaking across the top of the galaxy. [NASA, ESA, Pablo García Martín (UAM)]

In photographs, galaxies often appear static and isolated, surrounded by nothing but the blankness of space. This picture is deceiving: galaxies across the universe are dynamic, constantly compressing gas into new stars, expelling gas through supernovae and galactic winds, and replenishing their star forming material by channeling gas from cosmic filaments, accreting clouds from their galactic halos, or collecting previously expelled gas as it rains back down.

This interplay of inflowing and outflowing gas has important implications for a galaxy’s ability to form stars; without a way to resupply, galaxies would soon run out of star-forming gas. In a research article published this week, astronomers investigate one channel through which our own galaxy maintains its supply of star-formation fuel.

Quasars Lend a Hand

To learn more about how the Milky Way collects and incorporates new star-forming material, a team led by Hannah Bish (Space Telescope Science Institute; University of Washington) studied the low-velocity gas flowing into and out of our galaxy at the disk–halo interface. This interface is a transitional region in which gas from the Milky Way’s expansive, diffuse halo is incorporated into the disk, where it can condense into stars.

Hubble image of the quasar 3C 273

This Hubble image shows the quasar 3C 273. [ESA/Hubble & NASA; CC BY 4.0]

To study gas at the disk–halo interface, Bish’s team enlisted the help of some of the universe’s most luminous objects: quasars. A quasar is a bright, compact galactic center powered by accretion of gas onto a supermassive black hole. When the light from these distant, brilliant beacons passes through gas clouds like those enveloping the Milky Way, the composition, temperature, and velocity of the gas clouds leaves a spectral fingerprint on the light from the quasar.

Complex Inflows and Outflows

The team used spectra of 132 quasars and ground-based measurements of neutral hydrogen to identify and disentangle the motions of gases with different temperatures and compositions. Along 67% of the lines of sight, Bish and collaborators found evidence for both inflowing and outflowing gas, suggesting that these processes occur simultaneously.

infall velocities for gas accreting onto the Milky Way

Infall velocities for the northern (top) and southern (bottom) hemispheres of the Milky Way. The infall velocities in the southern hemisphere are roughly equal regardless of direction, denoted by color, or the ionization energy of the ion being studied. By contrast, the northern hemisphere shows dramatically different behavior with direction and ionization energy. Click to enlarge. [Bish et al. 2026]

They also found differences between inflows in different locations within the galaxy as well as for gas of different temperatures; in the Milky Way’s southern hemisphere, gas of all temperatures and coming from all directions flows inward at 5–10 km/s. In the northern hemisphere, things are more complicated: cooler gas flows inward at 5–10 km/s, while warmer gas streams toward our galaxy much faster. This trend also varies with location, appearing only in certain directions.

Bish’s team explored these results using a simple model of the Milky Way, which they found could largely reproduce the behavior in the southern hemisphere, but not the more complicated findings from the northern hemisphere. They suggested that the difference in behavior between the cooler and warmer gas could be evidence that accretion in the northern hemisphere occurs in patches, and that warm inflowing gas is piling up and cooling where it collides with denser gas at the disk–halo interface.

Looking forward, Bish’s team anticipates that high-resolution models — and more quasar observations to constrain those models — will be key to uncovering the causes of this curious behavior.

Citation

“Differential Accretion of Ionized Low-Velocity Gas at the Milky Way’s Disk–Halo Interface,” Hannah V. Bish et al 2026 ApJ 997 230. doi:10.3847/1538-4357/ae2741

3I/ATLAS

Editor’s Note: After a short hiatus, the Monthly Roundup is back! This series of articles, which began in 2023, examines multiple perspectives or findings on a single topic.

Interstellar object 3I/ATLAS's trajectory through the solar system

Interstellar object 3I/ATLAS’s trajectory through the solar system. Click to enlarge. [NASA/JPL-Caltech]

The discovery of interstellar object 3I/ATLAS on 1 July 2025 was one of last year’s top astronomy stories. First identified by the Asteroid Terrestrial-impact Last Alert System (ATLAS), 3I/ATLAS is just the third interstellar object that astronomers have seen speeding through our solar system; it was preceded by 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019.

