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Pulsar diagram

Pulsars, the best timekeepers in the universe, are key to many fields within astronomy. A new survey searching for pulsars has come online and is already making discoveries.

Searching for Pulsars

Pulsars, the extremely dense cores left over from some massive stars’ deaths, act as astronomical lighthouses — spinning fast and emitting jets of light from their magnetic poles, they send steady pulses of light to observers on Earth. Pulsars are particularly intriguing sources, providing insights into multiple areas of astrophysics like gravitational waves, general relativity, and high-density matter properties. While more than 3,700 pulsars have been discovered to date, increasing this number will enable further exciting science. 

Multiple pulsar surveys have utilized single-dish radio telescopes to cover portions of the sky, and to date, these surveys have primarily focused on finding pulsars near the dense stellar population in the Milky Way’s galactic plane. While these surveys have detected many pulsars, they are limited by their small fields of view, taking longer to cover large portions of the sky, and they miss the population of pulsars that must lie within globular clusters orbiting in the galactic halo. Some low-frequency detectors address these limitations but require intense computational resources to sift through the raw data to find pulsars. How can we search for pulsars efficiently across the sky?

Introducing CHAMPSS

CHAMPSS pointing map

CHAMPSS pointing map showing the survey’s planned sky coverage. The orange stars correspond to the newly discovered pulsar, and the pink points denote known pulsars within the commissioning surveys. Click to enlarge. [Andrade et al 2025]

Hoping to cover the gaps in pulsar surveys thus far, researchers turned to the Canadian Hydrogen Intensity Mapping Experiment (CHIME). CHIME is a unique instrument that utilizes a set of cylindrical antennas to monitor large portions of the sky for radio sources from 400 to 800 MHz. This instrument has already made great strides in detecting transient radio events and has discovered more than 80 pulsars since its first light in 2017. 

With a clear opportunity to advance the pulsar search, a group of scientists developed the CHIME All-sky Multiday Pulsar Stacking Search (CHAMPSS) — a radio pulsar survey that covers the full northern sky daily and uses long-term data stacking to detect irregular and faint sources. Through stacking observations across multiple days, CHAMPSS builds up signals to find faint sources that would otherwise go undetected. The data processing pipeline searches in real time for strong peaks that signify a transient radio event, and follow-up analysis determines if the event is a pulsar candidate. A strong candidate then moves through further stages to confirm it as a pulsar and determine its properties. 

Pulsar discoveries

The 11 pulsars discovered with CHAMPSS. Click to enlarge. [Andrade et al 2025]

First Discoveries and Looking Forward

What has CHAMPSS uncovered thus far? In testing the system through multiple commissioning surveys on a small subset of the sky, the CHAMPSS collaboration has discovered 11 new pulsars with periods ranging from 0.2 to 1.5 seconds. Additionally, the collaboration tested the sensitivity of their survey by searching for known pulsars within the observed area, and they found that their predicted and detected signal-to-noise ratios for known pulsars agree well. This confirmation allows the team to advance to the next phase of the survey, and it will soon be in full swing.

CHAMPSS complements and expands upon other radio pulsar searches, discovering faint and irregular sources that are missed without repeated observations. The future of pulsar astronomy is promising, and with the advent of this survey, a new collection of pulsars will be discovered. 

Citation

“CHIME All-sky Multiday Pulsar Stacking Search (CHAMPSS): System Overview and First Discoveries,” Christopher Andrade et al 2025 ApJ 990 50. doi:10.3847/1538-4357/adeb51

A spiral of bright material circling a black core.

Simulations suggest that there should be a million stars in our galaxy that once wandered so close to a supermassive black hole that they were nearly destroyed. What are the long-term effects of this encounter, and could astronomers observe the scars of these near-death experiences?

Moths to a Flame

center of the Milky Way

This infrared and X-ray image of the Milky Way’s center shows a swirl of hot gas surrounding Sagittarius A*, our galaxy’s central supermassive black hole. [NASA, ESA, SSC, CXC, STScI]

The center of our galaxy is a dangerous place to be a star. Like mosquitoes drawn to a light, millions of stars swarm around the supermassive black hole named Sagittarius A*, packing themselves in tight and kicking one another onto wild, eccentric, and ever-changing orbits. Every 100,000 years or so, one of these stars gets an unlucky push and finally gives in to the black hole’s irresistible pull. A nudge from a neighbor will send it hurtling beyond the point of safety, and in one intense flyby, it’ll be ripped apart by Sagittarius A*’s immense gravitational field.

