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Confirming Nunki as the closest core collapse progenitor candidate to the Sun

By combining VLTI/GRAVITY, VLTI/PIONIER, and MAPPIT observations to solve Nunki's orbit and characterize its spin-orbit misalignment, this study confirms it as the closest core collapse progenitor candidate to the Sun at approximately 69 pc, surpassing Spica and Bellatrix, while simultaneously ruling out a close equal-mass companion for Bellatrix.

Original authors: Idel Waisberg, Boaz Katz

Published 2026-03-19

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CC BY 4.0

Original authors: Idel Waisberg, Boaz Katz

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

Imagine the night sky as a vast, crowded neighborhood. For a long time, astronomers have been playing a game of "Who lives closest to us (the Sun) and is most likely to throw a massive, explosive party (a supernova) in the future?"

For years, the title of "Closest Potential Exploder" was held by Spica and Bellatrix, two bright stars about 77 light-years away. But a new study suggests we've been looking at the wrong neighbor. The real contender is Nunki (Sigma Sagittarii), a star system located just 69 light-years away.

Here is the story of how the authors, Waisberg and Katz, solved the mystery of Nunki, using a mix of high-tech detective work and cosmic logic.

1. The Mystery: Is Nunki a Single Star or a Double?

Nunki looks like a single, bright star to the naked eye. However, in 2025, a super-precise telescope called VLTI/GRAVITY (which acts like a giant, space-filling eye) peeked closely and saw a secret: Nunki isn't one star; it's a binary system. It's two stars dancing around each other, so close they are only about 0.6 astronomical units apart (closer than Venus is to our Sun).

Both stars are roughly the same size and weight (about 6.5 times the mass of our Sun). This is crucial because stars need to be heavy (over 8 solar masses) to eventually explode. Since these two are roughly equal, they might be hiding their true combined weight.

2. The Detective Work: Solving the Dance

To prove Nunki is the closest candidate, the team needed to know the exact rules of the dance between these two stars. They gathered data from:

Three old observations from 2017 (like finding old diary entries).
One new observation from 2024.
One ancient observation from 1991 (the "grandfather" of the data).

By plotting these points, they solved the orbit. They found the stars are on a very eccentric (oval-shaped) track. They swing close together, then swing far apart, like a rollercoaster that dips dangerously low before shooting up high.

The "Spin-Orbit" Twist:
Here is a weird detail: The stars are spinning incredibly fast (like a figure skater pulling in their arms), but the orbit they are dancing in is tilted at a weird angle. It's as if the two stars are spinning on their own axes, but the whole system is tilted sideways. This suggests they aren't perfectly aligned, which is a clue that they might have formed or interacted in a chaotic way.

3. The "Invisible" Twins

You might ask, "If they are two stars, why didn't we see them before?"
The authors explain this with a great analogy: Imagine two people running on a track. If they run at slightly different speeds, you can tell them apart. But if they are running so fast that their "blur" covers the whole track, and they are running almost side-by-side, you can't tell there are two of them.

Because these stars spin so fast, their light is smeared out (blurred). When the two stars pass each other, their speeds relative to us are so similar that their light waves merge into one. They are spectroscopically invisible as a pair. They are hiding in plain sight, disguised as a single star.

4. The Future: The Great Merger

This is where the story gets dramatic. Because the orbit is so oval-shaped, the stars get very close at their closest point.

The Scenario: In about 25 million years, the larger star will swell up like a balloon (becoming a Red Giant).
The Collision: Because the orbit is so stretched, the swollen star will reach out and touch its partner before the orbit can even out.
The Result: This triggers a chaotic event called Roche Lobe Overflow. The stars will likely crash into each other and merge into a single, massive monster star weighing over 10 times our Sun.

Why does this matter?
A single 6.5-solar-mass star isn't heavy enough to explode as a supernova. But if they merge, the new "Frankenstein" star will be heavy enough (over 10 solar masses) to collapse and explode.

5. Why Bellatrix Lost the Title

The paper also checked on Bellatrix, the other famous candidate. Some people thought Bellatrix might be a double star too. The authors used the same high-tech telescope to look closely and said, "Nope, it's a single star." Since Bellatrix is also slightly farther away than Nunki, it loses the title of "Closest."

The Final Verdict

Nunki is now the official champion of the "Closest Star Likely to Explode."

Distance: ~69 light-years (closer than Spica or Bellatrix).
The Plan: Two stars will merge into a giant.
The Payoff: That giant will eventually run out of fuel, collapse, and go BOOM in a supernova.

It's a cosmic drama playing out in our own backyard, waiting for the final act in about 25 million years. While that sounds like a long time, in the life of a star, it's just around the corner!

1. Problem Statement

The primary objective of this study is to identify the closest star to the Sun that is a candidate for a future core-collapse supernova. Historically, Spica ( $\alpha$ Virginis, $d \approx 77$ pc) and Bellatrix ( $\gamma$ Orionis, $d \approx 77$ pc) were considered the leading candidates due to their masses exceeding the $\sim 8 M_\odot$ threshold required for iron-core formation.

However, a recent interferometric detection suggested that Nunki ( $\sigma$ Sagittarii) is a close binary system at a distance of $d \approx 69$ pc, potentially making it the closest candidate. To confirm this status, the authors needed to:

Precisely determine the orbital parameters of Nunki to validate its nature as a close binary.
Assess the dynamical mass and future evolutionary path (specifically, whether a merger will occur to create a massive enough star for core collapse).
Rule out the possibility that Bellatrix is a close equal-mass binary, which would alter its mass estimate and evolutionary fate.

2. Methodology

The authors employed a multi-faceted approach combining interferometry, archival spectroscopy, and orbital dynamics modeling:

Interferometric Data Combination:
- VLTI/GRAVITY: Utilized a 2024 observation detecting a companion at a projected separation of 0.60 au with a K-band flux ratio of 89%.
- VLTI/PIONIER: Analyzed three archival epochs from 2017 (H-band) where Nunki was observed as a calibrator for $\lambda$ Sgr.
- MAPPIT: Included a 1991 archival measurement to extend the temporal baseline.
- Orbit Fitting: The team constructed a 3D grid search over period ( $P$ ), eccentricity ( $e$ ), and time of periastron ( $T_p$ ). They used Thiele-Innes elements to map orbital parameters ( $a, i, \omega, \Omega$ ) via linear regression, followed by non-linear least-squares fitting to find the global $\chi^2$ minimum.
Spectroscopic Analysis:
- Analyzed two high-resolution archival spectra (FEROS and HERMES) to search for double-lined spectroscopic binary (SB2) signatures.
- Modeled the spectra using the BSTAR2006 grid to determine effective temperature ( $T_{\text{eff}}$ ) and projected rotational velocity ( $v \sin i$ ).
Evolutionary Modeling:
- Used MESA Isochrones and Stellar Tracks (MIST) to model the future evolution of the primary and secondary stars.
- Calculated the conditions for Eccentric Roche Lobe Overflow (RLOF) and evaluated the energetics of a potential Common Envelope (CE) event versus a merger.
Bellatrix Verification:
- Conducted a new VLTI/GRAVITY observation of Bellatrix to search for close companions.

3. Key Results

A. Orbital Parameters of Nunki

The combined interferometric data yielded a robust orbital solution:

Orbital Period: $P = 134.779 \pm 0.025$ days.
Semi-major Axis: $a = 1.26 \pm 0.05$ au ( $18.0 \pm 0.2$ mas).
Eccentricity: $e = 0.492 \pm 0.003$ (highly eccentric).
Inclination: $i = 19.7 \pm 1.9^\circ$ (nearly face-on).
Periastron Distance: $r_p = 0.64 \pm 0.03$ au ( $\approx 138 R_\odot$ ).
Dynamical Mass: $M_{\text{dyn}} = 14.6 \pm 1.8 M_\odot$ . This is consistent with the sum of isochrone masses ( $M_{\text{Aa}} \approx 6.5 M_\odot$ , $M_{\text{Ab}} \approx 6.3 M_\odot$ ).

B. Spectroscopic "Invisibility"

Despite being an equal-mass binary, Nunki appears as a single star in spectroscopy.

The projected rotational velocity is high ( $v \sin i \approx 160$ km s $^{-1}$ ), resulting in broad spectral lines (FWHM $\approx 230$ km s $^{-1}$ ).
The maximum radial velocity separation between components is only $\sim 40$ km s $^{-1}$ .
Conclusion: The RV separation is significantly smaller than the line widths, rendering the binary spectroscopically undetectable. This explains why previous studies missed the companion.

C. Spin-Orbit Misalignment

The system exhibits a strong hint of spin-orbit misalignment:

Low orbital inclination ( $i \approx 20^\circ$ ) combined with high $v \sin i$ implies a true rotational velocity of $v \approx 490$ km s $^{-1}$ (90% of critical velocity) if aligned.
However, there is no evidence of a decretion disk (Be star signature).
The authors conclude the stellar spin axes are significantly misaligned with the orbital axis.

D. Bellatrix Verification

VLTI/GRAVITY observations of Bellatrix revealed no close companion with a K-band flux ratio $>1\%$ . This rules out the hypothesis that Bellatrix is a close equal-mass binary, confirming its status as a single star with a mass just above the core-collapse threshold.

E. Future Evolution and Merger Scenario

The high eccentricity ( $e \approx 0.5$ ) dictates the system's fate:

Eccentric RLOF: As the primary expands to $\sim 50 R_\odot$ (in $\sim 25$ Myr), it will fill its Roche lobe at periastron.
Dynamical Instability: The circularization timescale is extremely long ( $\sim 370$ Gyr), so mass transfer will begin while the orbit is still highly eccentric.
Merger Outcome: Modeling suggests the system will undergo "catastrophic" mass transfer, likely leading to a Common Envelope (CE) event.
- The binding energy of the primary's envelope ( $|E_{\text{bind}}| \approx 9.0 \times 10^{48}$ erg) exceeds the available orbital energy ( $|\Delta E_{\text{orb}}| \approx 2.4 \times 10^{48}$ erg) required to eject the envelope.
- Result: The envelope cannot be ejected; the stars will merge into a single massive star ( $M \gtrsim 10 M_\odot$ ).
Core Collapse: The merger product will be massive enough to develop an iron core and undergo core collapse.

4. Significance and Conclusion

Closest Progenitor: At a distance of $d \approx 69$ pc, Nunki is confirmed as the closest known core-collapse progenitor candidate to the Sun, surpassing Spica and Bellatrix ( $d \approx 77$ pc).
Merger-Driven Supernova: Uniquely, Nunki's path to core collapse is predicted to be via a stellar merger rather than single-star evolution. This highlights the importance of binary interactions in creating supernova progenitors.
Methodological Insight: The paper demonstrates that equal-mass binaries with rapidly rotating components can be "hidden" in spectroscopic surveys, necessitating interferometric observations for detection.
Bellatrix Status: The study definitively rules out Bellatrix as a close binary, refining our understanding of local massive star populations.

In summary, Waisberg & Katz provide the first robust orbital solution for Nunki, confirming it as a close, eccentric binary that will likely merge to form a massive star capable of a core-collapse supernova, making it the nearest such candidate to Earth.