Last summer, professional and amateur astronomers worldwide rolled out the red carpet in 3I/ATLAS’s honor, enlisting a pack of ground-based and spacefaring paparazzi to capture its every move. Observations in the first few months after 3I/ATLAS’s discovery revealed cometary features, including a fuzzy coma of gas and dust, a tail that trailed behind the object, and a plume of dust on the side facing the Sun.

With 3I/ATLAS having now made its closest approach to the Sun, venturing within 1.36 au of our star on 29 October 2025, the comet is wrapping up its tour of the solar system and heading back out to interstellar space — but the research continues. Today, we’re taking a look at five of the more than three dozen research articles published in the AAS journals that have examined 3I/ATLAS and its journey through our solar system. As always, this Monthly Roundup contains only a short summary of each article, so be sure to check out the full research articles linked at the bottom of this post!

Early Spectra of 3I/ATLAS

Bin Yang (Diego Portales University and the Planetary Science Institute) and collaborators obtained spectra of 3I/ATLAS less than two weeks after it was discovered, when it was 4.0–4.4 au from the Sun. Using the Gemini South telescope and the Infrared Telescope Facility (IRTF), Yang’s team sought to characterize its coma, the dusty envelope of gas that puffs out from the icy nucleus as the comet warms up.

3I/ATLAS spectrum and model fit

IRTF spectrum of 3I/ATLAS (black line) with the best-fitting model (red line). Click to enlarge. [Yang et al. 2025]

These observations showed that 3I/ATLAS has a relatively featureless, red-sloped spectrum from 0.5 to 0.8 microns, similar to certain asteroids and active comets in our solar system. In the near-infrared, 3I/ATLAS’s spectrum was flatter, with a broad absorption feature around 2.0 microns.

Yang and coauthors found that the near-infrared spectrum was well fit by a model including a blend of amorphous carbon and water ice (63% carbon and 37% water ice by volume) at a temperature of 120K. 3I/ATLAS’s spectrum bears certain similarities to well-studied solar system comets like C/2006 W3 and 6P/d’Arrest, which suggests that the particles in these objects’ comae are similar in size or composition. Ultimately, the discovery of abundant water ice in 3I/ATLAS’s coma, along with the carbon dioxide spotted in other studies, suggests that the comet formed in the cold, volatile-rich outer regions of its home system.

Unique Polarization Properties

3I/ATLAS is the second interstellar object for which astronomers have obtained polarization information. The first, 2I/Borisov, showed a high degree of positive polarization, comparable to the exceptional solar system comet Hale–Bopp.

plot of polarization as a function of phase angle

Comparison of 3I/ATLAS’s polarization (black) to measurements of 2I/Borisov (blue) as well as solar system comets and asteroids. Click to enlarge. [Gray et al. 2025]

Using the Very Large Telescope, the Nordic Optical Telescope, and the 2m Ritchey-Chrétien-Coude telescope to study 3I/ATLAS’s polarization between 17 July and 28 August 2025, Zuri Gray (University of Helsinki) and coauthors showed that 3I/ATLAS is also exceptional — but in the opposite direction. In contrast to 2I/Borisov’s strong positive polarization, 3I/ATLAS is strongly negatively polarized, with an unusually deep and narrow polarization profile. The narrowness of 3I/ATLAS’s polarization profile is similar to that of certain rare asteroids and cometary nuclei, but 3I/ATLAS is more strongly negatively polarized than these objects.

Gray’s team suggested that 3I/ATLAS’s extreme polarization behavior could be evidence for a mixture of large icy and dark particles in its coma. It’s also possible that it’s the first identified member of a new class of comets, distinct from both solar system comets and interstellar comet 2I/Borisov.

Is 3I/ATLAS Truly a Pristine Relic of the Star System It Came From?

Interstellar objects like 3I/ATLAS have been hailed as pristine beacons that grant us a rare glimpse into the conditions of other star systems. But as new work by Romain Maggiolo (Royal Belgian Institute for Space Aeronomy) and collaborators shows, 3I/ATLAS was likely greatly altered by its journey through interstellar space.

