Physics Photo of the Week

February 7, 2014

The Drinking Gourd

The Big Dipper - the prominent star formation in the northern skies - had been called "The Drinking Gourd" due to its resemblance to a natural utilitarian "dipper" made from a hollowed gourd.  This was the name given by mountain and plantation cultures in the pre-Civil War American South.  Other cultures have called it "The Plow" and "The Great Bear".  Click on the image for a larger photo.

The Drinking Gourd asterism is close to the north celestial pole, it never sets below the horizon, and is always viewable in the northern skies at all times of the year.  It is reputed that the highly recognizable star formation, visible at all times of the year, served to provide directions for slaves escaping to freedom via the Underground Railroad.  "Follow the Drinking Gourd" eventually became a popular folksong - "discovered" in 1912 by folklorist H. B. Banks and published by Banks in Publications of the Texas Folklore Society, Number VII, Austin, 1928.  The song has become popular during the civil rights movement of the 20th century and has enriched all our lives with the rich music of African American culture and reminds us of the tremendous debt we owe to African Americans for the inhuman treatment of these people.

Scientifically the Big Dipper/Drinking Gourd asterism has played an important part in measuring the atmospheric extinction of starlight when measuring the brightness of stars.  The stars of the Drinking Gourd asterism are well-known to possess standard brightness values - or magnitudes.  In February (and again in September) the Drinking Gourd spans a large elevation difference.  Dubhe, the topmost star in the photo, is about the same brightness as Alkaid, the lowest star at the end of the handle.  But because of the thicker atmosphere at low elevations, Alkaid actually looks fainter in the image.  By measuring the apparent brightness of the well-known stars at different elevations, we can measure the effect of the atmosphere and  account for the atmosphere effect in stellar photometry measurements.

The Drinking Gourd asterism is also one of the few star formations in which the stars are physically related to each other.  It is believed that the seven bright stars are actually members of an open cluster of stars that were formed at the same time out of a giant hydrogen cloud - similar to the Pleiades cluster (PPOW Nov. 22, 2013).  The Drinking Gourd stars are relatively close - less than about 100 light years compared to over 400 light years for the Pleiades and the seven stars are all moving relative to the other stars in relatively the same directions in space.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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