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galaxy merger

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

Merging Galaxies

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

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

Galaxy Interaction Simulations

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

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

torque star formation rate comparison

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

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

Observation and Simulation Comparison

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

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

Citation

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

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

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

The Growing Regulus Family

constellation Leo

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

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

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

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

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

A Potential Companion

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

near-infrared spectrum of SDSS J1007+1930

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

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

Assembling the Clues

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

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

Citation

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

Artist's impression of a pulsar

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

Stellar Speed Limit

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

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

Crust-Covered Pasta

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

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

plot of strain versus rotation rate

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

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

Continuous Gravitational Waves

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

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

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

Citation

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

cartoon of Thorne–Żytkow object formation

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

A Star Within a Star

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

illustration of an X-ray binary system

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

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

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

Diverging Paths

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

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

simulation snapshots showing gas density

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

Total Collapse of the Heart

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

plot of luminosity and duration of gamma-ray emission

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

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

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

Citation

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

Illustration of a Neptune-like exoplanet

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

Close-In Neptune, Breaker of Chains

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

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

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

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

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

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

Stability of Polar Neptunes

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

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

diagram showing the process of disk-driven resonance

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

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

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

Is K2-18b Covered in a Supercritical Ocean?

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

Possible structures for K2-18b

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

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

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

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

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

Citation

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

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

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

pre-supernova star

As scientists excitedly await the first light of the Vera C. Rubin Observatory, a recent study has projected that this facility will aid in identifying hundreds of massive stars on the cusp of death.

Before Detonation

Throughout their lifetimes, stars burn through hydrogen in their cores — million-degree furnaces smashing atoms together to form new ones. Massive stars, many times the mass of our Sun, have very high temperatures and pressures in their cores, causing them to live fast and die young. When all the fuel is burned, the star no longer produces enough thermal pressure to balance gravity, and the star dies in a rapid and massive explosion known as a Type II (or core-collapse) supernova. 

But the final stages of a massive star’s life are not yet fully understood. Observations of Type II supernovae show narrow emission lines that indicate that their progenitor stars were surrounded by circumstellar material — material that was shed from the star as it evolved. However, the exact mechanism through which these stars lose mass is unclear, but if we can catch a star nearing its end but before its deadly detonation, we can better understand these massive stars’ elusive final days. 

Rubin Observatory

Artist’s illustration of the Rubin Observatory observing the sky searching for supernovae. The large field of view of the telescope captures large areas of sky in a single image. Click to enlarge. [NOIRLab/NSF/AURA/P. Marenfeld; CC BY 4.0]

Rubin’s Remedy

Looking to explore many facets of the universe, the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), first light anticipated July 2025, will scan the Southern Hemisphere sky searching for transient events like supernovae. Recognizing the power of LSST, Alexander Gagliano (The NSF AI Institute for Artificial Intelligence and Fundamental Interactions) and collaborators run simulated observations to predict how many stars LSST will catch in their final days, before they explode as supernovae.

Previous observations of Type II supernovae reveal enhanced emission in the months to years prior to explosion, and as LSST monitors the sky, it will be able to capture this pre-explosion emission. The authors carefully model the expected light curves for various types of core-collapse supernova precursors based on the handful of pre-explosion emission events observed thus far. With these models, the authors simulate LSST observations, applying two methods that would allow for the detection of stars gearing up to explode. The first being single-visit observations in which the enhanced emission is detected using differential photometry prior to the star’s explosion, independent of detecting the subsequent supernova. The second method involves going back after a supernova has been detected. By performing binned photometry of the star with observations taken of it prior to its explosion, the preceding emission can be recovered. From here, the authors can predict how many events LSST will recover after it goes online. 

