Page 12 - Curriculum Visions Dynamic Book

 M15 Globular cluster in the constellation Pegasus. Cataloguing objects in the sky
It is easy to use names when you are talking about a small number of things, such as planets in the Solar System. But when you extend your study to every object in the night sky, the task of describing them is much more complex, and a special reference system
is needed. Two systems have been evolved: an earlier one, called the Messier system, and a more recent one, called the New General Catalogue (NGC).
The Messier Catalogue
At the end of the 18th century Charles Messier made
a list of 109 star clUsters, galaxies, and nebUlae. Each object was given an M nUmber based on the order in which he catalogued them. The Crab Nebula, our nearest major galaxy, is M1 (see pages 13 and 32), the Andromeda Galaxy is M31 (see pages 40–41), and so on. Note that only the most visible objects have M numbers because the resolving poWer of telescopes at the end of the 18th century was not as good as it is today.
New General Catalogue of nebulae and star clusters (NGC)
This cataloguing approach was developed by the Danish astronomer Johan Ludvig Emil Dreyer in 1888. He based it on information on stellar objects gathered by
the Herschel family in Britain.
The NGC is an alternative to the
Messier system for stellar objects.
An object can be classified in both systems. Like Messier, it does not cover all the objects that can be seen with modern equipment.
 M57 Ring Nebula in the constellation Lyra.
galaxy A system of stars and interstellar matter within the Universe.
m nUmbers In 1781 Charles Messier began a catalogue of the objects he could see in the night sky. He gave each of them a unique number. The first entry was called M1. There is no significance to the number in terms of brightness, size, closeness, or otherwise.
nebUla (pl. nebUlae) Clouds of gas and dust that exist in the space between stars.
resolving poWer The ability of an optical telescope to form an image of a distant object.
star clUster A group of gravitationally connected stars.
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