Editor’s Note: This week we’re at the 245th AAS meeting in National Harbor, MD, and online. Along with a team of authors from Astrobites, we will be writing updates on selected events at the meeting and posting each day. Follow along here or at astrobites.com for daily summaries, or follow @astrobites.bsky.social on Bluesky for more coverage. The usual posting schedule for AAS Nova will resume on January 21st.
Table of Contents:
- HEAD Bruno Rossi Prize Lecture: Martin Weisskopf (NASA Marshall Space Flight Center) and Paolo Soffitta (INAF-IAPS)
- Press Conference: Discovering the Universe Beyond Our Galaxy
- Plenary Lecture: Are We Alone? The Search for Life on Habitable Worlds, Giada Arney (NASA Goddard Space Flight Center)
- Press Conference: New Findings About Stars
- Plenary Lecture: Revealing the Solar Neighborhood’s Diversity and the Milky Way’s Substellar Halo, Aaron Meisner (NSF NOIRLab)
- Plenary Lecture: Newton Lacy Pierce Prize Lecture: The Evolution, Influence, and Ultimate Fate of Massive Stars, Maria Drout (University of Toronto)
HEAD Bruno Rossi Prize Lecture: Martin Weisskopf (NASA Marshall Space Flight Center) and Paolo Soffitta (INAF-IAPS) (by Jessie Thwaites)
Dr. Weisskopf says he expected to be surprised with the results from the Imaging X-ray Polarimetry Explorer (IXPE), but both he and Dr. Soffitta knew how important studying the polarization of high-energy objects in our universe would be to advancing X-ray astronomy. They, along with the entire IXPE team, are the recipients of this year’s High Energy Astrophysics Division (HEAD) Bruno Rossi Prize in recognition of the amazing science their instrument enables. (For a review of polarimetry, check out this Astrobites Guide!)
Dr. Weisskopf began the session by giving a brief history of how IXPE came to be. He realized that without considering polarization, the X-ray astronomy community would be missing a key element in their toolbox. They began by including a polarimeter on a rocket to look at Scorpius X-1, known to be a very bright X-ray source. With only 5 minutes to observe, they were able to prove the merit of their instrument on this short flight, which paved the way for new developments in the decades to come.
Next, they launched two types of polarimeter on the orbiting solar observer (OSO-8), and detected the Crab Nebula’s strong polarization, which is consistent with emission from relativistic electrons being bent by a magnetic field (synchrotron radiation). After this success, the team improved the instrument, and together the team proposed the mission to NASA. Although they weren’t initially selected, the mission that was selected ended up being cancelled due to budget issues, so the team re-submitted — and were selected! With one condition: that no data would be proprietary.
After launching the instrument in 2021, the surprises commenced; sources that were expected to have high polarization (from lots of synchrotron radiation) actually were weakly polarized, and vice versa. They observed a wide variety of sources, from magnetars and pulsars to supernova remnants and compact objects, and many more. They observed the bright magnetar 4U 0142+61, which has an energy dependence to its polarization, where both the polarization degree and angle change as a function of energy.
Dr. Soffitta begins his part of the lecture discussing the innovative techniques employed in the IXPE detector. They realized that they could harness the power of the photoelectric effect, where the electron would be emitted preferentially in the direction of polarization. This allowed them to build a broadband detector with sensitivity to many different source classes. Altogether, he says, they submitted the proposal for IXPE 13 times before it was finally selected to be built.
The results they find with this instrument are transformative to understanding these different source classes. They found that around 50% of the sources they studied had a polarization greater than 0, providing new insight into the magnetic field behavior in multiple source classes, and with important implications for particle acceleration happening in that source.