Power in Numbers

CCSN precursors from one year of synthetic LSST observations

All core-collapse supernova precursors for both detection methods from one year of synthetic LSST observations. Each color corresponds to each model light curve used. Click to enlarge. [Gagliano et al 2025]

Based on their analysis, the authors predict that LSST will detect ~150–240 Type II supernova progenitors per year with single-visit photometry. Over the course of the first three years of LSST, they anticipate 150–400 detections from the binned photometry. This projected frequency of detections will launch the study of late-stage stellar life to new levels, increasing the observed sample of Type II supernova progenitors astronomically. As these detections come in, the observations will reveal the properties and behaviors of these stars in their final days. This opens the door to understanding how end-stage massive stars lose mass and ignite into the most powerful events in the universe. 

Citation

“Finding the Fuse: Prospects for the Detection and Characterization of Hydrogen-Rich Core-Collapse Supernova Precursor Emission with the LSST,” A. Gagliano et al 2025 ApJ 978 110. doi:10.3847/1538-4357/ad9748

Ingenuity helicopter on Mars

Researchers have used an aircraft to measure the wind speed on Mars, marking the first time this method has been used on another planet. This groundbreaking measurement was made possible by the Ingenuity helicopter, which was active for nearly three years and spent, cumulatively, more than two hours in flight on the Red Planet.

A New Era of Aviation

photo taken by the Ingenuity helicopter

Ingenuity’s navigation camera snapped this photo of the Martian surface during the helicopter’s first flight. [NASA/JPL-Caltech]

In February 2021, Mars got two new inhabitants: the Perseverance rover and the Ingenuity helicopter, bundled together in NASA’s Mars 2020 mission. Ingenuity made history by being the first aircraft to carry out a powered and controlled flight on another planet.

The mission showed that flight is possible in the rarefied Martian air — more than 100 times thinner than Earth’s — and covered roughly 18 kilometers (11 miles) in total. Ingenuity, carrying no scientific instruments and weighing about as much as a Chihuahua, paved the way for future aircraft missions to other worlds, such as the highly scientifically capable, half-ton Dragonfly mission to Saturn’s moon Titan.

diagram of the Ingenuity helicopter with roll, pitch, and yaw labeled

Diagram showing the directions of roll, pitch, and yaw — the three dimensions used to describe the orientation of an aircraft. [Jackson et al. 2025]

Which Way the Martian Wind Blows

Now, almost exactly one year since its final flight, Ingenuity is still enabling more firsts. In a research article published today, a team led by Brian Jackson (Boise State University) described how they used information from Ingenuity to measure the speed and direction of Mars’s winds. Though Ingenuity did not carry any instruments capable of directly measuring the wind, the helicopter recorded its attitude, or orientation, as it flew.

Previously, Jackson had carried out field experiments on Earth with a small drone to show that wind parameters could be extracted from an aircraft’s attitude data. Building on that proof-of-concept study, Jackson’s team used models to understand how Ingenuity’s attitude would change in response to winds of varying speed and direction. From this modeling, the team reconstructed the winds that battered the tiny helicopter as it flew at altitudes spanning 3 to 24 meters (10 to 79 feet).

Comparing Conditions

The team calculated wind speeds ranging from 4.1 to 24.3 meters per second (9 to 54 miles per hour; that’s anywhere from a “gentle breeze” to a “strong gale,” to use the earthly terms). Compared to meteorological models, the measured speeds tended to be higher than expected and the wind directions did not always match. These differences might reflect the influence of localized geological features, like craters and scarps that whip the wind in highly variable directions, that the models do not fully capture.

plot of wind speed as a function of time

The wind speed calculated from Ingenuity’s motion (red circles) compared to the speed measured by the Perseverance rover’s instruments (blue line). Click to enlarge. [Jackson et al. 2025]

While the wind directions implied by the Ingenuity helicopter data generally agreed with measurements by the Perseverance rover, which measured the planet’s surface weather at an altitude of 1.5 meters (5 feet), Ingenuity measured higher wind speeds. Jackson’s team found it unlikely that the higher speeds measured at Ingenuity’s higher altitude were the result of random fluctuations; instead, they proposed a physical explanation rooted in the aerodynamic conditions upwind of the rover and helicopter.

