Bright & Nearby Stars · Constellation Taurus

Alcyone (Eta Tauri / 25 Tauri)

Coordinates, Gaia/VLBI distance (440 ly), magnitude 2.87, B7 IIIe gaseous decretion disk, quad companions, and Pleiades guide for Alcyone.

Right Ascension (J2000.0)03h 47m 29.08s
Declination (J2000.0)+24° 06′ 18.5″
Primary ClassificationB-type giant with equatorial Be emission disk (brightest star in the Pleiades)
Best Observing SeasonAutumn to Winter (October–March)

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.

Astrometric, Photometric, and Physical Parameters for Alcyone (Eta Tauri / 25 Tauri)
ParameterValueUncertaintyMethod & EpochQuality StatusSource Archive
Apparent visual magnitude (V)2.87 mag± 0.01 magJohnson UBV photoelectric photometrydefinitiveHipparcos New Reduction Catalog
Pleiades cluster distance (VLBI + Gaia DR3)136.2 pc± 1.2 pcVLBI radio parallax (Melis et al. 2014) + Gaia DR3 Pleiades cluster mean (444.2 ± 3.9 ly) (J2016.0)definitiveESA Gaia Archive — Data Release 3
Equatorial photospheric radius9.5 R☉± 0.6 R☉CHARA/NPOI optical interferometry (gaseous Hα Be disk extends to ~38 R☉)high-confidenceSIMBAD Astronomical Database (CDS)
Bolometric luminosity2400 L☉± 200 L☉UV/optical spectrophotometry at 136.2 pchigh-confidenceSIMBAD Astronomical Database (CDS)
Primary stellar mass5.9 M☉± 0.4 M☉~115 Myr Pleiades turnoff isochrone & interferometric modelinghigh-confidenceSIMBAD Astronomical Database (CDS)
Measurement Uncertainties & Competing Solutions: For 15 years the 'Pleiades distance controversy' pitted Hipparcos (120.2 ± 1.9 pc) against main-sequence fitting (~135 pc); VLBI radio interferometry and Gaia DR3 decisively resolved the dispute at 136.2 ± 1.2 pc (444.2 ly).

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Alcyone (Eta Tauri Pleiades Brightest Star) at RA 03h 47m 29.08s, Dec +24° 06′ 18.5″ within Taurus, 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.

J2000 Equatorial Sky Position (Taurus) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 74° 40°N Lat 36° 30°S Lat RA: 03h 47m 29.08s Dec: +24° 06′ 18.5″ Epoch: J2000.0 · V = 2.87 mag First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Astrophysical Context & Structure

Alcyone (Eta Tauri, 25 Tauri) is the brightest star in the Pleiades open cluster (Messier 45, the Seven Sisters), shining at apparent visual magnitude 2.87 near the heart of the cluster 136.2 ± 1.2 parsecs (444.2 light-years) away in Taurus. Weighing ~5.9 solar masses at an age of ~115 million years, Alcyone sits right at the main-sequence turnoff of the Pleiades: having nearly exhausted its core hydrogen, it has expanded to 9.5 solar radii and radiates 2,400 times the luminosity of the Sun at 12,300 K.

Spinning with an equatorial velocity of ~215 km/s, Alcyone is a classic 'Be' (B-emission) star: centrifugal force and non-radial pulsations fling ionized gas off its equator into a thin, glowing Keplerian decretion disk spanning roughly four stellar radii (~38 solar radii across), which imprints double-peaked hydrogen-alpha emission lines onto its spectrum.

In binoculars and small telescopes, Alcyone is surrounded by a charming quartet of co-moving cluster companions forming a miniature trapezoid ('Alcyone's Triangle'): 24 Tauri (Alcyone B, magnitude 6.28, A0 V) at 117 arcseconds, V647 Tauri (Alcyone C, magnitude 8.25, Delta Scuti variable) at 181 arcseconds, and Alcyone D (magnitude 8.70, F3 V, itself a tight double) at 191 arcseconds.

Practical Visual Observing Guide

Minimum InstrumentNaked eye (binoculars / 50 mm telescope for the Alcyone ABCD quartet)
Recommended Magnification25×–60×
Best Evening SeasonAutumn to Winter (October–March)
  • Anchors the bowl-handle transition of the 'miniature dipper' pattern inside the Pleiades (M45).
  • In any 50–80 mm telescope at 30×–60× (or mounted 15×70 binoculars), look immediately northwest of brilliant blue-white Alcyone to see the three 6th-to-9th magnitude companions (Alcyone B, C, and D) lined up in a neat arc 2 to 3 arcminutes away.

Catalog Cross-Identifications & Aliases

Common Aliases: η Tauri · HD 23630 · HIP 17702 · 25 Tauri

Cross-Catalog Identifiers for Alcyone (Eta Tauri / 25 Tauri)
Catalog AuthorityIdentifier
GAIADR365207800438662656
HIPHIP 17702
HDHD 23630
SIMBAD* eta Tau
WIKIDATAQ13043
Related Object · Taurus

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.

Related Object · Auriga

Capella (Alpha Aurigae Giant Binary)

Coordinates, Hipparcos distance (42.9 ly), twin G-giant 104-day interferometric binary orbit, and observing guide for Capella in Auriga.

Reference Guide

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.

Reference Guide

Stellar Spectral Classification (OBAFGKM)

Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.

Scientific Sources & Dataset Provenance

  • ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
  • 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)
  • 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