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Northern Rockies Skies for April: Gemini, the Twins

March 28, 2013

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

Located to the east of the Taurus-Orion line, Gemini was first cataloged by Ptolemy around 200 A.D. Its two brightest stars represent the mythological twins, Castor and Pollux.

Although there are many versions of the Gemini myth, most attribute Castor to the son of the king of Sparta, while Pollux is the son of Zeus. They grew up together and were part of the Argonauts’ search for the Golden Fleece. Due to their expert fighting and maritime skills, they are known as the patrons of sailors.

Castor, the second brightest star in Gemini, is actually a system of six stars all gravitationally bound to each other. Pollux, the brightest star in Gemini, is an orange giant about 34 light years from the sun. Its mass is approximately twice the mass of the sun.

Planet watch: Jupiter is close to Aldebaran in Taurus. Saturn rises around 9 p.m., so it can be seen during the entire night. Keep a close watch on April 14, when the moon and Jupiter are quite close.

Famous Astronomers: Fritz Zwicky
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Born in Bulgaria and educated in Switzerland, Fritz Zwicky (1898-1974) was one of the most prolific and unusual personalities in astronomy. In 1925, he immigrated to the United States on a fellowship at the California Institute of Technology, to work with Robert Millikan in experimental physics. Millikan, the first scientist to measure the charge of the electron, was working on cosmic rays and thought them to be gamma rays (very energetic photons). By the late 1920s, it was shown that cosmic rays are charged particles (in fact, very energetic ions, such as hydrogen and helium nuclei).

Motivated by the mystery of the origin of cosmic rays, Zwicky shifted to astronomy. In an extraordinarily insightful leap, he postulated that some stars catastrophically explode as "super-novae." This process accelerated charged particles -- the cosmic rays -- to very high energies, which produces a very dense "neutron star" in the accompanying implosion. The neutron itself had been discovered only a year earlier (1932). Thirty-four years later, the first rotating "light beacon" pulsars began to be discovered and, within a year, were proven to be the neutron stars postulated by Zwicky.

Very recent observations show, in fact, that supernovae are copious producers of cosmic rays. Thus, with the passage of time, Zwicky was proven correct on all three predictions. Zwicky and Walter Baade went on to search for and discover many supernovae at California’s Mt. Palomar. Zwicky’s record of 122 supernova discoveries is still unsurpassed.

To study the Coma cluster of galaxies, Zwicky arranged for a wide field-of-view, 18-inch Schmidt telescope to be built. Via spectroscopy of the individual galaxies, he was able to calculate their velocities (using the Doppler effect) and infer their masses. He concluded that the cluster "weighed" roughly more than 100 times the mass inferred from the cluster's total luminosity -- thereby making the first large-scale "dark matter" discovery in 1933.

Four years later, Zwicky proposed that a massive foreground galaxy, in near direct line of sight with a more distant galaxy, could "lens" the latter, causing a distorted image -- a gravitationally lensed image -- and that its image could be used to weigh the galaxy. The first such gravitationally lensed images weren’t discovered until 1979.

Zwicky predicted and discovered low-mass “dwarf” galaxies with the 100-inch Mt. Wilson telescope, and uncannily foresaw the discovery of quasars, predicting high-luminosity, compact blue galaxies that could be mistaken for stars.

Zwicky also developed many early jet engine designs (he held more than 50 patents). Sometimes called the "father of the modern jet engine," he was awarded the Presidential Medal of Freedom by Harry Truman for work on rocket propulsion. Zwicky himself acknowledged having a difficult personality, but he also was a humanitarian. He supported orphanages and secretly collected tons of books that he sent to destroyed libraries in Europe and Asia after World War II ended.

To view this month's sky chart, click here.

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