Hedorah, the first yellow supergiant Kaiju star candidate at z=3.7 revealed by JWST behind AS1063

arXiv:2601.11704 · astro-ph.GA, astro-ph.CO, astro-ph.SR · Submitted 2026-01-16 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Next we'll be talking about the paper "Hedorah, the first yellow supergiant Kaiju star candidate at z=3.7 revealed by JWST behind AS1063".

Jocelyn: The paper was written by J.M. Diego, J.M. Palencia, C. Goolsby, C.J. Conselice, D.J. Lagattuta et al. from Instituto de Física de Cantabria (CSIC-UC) and Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester and Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire and STAR Institute, Quartier Agora - Allée du six Août, 19c B-4000 Liège, Belgium. and Université Claude Bernard Lyon 1, CRAL UMR5574, ENS de Lyon, CNRS, and Department of Astronomy, University of Michigan and School of Physics and Astronomy, University of Minnesota.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Paper discussion segment 1: Vera: We've been looking at this incredible paper, "Hedorah, the first yellow supergiant Kaiju star candidate at z=three point one revealed by JWST behind AS1063," which has us all buzzing. Jocelyn, it's hard to not be fascinated by the fascination with the following pop culture reference to a monster-sized star.

Jocelyn: It really is, Vera. J.M. Diego and his team of researchers from institutions like the Instituto de Física de Cantabria in Spain and University of Manchester in UK, represent a paper that is a massive international collaboration. This kind of global teamwork is what it takes to analyze the such deep JWST data from the cluster AS1036.thought

Subrahmanyan: This paper's title isn't just being playful; it signals that we are looking at something extremely rare and sometimes quite extraordinary in the early universe. Finding a yellow supergiant at a redshift of three point one is essentially finding a finding a finding a finding a finding a finding a finding

Vera: Exactly, Subrahmanyan. Most of the lensed stars we've seen so far with JWST has been red or blue supergiants, so this "Hedorah" star candidate is an anomaly that challenges our understanding of stellar evolution at high redshift.

Jocelyn: And it's not just about one star; the title also mentions AS1063, which is the galaxy cluster acting as our natural telescope. By using this cluster to lens the background star, and its magnification, we little bit of a little bit of a little bit of a little bit of a little bit of a little bit of a little bit

Subrahmanyan: The implications for cosmology are massive. If Hedorah is indeed a yellow supergiant or hypergiant, it tells us about the mass and metallicity of the first generations of stars. It's like having a probe into the very early universe, scanning parsec-scale fluctuations in theaging process of stellar populations.

Vera: I love how they used that cluster as a magnifying glass to see something thats so distant it would otherwise be completely invisible. We're moving from just seeing galaxies to seeing individual stars across cosmic time.

Jocelyn: It's a huge leap in observational capability, Vera. I wonder what else is hiding in those deep JWST images?

Subrahmanyan: There is a whole spectrum of possibilities, from little-red-dots to extreme emission line galaxies.

Vera: That leads us perfectly into the next part of our discussion, where we'll look at the actual summary and findings of this study.

Paper discussion segment 2: Vera: Now that we've touched on the title and why it's so catchy, let's look at what J.M. Diego and his team actually discovered in "Hedorah, the first yellow supergiant Kaiju star candidate at z=three point one revealed by JWST behind AS1063."

Jocelyn: The summary of the paper is quite dense, but essentially, they've used ultra-deep JWST data to build a new free-form lens model for the cluster AS1063. This model is critical because it allows them to identify new counterimages and confirm previously known lensed systems.

Subrahmanyan: This modeling is the backbone of the science, Jocelyn. Without a highly precise lens model, you can't accurately determine magnification or geometric redshift. Without it, these "monster" stars are just blurry blobs in your data.

Vera: I was struck by their identification of a peculiar multiply lensed galaxy at z approximately seven point five with strong emission lines. They describe it as either an extreme emission line galaxy or a little-red-dot, which is such an intriguing find!

Jocelyn: And they also found that Hedorah itself has very unique photometry. It's undetected in the F090W filter and shows significant emission in the F150W band, which is consistent with a temperature of about six thousand five hundred Kelvin. That really points toward it being a yellow supergiant.

Subrahmanyan: The fascinating part is that Hedorah doesn't have a visible counterimage on the other side of the CC. The This could be due to microlensing or intrinsic variability, like if it's a Cepheid variable star.