Simulations suggest that most of the time, these hapless stars are not completely destroyed by their close encounters with the black hole. In principle, then, there should be about a million “survivor” stars wandering near the center of our galaxy. But, what would these survivors look like? And with careful measurements, could we tell them apart from their regular, unscathed counterparts?

Simulating a Star’s Worst Day

Recently, a team of astronomers led by Rewa Clark Bush (University of California Santa Cruz, Cabrillo College, Wesleyan University) tackled this question. First, the team needed to simulate a terrible day for a hypothetical star: the day it passes closest to the black hole. To compute what happens to the star in those intense couple of hours, they used an open-source hydrodynamic code called FLASH to track the state of the star in granular detail.

An 8-panel figure showing complex, asymmetric patterns of gas around a dense core.

Temperatures (top row) and diffusion timescales (bottom row) of four stars soon after their closest approach to the black hole. Click to enlarge. [Bush et al. 2025]

Then, once the star (or what was left of it) managed to flee back to safety, the team froze the simulations and took stock. They found that depending on the initial size of the star and the distance of its closest approach, the black hole stripped away anywhere from a few percent to almost two thirds of the star’s mass. The researchers then fed the stars into another code called MESA to simulate how the disturbed stars would settle into a new equilibrium and evolve over billions of years.

Back on Track

Plot comparing disturbed and undisturbed stars

Comparison of the original undisturbed stars (filled circles), the stars after a black hole encounter (open symbols), and other stars with different initial masses (black points). Click to enlarge. [Bush et al. 2025].

The team found that although these unlucky stars likely glow tens to hundreds of times brighter than usual after their flyby, this period of enhanced brightness lasts less than 100,000 years. For our galaxy, this translates to 1–10 stars currently in the galactic center that have been brightened by an encounter with a supermassive black hole.

Eventually, though, these stars pretty much return to a new normal. Though these stars would have had slightly different colors and luminosities had they not encountered the black hole, they end up looking similar to undisturbed stars with the same final mass.

Although detailed spectroscopic measurements of these remnants might reveal a strange composition thanks to all the stirring that happened during the flyby, it’d be hard to tell anything happened to these stars otherwise. Ultimately, life goes on, even for stars that were nearly destroyed by the most massive creature in the galaxy.

Citation

“Black Hole Survival Guide: Searching for Stars in the Galactic Center that Endure Partial Tidal Disruption,” Rewa Clark Bush et al 2025 ApJL 990 L7. doi:10.3847/2041-8213/adefde

A photograph of a heavily cratered moon on a black background.

By piecing together observations from dozens of radio telescopes in Chile, planetary scientists recently completed the first-ever map of Callisto’s emission at millimeter wavelengths.

Old and Cold

In striking contrast to Io’s frenzied volcanism and Europa’s cracking ice shell, Jupiter’s moon Callisto is much quieter and colder than its Galilean siblings. Though a liquid water ocean hypothetically sloshes beneath its frigid surface, it’d be hard to infer that just by looking at the moon. Callisto’s terrain is ancient, static, and bears the scars of the millennia of exposure to space. There are no volcanoes, plate tectonics, or any other features to disturb what researchers call “evolved ruins” of the numerous craters. These have names like Valhalla, Asgard, and Lofn, since Callisto’s geologic features are named after settings and characters in Norse mythology to further reinforce their freezing existence.

This never-reset canvas makes Callisto an ideal laboratory for studying the long-term effects of asteroid impacts, dust accumulation from nearby natural satellites, and other processes that modify a planetary surface over time. One way to infer the composition and structure of the first few meters of material is to observe the way the temperature changes over the course of a day. For example, ice and rock respond differently to temperature changes; similarly, small-grains of either material warm up and cool down at different speeds than their larger-grain counterparts.

An image of a purple-orange blob against a black background.

The residuals from one of ALMA’s observations of Callisto. [Adapted from Camarca et al. 2025]

In theory, planetary scientists can measure the temperature of a moon by noting how bright each patch of its surface appears in millimeter-wavelength observations. However, this is easier said than done, as only a handful of telescopes are capable of seeing Callisto as anything more than a fuzzy blob at those wavelengths. Back in 2016–2017, planetary scientists used the Atacama Large Millimeter/submillimeter Array (ALMA), a radio interferometer in Chile, to attempt the challenging feat of making a map of Callisto’s surface.

Mapping from Afar

A team led by Maria Camarca, California Institute of Technology, just finished churning through all of the data and assembling their map of Callisto’s temperature residuals. Though the final spatial resolution was at best a few hundred kilometers, meaning a comparable map of Earth would blur all of New England together, this represents the most detailed thermal map of Callisto yet. The map was detailed enough to easily spot the largest craters, and several impact craters including Valhalla stood out as being 3–5K colder than all of the surrounding terrain. The team spotted another anomalously cold region that coincides with the location of maximum carbon dioxide gas density recently seen with JWST.

