Variable Stars · Constellation Perseus

Algol (Beta Persei)

Coordinates, Hipparcos distance (90 ly), 2.8673-day eclipse cycle, Algol Paradox mass transfer, and visual timing guide for Algol.

Right Ascension (J2000.0)03h 08m 10.13s
Declination (J2000.0)+40° 57′ 20.3″
Primary ClassificationSemidetached eclipsing binary prototype in a hierarchical triple system
Best Observing SeasonAutumn to Winter (September–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 Algol (Beta Persei)
ParameterValueUncertaintyMethod & EpochQuality StatusSource Archive
Visual magnitude at maximum (out of eclipse)2.12 mag2.12 to 3.39 magPhotoelectric V-band photometry (primary eclipse drops to V = 3.39)definitiveGeneral Catalogue of Variable Stars (GCVS) & AAVSO VSX
Eclipsing orbital period (Aa1–Aa2)2.867328 d± 0.000005 dO−C eclipse minimum timing (2d 20h 48m 56s; undergoes small secular shifts from mass transfer)definitiveGeneral Catalogue of Variable Stars (GCVS) & AAVSO VSX
Heliocentric distance27.6 pc± 0.6 pcHipparcos + VLBI + CHARA interferometric 3D orbit (89.9 ± 2.0 ly) (J2000.0)definitiveHipparcos New Reduction Catalog
Primary B8V mass (Algol Aa1)3.17 M☉± 0.21 M☉CHARA interferometric + spectroscopic orbit (K0 IV secondary Aa2 = 0.70 ± 0.08 M☉)definitiveSIMBAD Astronomical Database (CDS)
Measurement Uncertainties & Competing Solutions: Although the orbital period (2.867328 d) is measured to fractions of a second at any epoch, ongoing Roche-lobe mass transfer and magnetic Applegate cycles cause the eclipse period to wander by several seconds over decades.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Algol (Beta Persei Eclipsing Binary) at RA 03h 08m 10.13s, Dec +40° 57′ 20.3″ within Perseus, 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 (Perseus) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 89° 40°N Lat 19° 30°S Lat RA: 03h 08m 10.13s Dec: +40° 57′ 20.3″ Epoch: J2000.0 · V = 2.12 mag First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Astrophysical Context & Structure

Algol (Beta Persei, from the Arabic Ra's al-Ghūl, the 'Demon's Head', representing the head of Medusa in Perseus) is the prototype of all Algol-type eclipsing binary stars (EA). Recorded as variable by Geminiano Montanari in 1667, its strict 2-day 20-hour 49-minute periodicity was measured in 1782 by 18-year-old deaf-mute English astronomer John Goodricke, who brilliantly deduced that an unseen darker body must be passing in front of the primary star every revolution.

Located 27.6 ± 0.6 parsecs (90 light-years) from Earth, the eclipsing pair consists of a hot, bright B8 V main-sequence star (Algol Aa1, 3.17 solar masses, 2.73 solar radii) and a larger, cooler, distorted K0 IV orange subgiant (Algol Aa2, 0.70 solar masses, 3.48 solar radii) orbiting only 0.062 AU apart. Every 2.8673 days, the dim K0 subgiant passes partially across the brilliant B8 primary for nearly 10 hours, causing Algol's naked-eye brightness to drop by 1.27 magnitudes (a 70% drop in light, from magnitude 2.12 down to 3.39).

For decades this configuration posed the famous 'Algol Paradox': how could the less massive 0.70-solar-mass K0 star already be an evolved subgiant while the 3.17-solar-mass B8 star is still on the main sequence? In 1955 John Crawford solved the puzzle: the K0 star was originally the more massive component, evolved first, filled its gravitational Roche lobe, and dumped most of its hydrogen envelope onto its companion—reversing their mass ratio.

Practical Visual Observing Guide

Minimum InstrumentNaked eye
Recommended MagnificationNaked eye (use Mirfak, Epsilon Persei, and Gorgonea Tertia as comparison stars)
Best Evening SeasonAutumn to Winter (September–March)
  • No telescope is needed to watch an eclipse of Algol: for 2 days and 11 hours it shines steadily at magnitude 2.12 (nearly as bright as Mirfak, α Persei, at 1.79), then fades over 5 hours to magnitude 3.39 (matching Rho Persei at 3.39 and Epsilon Persei at 2.89) before recovering over the next 5 hours.
  • The CHARA optical interferometer has actually filmed a 55-frame movie of the K0 subgiant transiting the 1-milliarcsecond disk of the B8 star.

Catalog Cross-Identifications & Aliases

Common Aliases: β Persei · Demon Star · HD 19356 · HIP 14576 · 26 Persei

Cross-Catalog Identifiers for Algol (Beta Persei)
Catalog AuthorityIdentifier
HIPHIP 14576
HDHD 19356
SIMBAD* bet Per
WIKIDATAQ13030
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Reference Guide

Apparent vs Absolute Magnitude Explained

Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.

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

  • 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