Spectroscopic measurements from JWST suggest that 3I/ATLAS has extremely high abundance of carbon dioxide and carbon monoxide compared to water. A comet’s chemical abundance ratios can be either inherited from the material from which the comet formed, or they can be due to processing after its formation. The team finds that comet 3I/ATLAS’s extreme abundance ratios can be attributed to the comet being pummeled by galactic cosmic rays — high-energy charged particles — as it cruised through interstellar space.

illustration of 3I/ATLAS

Illustration of the nucleus of 3I/ATLAS, featuring a pristine core encased in a crust that has been irradiated by cosmic rays. Click to enlarge. [Maggiolo et al. 2026]

This means that 3I/ATLAS’s current composition doesn’t match its initial composition at the time of its formation, and the cosmic-ray-altered layer of the comet’s nucleus may be 15–20 meters thick. Could volatile outgassing and dust ejection scratch away enough of the comet’s outer layers to expose its pristine interior? This depends on the level of activity the comet experienced as it passed close to the Sun in October 2025. If the comet outgassed strongly, it could have shed tens of meters from its irradiated shell, exposing the pristine core material. Under low to moderate outgassing scenarios, however, the pristine inner material remains hidden beneath the processed shell. Now past the comet’s closest approach to the Sun, we’ll soon know more about whether the composition of the outgassed material has changed as the comet has moved through the solar system.

3I/ATLAS as a Template for Future Encounters: Planning a Potential Rendezvous

It’s only a matter of time before the next interstellar object pays our solar system a visit, and researchers are already thinking about how they’ll study the next visitor. One of the best ways to learn about an interstellar interloper would be to send a spacecraft to rendezvous with it and collect data from up close.

To plan for possible future rendezvous, Atsuhiro Yaginuma (Michigan State University) and collaborators considered what it would have taken for an existing spacecraft to meet up with 3I/ATLAS shortly after it was discovered. The team considered ready-to-launch spacecraft on Earth as well as operational spacecraft currently orbiting Mars.

illustration of the trajectory of a spacecraft rendezvousing with 3I/ATLAS from Mars

Illustration of the trajectory for the minimum-energy rendezvous with 3I/ATLAS launched from Mars on the day the comet was discovered. [Adapted from Yaginuma et al. 2025]

Their calculations showed that meeting up with 3I/ATLAS from Mars would have required less energy than launching a new spacecraft from Earth, with Mars-originating flybys potentially possible with existing technologies. For Earth-originating flybys launched on or after the discovery date of 1 July, linking up with 3I/ATLAS is likely beyond current capabilities — but if the comet had been discovered earlier, a rendezvous launched from Earth may have been possible.

This highlights the importance of detecting interstellar objects as early as possible, as well as the advantage of stationing well-fueled spacecraft at key locations around the solar system; these spacecraft could be deployed rapidly after an interstellar object is discovered, enabling the collection of valuable data. (This is similar to the premise of the European Space Agency’s Comet Interceptor mission, which will wait at one of the Sun–Earth Lagrange points for the arrival of a long-period comet; such a mission could be repurposed to track down an interstellar comet.)

So, About Those Aliens…

Like its predecessor 1I/ʻOumuamua, 3I/ATLAS prompted speculation that it’s not a natural object, though there’s no compelling evidence that it’s anything but a comet. If 3I/ATLAS were an interstellar probe, we might be able to unmask it as such by intercepting its radio communications. In a recent Research Note, Ben Jacobson-Bell (University of California, Berkeley) and coauthors described their search for narrow-band radio signals — the communication medium for all spacecraft in our solar system — from 3I/ATLAS.

The Breakthrough Listen project pointed the 100-meter Green Bank Telescope in 3I/ATLAS’s direction, just one day before the object made its closest approach to Earth at a distance of 1.8 au. The team identified nine possible “events” in their data, all of which were ruled to be radio-frequency interference. Based on this non-detection, Jacobson-Bell and collaborators ruled out the presence of isotropic, continuously outputting radio transmitters with power greater than 0.1 watt. (For comparison, a cellphone puts out approximately 1 watt, and spacecraft radios tend to put out a few dozen watts.) Other searches for radio transmissions from 3I/ATLAS have similarly detected no signals.

