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Wellington Daily News - Wellington, KS
  • Looking Up: Stars are far, by any measure

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  • How can we tell how far away a star is from the Earth, let alone a galaxy? The easy answer is you look it up in a book or read a column and hope it’s right. That’s like asking how a radio works and you demonstrate by flipping a switch.
    For thousands of years, sky observers did not realize that the stars or the Milky Way Band varied in distance. They all appear to be fixed on the great inverted dome of the sky. It was clear that the wandering planets, the moon and the sun were closer, and by their motions and speed we could detect the order of distance, out to Saturn, the most distant planet known before the telescope was invented.
    Astronomers don’t limit themselves to expressing cosmic distance in millions or billions of miles or kilometers. Units of measurement include the astronomical unit (AU), which is the average distance of the Earth to the sun (93 million miles). This is handy for discussing distance within our solar system.
    The term "light year" is often misused, as it is not an expression of length of time. Instead, it is the distance that light travels in one Earth year, traveling at approximately 186,000 miles a second. A light year then is roughly 5.8 TRILLION miles. Space is really vast by our standards. Our planet, Earth, is not even quite 8,000 miles wide. The closest star system to the sun is Alpha Centauri, and that is 4.3 light years away.
    Distances within a few hundred light years are accurately measured by the parallax method. Using basic trigonometry, the star’s position is examined once and then again six months later, when the Earth is on the other side of the sun. This makes a rather big baseline (2 AU). From these two points, the star is seen to make a small shift in reference to the much farther background stars. How much of an angle of shift it makes is the star’s parallax. This is then used to compute the distance, since we already know how far we are from the sun.
    The farther the star, the smaller the parallax shift.
    Greater distances beyond the resolving power of our instruments, are figured through the shift of the colored spectrum of the starlight, or light from a galaxy.
    Observation of a class of variable star, the Cepheid variable, also tells us distance by measuring how bright the star appears. A Cepheid variable star changes by a predictable amount; distance to closer Cepheid stars are known by parallax. Distance to others much farther away can be shown then by seeing how faint the star appears compared to nearer examples. Cepheid variable stars have been detected in closer galaxies, giving us a way to extend our celestial tape measure beyond the Milky Way.
    Page 2 of 2 - First-quarter moon is on April 7.
    Notes are welcome at news@neagle.com.
    Keep looking up!

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