A multi-panel figure of a black and white surface beneath transparent red and blue blobs.

A map of Callisto’s thermal emission (red/blue) overlaid atop visible images of the surface collected by spacecraft. Click to enlarge. [Camarca et al. 2025]

The team also found that their data could not be explained by a simple model for Callisto’s surface that assumed one dominant material and structure. Instead, their images were better explained by more complex simulations that use two different thermal inertias. Though they are not sure yet why this may be, it is clear that Callisto, often overlooked compared to its flashier siblings, is a complex world worthy of further study. Luckily, the upcoming Jupiter Icy Moons Explorer mission and ALMA’s Wideband Sensitivity Upgrade will bring new knowledge of Callisto’s ancient surface.

Citation

“A Multifrequency Global View of Callisto’s Thermal Properties from ALMA,” Maria Camarca et al 2025 Planet. Sci. J. 6 183. doi:10.3847/PSJ/ade7ee

little red dots

Among the discoveries JWST has made since its 2021 launch, “little red dots” are one of the most perplexing. Named for their compact size and red color, the origins of these distant galaxies remain unknown. A recent article explores some little red dots’ spectral energy distributions and local environments to better understand what may be lighting up these tiny torches.

Little Red Dot Dilemma

Along with their size and color, little red dots exhibit “V”-shaped spectral energy distributions (how they emit light across wavelengths), broad hydrogen emission lines, and no observed X-ray emission. These properties land them in an untapped parameter space with some similarities to both active galactic nuclei (AGNs) and stellar populations. Some previous investigations have suggested that little red dots contain AGNs, reddened by a dusty accretion disk scattering or blocking AGN light. Other studies have found that models for stellar populations can also fit certain little red dot spectra well. 

Adding to the ambiguity, observations and theory predict AGNs to show broad spectral lines, which are present in little red dots — but if little red dots are AGNs, this implies a much higher density of AGNs in the early universe than previously predicted by ground-based surveys. Furthermore, AGNs are expected to emit in the X-ray and show photometric variability, but neither property has been detected definitively thus far for a little red dot. With a clear dilemma arising for the origins of little red dots, astronomers are still prodding at these curious sources.  

AGN or Not?

AGN vs non-AGN fits

Comparison of AGN versus non-AGN fits using the Bayesian information criterion (BIC). Positive values of ΔBIC favor a non-AGN fit, and ~70% of little red dots have positive ΔBIC. Click to enlarge. [Carranza-Escudero et al 2025]

Leveraging the wealth of data available from recent JWST surveys, María Carranza-Escudero (University of Manchester) and collaborators built a sample of 124 little red dots spanning redshifts of z ~ 3–10. The authors used both AGN and non-AGN models to fit the spectral energy distribution for each galaxy. 

Using a robust statistical analysis, the authors found that AGN models tend to “overfit” the data — with more free parameters, an AGN model can be tweaked in a way that may not actually be physical (e.g., fitting for extremely high dust extinction that would not be possible). Instead, models without AGN components appear to be more appropriate for about 70% of the little red dots in their sample, suggesting that these peculiar objects may have a significant star-forming component powering their emission. 

environments histograms

Histograms for two redshift windows showing that little red dots (red) tend to be found in less dense environments than other galaxies (blue) in the same redshift window. Click to enlarge. [Carranza-Escudero et al 2025]

Lonely Neighborhoods

In addition to characterizing little red dots’ emission, the authors analyzed the local environments to compare to other galaxies at similar redshifts. From their analysis, they found that little red dots tend to be found in sparser environments, generally isolated from other galaxies. One explanation for this could be that little red dots in higher-density environments evolve past this peculiar stage faster, which is supported by observations of high-density environments accelerating the evolution of other galaxy types at similar redshifts. However, further investigation is required to better understand the connection between the local environment and little red dot properties. 

More little red dots are yet to be discovered, and continued analysis of their emission and environments will uncover more intriguing characteristics. For now, it seems as though little red dots are still a mystery.

Citation

“Lonely Little Red Dots: Challenges to the Active Galactic Nucleus Nature of Little Red Dots through Their Clustering and Spectral Energy Distributions,” María Carranza-Escudero et al 2025 ApJL 989 L50. doi:10.3847/2041-8213/adf73d

Beta Pictoris debris disk

JWST observations of a nearby star’s debris disk recently revealed what may be one of the lowest-mass planets ever imaged.