Citation

“Spectroscopic Characterization of Interstellar Object 3I/ATLAS: Water Ice in the Coma,” Bin Yang et al 2025 ApJL 992 L9. doi:10.3847/2041-8213/ae08a7

“Extreme Negative Polarization of New Interstellar Comet 3I/ATLAS,” Zuri Gray et al 2025 ApJL 992 L29. doi:10.3847/2041-8213/ae0c08

“Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic-Ray Processing,” Romain Maggiolo et al 2026 ApJL 996 L34. doi:10.3847/2041-8213/ae2fff

“The Feasibility of a Spacecraft Flyby with the Third Interstellar Object 3I/ATLAS from Earth or Mars,” Atsuhiro Yaginuma et al 2025 ApJ 995 64. doi:10.3847/1538-4357/ae11b2

“Breakthrough Listen Observations of 3I/ATLAS with the Green Bank Telescope at 1–12 GHz,” Ben Jacobson-Bell et al 2025 Res. Notes AAS 9 351. doi:10.3847/2515-5172/ae3083

KPNO

After AAS 247 in Phoenix, Arizona, AAS Nova Editor Kerry Hensley and I had the chance to travel about 150 miles south for a tour of Kitt Peak National Observatory.

A Window into Kitt Peak

Located 60 miles southwest of Tucson, Kitt Peak National Observatory (KPNO) resides on a mountain top within the Tohono O’odham Nation reservation. Founded in 1958, Kitt Peak is home to nearly two dozen active telescopes ranging in wavelength coverage and scientific objectives. Ronald Proctor, Lori Allen, and Jacelle Ramon-Sauberan, all serving in crucial roles for the observatory, led our tour around the mountain.

Windows Center Lobby

The Windows On the Universe Center’s lobby exhibit displaying the Tohono O’odham language with words describing the land and astronomy. Jacelle Ramon-Sauberan, the Tohono O’odham Nation Education Liaison, is seen to the left. [KPNO/NOIRLab/NSF/AURA/R. Proctor; CC BY 4.0]

Our visit began in the new Windows on the Universe Center — the first science center inside of a telescope. Situated inside the retired McMath-Pierce Solar Telescope, the Windows Center takes visitors through exhibits exploring the universe, the telescope’s original control room, and the observatory’s connection to the Tohono O’odham Nation.

The center’s lobby introduced visitors to the beautiful land KPNO calls home. Working with Tohono O’odham linguists, Jacelle, the observatory’s Tohono O’odham Nation Education Liaison, curated the lobby exhibit that features the native language and the Nation’s connection to astronomy and the observatory. The science center serves as an educational resource and place of cultural exchange, expanding the relationship between the observatory and the Tohono O’odham Nation. 

Astronauts

Apollo astronauts in the control room of the McMath-Pierce Solar Telescope in 1964. [United States Geological Survey; CC BY 4.0]

After walking through an introduction to the universe exhibit, visitors enter the original control room of the telescope. Mirrors across the room’s ceiling focus the Sun’s light onto table screens where visitors can observe the Sun in real time without burning their eyeballs. In addition to observing the Sun, the solar telescope allowed astronauts in the 1960s to view a projection of the Moon’s surface and plan their landing site for the Apollo 11 mission. Visitors can stand in the same place and witness the instruments that made the moon landing possible.

At the end of the Windows Center is a Science On a Sphere Theater — a spherical display system that allows visitors to see planets, stars, moons, and even large-scale structures of the universe in 3D. Featuring multiple programs, the theater takes visitors through the universe and displays data collected right there on the mountain!

WIYN

The WIYN 3.5-meter telescope at sunset. [KPNO/NOIRLab/NSF/AURA/P. Marenfeld; CC BY 4.0]

A WIYN for Exoplanetary Science

Though not typically open to the public for tours, our next stop took us to the WIYN 3.5-meter telescope. This telescope is operated and owned by the WIYN Consortium, a partnership between the University of Wisconsin, Indiana University, NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory; formerly NOAO), Pennsylvania State University, and Princeton University. Operating since 1994 with a strong collection of instruments, WIYN has observed galaxies near and far, stars across their lifetimes, and exoplanets tugging on their host stars.

Lexi at WIYN

Me, Lexi Gault (AAS Media Fellow), in front of the WIYN 3.5-meter telescope. I have used WIYN’s SparsePak spectrograph to collect data for my dissertation research! [AAS Nova/Kerry Hensley]

WIYN’s newest instrument, NEID (pronounced “NOO-id”), has been advancing exoplanetary science since its first light in early 2020. Derived from the word meaning “to see” in the Tohono O’odham language, NEID is a high-resolution spectrograph that uses the radial velocity method to detect exoplanets. As a planet orbits its host star, its gravitational pull induces a wobble in the star, and NEID can detect these wobbles with unprecedented precision. This instrument allows astronomers to find exoplanets that were previously undetectable.

