Tuesday, September 29, 2020

The River Runs South

The Southeastern Sky 9:00PM December 3rd. Submitted

The Southeastern Sky 9:00PM December 3rd. Submitted

by Mike Usher

Tonight mighty Orion has heaved himself above the horizon, but first let’s look at the heavenly river Eridanus. Composed of mostly dim stars the Eridanus is supposed to represent the Nile or the Euphrates; it arises near the foot of Orion and winds thru many twists and turns into the deep southern sky terminating at the star Achernar.  This star is one of the very brightest in the sky but is dimed by the murk above the horizon. Achernar is never visible further north than Macon and in fact is mostly impossible to see north of the Florida border. Even here it only briefly pops above the horizon in late fall and early winter. Chances are you will not see it unless you have an unobstructed view of the southern horizon. Achernar is also one of the stranger stars in the sky – it rotates so fast that it is highly flattened. It’s equatorial diameter is 50% greater than it’s polar diameter, in other words it’s even more oval than an egg.

Orion is one of the more magnificent areas of the sky and we will return to it many times in future columns; tonight we will hit a couple highlights.

The stars in Orion are mostly very blue in color, particularly Rigel the brightest star in the constellation. The lone exception is Betelgeuse, one of two red supergiant stars visible to the naked eye. The name is commonly pronounced “Beetle Juice” in the U.S. and is about as close as Americans can come to the original Arabic pronunciation. Betelgeuse is rapidly nearing the end of it’s life and will explode into a supernova sometime in the future; perhaps 10,000 years from now, or maybe tomorrow – no one knows. A detonating supernova too close to Earth would be uncomfortable, but at 427 light years this one should be safe enough – and we will have an excellent view. It will easily be visible in the daytime and will compete with the full moon at night.

No mention of Orion is complete without mention of the Great Orion Nebula. To the naked eye it appears as a fuzzy star south of Orion’s belt. It’s actually a giant cloud of gas and dust that acts a stellar nursery and is one of the finest visible in our galaxy. Perhaps a thousand stars lie hidden inside the nebula, only a few are visible with binoculars. Four stars at the heart of the nebula give off copious UV radiation which causes the nebula to glow with a greenish light to the eye and reddish to old fashioned film cameras. Inside the nebula stars seem to be a little too crowded to have stable orbits; indeed several stars scattered across our sky are thought to have been ejected from the nebula in the past. See you next time!

Mr. Usher is Vice President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.

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