Dr. Soffitta describes a few of the important results so far with IXPE, starting with the polarization of active galactic nuclei. In NGC 4151, which is a Seyfert 1 galaxy, they find a polarization angle parallel to the radio jet, indicating that the corona is in the plane of the disk. On the other hand, in the Circinus Galaxy, which is a Seyfert 2 (meaning that we can’t see the central black hole but instead only see the surrounding torus), they find the polarization is instead perpendicular to the inner jet, which they interpret as the radiation being reflected. They find blazars to have 3–5 times higher polarization in X-rays than in lower wavelengths, which indicates the presence of electrons accelerated in the source emitting synchrotron radiation.
IXPE has enabled many possibilities for X-ray astronomy, including its fascinating results describing the mechanisms at play in many different source classes. Dr. Weisskopf leaves the audience with the directive to “enjoy wrestling with the scientific implications” of these astonishing results from IXPE.
Press Conference: Discovering the Universe Beyond Our Galaxy (by Lexi Gault) (Briefing video)
Introduction: 100 Years Since Hubble Discovered the Universe Beyond Our Galaxy (Press Release)
Jeff Rich (Carnegie Science Observatories)
Though we have marveled at faraway galaxies for quite some time now, just 100 years ago at an AAS meeting in Washington, DC, Edwin Hubble announced his discovery of galaxies beyond the Milky Way. Setting the stage for today’s press conference, Dr. Rich explained that Hubble had detected a Cepheid variable — a star whose brightness changes cyclically over time — in the Andromeda Galaxy. With this detection, Hubble could determine the distance to Andromeda, and for the first time, discovered a celestial object beyond our own galaxy. This discovery, as Dr. Rich reminded us, was made possible by the work of many astronomers, including Henrietta Swan Leavitt’s discovery of the period-luminosity relationship for Cepheid variables, and Harlow Shapley, who measured the size of the Milky Way that made Hubble’s distance calculation possible. With access to great telescopes and technology, astronomers have been able to make leaps and bounds in our understanding of the universe in a very short amount of time. Next generation instruments will continue to allow scientists in this generation and the next to flourish and discover more and more about the universe in which we live.
The Hubble Tension in Our Own Backyard
Daniel Scolnic (Duke University)
One of the most pressing issues in cosmology is the Hubble Tension, which is the discrepancy between the prediction of the current expansion rate of the universe based on the current standard cosmological model and what astronomers observe locally. In order to attempt to reconcile this tension, the Dark Energy Survey Instrument (DESI), as Dr. Scolnic explained, has built its own cosmological distance ladder to attempt to make a very precise measurement of the Hubble constant. Through observing supernovae in the Coma Cluster, DESI has been able to very precisely measure the distance to the Coma Cluster. However, the standard model predicts a distance farther away from us than has been measured by DESI and by many investigations prior. Objects in the universe are closer than our current cosmological standard model predicts, which, as Dr. Scolnic emphasized, has pulled the Hubble Tension taught, creating a crisis.
JWST Reveals the Early Universe in Our Backyard
Nolan Habel (NASA Jet Propulsion Laboratory)
Dr. Habel takes us to cosmic noon, the peak of star formation in the universe, which occurred when the universe was just 2.5 billion years old. At this time, a significantly smaller fraction of the universe’s gas had been processed into stars, meaning this star formation occurred with many fewer metals than exist in the universe today. Metals are a driver of the cooling of the interstellar medium, allowing the gas to collapse and coalesce into stars and planetary systems. But observing star formation under these conditions is difficult. Dr. Habel and collaborators have used JWST observations to study the nearby star-forming region NGC 346 in the Small Magellanic Cloud that has similar metallicity to that of cosmic noon. They are able to take spectra of individual protostars — stars early in their formation — probing the formation processes that shine a light on how stars and planets may have formed at cosmic noon.