The partnership between public and private universities and a government institution was the first of its kind. Through this consortium, hundreds of undergraduate and graduate students have conducted research with WIYN, and the telescope’s suite of instruments have enabled early career astronomers to explore the universe both near and far.

Mysteries of Dark Energy at Mayall

We concluded our tour at the Mayall 4-meter Telescope, the tallest telescope dome on the mountain at a staggering 18 stories high. This iconic telescope saw first light in 1973, and at the time, was the second-largest (in diameter) telescope in the world. Originally built for wide-field optical and infrared studies of the universe, the Mayall has enabled decades of critical scientific research.

Mayall dome

The Mayall dome across the mountain as seen from inside the WIYN dome. [AAS Nova/Lexi Gault]

Mayall is now home to the Dark Energy Spectroscopic Instrument (DESI), the most powerful multi-object survey spectrograph in the world. The DESI survey measures the impact of dark energy on the expansion of the universe through creating the largest ever 3D map of the universe. Mapping the distances to hundreds of millions of galaxies, DESI has revolutionized cosmology and enabled groundbreaking research into the origins of the universe. 

From the telescope’s massive dome, to the busy DESI control room, to the gallery floor with a nearly 360 degree view of the surrounding desert, the Mayall offers visitors a unique glimpse into Kitt Peak’s history and its future. While WIYN is a hidden gem, the Mayall 4-meter Telescope is open to the public for tours daily, so you can experience the exciting era of research and discovery at KPNO. 

Visit Kitt Peak

Interested in getting a peek yourself? KPNO is open to the public daily with multiple tour types to choose from, including both daytime and nighttime visits! Plan your visit to Kitt Peak here.

illustration of a gamma-ray burst

Discovered in July 2025, GRB 250702B is unlike any other gamma-ray burst astronomers have seen. New follow-up observations of the burst’s location have underscored the strangeness of this event.

Not Like Other GRBs

Gamma-ray bursts (GRBs) are intense flashes of gamma rays lasting anywhere from milliseconds to hours. The shortest of these bursts are thought to come from colliding neutron stars, while bursts lasting longer than a few seconds tend to come from collapsing massive stars, though notable exceptions have been discovered.

Even with known GRBs spanning such a broad range of timescales and arising from different sources, some bursts are truly exceptional. Such is the case for GRB 250702B, which was discovered on 2 July 2025 by the Fermi Gamma-Ray Burst Monitor. This event featured three separate bursts spread across several hours, as well as never-before-seen X-ray emission in the day leading up to the gamma-ray emission.

JWST on the Case

rest-frame spectrum of GRB 250702B

Rest-frame spectrum of GRB 250702B (blue) with the host galaxy spectrum of another burst, GRB 240825A, for comparison (orange). Click to enlarge. [Adapted from Gompertz et al. 2026]

Benjamin Gompertz (University of Birmingham) and collaborators used JWST to investigate GRB 250702B’s birthplace roughly 51 days after the burst was first detected. JWST spectroscopy revealed that GRB 250702B emerged from a galaxy at a redshift of z = 1.036, when the universe was roughly 6 billion years old.

This precisely measured redshift allowed the team to estimate the burst’s energy; in gamma rays alone, the event released at least 2.2 × 1054 ergs. This places GRB 250702B in the top 20 most energetic GRBs known, and the team estimates that events of this kind are 1,000 times less common than other long-duration GRBs and more than 100,000 times less common than typical core-collapse supernovae.

The burst’s host galaxy is also unusual. It’s larger, brighter, and dustier than other galaxies of similar redshift, and it’s unusually massive among GRB host galaxies. In the wavelength range studied here, it also appears to be the most luminous galaxy known to host a GRB.

What Caused GRB 250702B?

Now, to the cause of this unusual burst: is GRB 250702B just a very strange example of a long-duration burst arising from the collapse of a massive star into a black hole, or could it have an entirely different cause, like a star being ripped apart by a black hole?

plot of host galaxy subtracted spectra of GRB 250702B

The team placed limits on the supernova emission by subtracting a spectrum of the host galaxy from the event. The green lines show this subtracted spectrum under two different normalization conditions. The black lines show for comparison the emission from the bright supernova SN 2023lcr using two possible extinction values. Click to enlarge. [Adapted from Gompertz et al. 2026]

In the collapsing-massive-star scenario, the GRB would be accompanied by a supernova. Gompertz’s team was able to rule out bright supernova emission, but uncertainties in the level of dust within the host galaxy prevented them from excluding the possibility of a significantly fainter supernova hiding within the dust.

