Voyeurism is the last thing we need in sports

How do galaxies grow? A massive survey from JWST can help astronomers answer that fundamental question about galaxy evolution.

Time Machines

When astronomers look into the night, they are greeted by a sky full to bursting with galaxies. But immediately after the Big Bang, none of these grand structures could have existed in our universe: all of the matter was too hot and dense to collapse into the whirlpools and ellipses we see today. We know that it took time for dark matter to collapse into halos, for gas to cool and fall into those halos, and even more time for those halos to merge together and galaxies to form. However, quantifying that story is tricky. How long did it take for the gas to cool and collapse? How often did small galaxies merge? How did the birth and death of stars play into all of this?

One way astronomers could go about answering these deep, fundamental questions in galaxy evolution would be to look at many galaxies spread across different distances. Famously, since the speed of light is fairly slow (cosmically speaking), we can only see distant objects as they appeared in the past. (This is why any aliens looking toward Earth from 65 million light-years away — from within the Virgo Cluster, perhaps — would see our planet as it was when dinosaurs roamed free.) In principle, then, by looking at a progression of galaxies farther and farther away, astronomers could convert these still images into a sort of movie of how typical galaxies look moving further and further back in time.

Seeing Red

There is a complication to this strategy, however. The farther away an object is, the more the light it emits is stretched towards redder wavelengths. As a consequence, any telescope designed to detect visible light won’t see anything; to take pictures of these galaxies, we need to build detectors that are specifically sensitive to stretched-out, near-infrared light.

This is one of the primary reasons that JWST was designed, constructed, and flung into space. It was purpose-built to collect and analyze near-infrared light from distant galaxies, and now after several years of observations and data crunching, astronomers are finally getting some answers to those fundamental questions.

A Massive Survey

Recently, a team led by Lilan Yang, Rochester Institute of Technology, released their analysis of one of the earliest and largest surveys JWST has undertaken, a program called COSMOS-Web. The researchers mined a patch of sky about the size of the full Moon and extracted more than 30,000 distant galaxies, most of which had never before been seen by human eyes. They measured the brightness and size of each one and grouped them according to their redshift (a proxy for their distance) and whether they were actively forming stars or not.

The team found that, unsurprisingly, galaxies tend to grow over time. Excitingly, however, they also found that for star-forming galaxies, the relationship between a galaxy’s mass and its size is fairly constant for redshifts of z = 2–8. The scaling law between them also nicely matches up with measurements taken by the Hubble Space Telescope at smaller redshifts. Beyond that, though, for redshifts of z = 8–10, it’s possible that the scaling changes, though the authors emphasize that their sample is much smaller in these distant bins.

While more data are needed to confirm whether the youngest of these galaxies grow at a different pace than their later counterparts, we can be comforted that studies like this represent just our very first peek into a whole new era of galactic evolution, and that many more insights are sure to come as JWST keeps snapping images.

Editor’s Note: This article previously contained an error relating to the distance at which aliens would see Earth as it was when inhabited by dinosaurs. This error was corrected on 4 May 2026.

Citation

“COSMOS-Web: Unraveling the Evolution of Galaxy Size and Related Properties at 2 < z < 10,” Lilan Yang et al 2025 ApJS 281 68. doi:10.3847/1538-4365/ae0e1b

The post How Galaxies Grow Up appeared first on AAS Nova.

Roughly 25–50% of the white dwarfs in the solar neighborhood are “polluted,” showing signs of accretion of metal-rich planetary material. New JWST observations reveal the contents of the debris disk surrounding one of the most polluted white dwarfs in the solar neighborhood.

Heavy-Metal Pollution

White dwarfs are the remnants of low- to intermediate-mass stars that have ceased core fusion and evolved off the main sequence. The presence of metals in white dwarf atmospheres shows that it’s common for white dwarfs to accrete rocky materials from tidally disrupted bodies in their surroundings, allowing for a post-mortem investigation of the composition of rocky worlds in far-off planetary systems.