Growing in the Wind: Watching a Galaxy Seed Its Environment
David Rupke (Rhodes College)
Galaxies and their surroundings form an ecosystem in which gas travels into and out of galaxies, mixing materials, spreading metals, and enriching the circumgalactic medium. Dr. Rupke presented observations of Makani, the Hawai’ian word for wind, a galaxy actively seeding its environment with cool clouds of oxygen. The galaxy is surrounded by a nebula of gas extending 300,000 light-years that has been ejected by multiple hot winds that push material out of the galactic center. Simulations show that as these clouds travel outward, they cool, but this is difficult to observe directly as it occurs on very small scales. Using the Hubble Space Telescope, Dr. Rupke and collaborators imaged Makani in the far-ultraviolet, tracing the emission from oxygen that is five times ionized, which traces the gas where the cooling rate is at its peak. This imaging has set a benchmark for future observations of galaxies feeding their environments with future, stronger telescopes.
Three Quenched, Faint Dwarf Galaxies: New Probes of Reionization and Stellar Feedback (Press Release)
David Sand (University of Arizona)
Far from other galaxies, stranded out in the field, are three ultra-faint dwarf galaxies that have been discovered by Dr. Sand and his collaborators. Unlike the small faint satellites orbiting the Milky Way that are subject to life-altering interactions, these ultra-faint dwarfs are unperturbed, making them ideal targets for studying how the smallest structures in our universe form and evolve without complicated interactions with larger-scale structures. First discovered in images from the Dark Energy Camera Legacy Survey (DECaLS), the ultra-faint dwarfs were observed in more detail with the Gemini South telescope to really get a good picture of these fuzzy patches. From these observations, Dr. Sand reported, each of these galaxies has a single old, metal-poor stellar population, no recent star formation, and no gaseous components. This suggests that these galaxies either blew out their gas through supernova explosions or the reionization of the universe evaporated the gas from these galaxies. Further studies and discoveries of similar ultra-faint dwarf galaxies will provide more clues into the evolutionary conditions of these galaxies.
Plenary Lecture: Are We Alone? The Search for Life on Habitable Worlds, Giada Arney (NASA Goddard Space Flight Center) (by Lexi Gault)
During this plenary lecture, Dr. Giada Arney presented the science goals and plans for the Habitable Worlds Observatory (HWO). This mission aims to answer one of life’s fundamental questions: are we alone in the universe? This observatory will be a space-based, large-aperture telescope with a diameter between 6 and 8 meters and will observe through ultraviolet, optical, and near-infrared wavelengths (100–2500 nanometers). As Dr. Arney described, HWO will be a “super-Hubble,” capable of detecting Earth-like planets and characterizing their atmospheres, searching for signs of life.
The search for habitable worlds, as Dr. Arney explained, begins with liquid water. Though there is no guarantee that all possible lifeforms in the universe require liquid water to arise, we begin the search with what we know about life, and life on Earth needs liquid water. This requirement limits the search for habitable planets to Sun-like and lower-mass stars that are cool enough to host potential habitable zones. Searching for Earth-like planets around these stars will reveal whether planets like ours are common or rare, furthering our understanding of how common life may be.
Once we identify candidate planets, the search for life requires the identification of biosignatures: molecules that could not be explained without the presence of life. Dr. Arney took us through a tour of a few biosignatures that will be searched for through atmospheric composition analysis. The first biosignature, ozone (O3), is the product of photosynthesis. Though ozone can be produced in small amounts through starlight splitting water vapor apart, the lasting presence of atmospheric ozone is only known to be produced by photosynthetic life over a long duration of time. Another biosignature, methane, is primarily produced by microorganisms and builds up more through biotic (i.e., biological) processes than abiotic (i.e., physical rather than biological) processes.
However, detection of these biosignatures is not 100% indicative of life 100% of the time. There are multiple instances in which we could recover a false positive biosignature. In the case of oxygen, Earth produces very little abiotic oxygen, but this is not true for other planets. For example, Venus has a small amount of ozone, likely produced by oxygen interacting with the planet’s surface materials. In the case of methane, it can form abiotically through chemical interactions between water and iron-rich minerals. This is the case for Saturn’s moon Titan, which has an atmosphere made up of long-lasting methane that replenishes very slowly over time. With the chance of false positive biosignatures, Dr. Arney emphasizes the importance of careful observations of full stellar systems to accurately identify potential habitable worlds.
