This image shows a combined JWST and Hubble view of spiral galaxy NGC 1637, with the region of interest in the top right. The remaining three panels show a detailed view of a red supergiant star before and after it exploded. The star is not visible in the Hubble image before the explosion, but appears in the JWST image. The July 2025 view from Hubble shows the glowing aftermath of the explosion. Image Credit: NASA, ESA, CSA, STScI, Charles Kilpatrick (Northwestern), Aswin Suresh (Northwestern)
Can JWST Finally Solve the Red Supergiant Problem? Welcome, dear readers, to FreeAstroScience. Today we take on a surprisingly intimate cosmic mystery: why do the most massive red supergiants that should explode as supernovae seem to vanish from our pre-explosion images? In this article—written by FreeAstroScience only for you—we unpack a brand-new case where the James Webb Space Telescope (JWST) and Hubble teamed up to catch a doomed star in the act. Stick with us; by the end, you’ll see how a veil of carbon-rich dust can rewrite what we thought we knew about dying giants and the monsters they leave behind.
What did JWST and Hubble actually see?
In NGC 1637, ~12.0 Mpc away, a Type II supernova dubbed SN 2025pht was discovered on June 29, 2025 (ASAS-SN). Researchers then aligned post-explosion Hubble images with pre-explosion JWST and Hubble imaging to identify a credible progenitor candidate detected from 0.8 to 8 µm (HST/WFPC2 F814W through JWST/MIRI F770W). It’s the first JWST detection of a supernova progenitor star, and the longest-wavelength pre-explosion detection of any progenitor to date . Keck/LRIS spectroscopy on July 27, 2025 confirmed a Type II-P–like spectrum during the plateau phase, with Hα velocity ~6,800 km s⁻¹ and negligible Na I D host extinction along the line of sight .
The team modeled the pre-explosion spectral energy distribution (SED) and found a moderately luminous red supergiant (RSG) with
- log(L/L☉) ≈ 5.0, and
- heavy circumstellar extinction best explained by graphite-rich dust, yielding A_V ≈ 5.3 mag (preferred over silicate models, which overpredict the 7.7 µm flux) . The result: a bright star, largely hidden, now revealed in infrared light.
“It’s the first time JWST has identified a supernova progenitor star,” a milestone emphasized in coverage of the study, which also highlights the surprising carbon-rich nature of the dust around this RSG .
Why is carbon-rich dust such a big deal?
Most high-mass RSGs are expected to form oxygen-rich, silicate dust. Here, however, the data favor graphite (carbon-rich) dust—a rarer state in this luminosity range. The best-fit models consistently favor a graphitic shell with τ_V ~ 6.7 (and A_V ~ 5.3 mag), while silicate models require very high optical depths and still overshoot the mid-IR flux constraint at 7.7 µm .
This suggests the star’s envelope chemistry near collapse may have been carbon-enhanced, potentially by deep convective dredge-up or a “superwind” phase prior to explosion—ideas the authors explore as routes to producing the carbon-rich circumstellar medium (CSM) we now infer . Popular reporting on the result underscores the surprise: JWST’s bands overlap a silicate feature, yet the data point to carbon instead, implying a wind “very rich in carbon and less rich in oxygen” for a star of this mass .
Does this help solve the “red supergiant problem”?
Short answer: it strongly nudges the needle. For years, surveys struggled to find the most luminous RSGs (log L/L☉ ≳ 5.2) in pre-explosion images, raising fears that the most massive RSGs quietly “fail” and collapse into black holes without bright SNe. But a simpler culprit loomed: dust.
SN 2025pht adds weight to that dust scenario:
- The progenitor’s luminosity sits at log(L/L☉) ≈ 5.0 (≈ 15 M☉ at the host’s metallicity), but its optical output was heavily dimmed by A_V ~ 5.3 of local extinction—enough to misclassify or miss such stars in the optical alone .
- Because the SED is sampled from 0.8–8 µm, the bolometric correction is better constrained than for typical, single-band progenitor detections, reducing biases that plagued earlier samples .
Aha moment: What looked like “missing massive RSGs” may be, at least in part, “RSGs hidden under thick, carbon-rich blankets.” That’s not the whole story, but it’s a powerful, testable piece of it—and JWST is the right machine to test it further .
How did the team get these numbers?
Let’s unpack the essential modeling numbers, and place them in context.
What parameters describe the star and its dust?
Below is a compact, web-friendly table summarizing the preferred model for SN 2025pht (graphitic dust shell). Where relevant, I include simple formulas right after for intuition.
| Quantity | Symbol | Value | Notes |
|---|---|---|---|
| Bolometric luminosity | log(L/L☉) | 5.00+0.09−0.08 | ≈ 105 L☉ |
| Effective temperature (star) | Teff | 3030+210−270 K | Cool RSG photosphere |
| Dust shell temperature | Tdust | 790+40−30 K | Reprocessing IR emission |
| Visual optical depth | τV | 6.7 ± 0.4 | Graphitic dust preferred |
| Visual extinction | AV | ≈ 5.3 mag | From best-fit graphitic model |
| Distance to host galaxy | D | 12.03 ± 0.39 Mpc | Cepheid distance (NGC 1637) |
Sources: Model summary and discussion of dust composition and extinction (graphite favored; silicate disfavored at 7.7 µm) ; tabled parameters and modeling context ; observing setup and distance/reddening choices .
Quick math refreshers (with inline HTML formulas)
Luminosity scaling:
For log(L/L☉) = 5.0,Distance modulus:
With D = 12.03 Mpc,
(Note: the paper directly quotes A_V from the model; we use that reported value.)
How sure are we that this was the progenitor?
The alignment between the post-explosion HST frame and the pre-explosion JWST frame used 36 common sources, yielding an rms of 0.03″. The candidate’s centroid offset is only 0.017″ (~0.6 σ), squarely consistent with the SN location. The team also estimated a small chance-alignment probability and documented the source’s long-baseline variability (present in 2001; absent by 2024 in F814W), supporting association with the progenitor system .
What about the spectrum and the dust along our line of sight?
Keck/LRIS spectra on July 27, 2025 resemble SN 2004et near ~50 days into the plateau, and—crucially—host-galaxy Na I D is not detected, implying E(B–V) < 0.02 mag from the host. That’s why the analysis attributes the extinction to local circumstellar dust, not the interstellar medium of NGC 1637 .
So… what changes for astronomy now?
Three big shifts:
Infrared is king for progenitors. Without JWST’s reach into the mid-IR, we would have missed the heavy, carbon-rich veil hiding an otherwise luminous RSG. That changes progenitor demographics and bolometric corrections across the board .
Dust composition matters. It’s not just “how much dust,” but what kind. Graphite vs. silicate leads to different SED signatures; JWST/MIRI at ~8 µm clinched the case here by contradicting silicate-rich fits .
The “red supergiant problem” may be partly a dust problem. Optical surveys under-counted the brightest RSG progenitors because A_V ≳ 5 mag makes them look inconspicuously faint or even absent in the optical. JWST now reveals them—and their carbon-rich enshrouding—to be very much there .
As coverage of the study notes, this result inaugurates JWST-era progenitor work; with Roman’s wide-field power coming online, we could assemble large, time-domain progenitor samples that track dust, chemistry, and variability—before the bang .
What should we watch next?
- Time-variable SEDs: Restricting fits to tight time windows can expose changes in dust geometry or temperature. The authors outline next-generation modeling with mixed dust compositions, ranged shell geometries, and time-sliced fits—a roadmap for the field .
- Broader samples: Mid-IR–bright RSGs in nearby galaxies are ripe targets. JWST + Roman could map which RSGs are “superwind” or post-outburst candidates—those most at risk of imminent collapse .
Conclusion: Are we witnessing the end of “missing” massive stars?
We might be. In SN 2025pht, JWST saw through a thick, carbon-rich cloak and found a luminous red supergiant moments (cosmically speaking) before it blew. The evidence points to heavy local extinction and a graphitic dust shell that would have sabotaged optical searches. That clarity reframes older puzzles and opens a sharper, infrared-lit path forward: find the dust, find the stars, and watch them die.
Come back to FreeAstroScience.com as we keep tracking these revelations across the sky. This post was written for you by FreeAstroScience.com, which specializes in explaining complex science simply—to inspire curiosity and to remember that the sleep of reason breeds monsters.
Key Sources: Kilpatrick et al. 2025, ApJL, SN 2025pht progenitor analysis and dust modeling (graphite favored; A_V ~ 5.3; log L/L☉ ~ 5.0; NGC 1637 at 12.03 Mpc; first JWST progenitor detection) .
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