Algol (Beta Persei) is the star that gave eclipsing binaries their name: watch its hot primary star and cooler, larger companion pass in front of each other, and read the live light curve as it dims.
Published figures: orbital period 2.867 days; magnitude 2.1 (normal) to 3.4 (primary eclipse minimum); primary eclipse duration about 10 hours; distance about 90 light-years (AAVSO / published eclipsing-binary references).
Drag to orbit and scroll or pinch to zoom. Scrub the phase slider, press Play orbit, or switch views.
Algol Eclipsing Binary 3D Explorer
This browser explorer shows Algol (Beta Persei), the star that gave eclipsing binaries their name: a hot primary star and a cooler, larger companion pass in front of each other every 2.867 days, and a live light curve plots the brightness dip as it happens.
Algol normally shines at magnitude 2.1, but every 2.867 days a partial eclipse dims it to magnitude 3.4 for about 10 hours as the cooler, larger companion star (Algol B) passes in front of the hotter primary (Algol A). A much shallower secondary dip happens half an orbit later, when the primary passes in front of the companion. Scrub the phase slider or press Play to watch both eclipses and read the matching light-curve dip.
- Scrub the phase slider or press Play to run one full 2.867-day orbit
- Watch the light curve dip deeply at the primary eclipse and only slightly at the secondary eclipse
- Switch between Eclipse view (what an observer on Earth measures) and Top view (why the eclipse happens)
- Read the real masses, radii, and spectral types of both stars
- Drag to orbit, scroll or pinch to zoom
- Runs fully in the browser with the vendored three.js engine - no account, no upload
| Figure | Published value |
|---|---|
| Orbital period | 2.867 days (2 days 20 h 49 min) |
| Magnitude range | 2.1 (normal) to 3.4 (primary eclipse minimum) |
| Primary eclipse duration | about 10 hours (partial) |
| Distance | about 90 light-years (28 parsecs) |
| Algol A (primary) | ~3.17 solar masses, ~2.73 solar radii, spectral type B8V |
| Algol B (companion) | ~0.70 solar masses, ~3.48 solar radii, spectral type K0IV (subgiant) |
| Discovery | variability noted 1600s (Montanari); eclipsing-binary explanation 1783 (Goodricke); confirmed spectroscopically 1889 (Vogel) |
Algol B is less massive than Algol A but physically larger - it has swelled off the main sequence and overflows its Roche lobe, an evolutionary puzzle called the Algol paradox, resolved by mass transfer from B onto A that reversed their original mass ratio.
For orbital dynamics and barycenters instead of an eclipse light curve, open the Binary Star System 3D Explorer.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached.
This is an educational approximation - star sizes, the orbit separation, and the light-curve shape are simplified for teaching, not a photometry pipeline or tonight's sky.
Frequently Asked Questions
What is Algol?
Algol (Beta Persei) is the brightest eclipsing binary star in the night sky and the prototype of the class - a close pair of stars that pass in front of each other every 2.867 days, dimming the combined light we see.
Why does Algol's brightness change?
Every 2.867 days the cooler, larger companion star (Algol B) passes in front of the hotter primary star (Algol A) as seen from Earth, blocking most of the primary's light for about 10 hours and dropping the combined magnitude from 2.1 to 3.4.
How deep are the two eclipses?
The primary eclipse (Algol B in front of Algol A) is deep because A supplies most of the light. The secondary eclipse (Algol A in front of Algol B), half an orbit later, is much shallower - detectable with precise photometry rather than the naked eye.
Who discovered Algol was an eclipsing binary?
Brightness changes were noted in the 1600s by Italian astronomer Geminiano Montanari. In 1783, 18-year-old English astronomer John Goodricke proposed to the Royal Society that an orbiting companion caused the dimming, winning the Copley Medal; the binary nature was confirmed spectroscopically by Hermann Vogel in 1889.
Why is the cooler star bigger than the hotter one?
Algol B has less mass than Algol A but is physically larger because it evolved faster and swelled off the main sequence, overflowing its Roche lobe. This mismatch - the less massive star being more evolved - is called the Algol paradox, resolved by mass transfer that reversed the pair's original mass ratio.
Is this a physical simulation?
No. It is an educational explorer using published magnitude, period, mass, and radius figures - star sizes, the orbit view, and the light-curve shape are teaching approximations, not a photometry pipeline or tonight's sky.