How to Find a Small Planet

Though it remains a formidable technical challenge, astronomers have gotten fairly good at taking images of planets around other stars by carefully blocking the light of the host and searching for the small points of light that remain. However, though this technique is well-suited for discovering large, bright, high-mass planets, their lower-mass cousins below the size of Jupiter remain challenging to detect. To get pictures of these smaller worlds, astronomers must resort to detective work and search for signs of their presence through indirect means.

One promising approach is to look not for the planet itself, but rather its effect on the dusty disk of material around the star. These structures, called debris disks, are constantly replenished by planetesimals colliding and grinding one another to dust. If a planet happens to orbit within this disk, it will “stir” the dust into distinctive patterns including rings and spiral arms. If astronomers observe that a star has a debris disk with gaps or spirals, they can analyze those substructures and deduce where a planet might be hiding.

The Star of this Show: TWA 7

TWA 7 debris disk and candidate planet

A mid-infrared image of TWA 7’s debris disk and candidate planet. [ESA/Webb, NASA, CSA, A.M. Lagrange, M. Zamani (ESA/Webb); CC BY 4.0]

TWA 7 is a tiny M-dwarf star just over 100 light-years from Earth. Data from the Spitzer Space Telescope revealed that TWA 7 was unusually bright when observed at infrared wavelengths, which hinted that there might be a warm, dusty disk surrounding the star. Follow-up observations with the Hubble Space Telescope and several major ground-based facilities confirmed that this star successfully met all the conditions listed above: TWA 7 is surrounded by a face-on debris disk with rings and a faint spiral arm. Using all of this information, astronomers predicted that a Saturn-mass planet might lie in a low-density pocket of the disk just beside the star.

This prediction led to a search with JWST last summer. Initial observations taken at mid-infrared wavelengths revealed a bright dot sitting near the predicted location of the planet. However, with just these observations, it was hard to confidently say that this source wasn’t just a distant background galaxy that happened to appear there by chance. To help settle the matter, JWST took another look in two different near-infrared wavelength bands a few weeks later.

JWST observations of TWA 7

New JWST observations of TWA 7. The sources labeled C5 and C6 are planet candidates. C6 is located at the same place as the planet candidate identified in mid-infrared observations, making it a strong candidate. C5 requires further observations to understand if it is real or an artifact. Click to enlarge. [Crotts et al. 2025]

Revisiting TWA 7

A team led by Katie Crotts (Space Telescope Science Institute) recently published these later observations. These new data show a dot in the exact same place as before, and with a color that’s much more planet-like than galaxy-like.

While this adds plenty of evidence to the planetary interpretation, the team cautions that one more set of follow-up observations is needed to be confident that this is, in fact, a planet. Assuming future observations back up these first hints, however, this would be the lowest-mass planet ever imaged, and a happy conclusion to a detective story that started with dust.

Citation

“Follow-Up Exploration of the TWA 7 Planet–Disk System with JWST NIRCam,” Katie Crotts et al 2025 ApJL 987 L41. doi:10.3847/2041-8213/ade798

JWST images of Earendel

What’s the nature of the distant source Earendel, which appears as a point of light in a dramatically gravitationally lensed galaxy?

Record-Breaking Discovery

Hubble image of the Sunrise Arc and Earendel

Hubble image zooming in on the Sunrise Arc and Earendel. The two images of the mirrored star cluster are called 1a and 1b. [NASA, ESA, Brian Welch (JHU), Dan Coe (STScI); Image Processing: Alyssa Pagan (STScI)]

In 2022, astronomers using the Hubble Space Telescope reported the discovery of the most distant single star candidate ever seen, now pinpointed to have a redshift of z = 5.926. The star, named Earendel, is an incredible beacon from the first billion years of the universe, standing out brilliantly from the red smear of its host galaxy, the Sunrise Arc.

But there’s a catch — at the distances involved, distinguishing between one star and many isn’t easy, and Earendel might not actually be just one star. New research uses stellar population modeling to explore the possibility that what has been touted as a single star is really a cluster.

The Light of Earendel, Our Most Beloved Star… Cluster?

The question sounds simple: does the light from Earendel resemble that of one star, or does it more closely align with the emission from a collection of many stars? What complicates matters is that Earendel’s light has been warped and magnified by an intervening galaxy cluster in a process called gravitational lensing. Because the degree of magnification isn’t known precisely, it’s not clear exactly how large the source is — leaving wiggle room for Earendel to be one or many stars.