One of the most impressively metal-polluted white dwarfs known is GD 362, which is surrounded by a dusty debris disk and shows evidence for at least 16 accreted elements. GD 362 is a helium-atmosphere white dwarf, but previous observations have also shown that it has an unusually large amount of hydrogen in its atmosphere — a possible sign that the star has consumed one or more bodies containing water or ice.

Disk Investigation

Now, a team led by William Reach (Space Science Institute) has obtained JWST spectra of GD 362 to examine the material in its disk directly. Using both the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI), the team collected spectra of the white dwarf from 0.6 to 17 microns, with additional photometric measurements at 18, 21, and 25.5 microns. The JWST spectra show bright emission from the white dwarf at wavelengths shorter than 2.5 microns, with a clear signature of disk material at longer wavelengths, including a strong emission feature at 9–11 microns.

Reach’s team modeled the JWST spectra to constrain the mineralogy of the disk and its physical attributes. They found that the disk likely stretches from 140 to 1,400 times the white dwarf’s radius and contains both silicates and carbon, with the prominent emission feature pointing to olivine and pyroxene silicate minerals. The elements seen in the white dwarf’s atmosphere are generally similar to those observed in the disk, strengthening the connection between accretion from the disk and the metals present in the white dwarf’s spectrum.

Hydrogen Mismatch

The new JWST data furthered the investigation of GD 362’s atmospheric hydrogen and the possibility that it’s accreting water- or ice-rich material. Though water molecules have spectral features in the range studied, none of these features appeared in the JWST spectra of GD 362. This isn’t necessarily a surprise; the team showed that water molecules in the disk would quickly be split apart by GD 362’s high-energy radiation.

Curiously, though, Reach’s team found little evidence for hydrogen-rich material within the disk that could be the source of the white dwarf’s atmospheric hydrogen. Because accreted hydrogen would remain in the star’s atmosphere indefinitely, the presence of atmospheric hydrogen could be evidence that the white dwarf accreted water-rich material from its debris disk in the past, though the materials currently present in the disk are dry.

In addition to investigating GD 362’s disk, Reach and collaborators searched for planets orbiting the white dwarf. Though they found none, they were able to rule out the presence of companions with masses greater than 25 times the mass of Jupiter.

Citation

“Composition of Planetary Debris Around the White Dwarf GD 362,” William T. Reach et al 2025 ApJ 994 195. doi:10.3847/1538-4357/ae11a9

The post Investigating a Polluted White Dwarf appeared first on AAS Nova.

Editor’s Note: For the remainder of 2025, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 2nd.

The SOFIA Massive (SOMA) Star Formation Survey. V. Clustered Protostars

Published June 2025

Main takeaway:

A team led by Zoie Telkamp (University of Virginia) used the Stratospheric Observatory for Infrared Astronomy, SOFIA, to study massive protostars and test theories of high-mass star formation. Contrary to the predictions of some star-formation models, the team found no evidence that massive protostars require a certain surface mass density to form. Formation in a cluster environment, however, may limit the formation of the most massive protostars.

Why it’s interesting:

Massive stars are rare, short-lived, and luminous. They influence their environments across a vast range of spatial and temporal scales, from advancing the epoch of reionization in the early universe to impacting the formation of individual planetary systems in the present-day universe. The fundamental question of how high-mass stars form is still unsettled. Theories of high-mass star formation range from scaled-up versions of low-mass star formation to scenarios involving collisions between protostars.

More about this massive-star study and the potential impact of a cluster environment:

Researchers developed the SOFIA Massive Star Formation Survey to investigate the origins of massive stars, targeting roughly 50 high-mass and intermediate-mass protostars in a wide variety of environments in our galaxy. This particular work, the fifth in a series of articles reporting the survey’s findings, described the team’s study of massive protostars forming in cluster environments. Using the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST), Telkamp and collaborators identified 34 protostars in seven star-forming regions and estimated their masses and other physical properties. The team noted a lack of protostars above 30 solar masses in these cluster regions, which might be evidence that competition for gas in dense environments can prevent the formation of more massive protostars. More work is needed to confirm this finding.

Citation

Zoie Telkamp et al 2025 ApJ 986 15. doi:10.3847/1538-4357/adcd79

The post Selections from 2025: The Formation of Massive Stars appeared first on AAS Nova.