The observations also place constraints on a tidal disruption event scenario, in which a star is ripped apart by a black hole. The data strongly favor a jetted, relativistic tidal disruption event, which makes it difficult to pin down the mass of the black hole involved. The timescales are consistent with a white dwarf spiraling around an intermediate-mass black hole, and other factors suggest that the black hole could be no more than a million solar masses, placing it firmly in the intermediate-mass regime, should this explanation prevail.

With GRB 250702B’s source still unsettled, astronomers will continue to study the burst and its unusual host galaxy. Gompertz and collaborators anticipate that follow-up spectroscopy will provide more clues in this case.

Citation

“JWST Spectroscopy of GRB 250702B: An Extremely Rare and Exceptionally Energetic Burst in a Dusty, Massive Galaxy at z = 1.036,” Benjamin P. Gompertz et al 2026 ApJL 997 L4. doi:10.3847/2041-8213/ae2ed9

Researchers have recently discovered a number of ultra-long-period pulsars that are difficult to explain with typical pulsar models. A new article explains how these pulsars might arise from massive stars in close binary systems.

Strangely Slow Sources

Crab Nebula

A multi-wavelength view of the Crab Nebula, the remnant of a supernova that birthed a neutron star. The neutron star powers a pulsar wind nebula, shown in blue. [X-Ray: NASA/CXC/J.Hester (ASU); Optical: NASA/ESA/J.Hester & A.Loll (ASU); Infrared: NASA/JPL-Caltech/R.Gehrz (Univ. Minn.)]

When massive stars go supernova, the explosion can leave behind the condensed stellar core in the form of a neutron star. Neutron stars that spin rapidly become pulsars, whose fast rotation powers beams of radio emission along the star’s poles.

Until not too long ago, pulsars appeared to have periods no longer than 12 seconds. Above that “sluggish” rotation rate, researchers suggested, pulsars could no longer produce electron–positron pairs and channel them along polar magnetic field lines to generate their characteristic radio signals.

So it seemed, until unexpectedly slow pulsars began to crop up. The first trend-breaking pulsars had periods of a few dozen seconds, and objects with pulse periods of minutes or hours have now been found. What could these strange objects be?

Neutron Star Origin Story

In a recent article led by Savannah Cary (University of California, Berkeley), researchers proposed that these strange radio sources are pulsars, but pulsars that rotate slowly and have magnetically powered (rather than rotationally powered) radio emission.

The proposed origin story for these ultra-long-period pulsars begins with a binary system containing a massive star and a close stellar companion. As the massive star evolves, it transfers its outer layers to the companion and expires as a stripped-envelope supernova that births a rapidly spinning neutron star. As the supernova ejecta crashes into the companion star, the star heats up and expands to 5–100 times its original radius.

simulation of supernova ejecta colliding with a star

Simulation snapshot taken one hour after the supernova explosion. The black dashed line shows the path the neutron star (NS) will take through the companion star’s outer layers. Click to enlarge. [Adapted from Cary et al. 2026]

Meanwhile, the explosion delivers a ferocious kick to the newborn neutron star, likely unbinding the binary system. This sets the neutron star on a new course that could take it through the puffed-up outer layers of its former companion, where it may collect gas from the star and ejected material from the supernova into an accretion disk.

Slowing Down

Cary’s team explored the outcomes of this scenario with a trio of simulations: hydrodynamical models for the shock heating of the companion star, numerical models for the formation of the disk around the neutron star, and analytical models for the interaction between the disk and the neutron star.

For a binary system containing stars of 6.4 and 4.0 solar masses and a separation of 20 solar radii — values drawn from observations and simulations — the resulting unbound neutron star collects a disk in 8–10% of cases. The neutron star’s mass, radius, rotation rate, and magnetic field strength determine whether the neutron star and the disk will interact. If they don’t, the neutron star remains rotating rapidly like a typical pulsar. If they do, the neutron star’s rotation will slow to a relative crawl over about a million years. For modest magnetic fields between 1013 and 1014 Gauss, this means rotation periods in the realm of 102 seconds, whereas stronger magnetic fields, above 1015 Gauss, yield periods of roughly a day.