This study highlights both the challenge and potential of measuring winds with an aircraft, and Jackson’s team plans for future work to refine the method. Accurate measurements of wind speeds on Mars can help scientists investigate our neighboring planet’s surface processes and dust transport, as well as help to plan safe entry, descent, and landing for future missions.

Citation

“Profiling Near-Surface Winds on Mars Using Attitude Data from Mars 2020 Ingenuity,” Brian Jackson et al 2025 Planet. Sci. J. 6 21. doi:10.3847/PSJ/ad8b41

JWST Field - 20,000+ galaxies

A new study explores the conditions in an extremely distant galaxy to understand how early galaxies may evolve to form the objects we see now in the local universe.

Early Universe Galaxies

Over the past few years since JWST went online, astronomers have discovered some of the most distant galaxies in the universe. To be visible from over 13 billion light-years away, these galaxies must be extremely bright and thus powered by an intense energy source — typically either active massive central black holes known as active galactic nuclei (AGN) or very strong star formation that causes a burst in stellar light.

The universe’s earliest galaxies are astronomical fossils, providing clues to how nearby galaxies and objects have evolved to their current state. These high-redshift galaxies may be progenitors of globular clusters or beginnings of growing galaxies — objects whose origins are not well understood. Further characterizing the newly discovered galaxies may reveal how objects in the local universe came to be.

ALMA detection of 88 micron line

ALMA detection of the 88-micron emission line from doubly ionized oxygen. The left-hand panel plots the intensity map of GHZ2 as seen on the sky, where the yellow in the center corresponds to the galaxy. The right-hand panel shows the ALMA spectrum with the 88-micron emission line highlighted in yellow. Click to enlarge. [Zavala et al. 2024]

Star Formation or AGN?

One such high-redshift galaxy is GHZ2, which was recently discovered with JWST and lies at a redshift corresponding to ~400 million years after the Big Bang. To complement the JWST spectroscopic observations, Jorge A. Zavala (National Astronomical Observatory of Japan) and collaborators observed the galaxy using the Atacama Large Millimeter/submillimeter Array (ALMA). These observations aimed to measure far-infrared emission lines that are critical in determining if a galaxy is powered by star formation or an AGN. The authors targeted and successfully recovered the 88-micron (1 micron = 10-6 meter) emission line from doubly ionized oxygen — an emission line known to correlate with a galaxy’s star formation activity. From the detection, the authors derived a redshift that agrees with the redshift determined from JWST.

With the ALMA observations and prior measurements from JWST, the authors compare GHZ2 to other galaxies. They find that GHZ2 exhibits characteristics similar to other galaxies that are dominated by star formation. Additionally, this very high-redshift galaxy appears to share characteristics with giant star-forming regions, which further suggests that GHZ2 is likely driven by extreme star formation. The authors find that, though some AGN contribution fraction cannot be definitively ruled out, GHZ2 is most likely dominated by metal-poor and young stellar populations.

How Will GHZ2 Evolve?

stellar mass-velocity dispersion relationship

The stellar mass–velocity dispersion relationship with data points plotted for different object types including globular clusters (light-purple circles), ultra-compact dwarf galaxies (solid purple circles), dwarf ellipticals (down-pointing triangles), compact ellipticals (blue solid circles), and elliptical and S0 galaxies (green squares). The target galaxy GHZ2 is shown with a yellow star, which falls in closest agreement with compact ellipticals and ultra-compact dwarf galaxies in this relationship. Click to enlarge.
[Zavala et al. 2024]

What does the dominant star formation in GHZ2 imply about its evolution? With powerful star formation, GHZ2 could be a proto-globular cluster, though the overall mass of the galaxy is very high compared to typical globular clusters seen in the local universe. Along with its high mass, GHZ2’s size is also a bit larger than reported for other proto-globular clusters. Given the galaxy’s high mass and extended radius, the authors suggest that GHZ2 could be composed of multiple massive star clusters that could evolve into multiple globular clusters, or it could be the compact core of a growing, more massive galaxy.