To wrap up her talk, Dr. Arney highlighted the breadth of the HWO’s science. Though intended to search for life, HWO will be a very powerful facility that will seek to answer questions across a wide range of topics in astronomy including galaxy formation and evolution, evolution of elements over time, and detailed studies of our solar system. Even more, Dr. Arney urged, HWO will answer questions we have not yet thought to ask.
Press Conference: New Findings About Stars Near and Far (by Archana Aravindan) (Briefing video)
A Predicted Great Dimming of T Tauri: Has It Begun? (More information)
Tracy Beck (Space Telescope Science Institute)
Dr. Beck studies “the” T Tauri, which is the prototype for an entire class of Sun-like stars in our galaxy. Originally thought to be a single star, T Tauri consists of two distinct systems: T Tauri North, commonly visible in optical wavelengths, and T Tauri South, a binary system obscured in the optical by a dusty circumbinary disk and located in the foreground of the northern system. In 2017 and again in 2021, T Tauri North experienced an unprecedented dimming of about 2 magnitudes — the first such event observed in over a century. Dr. Beck proposes that this dimming is caused by the T Tauri South binary system, which is moving along its wide circumbinary orbit toward T Tauri North. As it progresses, dust from the circumbinary disk is gradually obscuring the northern star along our line of sight. This process will eventually cause T Tauri North to disappear entirely from view.
A Super Star Cluster Is Born: JWST Reveals Dust and Ice in a Stellar Nursery (Press release)
Omnarayani Nayak (NASA Goddard Space Flight Center)
Recent observations from JWST by Dr. Nayak and team provide new insights into super star clusters, key sites of intense star formation 6–7 billion years ago. In the Large Magellanic Cloud, N79, a young super star cluster less than 100,000 years old, forms stars at twice the rate of the Milky Way, with JWST’s NIRCam instrument identifying more than 1,500 protostars and MIRI revealing younger stars forming centrally. Lower-mass stars form farther from the core, where five massive protostars, including one 40 times the Sun’s mass, dominate. Recently launched outflows (< 100,000 years ago) are detected by the Atacama Large Millimeter/submillimeter Array, while Chandra X-ray Observatory data confirm that the age of the super star cluster is close to 100,000 years. This super star cluster, forming stars 2–4 times more efficiently than 30 Doradus and twice as efficiently as the Milky Way, offers a vital glimpse into the earliest stages of protostar formation.
A Discovery of Ancient Relics in a Distant Galaxy
Kate Whitaker (Umass Amherst)
Globular clusters are ancient relics of the universe, formed between 13 and 11 billion years ago, often found in association with more massive galaxies. Early globular clusters were young and blue, becoming older and redder over time. Observations of galaxies 11 billion years ago provide a unique perspective on both old and young globular clusters, offering a glimpse into their evolution. Using the JWST-UNCOVER treasury program, Dr. Whitaker and team identified a galaxy with a smooth elliptical light profile (indicating a past merger), which they nickname the Relic. This galaxy hosts a mix of young, intermediate-mass, and old associated star clusters with many remaining undetected. The findings suggest old clusters formed in situ, intermediate ones arose from tidal interactions and accretion, and young clusters also likely formed from accretion events. These clusters reveal the formation history of the Relic galaxy, offering a unique laboratory to test and improve theories of globular cluster formation.
Exploring the Sun’s Active Regions in the Moments Before Flares
Emily Mason (Predictive Science Inc.)
Solar flares, which are powerful bursts of energy from the Sun, occur when magnetic energy in the Sun is converted into light and motion. They can cause significant damage to satellites and communication systems around the Earth and their timing remains difficult to predict. Using data from the Solar Dynamics Observatory across four channels (which will allow for a range in temperatures/wavelengths), Dr. Mason and team analyzed coronal loops above active regions in the Sun. Observations focused on the active regions at the solar limb, avoiding overlapping active regions and ensuring no major flares occurred in the six hours prior for clean detection. Comparing light variability over six hours revealed that loops above active regions were more active, with fluctuations peaking 1-2 hours before a flare, particularly in cooler plasma. Rapid, small-scale brightness spikes often preceded confined flares without coronal mass ejections (CMEs). This method shows promise for predicting flares and is now being tested on more complex and blind cases to determine the accuracy of the predictions as well as to investigate the drivers of these small-scale variabilities.
Plenary Lecture: Revealing the Solar Neighborhood’s Diversity and the Milky Way’s Substellar Halo, Aaron Meisner (NSF NOIRLab) (by Lindsey Gordon)
Dr. Meisner works on a census of the local solar neighborhood, with a focus on brown dwarfs. Brown dwarfs fill in the mass range between red dwarfs, the smallest “real” stars, and large planets. They’re sometimes called “substellar objects,” and they most likely formed like stars do. These brown dwarfs are cold, old, and have low metallicities. They can be hard to study because there is a lot of degeneracy between their temperatures, masses, and ages, as they cool very slowly over billions of years. Their spectra have a lot of humps due to molecular absorption in their atmospheres, and they fall into the L, T, and Y spectral classes.
Why study brown dwarfs? There are lots of them and they’re near us. They can be used as an easier-to-study analog of exoplanetary atmospheres and to contextualize our Jovian planets. With long lifespans, they trace early galactic structure and the Milky Way’s initial mass function.
There are no known Jupiter-temperature brown dwarfs, but we think they should exist. WISE 0855 is the coldest known brown dwarf (about 100K hotter than Jupiter) and is only 2 parsecs away from us, so we thought we would’ve found more of them. In the search for Jupiter-temperature brown dwarfs, Dr. Meisner’s team instead found new classes of brown dwarfs in the thick disk and halo of our galaxy.
The primary data these studies used was from the WISE mission, which is an infrared wide-field survey satellite. With more than 60 million exposures to look through, using citizen science / participatory science was necessary to identify the brown dwarfs in the data. Brown dwarfs are identified by their color — red in WISE data — and their rapid motion. The team produced mini movies of the WISE observations that volunteers from around the world classified through the Backyard Worlds Zooniverse program. Since 2017, volunteers have performed 11.5 million classifications and found 4,200 candidate brown dwarfs. The team also launched the Backyard Worlds: Cool Neighbors program, which uses AI and human co-classification. A neural network preselects the most likely candidates to show to the volunteers, and this method has given them a 3x boost in classifications. The incorporation of AI doesn’t take away from the human element of a participatory science project. The team has a big emphasis on community building; the science team meetings are attended by volunteers to give feedback and contribute.
Three types of progressively cooler brown dwarfs and their characteristics (click to enlarge). Y dwarfs are the coldest of substars. [NASA/JPL-Caltech with annotations from the Backyard Worlds project]
Then there is “The Accident.” WISEA 1534-1043 is a very cold (~500K) Y-type brown dwarf that was found purely by accident during Backyard Worlds. It’s a 6–8 magnitude outlier from other known brown dwarfs and has been confirmed by WISE, Gemini, Hubble, Spitzer, and JWST to be a brown dwarf just 16 parsecs away. It has a crazy spectrum compared with other brown dwarfs, which is part of what led them to add metallicity into their classification scheme.
JWST spectra of brown-dwarf atmospheres see molecular atmospheric absorption lines including methane, ammonia, and water. Under review for publication are the first ever detection of phosphine in the atmosphere of Wolf 1130 C, a 620K brown dwarf, and the first ever detection of silane (SiH4) in the atmosphere of The Accident. We’ve been expecting these features and trying to find them in our own solar system, but this might be the first time we’ve actually seen them.
