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Northern Rockies Skies for November: Cassiopeia, the Queen

October 30, 2014

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

In Greek mythology, queen Cassiopeia was the boastful wife of King Cepheus, who was punished for saying she was more beautiful than the Nereid sea nymphs. In the sky, Cassiopeia takes the shape of a stretched out “W” (or “M”) and is located directly opposite the Big Dipper, with Polaris and the Little Dipper equidistant from both.

In this month’s sky map, we have inserted an additional map of Cassiopeia with the five brightest stars labeled. Alpha Cass (or Schedar), which means the breast of heart, is an orange giant. Beta Cass (or Caph) is a delta Scuti type star located about 55 light years from the sun. Gamma Cass, the central star of the bright five, is 60 light years away and is more than 40,000 times more luminous than the sun.

The U.S. astronauts used this star, which they called NAVI (Ivan spelled backwards), as a navigational reference. Delta Cass (Ruchbah) and epsilon Cass (Achird) are about the same brightness, but Achird is closest to the sun (20 light years) and similar to the sun in temperature and size.

Planet Alert: Mars is on the southwest horizon right after sunset; Jupiter rises after midnight. The Leonid meteor showers peak around Nov. 17.  

The Kepler Spacecraft: Earth-like Planet Finder

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The first confirmed discoveries of planets in stellar systems beyond our own solar system were made using ground-based telescopes in the late 1980s into the 1990s. The first found "exoplanets" tended to be Jupiter-sized objects orbiting near their parent stars, which results in relatively large, detectable gravitational perturbations on the stars' common orbital motion with the planet.

These initial discoveries motivated design of a space mission to find planets with masses and orbits more like Earth's and with parent stars similar to the sun. The mission was named Kepler, after the great astronomer Johannes Kepler (1571-1630), who enunciated the laws of planetary orbits.

Kepler was launched in 2009 into an orbit around the sun, and the initial planet-finding mission lasted for about five and a half years, although the spacecraft is still functioning in a secondary mission mode. The spacecraft has a mass of about one ton and carries a primary mirror of diameter 1.4 meters with a field of view of about 12 degrees diameter. The telescope points at a sky field containing parts of the constellations of Cygnus, Lyra and Draco -- perpetually monitoring brightnesses of more than 145,000 stars.

Kepler's orbit around the sun avoids occultations of this sky field, which would occur in an Earth orbit. The experiment's 95-megapixel camera measures changes in star brightnesses of about 30 parts per million. In some cases, a fraction of a star's brightness variation is due to an Earth-like planet passing in front of the star's disk (like the recent Venus transits across the sun in 2004 and 2012, visible from Earth).

Repeated periodic decreases in brightness confirm that a planet orbiting the star is the cause; additional ground-based observations are often used for confirmation. During the Kepler mission, about 1,000 exoplanets -- mostly Earth-like in sun-like systems -- have been discovered and confirmed in as many as 400 stellar systems. A few thousand more candidates found by Kepler remain to be confirmed.

The larger import of Kepler's legacy, inferred by planetary astronomers, is that approximately 50 billion Earth-like planets in habitable orbits around sun-like and smaller stars exist in our Milky Way galaxy, and perhaps a few tens of thousands of these planets lie within a distance of a thousand light-years.

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

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