This work suggests that close binary systems could be the source of ultra-long-period pulsars, of which Cary’s team estimates there may be 10–1,000 in the Milky Way. Future work will explore this possibility further, examining broader swaths of parameter space to learn more about how this scenario could contribute to the population of ultra-long-period pulsars in our galaxy.

Citation

“Accretion from a Shock-Inflated Companion: Spinning Down Neutron Stars to Hour-Long Periods,” Savannah Cary et al 2026 ApJ 996 141. doi:10.3847/1538-4357/ae1d43

Illustration of the solar wind interacting with Earth's magnetic field

What happens when pileups of solar wind plasma collide with Earth’s protective magnetosphere? New work uses machine learning to examine how strongly these events affect our planet’s magnetic field.

Plasma Pileups

photograph of a coronal mass ejection

The Solar & Heliospheric Observatory (SOHO) took this coronagraphic image of a coronal mass ejection on 20 April 1998. [SOHO (ESA & NASA)]

Geomagnetic storms driven by solar activity paint night skies with glowing aurorae, but they also threaten spacecraft electronics with showers of high-energy particles. While immense eruptions of solar plasma and magnetic fields called coronal mass ejections are the most infamous example of solar activity, a team led by Yudong Ye (Sun Yat-Sen University) recently focused on another, less destructive form of activity: stream interaction regions.

Stream interaction regions arise when slow-moving solar wind is struck from behind by faster-moving solar wind emitted later. The collision of the two solar wind streams creates a tangle of compressed plasma and strong magnetic fields capable of peeling back Earth’s protective magnetosphere and dumping in high-energy charged particles, with beautiful yet harmful results.

Machine Learning Method

Though stream interaction regions are less disruptive than coronal mass ejections, they’re far more common; they frequently needle Earth’s magnetosphere, especially during the calmer years of the Sun’s activity cycle. Predicting how strongly a stream interaction region will influence Earth’s magnetosphere — in other words, how geoeffective it is — is challenging, however. When two streams of solar wind collide, their properties combine in complex and nonlinear ways that traditional statistical investigations have struggled to pin down.

Now, Ye and collaborators have used machine learning to study the properties and impact of stream interaction regions in a physically meaningful way. They performed their study on a sample of 879 stream interaction events for which there is abundant information, such as temperature, magnetic field strength and direction, and solar wind conditions before and after the event.

illustration of the support vector machine framework

Illustration of the authors’ support vector machine framework. The optimal hyperplane is the boundary that best divides the data by maximizing the distance between the boundary and the data points nearest to it; these points are called support vectors. Square and triangle symbols represent two classes of data. Click to enlarge. [Ye et al. 2025]

Ye’s team based their framework on a support vector machine classifier: a classical machine learning algorithm that draws a mathematical boundary between groups of data while maximizing the distance between the boundary and the data points nearest to the dividing line. The support vector machine algorithm is well-suited to the task of modeling the geoeffectiveness of stream interaction regions because it doesn’t require a particularly vast dataset, can tolerate misclassified events, and allows for a physical interpretation of the results.

A Physical Interpretation

The team first reined in the model’s complexity by identifying the most important features in the dataset. They then determined which features or combination of features had the largest contribution to the output — in other words, which physical parameters most strongly determined the geoeffectiveness of the event.

illustration of magnetic reconnection in Earth's magnetosphere

Illustration of how the interplanetary magnetic field (IMF) interacts with Earth’s magnetosphere. When the IMF points southward, as it does in this diagram, the impact on Earth’s magnetosphere is increased, with magnetic reconnection occurring in the red areas. Click to enlarge. [NASA]

Ye and collaborators found that the strongest determinants of an event’s geoeffectiveness were how long the solar wind was directed southward, the strength of the solar wind electric field, and the average and minimum strengths of the southward-pointing solar wind magnetic field. These results align with the current understanding of how energy is transferred from the solar wind to Earth’s magnetosphere through magnetic reconnection, a release of magnetic energy driven by rearrangement of magnetic fields. This shows how classical machine learning methods can enhance our ability to predict the outcome of oncoming space weather while simultaneously examining the physical drivers of the event.

Citation

“Assessing the Geoeffectiveness of Stream Interaction Regions Through Physically Interpretable Machine Learning,” Yudong Ye et al 2025 ApJ 993 10. doi:10.3847/1538-4357/ae0454

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