Many stars on the border-line of invisibility send us flickering glints of light although seldom can we clearly see more than two thousand at one time and usually many of these are blotted out by the thick veil of atmosphere which surrounds the earth.
Among the exceptionally bright stars which may be seen from the northern hemisphere are:
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Arcturus (in Boötes)
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Regulus (in Leo)
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Yet even these cannot be seen at the same time but must be viewed from different parts of the earth's journey around the sun.
These brightest stars, which are called first magnitude stars, should be among one's first star acquaintances, for they serve conveniently as guides to locate the other stars, which are also classified according to magnitudes. The second, third, fourth and lesser magnitudes are each progressively two and a half times lower in the scale of brightness. The smallest stars discernible to the unaided eye of ordinary vision are stars of the fifth magnitude, although a sharp eye can discern those of the sixth and even of the seventh magnitude. All below this are telescopic stars. With a 60-inch reflector 219,000,000 stars are visible. The Mount Wilson 101-inch telescope brings the number up to 319,000,000. Stars are most dense in the region of the heavens called the Milky Way. Sir William Herschel observed 116,000 go past the field of his telescope in a quarter of an hour while directed at the densest part of the Milky Way.
This vast collection of stars differ not only in brightness but also in color.
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