Though the exact evolution of GHZ2 is unclear, this study underscores the significance of ALMA and JWST as a powerful pair of instruments capable of characterizing the most distant galaxies in the universe. Future observations with both instruments will continue to clarify what the early universe looked like and how it has evolved over time. 

Citation

“ALMA Detection of [O III] 88 μm at z = 12.33: Exploring the Nature and Evolution of GHZ2 as a Massive Compact Stellar System,” Jorge A. Zavala et al 2024 ApJL 977 L9. doi:10.3847/2041-8213/ad8f38

globular cluster Terzan 5

The star SOS1 is not like its neighbors. Using chemical and dynamical data, stellar sleuths have tracked this star from its current home in the Milky Way’s central bar back to its likely origin in one of the most massive globular clusters in our galaxy.

A Star in a Bar

barred spiral galaxy NGC 1300

The Milky Way, like the galaxy NGC 1300 shown here in an image from the Hubble Space Telescope, has a central bar of stars. [NASA, ESA, and The Hubble Heritage Team (STScI/AURA); Acknowledgment: P. Knezek (WIYN)]

Today, the Milky Way has an intricate and interlocking structure: thin and thick disks of stars surrounded by an extended halo, with a bulge of old stars at the center. A bar of stars cuts across the center of our galaxy, and globular clusters — ancient collections of thousands to millions of stars — dot the galactic bulge and halo. These structures didn’t always exist, and a major goal for galactic research is understanding when and how the many components of our galaxy were assembled.

One piece of the puzzle might be provided by the star 2M17454705-2639109, also known as SOS1. This star is located in the busy galactic downtown of the Milky Way’s center, orbiting within the central bar of stars. Data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) show that SOS1 has a curious chemical composition that sets it apart from its neighbors. Now, researchers have shown that these chemical differences may be evidence that SOS1 originated far from its current location — and that many other stars in the galactic bar might have completed similar journeys.

Globular cluster Liller 1

The reddish stars of the globular cluster Liller 1 glow behind bright blue stars in the foreground. [ESA/Hubble & NASA, F. Ferraro; CC BY 4.0]

SOS1: Far from Home?

A team led by Stefano Souza (Leibniz Institute for Astrophysics Potsdam; University of São Paolo; Max Planck Institute for Astronomy) investigated SOS1’s origins by first comparing its chemical abundance pattern to those of different populations of stars in the Milky Way. The observed pattern of low carbon, high nitrogen, and high aluminum matches expectations for second-generation stars in globular clusters: densely packed, roughly spherical collections of thousands to millions of stars.

But how would a star born in a globular cluster end up in the Milky Way’s central bar? Souza’s team highlighted two possible scenarios: SOS1 might have been ejected from its home cluster by a gravitational interaction with a binary star system, or — deemed more likely — it could have been stolen from its home cluster by the tidal forces of the Milky Way.

Candidate Clusters

Souza’s team used N-body simulations to determine if SOS1 once called one of the existing globular clusters home. (The team notes that it’s possible that SOS1’s parent star cluster no longer exists, having been pulled apart by the Milky Way’s powerful tidal forces.) The likeliest candidate is Terzan 5, which is among the most massive and most ancient globular clusters in the Milky Way. The simulations suggest that SOS1 might have been bound to this cluster 353 million years ago.

chemical and age comparison between SOS1 and Terzan 5

Chemical (left) and age (right) comparison between SOS1 and stars in the globular cluster Terzan 5. Click to enlarge. [Souza et al. 2024]

The chemical abundances of SOS1 support this hypothesis, since SOS1’s curious chemical makeup is consistent with that of the oldest and most metal-poor stars in the cluster. The final clue would be a comparison of the ages of Terzan 5 and SOS1. Though the data did not allow for a precise determination of the star’s age, the preliminary analysis suggests that it is of a similar age to the cluster.