JWST spectra of Earendel and 1b

JWST spectra of Earendel (top) and 1b (bottom), along with the best-fitting models. Click to enlarge. [Pascale et al. 2025]

To investigate Earendel’s identity, Massimo Pascale (University of California, Berkeley) and collaborators fit a simple stellar population model to JWST Near-Infrared Spectrograph (NIRSpec) spectra of both Earendel and another source in the Sunrise Arc called 1b, which is widely accepted to be a star cluster. The model varied the age of the cluster, its metallicity, the amount of dust it contains, and other factors. To make the modeling more rigorous, the team also used three different stellar population model libraries.

Both Earendel and 1b were well fit by all three stellar population models, supporting the hypothesis that Earendel is a cluster. Earendel and 1b share certain similarities, such as metallicity (less than 10% of the Sun’s), stellar surface density (high, rivaling the maximum density seen in the local universe), and age (more than 30 million years old).

Cluster Comparison

age and metallicity of star clusters in the nearby universe and at high redshifts

Metallicity and formation age of star clusters in the local universe, in the Milky Way and Magellanic Clouds, and at high redshifts. Click to enlarge. [Pascale et al. 2025]

Given the potential ages and metallicities of the two sources, it’s possible that both Earendel and 1b are the precursors to today’s globular clusters. These clusters may fit into an evolutionary sequence that connects other lensed star clusters, such as the redshift z = 10.2 Cosmic Gems clusters and the z = 1.4 Sparkler clusters.

While this work demonstrates that Earendel could be a cluster, it doesn’t prove that it is. Doing so is challenging, especially since certain features predicted to exist for a single star might be beyond our observational capabilities, or they could be reproduced by clusters with certain properties. The authors pointed to one smoking-gun signal for Earendel being a single, massive star: brightness fluctuations due to microlensing by stellar winds. So far, no such variability has been found, and the cluster hypothesis remains viable.

Citation

“Is Earendel a Star Cluster?: Metal-Poor Globular Cluster Progenitors at z ∼ 6,” Massimo Pascale et al 2025 ApJL 988 L76. doi:10.3847/2041-8213/aded93

An artist's impression of a planet beside a sun-like star.

By pushing JWST to its limits, astronomers may have just spotted a planet around the nearest Sun-like star, Alpha Centauri A. Though another look at the star is needed next year to confirm the finding, if the detection holds up, this discovery easily earns several superlatives: the planet would be the nearest, oldest, coldest, and lowest-mass world ever directly imaged beyond our solar system.

Hi, Neighbor!

The scale of our Milky Way galaxy is hard to comprehend with our Earth-bound minds that are more attuned to measuring distances in miles than in light-years. However, even though our suburban stellar neighborhood is tens of thousands of light-years from the galactic downtown, it is not empty: there are a handful of nearby systems that offer unique opportunities to study stars and their potential planet children “up close.”

An image of two bright stars atop many faint stars, with one faint star circled in red.

Alpha Centauri A and B (bright stars), along with Proxima Centauri, circled in red. [Wikipedia; CC BY-SA 3.0]

The nearest star system to the Sun at just over 4 light-years away is called Alpha Centauri. It’s a star “system” since what appears to the naked eye to be one star is actually three objects orbiting each other: Alpha Centauri A and Alpha Centauri B are two Sun-like stars that circle each other every 80 years or so, while Alpha Centauri C, better known as Proxima Centauri, circles the inner pair on a much wider orbit.

Though Proxima Centauri is the most famous of the trio, since it’s technically the closest star to the Sun, its larger siblings are arguably better targets for searching for planets. Since these stars are more massive, their habitable zones are large enough that astronomers can block out the star itself and search the surrounding area for planets using a tool called a coronagraph.

Challenging Observations

Back in 2019, a team of astronomers using a coronagraph on the Very Large Telescope uncovered tantalizing evidence of a giant planet orbiting Alpha Centauri A. However, confirming or rejecting this initial peek using telescopes on Earth’s surface was challenging. When JWST finally launched in 2021, following up on this hint was a top priority. Astronomers first attempted to point JWST toward the Alpha Centauri system in 2023, but unfortunately, the observations from that attempt were unusable.

Though one might think aiming a telescope at a nearby star is easier than taking a look at something more distant, this is unfortunately not the case. For one thing, the stars of the Alpha Centauri system are bright enough to overwhelm JWST’s sensitive detectors without careful calibration. For another, keeping the telescope locked on target and the star behind the coronagraph requires continuous tweaks to account for the Sun’s motion relative to Alpha Centauri, JWST’s motion relative to the Sun, and Alpha Centauri A’s motion relative to Alpha Centauri B. The team regrouped, tested a new observing strategy in mid-2024, then took three long observations in August 2024, February 2025, and April 2025.

