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Hubble as a Unique Discovery Engine of the Fate of Massive Stars and Black Hole Formation

This paper proposes extending Hubble Space Telescope operations into the 2030s with a dedicated UV survey to directly detect disappearing hot massive stars, thereby addressing the critical gap in understanding how stellar-mass black holes form.

Original authors: Avishai Gilkis, Eva Laplace, Charles D. Kilpatrick, Maria R. Drout, Anna J. G. O'Grady, Christopher A. Tout

Published 2026-06-10

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CC BY 4.0

Original authors: Avishai Gilkis, Eva Laplace, Charles D. Kilpatrick, Maria R. Drout, Anna J. G. O'Grady, Christopher A. Tout

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Mystery: Where Do Black Holes Come From?

Imagine the universe as a giant construction site. We know that massive stars are the "bricks" that eventually turn into neutron stars or black holes. But there is a huge gap in our knowledge: We don't know exactly which types of stars turn into black holes, or how they do it.

Sometimes, a star might try to explode (like a supernova) but fail, collapsing silently into a black hole instead. Scientists call this a "failed supernova." The problem is, we haven't seen many of these events yet. It's like trying to figure out how a specific type of car breaks down, but you've only ever watched the ones that crash loudly. You might be missing the ones that just quietly stop working.

The Missing Clue: The "Hot" Stars

For the last 20 years, astronomers have been looking for these disappearing stars, but they've been looking in the wrong place. They've mostly been watching cool, red, giant stars (like looking for a red balloon in a dark room).

However, new computer models suggest that the stars most likely to turn into black holes are actually hot, blue, and glowing with ultraviolet (UV) light. These stars are like "blue flames" in a dark room. If you only look for red balloons, you will miss the blue flames entirely. Furthermore, these hot stars often live in crowded neighborhoods (dense star clusters), making them very hard to spot if your vision isn't sharp enough.

Why Hubble is the Only Detective That Works

The paper argues that to solve this mystery, we need a specific set of tools that only the Hubble Space Telescope (HST) has. Think of Hubble as a detective with a unique toolkit that no other current or planned telescope possesses:

UV Vision: Hubble can see ultraviolet light. Other powerful telescopes like the James Webb Space Telescope (JWST) are amazing at seeing infrared (heat) and optical light, but they are "blind" to the UV light that these hot, disappearing stars emit. It's like trying to find a UV-blacklight poster using only a regular flashlight; you won't see it.
Super-Sharp Eyes: Hubble has incredibly high resolution. It can separate individual stars in crowded galactic neighborhoods. Other telescopes, like the upcoming Rubin Observatory (LSST), have wide views but their "eyes" are a bit blurry when looking at distant galaxies. They would see a blurry blob of stars instead of a single disappearing star.
The Time Machine: Hubble has been taking pictures of the same galaxies for decades. This allows astronomers to compare a photo from 20 years ago with a photo taken today. If a bright blue star is there in the old photo but gone in the new one, Hubble can spot the disappearance. No other telescope has this long, continuous history of high-quality UV photos.

The Plan: A Roadmap for the 2030s

The authors propose a specific plan to keep Hubble working into the 2030s to hunt for these black hole births:

The "Before and After" Strategy: They plan to re-image nearby galaxies in UV light. By comparing these new images with old Hubble archives, they can spot stars that have vanished.
The "Follow-Up" Strategy: When other telescopes (like Rubin) spot a strange, faint event, Hubble can zoom in with its sharp UV eyes to confirm if it's a hot star disappearing.

The Expected Result

Based on their calculations, if they look at the nearby galaxies Hubble can reach, they expect to find about one star disappearing into a black hole every year.

The Bottom Line

The paper concludes that extending Hubble's life into the 2030s is not just about taking pretty pictures; it is the only way to solve the puzzle of how black holes are born. Without Hubble's unique combination of UV vision, sharp focus, and long-term memory, we will likely miss the majority of these events, leaving a critical piece of the universe's story unsolved.

Technical Summary: Hubble as a Unique Discovery Engine for Massive Star Fate and Black Hole Formation

Problem Statement
The formation mechanism of stellar-mass black holes (BHs) remains an open question in astrophysics, characterized by a lack of observational constraints. It is currently unknown which stellar types preferentially produce black holes or whether the formation process is accompanied by strong electromagnetic transients, weak transients, or no emission at all. While gravitational wave detections have confirmed the existence of merging BHs, the specific evolutionary pathways leading to their formation are poorly constrained.

A critical gap exists in the search for "failed supernovae," where massive stars collapse directly into black holes without a successful explosion, appearing observationally as disappearing stars. Previous searches over the last two decades have focused primarily on luminous cool supergiants monitored in optical bands. However, theoretical models suggest that a substantial fraction of BH progenitors are hot, blue, UV-luminous stars (including Wolf–Rayet-like and stripped-envelope phases). These progenitors are intrinsically faint in the red optical bands used by current surveys and often reside in crowded star-forming environments, leading to systematic incompleteness in existing searches.

Methodology and Observational Strategy
The paper proposes a roadmap to extend the Hubble Space Telescope's (HST) role into the 2030s to address this gap. The methodology relies on HST's unique combination of three capabilities that no existing or planned facility possesses simultaneously:

Deep UV Sensitivity: Essential for detecting hot, massive progenitors that emit most strongly in the ultraviolet.
Sub-arcsecond Imaging: Required to resolve individual stars within crowded star-forming regions and separate them from companions or clusters.
Decade-long Temporal Continuity: Necessary to perform precise image subtraction between epochs to identify stars that have vanished or significantly faded.

The proposed strategy involves two complementary observing modes:

Archival Re-imaging: Re-observing nearby galaxies (within $\sim$ 10–20 Mpc) that possess existing HST UV observations. This allows for the direct identification of stars that have disappeared or undergone significant UV fading over baselines of a decade or more.
Targeted Follow-up: Utilizing HST's high-resolution UV imaging to characterize candidate disappearing stars and unusual low-luminosity transients identified by wide-field surveys such as Rubin/LSST, Roman, and JWST. While these future facilities will discover unprecedented numbers of transients, they lack the specific UV sensitivity and resolution required to isolate and confirm hot massive star progenitors.

Key Contributions and Theoretical Basis
The paper outlines a theoretical framework based on detailed stellar and binary evolution calculations combined with physically motivated core-collapse prescriptions.

Progenitor Characteristics: Theoretical models predict that BH-forming endpoints include single stars and binary systems (with a 70% binary fraction assumption) that are hot and UV-luminous. Color–magnitude diagrams indicate these progenitors are distinct from the cool supergiants (RSGs) typically targeted in optical surveys.
Event Rate Estimates: By weighting synthetic populations by the initial mass function and empirical binary distributions, the authors estimate a direct-collapse rate of approximately 0.4 events per century for a galaxy forming stars at $1 M_{\odot} \text{yr}^{-1}$ .
Projected Yield: Considering the combined star-formation rate of the nearby galaxy population accessible to HST, the paper estimates an expected detection rate of order one detectable BH-forming disappearance event per year during the 2030s.

Results and Comparative Analysis
The paper argues that HST is uniquely positioned to detect these events where other facilities cannot:

Rubin/LSST: Offers wide-field transient discovery but suffers from ground-based resolution limits ( $\sim$ 0.7" FWHM), which blend stellar populations at distances of 10–20 Mpc, preventing the isolation of individual massive stars.
Roman and JWST: Provide exceptional infrared sensitivity and resolution but lack access to the UV wavelength range where hot massive stars emit most strongly.
Future UV Missions: Will offer large-area monitoring but lack the sub-arcsecond resolution required to deblend crowded environments.

Comparative imaging simulations (e.g., of NGC 1494) demonstrate that HST's combination of $\sim$ 0.1" resolution and UV/optical depth allows for the detection and deblending of significantly more stars than optical or IR-only facilities.

Significance and Claims
The paper claims that extending HST operations into the 2030s is critical for solving the unsolved problem of black hole formation. By enabling direct searches for disappearing hot massive stars, HST can provide:

Statistical Constraints: The ability to statistically constrain the channels of black hole formation, specifically testing the prevalence of direct collapse versus failed supernovae.
Complementarity: A vital link in the multi-messenger and multi-wavelength ecosystem, complementing the discovery capabilities of Rubin/LSST, Roman, JWST, and future UV missions by providing the necessary UV identification and characterization of candidates.
Unique Capability: The paper asserts that no other facility combines the necessary UV sensitivity, high angular resolution, and long-term temporal baseline, making HST the sole instrument capable of executing this specific discovery program in the near future.