Big and Little Dippers
The Big Dipper is an asterism that makes up part of the constellation of Ursa Major, the Big Bear. It is seen here at the top right of the image. The Little Dipper, part of the constellation of Ursa Minor, the Little Bear, is seen at the lower center. Polaris, the North Star, is at the end of the handle of the Little Dipper. The two stars at the end of the bowl of the Big Dipper, Merak and Dubhe, are called the Pointer Stars because a line drawn between them points to Polaris. Hold your mouse cursor over the image to see the lines that make up the Big and Little Dippers, and the line from the pointer stars to Polaris.
Polaris (Alpha Ursae Minoris) is called the North Star because it is currently very close to the north celestial pole, the point in space where the spin axis of the Earth points in the northern hemisphere. This is the point where all of the northern celestial objects in the sky appear to rotate around because of the Earth's rotation on its axis. Polaris is a multiple star system consisting of Polaris A, the bright star we see with our unaided eyes, Polaris B, a 9th magnitude star that is visible in small telescopes 18 arcseconds away, and two other fainter more distant companions. Polaris shines at 2nd magnitude, and is also a Cepheid variable star that varies slightly in brightness over a 4-day period. It is a spectral-type F7 supergiant star with 2,500 times the mass of our Sun. It is located 430 light-years away from us. The Big Dipper and Polaris play an important part in the story of the Underground Railroad which helped slaves escape their captivity in the southern states of the United States before the Civil War by fleeing north to Canada. The folk song Follow the Drinking Gourd (another name for the Big Dipper) was a coded song that gave directions on the escape route from Alabama and Mississippi. While traveling on their long journey, they could always tell which way was north by the location of Polaris, which they could find by the pointer stars in the Big Dipper. The Big Dipper is a circumpolar asterism for most of the United States. This means it stays above the horizon all night long as it apparently rotates slowly counterclockwise during the night around Polaris due to the Earth's rotation. It is also comprised of very bright stars in an easy-to-locate pattern. The Little Dipper, on the other hand, is comprised of fairly faint stars that do not really stand out, except for second-magnitude Polaris. This image shows what the northern sky looks like at midnight on March 21, the first day of spring.
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