31-03-2007 April night sky (Article 79)

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Simon Kenny
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Location: Shannon, Co. Clare, Ireland

31-03-2007 April night sky (Article 79)

Post by Simon Kenny » Wed Feb 04, 2009 10:50 am

April night sky (Article 79)

Though poorly stocked with spectacular stars that you can see without an instrument, April’s night sky does contain two stars that prompt thoughts on the movements involved within our Galaxy.

Brilliant Arcturus, easily found as the brightest star seen by following the curve of the handle of the Plough downwards, now lies high in the east. One of the nearest stars at 37 light years, it is the brightest star in the often overlooked constellation of Bootes, the Herdsman. The 4th brightest star seen from Earth, this orange giant near the end of its life is 25 times larger and 110 times more luminous than our Sun. Contrary to most stars within our Galaxy, Arcturus does not revolve around the Galaxy’s centre within the galactic plane. Instead, it is one of several stars that inhabit a region around our Galaxy we call the halo, and Arcturus’ path takes it across the galactic equator. This direction gives it a high space velocity, a motion across our sky of 2 arc seconds per year, equivalent to Moon’s apparent diameter in 900 years.

Due south, the constellation Leo dominates, its heart marked by Regulus, the 21st brightest star in our sky. At 78 light years it too is relatively near to us, and persistent observations of this star over more than 2000 years ago allowed the Greek mathematician, Hipparchus, to discover the precession of the equinoxes in 130 BC. This is the wobbling effect (like an old fashioned child’s spinning top) of our planet over a period of almost 26000 years, whereby our poles scribe a large circle against the background sky. Consequently we have different Pole Stars over time. Today it’s Polaris, for the ancient Egyptians it was a faint star called Thuban, and in millennia to come it will be the brilliant Vega. Regulus has a wonderfully coloured companion star just visible in small telescopes. Being white we know Regulus has a scorching surface temperature, over 13000 degrees C, 5 times larger and 140 times more luminous than our Sun.

This month, a resplendent Venus dazzles the evening sky. It lies not far from the Pleiades during second week. A fading Saturn remains in Leo with the Moon between it and Regulus on 25th. Jupiter rises before midnight by month’s end. The 22nd sees maximum of Lyrid meteor shower, offspring of Comet Thatcher. This small shower is active from 19th to 25th. Our April meeting on 4th is an open forum night, open to the public Clear skies!

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