A Planet Candidate

JWST coronagraph image of the Alpha Centauri system

JWST coronagraph image of the Alpha Centauri system. Alpha Centauri B is the bright labeled star at the top, while Alpha Centauri A is labeled but hidden behind the coronagraph. The inset shows the region around Alpha Centauri A during the three different observations. The planet candidate, labeled S1, is only visible in the August 2024 observation. Click to enlarge. [Beichman & Sanghi et al. 2025]

This time, the observations went off nearly without a hitch. After careful data processing, the team led by Charles Beichman and Aniket Sanghi of the California Institute of Technology found a bright blob positioned right beside the star in their August data. Interestingly, the blob disappeared in the next two observations. Working on the hunch that this blob was the same object as the planet candidate seen by the Very Large Telescope, the team realized that most allowed orbits would place the planet too close to the star to be visible in those later observations. In other words, the planet might be playing hide-and-seek, peeking out from behind the star in one month only to vanish in the next.

The authors estimated that the source is a 1–1.1 Jupiter radius planet with a mass of 90–150 Earth masses (0.28–0.47 Jupiter mass) on a 2–3-year orbit. The team stressed that with just one detection, this should still be regarded as a planet candidate, not a confirmed discovery. If the planet is eventually verified, however, it stands out among its peers as the first mature, cold planet ever imaged within its star’s habitable zone. Future detectors may be able to collect spectra of the planet itself, unlocking new insights into giant planet evolution and composition in multiple-star systems with nested orbits, like the Alpha Centauri system.

Now that JWST has a proven strategy for observing this challenging system, it should be possible to take follow-up images. If the planet is real and traveling on its expected orbit, it should reappear beside Alpha Centauri A in August 2026. Hopefully, JWST will be looking when it does, and we can finally confirm that we have a planetary neighbor next door.

Citation

“Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits,” Charles Beichman et al 2025 ApJL 989 L22. doi:10.3847/2041-8213/adf53f

“Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis,” Aniket Sanghi et al 2025 ApJL 989 L23. doi:10.3847/2041-8213/adf53e

Current gravitational wave detectors are primarily sensitive to powerful sources like black hole–black hole mergers — what types of events may next-generation gravitational wave detectors reveal? A recent study explores stripped subgiant stars around supermassive black holes and how their gravitational waves may be detected in next-generation missions. 

Inspiraling Stars and Gravitational Waves

The immense gravity of supermassive black holes at the centers of galaxies drives complicated dynamics that could generate so far undetected gravitational wave signals. One predicted source of gravitational waves is a rare event known as an extreme-mass-ratio inspiral (EMRI) — a prolonged inspiraling of a stellar-mass object around a supermassive black hole, driven inward by gravitational waves. EMRIs are thought to arise when a stellar-mass object is captured into a close-in nearly circular orbit around a supermassive black hole, emitting gravitational waves due to the object’s proximity to the black hole. 

To date, most studies have explored EMRIs involving compact objects like black holes, but an EMRI with an inspiraling star is possible, though not yet well explored nor understood. In addition to generating gravitational waves, a stellar EMRI will also emit light as the supermassive black hole rips up siphoned material from the star. These events may explain recent observations of peculiar X-ray transients, but their gravitational wave counterparts remain undetected as EMRIs’ predicted signals fall outside the sensitivity range of current ground-based gravitational wave detectors. However, the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector expected to launch in the mid-2030s, will be sensitive to the gravitational waves of EMRIs. In order to properly identify them, we must better understand what to expect.

Diagram showing the evolutionary steps of a supermassive black hole capturing a subgiant star that subsequently undergoes mass transfer and gravitational wave-driven inspiral. Click to enlarge. [Olejak et al 2025]

Modeling a Subgiant Inspiral

Seeking to better predict what the gravitational waves of stellar EMRIs may look like and what LISA may detect, Aleksandra Olejak (Max Planck Institute for Astrophysics) and collaborators used stellar evolution code to simulate the evolution of a subgiant star as it transfers mass to a supermassive black hole and gravitational waves draw in its orbit. In their models, the supermassive black hole captures the star through scattering between stars or tidal interactions with a stellar binary in which the black hole catches one star and ejects the other.

Once the star is captured, its orbit becomes circularized around the black hole, pulled in closer over time until mass transfer of the star’s outer envelope begins. During this stage, the gravitational wave signal remains too weak to be detected. However, as the star is stripped down to just a helium core, it contracts and experiences an extended gravitational wave–driven orbital decay.

