Algol, or Beta Persei, appears to dim for several hours about every 2.87 days because two close stars repeatedly pass in front of one another from Earth’s point of view. The change is large enough to notice without a telescope. The system is also a triple star—and its fainter, less massive close companion is the more evolved star, a seeming reversal explained by an earlier exchange of matter.
Why does Algol appear to wink?
Algol is a multiple-star system in Perseus. Its familiar, repeating change in brightness comes from an eclipsing pair: as the fainter companion crosses in front of the brighter star, it blocks some of the light reaching us. NASA’s NICER/ISS science note dated July 12, 2018, describes the naked-eye star as fading by about one-third for several hours every 2.87 days. Those are approximate visible-light changes, not a claim that the star disappears.
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The cycle is regular, but the eclipse itself lasts hours. AAVSO’s educational profile gives a duration near 10 hours and rounded apparent magnitudes of about 2.1 at maximum and 3.4 at the primary minimum. These are reference values from that chapter, not precision measurements. AAVSO, Variable Star Astronomy, Chapter 13
What stars make up the Algol system?
The close pair combines stars with contrasting appearances and evolutionary states. In the AAVSO profile, the brighter star is a B8 main-sequence star; its companion is a larger, dimmer, less massive K2 subgiant. A more distant F1 main-sequence star makes Algol a triple system. The close pair’s eclipses account for the prominent repeating variation seen from Earth.
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Why is the less massive star more evolved?
In an isolated pair, a more massive star would generally be expected to evolve more quickly. Algol’s close pair looks “backwards”: the K2 subgiant is now less massive than the bright B8 star, yet it is further along in its evolution. The accepted explanation is that the stars exchanged mass. The subgiant began as the more massive star and shed material to its companion, changing their present-day mass ranking. NASA’s account describes this as an evolutionary paradox resolved by prior mass exchange; AAVSO also describes material transfer from the K2 component to the B8 star.
NASA’s 2018 note also reports a separate observation by the NICER X-ray telescope: the system’s X-ray brightness fell by about half for several hours, at a time consistent with the X-ray-emitting region of the less massive star being occulted. That figure refers to the X-ray signal in that observation, not the visible-light eclipse’s roughly one-third fade. NASA HEASARC, NICER / ISS Science Nugget, July 12, 2018
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Can you see Algol’s eclipse without a telescope?
Yes. AAVSO says the primary eclipse can be seen with the unaided eye, provided you look at the right time and have suitable observing conditions. The change is not a sudden wink: compare Algol’s brightness with nearby stars over the hours around an eclipse. A telescope is not needed to appreciate the main fade; telescopic or photometric observing is useful when the goal is to record a light curve or study finer detail.
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What is known about Algol’s history?
Algol is commonly rendered from the Arabic Al Ra’s al Ghul as “The Demon’s Head.” Its ominous traditional name has encouraged speculation that people noticed its variability in antiquity, but a name is not proof of an observation. A 2013 paper asks whether ancient Egyptian calendars recorded the binary’s period; the proposal remains a hypothesis, not an established discovery. The Astrophysical Journal, “Did the ancient Egyptians record the period of the eclipsing binary Algol—the Raging One?”
The historical record cited by AAVSO places the first written account of Algol’s variability in 1667, by Geminiano Montanari. AAVSO credits John Goodricke with establishing the period in 1782; historical accounts differ on whether that work is dated to 1782 or 1783. Edward Pickering presented evidence for an eclipsing binary in 1881. NASA describes Algol as the first recognized eclipsing stellar system. AAVSO, Variable Star Astronomy, Chapter 13
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