Betelgeuse Companion: Siwarha

• Siwarha is the officially recognized companion of Betelgeuse, detected through photometric, spectroscopic, astrometric, and direct imaging methods.
• The companion orbits at approximately 2.3 stellar radii with a near-circular 6-year period, influencing Betelgeuse’s circumstellar environment via plasma interactions.
• This discovery offers a unique laboratory for studying binary interactions in evolved massive stars, enhancing our understanding of stellar evolution and mass loss.

Betelgeuse, the red supergiant α Orionis (HD 39801), is now established to possess a close stellar companion orbiting deep within its extended chromosphere. The companion, detected through an overview of photometric, spectroscopic, astrometric, and direct imaging data, is formally named Siwarha under the auspices of the International Astronomical Union (IAU), although the informal moniker “Betelbuddy” has also appeared in outreach contexts. The system stands as a unique laboratory for binary interaction at extreme mass ratios and radii, in the late stages of massive stellar evolution.

1. Discovery and Multi-modal Detection

Photometric, radial velocity, and astrometric anomalies in Betelgeuse have been chronicled since the 1970s, particularly the “long secondary period” (LSP, $P\approx2000$ days) in its variability (Dupree et al., 1 Jan 2026). Goldberg et al. (2024) and MacLeod et al. (2025) independently modeled these signatures with a binary scenario, corroborated by HIPPARCOS/Gaia astrometry and over a century of archival data. On 9 December 2024, the first direct imaging was achieved via the ‘Alopeke’ speckle imager on Gemini North, revealing an optical companion at angular separation $\theta=52$ mas and position angle $\mathrm{PA}=115^\circ$ east of north (Hasan, 5 Jan 2026). The companion’s brightness is $\sim6$ magnitudes fainter than Betelgeuse at $466$ nm ($\Delta m\approx6$), corresponding to a flux ratio of $\sim 1/250$.

The detection, while initially at $1.5\sigma$ significance, exhibits congruence in position, angular separation, and photometry with dynamic predictions from binary fits (Hasan, 5 Jan 2026). This multi-tracer evidence—LSP photometry, Doppler velocity modulations, phase-locked circumstellar absorption (optical Mn I), UV chromospheric line variations (Fe II, Si I, Mg I, Mg II), and astrometric periodicity—strongly indicates a physical companion in a $\sim6$-year, near-circular orbit at $\sim2.3$ photospheric radii (Dupree et al., 1 Jan 2026).

2. Physical and Orbital Parameters

The physical separation corresponding to the observed $\theta$ is $a\approx11.1$ AU at a distance $D=214$ pc (using $a[\textrm{AU}] = \theta["]\times D[\textrm{pc}]$), situating the companion well within Betelgeuse’s chromosphere. Kepler’s third law,

$P^2 = \frac{4\pi^2 a^3}{G(M_1+M_2)}$

yields a period $P = 2109\pm9$ days ($\sim 5.8$ years) for a system mass $M_1 \approx 15$–$17 M_\odot$ (Betelgeuse), $M_2\approx 0.6$–$1.5 M_\odot$ (companion) (Dupree et al., 1 Jan 2026, Hasan, 5 Jan 2026). The mass and spectrum of the companion suggest a late-B to early-A pre–main-sequence star.

Recent spectroscopic observations quantify Doppler shifts up to $|v|\sim15$ km/s in circumstellar Mn I lines during conjunction phases and document outflows tracking the orbital cycle (Dupree et al., 1 Jan 2026). The predicted orbital velocity is $v_{\rm orb} \approx 43$ km s$^{-1}$ (for $a \approx 2.3\,R_\star$).