In the new era of Big Data and next-generation surveys, participatory science is going to become ever more important for astronomers. Extremely large telescope programs like Rubin, LSST, and Euclid should increase our count of brown dwarfs by a factor of 15 and help fill out the census of our neighbors.
Newton Lacy Pierce Prize Lecture: The Evolution, Influence, and Ultimate Fate of Massive Stars, Maria Drout (University of Toronto) (by Lindsey Gordon)
Dr. Maria Drout was awarded the 2024 Newton Lacy Pierce Prize for “revealing discoveries of the evolution, influence, and end states of massive stars through the study of explosive transients and resolved stellar populations.”
Massive stars are objects that every field of astronomy, from planetary science to cosmology, could use a better understanding of. Multi-star systems are ubiquitous in astronomy, and massive stars almost always form with at least one companion, with two-thirds of those systems interacting with their companion. There are many possible evolutionary pathways for these binary systems, the steps along which Dr. Drout and her team investigate. With the advent of gravitational wave astronomy, mergers between the possible compact object (neutron star or black hole) remnants of these systems are now observable in multiple messengers.
Dr. Drout discussed a potential scenario where two massive stars in a binary evolve into a binary pair of neutron stars. Over the course of their lifetimes, one star may go off as a supernova, the outer layers of one star may be stripped away by the other, the two may evolve together in a common envelope, and finally the second star must go off in a unique type of supernova.
The first supernova in the system can be found through the rise of wide field time-domain monitoring surveys, which now give us some 20,000 events per year. Recent studies have shown the progenitors of these core-collapse supernovae have properties that we didn’t expect and may in fact be hydrogen-poor core-collapse supernovae. These have low ejecta masses and high rate, which suggest a contribution from a (comparatively) lower-mass binary companion. We expect a lot of these stars to exist, but need to understand just how many of them there are, their metallicities, and their explosion properties. They also need better constraints on the fate of the surviving companion star.
At some point in the system’s lifespan one or both stars becomes a stripped helium star (He stars). The outer hydrogen envelope is removed and the hot helium core is revealed. Theories of binary evolution suggest they should be common, and they’re the favored progenitor for hydrogen-poor core-collapse supernovae. Dr. Drout’s group found the first population of these stars by reprocessing Swift Ultraviolet/Optical Telescope photometry data and looking for hot, ultraviolet-bright sources. Follow-up spectra on some of these systems showed deep He II absorption lines, indicating very hot, low luminosity stars with a lot of helium. From here, they’re looking to form a more complete sample of these stars to do population statistics with. Within the sample they have additional data on, there is at least one system that has an orbital period indicative of a binary between a stripped helium star and a compact object, which could be on its way to becoming a neutron star binary.
The common stellar envelope stage of a system occurs when one star fills its Roche lobe and begins to unstably transfer mass to its companion. The two stars can then either merge or eject the outer envelope and form a tighter binary system. Numeric simulations of these systems are tough, and there are no systems where we have measurements of the system both before and after it entered a common envelope. We mostly think these systems are a white dwarf and a main-sequence star, but knowing for sure if the two evolved together or just got caught by gravity is tricky. Stellar clusters — where all the stars are about the same age — present a viable opportunity for finding systems that co-evolved. The team was able to find 55 candidates for white dwarf–main-sequence binaries in clusters for future study of this phase of evolution.
At some point, a second supernova has to go off in the system to end up with two compact objects for a neutron star binary. This second supernova is also hydrogen-poor and has been through a lot in its evolution. These “ultrastripped” supernovae have very low masses and eject only about one solar mass of material. They are fainter and evolve more rapidly than other core-collapse supernovae, which makes them difficult to observe. However, the new wide-field surveys are finding so many transient events that we now have a small population of candidates for this type of supernova.
These events are just possible landmarks along the way to the compact object binaries lighting up the gravitational wave sky. Dr. Drout’s work represents the exciting new stellar populations that are still very much in the discovery phase of our understanding.