The chemical similarities and dynamical properties make it likely that SOS1 once resided in a globular cluster, possibly Terzan 5. Its current residence in the Milky Way’s central bar supports the idea that ancient globular clusters contributed stars to the bar through tidal stripping.

Citation

“Tracing Back a Second-Generation Star Stripped from Terzan 5 by the Galactic Bar,” Stefano O. Souza et al 2024 ApJL 977 L33. doi:10.3847/2041-8213/ad91af

36 images of supernova host galaxies

Another year is drawing to a close, and we’re looking back on all the discoveries that we’ve covered on AAS Nova this year. The top stories offer an astronomical smorgasbord and an in-depth look at some of the most recognizable objects in our galaxy. Without further ado, here are the top 10 most-read posts of 2024:

the Milky Way’s central supermassive black hole in polarized light

The first image of the Milky Way’s central supermassive black hole in polarized light. [EHT Collaboration; CC BY 4.0]

10. A New Way of Looking at the Milky Way’s Supermassive Black Hole

In 2022, the Event Horizon Telescope collaboration released the first image of the Milky Way’s supermassive black hole, Sagittarius A*. But the collaboration’s work didn’t stop there, and two years later they released an image of our hometown black hole in an entirely new light: polarized light, to be exact. The polarization of light — the orientation of the light waves as they travel through space — informs researchers as to the magnetic field conditions close to the black hole as well as in between the black hole and Earth. These results revealed a high degree of linear polarization as well as a lesser amount of circular polarization, suggesting a strong, orderly magnetic field.

An illustration of an exoplanet being engulfed by its home star, as 8 UMi b somehow has not been

An illustration of an exoplanet being engulfed by its home star. The planet 8 Ursae Minoris b has somehow escaped this fate. [International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani; CC BY 4.0]

9. Astronomers Reopen the Mystery of a Planet That Shouldn’t Exist

Researchers thought they had solved the mystery of the exoplanet 8 Ursae Minoris b: unsure how the planet had survived its host star ballooning into a red giant, they proposed that the star had swallowed its one-time stellar companion, changing its evolution in a way that saved its planet from certain doom. But new data analyzed by Huiling Chen and collaborators upended this tale of a planet saved by a stellar merger: Ursae Minoris b is simply too young to have merged with a companion star. Luckily, the team’s research brought to light another possible resolution to the mystery.

supernova SN 1994D in the galaxy NGC 4526

The supernova SN 1994D illuminates the outskirts of its host galaxy, NGC 4526. [NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team]

8. The Quest to Watch a Supernova in Real Time

The sooner researchers spot a supernova explosion, the more they can learn from it — but is there a way to know when a supernova is coming? The ejecta from a supernova explosion is so dense that light from the explosion is delayed on its way to our telescopes, but nearly massless particles called neutrinos can escape the blast and, in theory, announce the explosion before the light reaches us. Yuri Kashiwagi and collaborators examined how the upgraded Super-Kamiokande detector can be used to alert astronomers of an impending supernova, enhancing our ability to learn from these explosions.

7. How Common Are Solar Systems Like Our Own?

illustration of the planets in our solar system and their orbits

Illustration (orbits not to scale) of the planets in our solar system. [NASA/JPL]

When researchers began to discover exoplanet systems, it immediately became clear that not all systems are arranged like our own. What remains unclear is how many exoplanet systems are similar to the solar system. One important feature of our solar system is the presence of small planets — like Earth — orbiting interior to large planets, like Jupiter. Astrobites’s Jack Lubin reports on work by Marta Bryan and Eve Lee that searches for this configuration in distant planetary systems, helping to understand how common solar system–like arrangements are in the galaxy.

An icy surface overlaid by a liquid water ocean. Beams of light are streaming through the water from the ocean's top towards the ice below.

