Betelgeuse (Alpha Orionis)
Coordinates, competing optical vs radio distances (168 vs 222 pc), radius, Great Dimming physics, and observing guide for Betelgeuse.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Distance (Combined Hipparcos + radio + seismic solution) | 168 pc | +27 / −15 pc | Combined astrometric & asteroseismic hydrodynamic modeling (Joyce et al. 2020) (J2016.0) | disputed | Hipparcos New Reduction Catalog |
| Distance (VLA/ALMA radio continuum + Hipparcos solution) | 222 pc | +48 / −34 pc | Submillimeter/radio astrometry combined with Hipparcos intermediate data (Harper et al. 2017) (J2000.0) | disputed | SIMBAD Astronomical Database (CDS) |
| Visual magnitude range (V) | 0.5 mag | 0 to 1.61 mag | AAVSO & GCVS visual/photoelectric monitoring (reached V=1.61 in Feb 2020 Great Dimming) | definitive | General Catalogue of Variable Stars (GCVS) & AAVSO VSX |
| Fundamental radial pulsation period | 416 d | ± 24 d | Optical photometry & radial-velocity oscillation (plus ~2,100 d long secondary period) | definitive | General Catalogue of Variable Stars (GCVS) & AAVSO VSX |
| Photospheric radius | 764 R☉ | +116 / −62 R☉ | Derived from ~42 mas limb-darkened diameter at 168 pc (scales to ~950–1,000 R☉ at 222 pc) | model-dependent | SIMBAD Astronomical Database (CDS) |
| Effective temperature | 3600 K | ± 200 K | Near-IR & TiO band spectroscopy (dropped to ~3,450 K during 2020 dimming) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Betelgeuse (Alpha Orionis Red Supergiant) at RA 05h 55m 10.31s, Dec +07° 24′ 25.4″ within Orion, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Betelgeuse (Alpha Orionis) is a massive M1–M2 Ia–Iab red supergiant anchoring the northeastern shoulder of Orion. If placed at the center of our Solar System, its bloated convective photosphere (~764 to 1,000 solar radii, depending on the adopted distance) would engulf the orbits of Mercury, Venus, Earth, Mars, and the main asteroid belt out toward Jupiter. Having begun life roughly 8 to 10 million years ago as an ~16-to-19 solar-mass O-type star, Betelgeuse is currently fusing helium into carbon and oxygen in its core and is expected to explode as a Type II-P core-collapse supernova within the next 100,000 years.
In 1920 Albert Michelson and Francis Pease used the 20-foot beam interferometer on the Mount Wilson 100-inch telescope to measure Betelgeuse's angular diameter (~47 mas)—the first star beyond the Sun ever to have its physical size measured. Paradoxically, that huge angular diameter makes Betelgeuse's trigonometric parallax famously difficult to measure: giant convective plumes spanning a quarter of the stellar disk brighten and fade over months, shifting the optical photocenter back and forth by several milliarcseconds and producing competing optical/seismic (168 pc) and radio VLA/ALMA (222 pc) distance solutions.
Between October 2019 and February 2020, Betelgeuse underwent the historic 'Great Dimming,' plunging to magnitude 1.61—its faintest level in modern photometric records. High-resolution VLT/SPHERE imaging and Hubble ultraviolet spectroscopy proved that a coincidental alignment of its 416-day fundamental pulsation and ~2,100-day long secondary cycle launched a massive gas plume from its southern hemisphere; as the outflow cooled above a dark convective cool spot, it condensed into an opaque cloud of silicate dust directly along Earth's line of sight.
Practical Visual Observing Guide
- Easily visible to the naked eye even from downtown city centers as the deep orange-amber star at the top-left corner of Orion.
- You can monitor Betelgeuse's semiregular brightness changes with the unaided eye using the Argelander step method: compare its brightness against Procyon (mag 0.37), Aldebaran (mag 0.86), Pollux (mag 1.14), and Bellatrix (mag 1.64).
- Avoid staring too long when comparing orange Betelgeuse to blue-white Rigel or Bellatrix, as the Purkinje / Kohlrausch effect causes red stars to appear artificially brighter to dark-adapted rod cells if stared at continuously.
Catalog Cross-Identifications & Aliases
Common Aliases: α Orionis · HD 39801 · HIP 27989 · 58 Orionis
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 27989 |
| HD | HD 39801 |
| SIMBAD | * alf Ori |
| WIKIDATA | Q12124 |
Related Objects, Comparisons & Guides
Rigel (Beta Orionis Blue Supergiant)
Coordinates, Hipparcos distance (264 pc), luminosity (120,000 L☉), Rigel BC companion, and observing guide for Rigel in Orion.
Orion Nebula (Messier 42 / NGC 1976)
Coordinates, VLBI/Gaia distance (412 pc), Trapezium ionizing cluster, and visual observing guide for the Orion Nebula (M42) in Orion.
Aldebaran (Alpha Tauri Orange Giant)
Coordinates, Hipparcos distance (66.6 ly), 45.1 R☉ radius, Hyades foreground alignment, and observing guide for Aldebaran in Taurus.
Mira (Omicron Ceti Pulsating Giant)
Coordinates, Hipparcos distance (100 pc), 332-day pulsation cycle (mag 2.0 to 10.1), white dwarf companion, and observing guide for Mira.
Eta Carinae (Luminous Blue Variable Binary)
Coordinates, Gaia/expansion distance (2.35 kpc), 5 million L☉ luminosity, 1843 Great Eruption Homunculus, and observing guide for Eta Carinae.
Betelgeuse vs Rigel Supergiant Comparison
Compare Orion's two flagship supergiant stars—red supergiant Betelgeuse (Alpha Orionis) and blue supergiant Rigel (Beta Orionis)—in temperature, size, and mass.
Tracking Bright Variable Stars Observing Guide
How to estimate visual magnitudes and light curves of naked-eye and binocular variable stars: Algol, Delta Cephei, Betelgeuse, Mira, and Eta Carinae.
Winter Circle, Orion & Taurus Observing Guide
Winter sky observing tour of Orion, Taurus, Canis Major, Auriga, and Perseus: bright OB/M supergiants, open clusters, M42, and the Crab Nebula (M1).
Measurement Uncertainty & Optics in Astronomy
How random vs systematic errors, asymmetric error bars, model dependencies, and optical aperture/exit-pupil limits shape astronomical measurements.
Trigonometric Parallax, Parsecs & Gaia DR3
How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
Scientific Sources & Dataset Provenance
- Hipparcos New Reduction Catalog (2007 New Reduction): van Leeuwen, F. 2007, A&A, 474, 653 (Validation of the new Hipparcos reduction) — ESA / van Leeuwen (2007)
- General Catalogue of Variable Stars (GCVS) & AAVSO VSX (v5.1 (2026 update)): Samus, N. N., et al. 2017, Astronomy Reports, 61, 80 (General Catalogue of Variable Stars: Version GCVS 5.1) — Sternberg Astronomical Institute / AAVSO
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg