D Deep sky or Deep Sky Object (DSO, for short): which differs from deep space, is a term used by amateur astronomers to describe mostly faint astronomical objects outside the solar system, such as star clusters or galaxies. These objects are hundreds to billions of light-years distant from Earth. Nearly all clusters and nebulae are contained within galaxies, and there are a number of galaxies visible to the naked eye. They are, in order of closeness, the Milky Way, the Large Magellanic Cloud (approximately 160,000 light-years away), the Small Magellanic Cloud (about 200,000 lightyears away), and the Andromeda Galaxy, (about 2.5 million light-years away).
E Eclipse: the obscuration of the light of the moon by the intervention of the earth between it and the sun (lunar eclipse) or the obscuration of the light of the sun by the intervention of the moon between it and a point on the earth (solar eclipse).
G Globular Cluster: A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin globulusâ€”a small sphere. A globular cluster is sometimes known more simply as a globular.
Galaxies: A galaxy is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter. The name is from the Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with one trillion (1012) stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
N Nebulae: A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy was referred to as the Andromeda Nebula before galaxies were discovered by Edwin Hubble). Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.
O Open Clusters: An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters.