Betelgeuse is so bright its companion is almost impossible to find. (ESO/Digitized Sky Survey 2/Davide De Martin)
Why did astronomers suspect a companion in the first place?
Because Betelgeuse’s brightness and radial velocity show a slow, repeating rhythm—about **2,170 days (~6 years)**—that lines up with a binary orbit. That ephemeris placed the companion at maximum apparent separation (quadrature) in early December 2024, the ideal window to try to see it directly and to test what it is.
- Orbital period: ~2,170 days
- Separation:
1,850 R☉ (9 au) - Betelgeuse’s size: ~764 R☉
- Distance: ~168 pc
- Star’s mass: ~18 M☉ (±1) All from the new analysis and prior constraints synthesized in the ApJ paper.
What did Chandra actually do—and find?
A Director’s Discretionary Time campaign on Chandra/HRC-I observed Betelgeuse over seven epochs between Dec 9–15, 2024, totaling 41.85 ks—the deepest X-ray look yet. The team detected no X-ray point source at Betelgeuse’s corrected position. That “no” is powerful because hard X-rays cut through gas and dust better than optical light.
Here are the essentials:
| Parameter | Value | Notes |
|---|---|---|
| Total exposure | 41.85 ks | Seven epochs (Dec 9–15, 2024) |
| Count-rate 3σ limit | < 3.7 × 10−4 s−1 (0.1–10 keV) | HRC-I wide band |
| Flux & LX limits (0.5–8 keV) | Model + NH-dependent | See table below |
| Conclusion | No X-ray source detected | Rules out accreting compact object |
Key point: turning counts into intrinsic luminosity needs an absorption estimate (NH). Betelgeuse’s clumpy wind dominates that, not the interstellar medium. The team bracketed plausible NH from ~6×10²¹ to 3×10²³ cm⁻² and translated the limit for two standard spectra (thermal plasma ~10 MK, or a power law Γ≈2).
| Assumption | NH (cm−2) | Spectral model | LX,lim (erg s−1) |
|---|---|---|---|
| Lower absorption case | 6.2 × 1021 | Thermal (~10 MK) | < 4.4 × 1028 |
| Wind-based estimate | 5.7 × 1022 | Power law (Γ≈2) | < 4.5 × 1029 |
| Very high absorption | 3 × 1023 | Thermal (~10 MK) | < 5.6 × 1031 |
These are the headline numbers we’ll use to test each hypothesis.
Could the companion be a white dwarf?
If the companion were a white dwarf (WD), Betelgeuse’s wind would feed it. Accreting WDs in symbiotic systems are X-ray bright, typically 10³⁰–10³⁴ erg s⁻¹—often with hard spectra. That blows past Chandra’s limits here by large margins under most reasonable NH. The paper further argues it’s very hard to form a supergiant–WD system with these orbital properties in population synthesis. Verdict: strongly disfavored.
What about a neutron star?
A neutron star (NS) would be even brighter in X-rays when wind-fed. Using standard Bondi–Hoyle accretion, the expected luminosity is immense.
Wind accretion and X-ray power (as used in the paper):
Plugging in the system’s numbers (a ≈ 1,850 R☉, v_rel ≈ 45 km s⁻¹, v_w ≈ 15 km s⁻¹, ṁ_w ≈ 10⁻⁶ M☉ yr⁻¹, M_NS ≈ 1.4 M☉, η≈0.1), the paper obtains LX ≈ 6 × 10³⁷ erg s⁻¹—millions of times brighter than Chandra’s limits. Even extreme “propeller” states or other throttles can’t plausibly dim this by the seven orders of magnitude required. Verdict: ruled out.
Is a young, Sun-like star the best fit?
Yes. A young stellar object (YSO) of roughly solar mass at ~10 Myr can produce LX ≈ 10²⁹–10³¹.⁵ erg s⁻¹, with big scatter from magnetic activity and rotation. That sits inside the upper limits once you allow for Betelgeuse’s wind absorption. The non-detection, therefore, is consistent with a low-mass, still-settling star—likely F-type, possibly still approaching the main sequence. This is also how the discovery press summary framed it, using the nickname Siwarha for the companion.
Quick scoreboard: who survives the X-ray test?
| Hypothesis | Typical LX (erg s−1) | Chandra limits | Verdict |
|---|---|---|---|
| Accreting White Dwarf | 1030–1034 | <1029–31 (depends on NH) | Disfavored by orders of magnitude |
| Accreting Neutron Star | 1032–1036 (theory ~1037) | <1029–31 | Ruled out (theory & data) |
| Young solar-mass star (YSO) | 1028–1031.5 | <1029–31 | Consistent |
Numbers compiled from the Chandra analysis and standard ranges summarized therein.
But isn’t the mass ratio… weird?
It is, and that’s the fun part. Binary formation prefers more equal masses at close separations. Here we’re seeing an extreme mass-ratio system: a hulking red supergiant with a small, likely Sun-like partner at ~9 au. That’s a rarely explored corner of parameter space because such systems are hard to predict and even harder to find. This object opens that door.
What about the Great Dimming—was that the companion?
No. The late-2019/early-2020 Great Dimming came from a dust cloud briefly blocking part of Betelgeuse’s disk, not from binary shenanigans. Still, the dust and wind matter for X-rays; they set the absorption that the team carefully modeled when converting count limits to LX.
What’s next—can we ever “see” the companion directly?
We partly did already at favorable geometry around December 2024, when several facilities targeted the system. The X-ray non-detection does the heavy lifting for identity, while HST-UV spectroscopy and future interferometry will refine the companion’s nature and the wind environment. The orbit cycles on roughly a 6-year clock; the next approaching quadrature happens in late 2027, another window to test models.
Takeaway you can share
- Chandra’s deepest look found no X-ray source at Betelgeuse’s position in Dec 2024.
- Accreting compact objects (WD/NS) would be blazingly X-ray bright here; they’re out.
- A young, low-mass star—likely F-type, “baby Sun” territory—fits the data and the system’s age.
- This system probes extreme mass ratios in binaries at ~au separations.
- The story isn’t finished; 2027 will be another key checkpoint.
Conclusion: what does this change?
If Betelgeuse’s partner is a young, Sun-like star, then we’re witnessing a giant in its twilight sharing space with a teenager just settling in. That juxtaposition sharpens our models of binary birth, wind interactions, and how much a supergiant’s envelope can hide. It also reminds us that non-detections can be thunderously informative—today’s silence speaks volumes.
This post was written for you by FreeAstroScience.com, where we explain complex science simply to spark curiosity—because the sleep of reason breeds monsters.
Sources
- O’Grady et al. (2025), ApJ 992:107 — Chandra non-detection, luminosity limits, accretion arguments, and conclusion favoring a YSO.
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