Friday, October 28, 2011

Night Sky Tour: October 28 - November 3, 2011

Venus and Mercury are visible in the west during the evening twilight. Mercury sets just before 7:00 PM and Venus just after 7:00 PM all week long. Jupiter rises in the east at 6:40 PM on October 28th in the constellation Aries. Each night he will rise a bit earlier. By November 4th Jupiter will rise around 6:20 PM. Jupiter will be visible all week night setting shortly before sunrise. Mars rises around 2:00 AM on October 28th and like Jupiter will rise a bit earlier each night. On November 4rd Mars rises around 1:50 AM. Mars is traveling through the constellation Leo above Regulus. Saturn rises just before the dawn in the constellation Virgo above Spica. Saturn rises around 6:45 AM on October 28th and around 6:25 AM on the morning of November 4th.

I talk about apparent magnitude in this blog as I compare Saturn’s brightness to Spica’s. Apparent magnitude is how bright planets and stars appear from Earth - the brighter the object the lower its numerical value of magnitude. So Saturn at 0.73 magnitude is brighter than Spica at 0.96 magnitude. The sun, the brightest object in our sky has an apparent magnitude of -26.7. Venus is the brightest planet and her brightness varies based on her phase and proximity to earth which has to do with proximity to superior & inferior conjunction. In fact all planets have varying degrees of brightness when observed from Earth for similar reasons. Right now Venus’s apparent magnitude is -3.81. Jupiter is next brightest with an apparent magnitude of -2.76. Mercury has an apparent magnitude of 0.22. Mars is currently the dimmest planet with an apparent magnitude of 1.11. This is different than absolute magnitude.

Absolute magnitude is how bright stars would be if they were all placed 10 parsecs from Earth. A parsec is 3.262 light years, or about 19 trillion miles or 31 trillion kilometers. Right we can’t even fathom that distance! In fact there is no star other than our Sun that is within a parsec of Earth. Absolute magnitude is where the Hertzsprung-Russell Diagram comes in. This diagram measures the temperature of the star and its spectral class along the bottom. These two factors are related because blue stars are brighter than red stars, just like the bluish-white tips of any flame is hottest. Down the side it measures a stars luminosity and absolute magnitude. What emerges when we plot all the stars on this graph is a main sequence of stars. Most stars, like our Sun, are main sequence stars. However we also get super giants like Antares and Betelgeuse, giants like Aldebaran, and white drwarfs like Sirius B. The Sirius we know as the dog star of summer is a main sequence star.

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