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Download 3.1e Finding Polaris and Sirius
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3.1e demonstrate the use of ‘pointers’ and other techniques to find other celestial objects, including: i Arcturus and Polaris from the Plough ii Sirius, Aldebaran and the Pleiades from Orion iii Fomalhaut and the Andromeda Galaxy from the Great Square of Pegasus i Arcturus and Polaris from the Plough Pointers are used by astronomers to help guide people to new regions of the night sky. There are a number of excellent imaginary lines that can guide a person in a straight line to locate an important star. Finding Polaris, the Pole Star (to point north) is an excellent example:- Finding North, the Pole Star (Polaris) from the Plough POLARIS - The Pole Star (part of Ursa Minor) DUBHE THE PLOUGH (part of Ursa Major) MERAK To find North, the two stars of the PLOUGH, Dubhe and Merak, are used as ‘pointers’. Following along the line from Merak through Dubhe across the sky will bring you close to Polaris – the first bright star in the region. The North Star is not directly on the line because PRECESSION means that Polaris will move away from being the ‘North Star’ in the future. Sometimes a straight line will not work and a smooth curve is used. Arcturus from the Plough is a good example of this:- Finding Finding Arcturus, in the constellation Boötes, from the Plough To Polaris Boötes Arcturus Using the handle of the ‘Big Dipper’ or the ‘saucepan’ gives a natural curve from the Plough. This brings you down to the brightest star in the constellation of Boötes, which is the magnitude -0.05 star Arcturus - the brightest star of the northern hemisphere and the fourth brightest star in the night sky. Many people are told of the technique to find north using the ‘pointer’ stars of the Plough. The same two ‘pointer’ stars can be used to find another important area of the night sky – particularly useful in spring and summer. If you use the stars as your line, but this time move in the opposite direction (the same distance that was used to reach the Pole Star), you come to the constellation of Leo. ii Sirius, Aldebaran and the Pleiades from Orion The Pleiades (M45) Aldebaran in the constellation Taurus, the Bull Orion, the Hunter Orion’s Belt Sirius To find Sirius, the Dog Star and the brightest star in the night sky, the three stars of Orion’s Belt are used as ‘pointers’. Following along the line down from Orion’s Belt across the sky will bring you to Sirius. Using Orion’s Belt as ‘pointers’, but this time moving in the opposite direction, you pass through part of the constellation of Taurus which is shaped like a letter V. The brightest star of the V is Aldebaran – an orange giant star. Carrying further along the line brings you to the bright Open Cluster (M45), the Pleiades. iii Fomalhaut and the Andromeda Galaxy from the Great Square of Pegasus Star hopping is used to find the Andromeda Galaxy (M31). If you live where you have very dark skies, the Andromeda Galaxy is the furthest object that you can see with your naked eye – 2.4 million light years away! The galaxy appears as a small, white, fuzzy patch. When you have found the Great Square of Pegasus, you need to find the top left hand star of the square (the star does not belong to the constellation of Pegasus – it actually forms part of the Andromeda constellation). There are two faint pairs of stars that appear in the order as shown below. If you move past the first pair and concentrate on the second pair – moving from the bottom star of the pair to the top star of the pair, move up the same sized gap again in the same line and you are in the area of the Andromeda Galaxy. Using a pair of binoculars will allow you to see the large galaxy easily. Andromeda Galaxy (M 31) Great Square of Pegasus Fomalhaut (in the Southern Fish constellation) The star is at declination -30˚ and so the star will only be seen easily in the summer Using the two stars on the right of the Great Square of Pegasus - follow the line down. The first bright star that you meet is Fomalhaut.