3. Spectroscopic and Imaging Signatures

Phase-dependent absorption and emission phenomena in Betelgeuse’s circumstellar environment are synchronized with the companion’s orbit. The following observational features support this:

• Optical Mn I absorption: Equivalent widths and velocities vary sinusoidally with $P=2109$ days, indicating plasma interactions trailing the companion.
• UV chromospheric lines (Mg II, Fe II, Si I, Mg I): Blue/red peak flux ratios track the binary phase, with maxima post-transit.
• Speckle imaging: Confirmation of the companion at $\sim2.3\,R_\star$ (at $466$ and $562$ nm), matching orbital phase predictions.
• Long-term photometric modulations: The LSP aligns with orbital predictions from the binary solution (Dupree et al., 1 Jan 2026, Hasan, 5 Jan 2026).

Following the companion’s transit, observed circumstellar absorption and outflows increase, then gradually subside in advance of the next transit, consistent with an expanding wake structure induced by the embedded orbiting star.

4. Nomenclature: Siwarha Versus Betelbuddy

The formal name, Siwarha (Arabic: سِوَارَة, “her bracelet”), was proposed by Howell et al. (2025) in adherence to the Arabic origin of “Betelgeuse” (yad al-jauzā’, “Hand of Orion”), emphasizing cultural and etymological continuity (Dupree et al., 1 Jan 2026). The IAU’s Working Group on Star Names (WGSN) vetted and adopted “Siwarha” as the unique, proper designation for the companion (also α Ori B) according to their criteria: historical significance, cultural representation, and avoidance of colloquial or commercialisms.

The alternative, “Betelbuddy,” is a modern, informal coinage tailored for outreach, combining “Betelgeuse” with “buddy.” While it possesses immediate public appeal in English-speaking contexts, no peer-reviewed literature or IAU documentation recognizes “Betelbuddy” as an official or alternate name (Dupree et al., 1 Jan 2026, Hasan, 5 Jan 2026). All major catalogues and naming authorities, as well as the referenced research corpus, consistently endorse “Siwarha.”

Name	Origin/Meaning	Status
Siwarha	Arabic, “her bracelet”	Official (IAU WGSN)
Betelbuddy	Modern, English-language pun	Informal/Outreach

5. Implications for Supergiant Stellar Astrophysics

The Betelgeuse–Siwarha system furnishes a compelling laboratory for investigating binary interactions within a supergiant envelope. Plasma diagnostics reveal complex, phase-locked outflows and absorption structures associated with the companion’s orbital wake. This system exemplifies a late-stage massive star with a low-mass interior companion, raising prospects for constraining evolutionary scenarios involving mass transfer, envelope ejection, and future core-collapse pathways.

The detection and characterization protocols—integrating speckle imaging, multi-band photometry, high-resolution spectroscopy, and astrometric time series—demonstrate the power of convergent multi-modal diagnostics for binaries embedded in extended stellar atmospheres. A plausible implication is that other red supergiants with unexplained LSPs might harbor yet-undetected close companions.

6. Future Prospects

Refined direct imaging at the next orbital maximum (November 2027) promises higher signal-to-noise (statistical significance moving beyond $3\sigma$), improved multi-band photometry (0.5–2.2 μm), and full dynamical orbit solutions, potentially measuring Betelgeuse’s mass to $\sim5\%$ precision (Hasan, 5 Jan 2026). Planned campaigns with advanced adaptive optics facilities (e.g., GeMS/GSAOI, VLT SPHERE) will constrain temperature, luminosity, and further elucidate mass-loss dynamics. Long-term monitoring will clarify the stability and evolutionary fate of the system.

7. Summary

Spectroscopic, photometric, astrometric, and direct-imaging evidence robustly establish the presence of a close-in ($\sim2.3\,R_\star$) companion orbiting Betelgeuse with a period of $\sim2100$ days. The IAU’s formal designation for the companion is Siwarha, selected for its etymological and cultural ties to Betelgeuse’s historical nomenclature. “Betelbuddy” remains an informal, outreach-driven nickname without official status. The continued study of the Betelgeuse–Siwarha binary is expected to yield significant advances in understanding post-main-sequence stellar evolution, binary interactions within extended envelopes, and the end-of-life pathways of massive stars (Dupree et al., 1 Jan 2026, Hasan, 5 Jan 2026).