Galaxies are massive, gravitationally bound systems consisting of stars, an interstellar medium of gas and dust, and dark matter. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The word Galaxy is derived from the Greek word for milk, a reference to our own Milky Way.

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Nebulae are interstellar clouds of dust, hydrogen gas and plasma. It is the first stage of a star's cycle. The word Nebula is derived from the Latin work for mist.

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Star clusters are groups of stars which are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young.

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A Quasar is an extremely bright and distant active nucleus of a young galaxy with a compact halo of matter surrounding a central supermassive black hole. A star is a massive, luminous ball of plasma. A star shines because nuclear fusion in its core releases energy which traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created inside the cores of stars.

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