Vera: That's a great point, Subrahmanyan. They even explore the possibility that Hedorah is part of a small group of stars rather than a single star, though they argue that's less likely given its colors.

Jocelyn: It sounds like the data is incredibly revealing, but I wonder if there we are seeing something more exotic than just a single star.

Subrahmanyan: That could be the case; they even discuss how fuzzy dark matter could explain the lack of a counterimage by creating parsec-scale fluctuations in magnification.

Vera: We'll have to dig into that in our next segment when we talk about the improvements and implications of their work.

Paper discussion segment 3: Vera: We've just talked about the discoveries and the strange nature of Hedorah, so let's look at how this paper, "Hedorah, the first yellow supergiant Kaiju star candidate at z=three point one revealed by JWST behind AS1063," suggests improvements for our future studies.

Jocelyn: One of the biggest improvements is the method they used to build their lens model. They used a hybrid type of modeling technique that combines a smooth, large-scale component with small-scale contributions from cluster galaxies. This provides much more detail than older models could offer.

Subrahmanyan: This precision is vital for future work, Jocelyn. As we get more data from JWST and eventually the ELT, we'll be able to to use these lensed stars as "pencil beams" to scan the small-scale fluctuations in the lensing potential, which can tell us about dark matter on a parsec scale.

Vera: I love how they highlight that finding a Cepheid at high redshift would be a unique opportunity for cosmology. If we can confirm Hedorah is a Cepheid, it would provide an independent constraint on H0 that doesn't rely on SNe.

Jocelyn: But as the paper points out, using lensed Cepheids as standard candles has its own set of challenges, specifically the stochastic distortion in magnification from microlenses.

Subrahmanyan: Exactly, Jocelyn. The magnification is a huge variable, and you have to account for both the macromodel and the microlensing effects if you want to use them as standard candles.

Vera: They also suggest that more deep multi-epoch observations of AS1063 with JWST are needed to confirm whether Hedorah is a microlensing event or an intrinsically variable star.

Jocelyn: So, the path forward involves more time and more deep imaging, but it's a wonderful start.

Subrahmanyan: It's the beginning of a new era where we can see individual stars in the distant universe.

Vera: And that brings us to our final segment where we'll wrap everything up.

Conclusion: Vera: We've had such a fascinating discussion today about "Hedorah, the first yellow supergiant Kaiju star candidate at z=three point one revealed by JWST behind AS1063." It's been an absolute pleasure walking through this research.

Jocelyn: It really has, Vera. The way they used the cluster AS1063 as a natural telescope to find Hedorah is just incredible. Seeing a single star at such a high redshift is something we truly've never seen before.

Subrahmanyan: This paper marks an important milestone in our understanding of stellar evolution and the possibility of using lensed stars as cosmological probes. Whether Hedorah is a single monster star or a group of stars, it's teaching us something new about the cosmic history of the scale.

Vera: And if it's confirmed as a Cepheid, it's an incredible discovery that could reshape our understanding of the cosmic distance ladder.

Jocelyn: I can't wait to see what future observations confirm these findings. The science is truly moving at a very rapid pace with JWST.

Subrahmanyan: It's a wonderful time to be an astronomer, indeed.

Vera: Thanks for all our listeners out today. We're moving on to the next paper soon!

Jocelyn: Goodbye for now!

Subrahmanyan: See you in the next one!

J.M. Diego, J.M. Palencia, C. Goolsby, C.J. Conselice, D.J. Lagattuta, G. Mahler, J. Richard-5, K. Sharon-6, and L.L_R_ Williams-7

Instituto de Física de Cantabria (CSIC-UC) · Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester · Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire · STAR Institute, Quartier Agora - Allée du six Août, 19c B-4000 Liège, Belgium. · Université Claude Bernard Lyon 1, CRAL UMR5574, ENS de Lyon, CNRS, · Department of Astronomy, University of Michigan · School of Physics and Astronomy, University of Minnesota

astro-ph.GA, astro-ph.CO, astro-ph.SR

Submitted: 2026-01-16

Updated: 2026-09-11

Comments: Version with severeal improvements including reference to Furtak et al (2026) with a posteriori spectrocopic confirmation of Hedorah's hots galaxy