Possible Detections with LISA

The strain spectral density as a function of the detected frequency of the gravitational wave. The colorful line indicates the signal from a stripped subgiant at the Milky Way’s center, and the gray lines are what would be observed at greater distances. LISA’s sensitivity to gravitational wave signals is shown with the black and red lines with detectable sources falling to the right of or above the line. Click to enlarge. [Olejak et al 2025]

With a better understanding of this process, the authors explored how such a signal compares to LISA’s sensitivity at various distances away from Earth. They found that as a star spirals inward, the gravitational wave signal grows stronger and enters LISA’s detectable frequency range. Stellar EMRIs occurring closer to Earth will result in higher signal-to-noise detections, and the gravitational waves from those occurring within the Milky Way would remain detectable for hundreds of thousands of years. The authors’ analysis revealed that, given their simulations, LISA could detect such an event out to ~3 billion light-years away. They estimate that a few systems involving similarly stripped subgiant stars could be detectable by LISA during a four-year mission, with a 1% chance of our galactic center hosting one.    

This study provides exciting potential for what is possible with LISA, and future detections of gravitational waves will shine further light onto the complicated dynamics of galactic centers and the evolution of stars around supermassive black holes.

Citation

“Supermassive Black Holes Stripping a Subgiant Star Down to Its Helium Core: A New Type of Multimessenger Source for LISA,” Aleksandra Olejak et al 2025 ApJL 987 L11. doi:10.3847/2041-8213/ade432

Illustration of an accreting supermassive black hole shrouded by dust

Astronomers have confirmed the discovery of a little red dot galaxy from when the universe was roughly half a billion years old. The galaxy, CAPERS-LRD-z9, is the most distant object to show the tell-tale broad emission lines of gas spiraling around a black hole, opening a new window onto black hole growth in the early universe.

Little Red Dots in the Spotlight

Images of six "little red dot" galaxies from JWST

Images of six “little red dot” galaxies from JWST. [NASA/ESA/CSA/I. Labbe]

Of all the discoveries JWST has enabled since its launch, none seems as enduringly mysterious as the tiny, distant galaxies nicknamed “little red dots.” These early universe objects are characterized by their small sizes, red color in JWST images, and V-shaped spectra.

But what are little red dots? Are they growing supermassive black holes busily amassing gas? Compact collections of old stars? Shredded stars spoon-feeding baby black holes? As reported today in the Astrophysical Journal Letters, new JWST data provide answers for one particularly distant dot.

JWST Takes Another Look

CAPERS-LRD-z9 was first identified as a possible high-redshift little red dot when it was observed by the Public Release IMaging for Extragalactic Research (PRIMER) survey with JWST’s Near-Infrared Camera (NIRCam). Anthony Taylor (The University of Texas at Austin) and collaborators followed up on the discovery with JWST Near-Infrared Spectrograph (NIRSpec) observations from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS). This spectrum pinned the object’s redshift at z = 9.288, corresponding to when the universe was only about half a billion years old.

With CAPERS-LRD-z9 placed along the cosmic timeline, Taylor and coauthors turned to the question of its identity. The JWST spectra revealed a broad emission line from hydrogen gas moving at thousands of kilometers per second — evidence that CAPERS-LRD-z9 harbors an accreting supermassive black hole (an active galactic nucleus or AGN) that is spinning gas into a frenzy around it. CAPERS-LRD-z9 is the most distant object known to show this characteristic signature of a growing black hole.

JWST spectrum of CAPERS-LRD-z9

JWST NIRSpec spectrum of CAPERS-LRD-z9 with the fit from the AGN (red) plus stars (blue) model. Click to enlarge. [Taylor et al. 2025]

Next, the team examined the little red dot’s spectral energy distribution, using a model that includes an AGN and a stellar population. To reproduce the step-like discontinuity seen in CAPERS-LRD-z9’s spectrum, they used a model in which the supermassive black hole is shrouded in a shell of dusty gas. The model fit well with a black hole mass of 38 million solar masses (though the authors estimate the mass could be anywhere in the range of 4.5–316 million solar masses), an upper limit of 1 billion solar masses on the stellar mass, and a gas density of 1010 per cubic centimeter surrounding the black hole.

Seeding a Black Hole

plot of black hole mass versus redshift for detected AGN and quasars

Observed black hole masses and redshifts of several sources, including CAPERS-LRD-z9 (large red star), quasars at redshifts of z > 6 (blue squares), massive spectroscopically confirmed AGN with broad emission lines (filled symbols), and the highest-redshift AGN detected to date via X-ray or ultraviolet emission. The shaded areas show the black hole masses achievable through accretion at the Eddington rate onto a massive (red) or stellar-mass (purple) seed. Click to enlarge. [Taylor et al. 2025]

Since successfully fitting one model doesn’t immediately rule out others, Taylor’s team also applied a stars-only model to the spectrum. This model fit poorly, and combined with the presence of broad hydrogen lines from fast-moving gas, this is strong evidence that CAPERS-LRD-z9 contains an accreting supermassive black hole.

