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Everyone has seen stars twinkling in the night sky. The flickering is produced by turbulence in Earth's atmosphere that temporarily deflects the narrow beams of starlight before they reach our eyes. Twinkling occurs on timescales of fractions of a second. The phenomenon is strictly due to the air we're looking through — we're not seeing any changes in the stars themselves.
But a large percentage of stars in the sky actually vary the amount of light they emit, resulting in changes to their visual brightness on timescales of hours, days or even years. In most cases, the changes are subtle. In fact, they weren't detected until astronomers began to take quantitative measurements of stars' brightness and saw differences from one night to another. These stars became known as variable stars.
Handful - Stars - Millennia - Edition - Mobile
A handful of obviously variable stars have been known for millennia. In this edition of Mobile Astronomy, we'll look at an eclipsing binary star that changes in brightness enough for everyday skywatchers to detect it easily. It's Algol, nicknamed the "Demon Star," and it lies nearly overhead for mid0northern latitude skywatchers during early evenings.
Perseus is a northern constellation that occupies the sky between the Pleiades cluster in Taurus and the W-shaped constellation Cassiopeia, high in the western evening sky during winter. It is shown here at 9 p.m. in your local time zone.
Algol - Beta - Persei - Star - Constellation
Algol, also designated Beta Persei, is the second-brightest star in the constellation Perseus, the hero. This time of year, Perseus is already near the zenith (i.e., directly overhead) as soon as the sky becomes fully dark in early evening. It occupies the sky between the bright little Pleiades cluster in Taurus the bull and the distinctive W-shaped constellation of Cassiopeia, the queen. Astronomy sky-charting apps such as SkySafari 6 for iOS and Android will help you find Algol...
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