Study GuideAstronomy–ObservaƟonal ProperƟes of Stars1. Stellar Parallax and DistancesWhen astronomers want to find the distance to stars that are close to Earth, they use a method calledstellar parallax. This method uses simpletrigonometryand the size of Earth’s orbit around the Sunto calculate the star’s distance.1.1What Is Parallax?Imagine looking at a nearby object with one eye closed, then switching to the other eye. The objectseems to shift its position against the distant background. This apparent shift is calledparallax.For stars, the two “eyes” are the opposite points in Earth’s orbit, six months apart. The baseline is thediameter of Earth’s orbit around the Sun.Theparallax angle(usually written asp″and measured in seconds of arc) is half the angle betweenthe star’s apparent positions at these two points.Because stars are so far away, the parallax angle is very small, and the triangle formed by Earth’sorbit and the star is very long and skinny.1.2How Does Parallax Relate to Distance?The distancedto a star (measured in astronomical units, AU) is related to the parallax angle by thisformula:This means:•If a star has a parallax angle of 1 second of arc (1″), it is 206,264 AU away.•To make things easier, astronomers use a special unit called theparsec(pc).•One parsecis defined as the distance where the parallax angle is exactly 1″. So:1 parsec = 206,264 AUPreview Mode
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