This finding raises the question of how a black hole can grow to millions of solar masses in just 500 million years. The authors showed that this is possible in two scenarios: either the black hole began as a >10,000-solar-mass “seed” that grew at the Eddington rate — the hypothetical limit at which a black hole can accrete matter — or it started out as a smaller, ~100-solar-mass seed that grew at a super-Eddington rate. The observations rule out the possibility that the black hole grew from a ~100-solar-mass seed accreting at or below the Eddington rate.

In addition to setting the record for most distant broad-line AGN known, CAPERS-LRD-z9 gives new intel on the lives of black holes in the early universe. This won’t be the last we hear about this little red dot!

Citation

“CAPERS-LRD-z9: A Gas Enshrouded Little Red Dot Hosting a Broad-Line AGN at z = 9.288,” Anthony J. Taylor et al 2025 ApJL 989 L7. doi:10.3847/2041-8213/ade789

NGC 4151

Astronomers have known about NGC 4151’s X-ray-emitting active galactic nucleus since the 1970s. Now, observations from the X-Ray Imaging and Spectroscopy Mission (XRISM) show it in a whole new light.

Getting Feedback

illustration of active galactic nucleus winds

Illustration of the winds and accretion disk of an active galactic nucleus. [NASA, and M. Weiss (Chandra X -ray Center)]

Active galactic nuclei (AGNs) are extremely luminous galactic centers powered by accretion of matter onto a supermassive black hole. These nuclei provide a venue for galaxies and their supermassive black holes to interact. This interaction goes both ways, with the galaxy providing fuel to the supermassive black hole, and the AGN injecting material, momentum, and energy back into the surrounding galaxy in a process known as feedback.

AGN feedback can dramatically reshape a galaxy’s star formation and evolution. One of the most powerful ways an accreting supermassive black hole can influence its host galaxy is through ionized winds, which can scour the surrounding galactic bulge clear of star-forming material. There is much still to learn about these winds, including how they are structured and what physical processes drive them.

New Insights from XRISM

NGC 4151 provides an excellent opportunity to learn about active galactic nucleus winds. Only about 60 million light-years away, NGC 4151 hosts an accreting 34-million-solar-mass black hole that is luminous, highly variable, and shows a complex absorption spectrum that suggests that the AGN’s intense radiation is filtering through outflowing winds.

XRISM spectrum of NGC 4151

XRISM spectrum of NGC 4151, showing the multi-component model fit. Click to enlarge. [Xiang et al. 2025]

To study the winds of NGC 4151, a team led by Xin Xiang (University of Michigan) obtained X-ray spectra of NGC 4151 from XRISM on five dates spanning a period of about 6 months.

Xiang’s team modeled the array of absorption and emission lines in the XRISM spectra to tease out the properties of NGC 4151’s winds. They found that the best match to the spectra required up to six layers of absorbing material, showing that NGC 4151’s winds are highly structured. These absorbing layers took the form of slow warm absorbers (velocities between 100 and 1,000 km/s), very fast outflows (1,000–10,000 km/s), and ultra-fast outflows (10,000–100,000 km/s; up to one-third of the speed of light). While most of these components carry away mass as fast as or faster than the black hole accretes, some appeared to lack the oomph to escape the oncoming accretion flow, suggesting that they might be “failed” winds that curl back toward the AGN.

Being Blown Away

The data showed that the winds are likely magnetocentrifugally driven, meaning that material is lifted and accelerated from the surface of the disk along magnetic field lines as the disk rotates. For some of the wind components, especially warm absorbers emerging farther from the black hole, radiation pressure may be a driver as well.

Diagram of the multiple wind components of NGC 4151

Diagram of the wind components of NGC 4151, including ultra-fast outflows (UFOs), very fast outflows (VFOs), and warm absorbers (WAs). [Xiang et al. 2025]

Altogether, the modeling paints a picture of a complex, asymmetric, time-variable, and clumpy set of winds that carry significant mass away from the AGN. Several fast-moving outflow components appear to exceed the threshold luminosity necessary to blow star-forming gas out of the galactic bulge, suggesting that star formation in NGC 4151’s galactic bulge may someday be halted entirely by these winds.

Citation

“XRISM Spectroscopy of Accretion-Driven Wind Feedback in NGC 4151,” Xin Xiang et al 2025 ApJL 988 L54. doi:10.3847/2041-8213/adee9b

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