Let's Talk Physics

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Algol, the binary star system

Algol, or beta Per, is a star in the constellation of Perseus, and is a triple star system. It’s located roughly 93 lightyears (or 879,800,000,000,000 km) away from Earth. It gained the nickname Demon Star as, when first observed around the 18th century, the astronomers of the day noticed that it would twinkle every 3 days or so. Astronomers back then guessed that this was due to a dark body passing in front of the star. Not a bad guess, but not the whole truth either.

Algol, in the constellation of Perseus

Algol, in the constellation of Perseus

It turns out that Algol is a triple star system. There is a large, hydrogen burning (main sequence) star called Algol A, the sub-giant Algol B (which has moved on to burning Helium and heavier elements) and Algol C, a star only recently discovered in the system. The twinkling that astronomers observed in the 18th century is due to Algol B passing in front of Algol A and blocking some of the light, and since Algol B is an a cooler star than Algol A and doesn’t emit as much light, it seems to us on Earth that the tiny dot of light dims on its own. This eclipse of Algol A is observable by eye and happens ever 3 days (approx, details for each eclipse can be found here), and if you’re ever outside, look up at Perseus, because you might just see it wink at you.

Now, when it was discovered that Algol was at least a binary system, consisting of a large Main Sequence star and a smaller sub-giant star, astrophysicists become very confused. You see, in normal binary star systems, the larger star evolves a lot faster than the smaller stars – hence, when astronomers found that Algol A was larger than Algol B, and yet earlier in its stage of evolution, they began screaming in panic (not really, they more than likely sat around, drinking coffee and stroking the long beards that were the fashion at the time). But soon, an answer presented itself – Algol A had simply ate some of the mass of Algol B.

So this is the current picture – Algol B was, a long time ago, much bigger than Algol A and burning Hydrogen. But when that Hydrogen ran out, it grew, and suddenly, quite a lot of it’s mass got pulled over into Algol A, making it the bigger star in the system. Hence, the final piece of the puzzle for Algol was put into place. It’s a binary star system with a period of 3 days, and one of the stars has an appetite for eating other stars.

The Universe can be a strange place sometimes.


About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

One comment on “Algol, the binary star system

  1. Pingback: How it’s All Going to End: The Wandering Black hole (Part 4) | Let's Talk Physics

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This entry was posted on January 27, 2013 by in Astrophysics and tagged , , , .


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