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 73/100

The gist: This paper presents a new free-form lens model for the galaxy cluster AS1063 (z = 0.348) using "ultra-deep JWST data from the GLIMPSE program" and previously spectroscopically confirmed lensed

Key concepts

Yellow Supergiant: A star type that is being considered an anomaly at high redshift (z=3.7). Hedorah shows unique photometry, being undetected in the F090W filter but showing significant emission in the F150W band, consistent with a temperature of about six thousand five hundred Kelvin.
Galaxy Cluster AS1063: This cluster acts as a natural telescope by lensing background stars. Researchers used it to magnify and observe distant objects that would otherwise be invisible, allowing them to study individual stars across cosmic time.
Lens Model: A model built using ultra-deep JWST data to accurately map the cluster AS1063. This precise modeling is critical for determining magnification and geometric redshift, which are necessary to interpret the observations of distant 'monster' stars.
Redshift (z=3.7): A measure used in cosmology indicating how far away an object is in the early universe. Finding a yellow supergiant at this high redshift provides information about the mass and metallicity of the first generations of stars.

Terminology

Microlensing — 26× in this paper · explained in 2 episodes
Adam — 11× in this paper · explained in 2 episodes
Alignment — 3× in this paper · explained in 1 episode
Consistency — 3× in this paper · explained in 3 episodes
Core — 2× in this paper · explained in 2 episodes
Metallicity — 2× in this paper · explained in 3 episodes
AGN — 1× in this paper · explained in 1 episode
Aperture — 1× in this paper · explained in 1 episode
Calibration — 1× in this paper · explained in 7 episodes
Fine-Tuning — 1× in this paper · explained in 5 episodes
Gravitational Lensing — 1× in this paper · explained in 2 episodes
Hubble Frontier Fields — 1× in this paper
Hubble Space Telescope — 1× in this paper

Summary

This paper presents a new free-form lens model for the galaxy cluster AS1063 (z = 0.348) using ultra-deep JWST data from the GLIMPSE program and previously spectroscopically confirmed lensed galaxies. The researchers utilized the WSLAP+ algorithm, a hybrid modeling technique that combines a free-form decomposition of the smooth, large-scale component (DM, gas, and lower-density stellar contribution to the intracluster light) with a small-scale contribution from cluster galaxies. This model was employed to confirm many of the previous lensed system candidates and discover new lensed system candidates in the JWST images, allowing for the computation of geometric redshifts, time delays, and magnification for all counterimages.

The central discovery of the paper is Hedorah, a yellow supergiant lensed star candidate at z ≈ 3.1. Hedorah is an unresolved source located within a caustic crossing galaxy (system 60) that exhibits a very peculiar yellowish color (in the rest frame) that does not resemble any of the point sources nearby, including the GCs. Photometric data shows that its photometry is consistent with that of a single star at the redshift of the arc (z = 3.1) and temperature T = 6500 K, identifying it as a yellow supergiant star or yellow hypergiant star. The authors note that because previous lensed stars at z > 0.1 have been either blue or red supergiants, Hedorah [is] the first yellow supergiant discovered beyond z = 0.1 and confirming that, despite their rarity, they can also be found at these redshifts.

The paper investigates several possible explanations for Hedorah's observed properties:

Star Clump and Substructure:

One possibility is that Hedorah is a clump with very peculiar color in the lensed arc at z = 3.1, with a counterimage hidden by an invisible substructure (such as a millilens). However, the authors conclude this scenario is highly unlikely due to the low probability of perfect alignment and the difficulty of fine-tuning a group of stars to match such a unique spectrum.

Yellow Supergiant/Hypergiant or Cepheid:

The most plausible interpretation is that Hedorah is a yellow supergiant or hypergiant star observed during a maximum in brightness due to intrinsic variability (for instance a Cepheid star) or a microlensing event. While the authors consider if Hedorah could be the first Cepheid discovered at cosmological distances, they conclude it is more likely a hypergiant yellow star approaching the end of its life. If it is indeed a yellow hypergiant, it could be "approaching its final stages before going supernova (for as instance IRC+10420), thus offering the unique opportunity to study a SN precursor at z > 3."

Alternative Dark Matter Scenarios:

The authors also discuss how fuzzy dark matter (ultralight particles) could create parsec-scale fluctuations in the lensing potential, which might explain the lack of a visible counterimage for Hedorah by causing parsec scale distortions in the magnification.

In addition to Hedorah, the study identifies several other significant lensed systems:

High-Redshift Galaxy:

A peculiar multiply lensed galaxy with a strong emission line at ≈ 4 µm that likely corresponds to H−β and/or OIII at z ≈ 7.5, which may be an extreme emission line galaxy (EELG) or a little-red-dot (LRD).

Highly Magnified Galaxy:

A singly imaged but highly magnified galaxy at z ≈ 1.85 that is stretched by a large factor surpassing 100 in the central region.

Caustic Crossing Arc:

A caustic crossing arc at z = 0.73 (system 19) which shows tantalizing evidence of individual red giant stars being present as unresolved red sources near the cluster CC.

Improvements for AI systems

Direct Integration of Hybrid Mass Models into Physics-Informed Neural Networks (PINNs) for High-Resolution Source Reconstruction:

By incorporating the hybrid modeling approach used in WSLAP+ (combining free-form large-scale mass distributions with compact stellar component distributions), an AI system can move beyond standard convolutional architectures. This improved system can perform ultra-high-resolution deconvolution of distorted background objects (e.g., reconstructing a galaxy at z ≈ 1.85 from a highly elongated arc) by solving the gravitational lens equation as a differentiable layer within the network, effectively achieving sub-milliarcsecond resolution from pixelated data.

Multi-Modal Bayesian Anomaly Detectors for Lensed Object Classification:

This improvement involves fusing morphological features (point-source vs. extended) with high-dimensional Spectral Energy Distribution (SED) fitting and local density statistics. The system can autonomously differentiate between single highly magnified stars, small stellar clumps, and globular clusters in extremely crowded or high-magnification fields. It would specifically utilize the counterimage consistency check—validating candidates by their ability to satisfy a predicted geometric redshift and magnification profile across multiple images—to eliminate false positives in massive survey datasets.

Spatiotemporal Flux Synchronizers for Microlensing and Intrinsic Variability Differentiation:

By utilizing predicted geometric time delays and magnification maps, an AI system can synchronize multi-epoch observations of lensed candidates. This allows the system to distinguish between intrinsic stellar variability (e.g., Cepheid pulsations) and extrinsic microlensing events by analyzing the light curve's symmetry, timescale, and chromaticity relative to the predicted time delay between counterimages.

Stochastic Magnification Profilers for Dark Matter Model Discrimination:

This improvement implements a specialized inference engine designed to detect parsec-scale magnification fluctuations. By analyzing the statistical distribution of flux ratios and magnification discrepancies between counterimages (as seen in the comparison between CDM and Fuzzy Dark Matter), the system can perform high-precision Bayesian model selection to determine if dark matter exhibits wave-like, ultralight particle properties (ψDM) or standard cold dark matter (CDM) behavior.

Abstract

We present a new free-form lens model for the z=0.348 galaxy cluster AS1063, based on previously spectroscopically confirmed lensed galaxies and new JWST images from the GLIMPSE program. We use the ultra-deep JWST data to identify new counterimages for previously confirmed (spectroscopically) lensed systems. We use the full set of spectroscopically confirmed systems to derive a new lens model, which is later used to confirm many of the previous lensed system candidates and discover new lensed system candidates in the JWST images. We compute the geometric redshifts, time delays, and magnification for all counterimages (confirmed and not confirmed). Among the new systems, and based on photometry, we find a peculiar multiply lensed galaxy with a strong emission line at about 4, μ m that likely corresponds to H-β and/or OIII at z about 7.5. This galaxy could be a little-red-dot or an extreme emission line galaxy. We also identify a yellow supergiant lensed star candidate at z about 3.7. This star shows some similarities with previous Kaiju stars and we nickname it "Hedorah", in honor of the famous yellow-eyed Kaiju. Previous lensed stars at z>0.1 are either blue supergiants or red supergiants, making Hedorah the first yellow supergiant discovered beyond z=0.1 and confirming that, despite their rarity, they can also be found at these redshifts. Since many Cepheid stars are yellow supergiants, we consider the possibility that Hedorah could also be the first Cepheid discovered at cosmological distances, but we conclude that Hedorah is more likely a hypergiant yellow star approaching the end of its life. Alternatively, Hedorah could be a small group of stars, although this is less likely based on Hedorah's peculiar colors and additionally may require the more exotic fuzzy dark matter to help explain the lack of counterimage.