An artist’s impression of light shining through the ocean of a “water world,” possibly like LHS 1140b. [NASA]

6. What Kind of World is LHS 1140b?

LHS 1140b is a bright, nearby star that is known to host two planets. The nature of the inner of the two planets, LHS 1140b, is unclear, despite the bevy of telescopes that have observed this world. As Charles Cadieux and collaborators have shown, LHS 1140b might be the smallest known mini-Neptune exoplanet, or it might be a water world with a surface of ice and oceans.

illustration of Betelgeuse

Artist’s impression of the red supergiant star Betelgeuse. Though depicted solo, new research suggests that Betelgeuse might have a tiny companion. [Adapted from ESO/L. Calçada; CC BY 4.0]

5. Hiding in Plain Sight: Betelgeuse’s Binary Buddy

Betelgeuse is a highly recognizable red supergiant in the constellation Orion. Astrobites’s Alexandra Masegian reports on two AAS journal articles that independently arrived at the same conclusion: that Betelgeuse is not a single star, but rather a member of a binary system. While the two articles find slightly different masses and orbital separations for the proposed companion star, they both point to Betelgeuse’s long secondary pulsation period as evidence of the companion.

cartoon showing different types of exoplanets

Cartoon showing a variety of exoplanet types. Figuring out whether a planet is rocky or gaseous can be a challenge, as is the case for K2-18b. [NASA/JPL-Caltech/Lizbeth B. De La Torre]

4. K2-18b May Not Be Habitable After All

The 8.6-Earth-mass exoplanet K2-18b made headlines when researchers reported that the planet might be a rocky world covered in oceans. Even more eyebrow raising was the potential detection of a faint signal from dimethyl sulfide, a compound that on Earth is only associated with the presence of life. Nicholas Wogan and coauthors used models to interpret JWST data of K2-18b, finding that the planet is instead most likely an uninhabitable gas-rich world — though they didn’t entirely rule out the inhabited ocean world scenario.

The Cassiopeia A supernova remnant as seen by JWST

The Cassiopeia A supernova remnant as seen by JWST. [Rho et al. 2024]

3. Featured Image: A New Portrait of Cassiopeia A

The Cassiopeia A supernova remnant has sat for countless astronomical portraits, each of which reveals new details about this remnant of an exploded star. This portrait from JWST’s Mid-Infrared Instrument and Near-Infrared Camera highlighted electrons spiraling around magnetic field lines as well as light from argon and carbon monoxide, allowing a team led by Jeonghee Rho to study the connections between the formation of molecules like carbon monoxide and the creation of cosmic dust.

infrared image of the supergiant star Betelgeuse and its surroundings

Red supergiant star Betelgeuse, pictured here in an infrared image from the Herschel Space Observatory, has ejected a considerable amount of material. [ESA/Herschel/PACS/L. Decin et al.]

2. Monthly Roundup: Betelgeuse, Betelgeuse, Betelgeuse

Everyone’s favorite red supergiant makes two appearances in this list! This article summoned three perspectives on Betelgeuse, which has made frequent news appearances over the past few years because of its pronounced, prolonged dimming episode in 2019–2021. Though the star has returned to its normal brightness, questions linger about the star’s future behavior and its uncertain past — including whether Betelgeuse is the product of a stellar merger.

1. The Odds of the Unthinkable

Radar images of the near-Earth object Apophis

Radar images of the near-Earth object Apophis. [NASA/JPL-Caltech and NSF/AUI/GBO]

By far the most widely read article on AAS Nova in 2024 concerned an asteroid with an unsettling name: Apophis, named for the Egyptian deity that embodies disorder, destruction, and darkness. On 13 April 2029 — for the superstitious among you, the 13th happens to be a Friday — Apophis will zoom between Earth and the Moon. Apophis’s passage bears no threat to Earth, but an article by Paul Wiegert explored the possibility that gravitational nudges from other asteroids could change all that, sending Apophis careening catastrophically toward our planet in the future.

Thank you for joining us for another year of astronomy news — we hope to see you in 2025 for more discoveries. Happy New Year!

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