Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body. Typically an astronomical (celestial) body refers to a single, cohesive structure that is bound together by gravity (and sometimes by electromagnetism). Examples include the asteroids, moons, planets and the stars. Astronomical objects are gravitationally bound structures that are associated with a position in space, but may consist of multiple independent astronomical bodies or objects. These objects range from single planets to star clusters, nebulae or entire galaxies. A comet may be described as a body, in reference to the frozen nucleus of ice and dust, or as an object, when describing the nucleus with its diffuse coma and tail.
The universe can be viewed as having a hierarchical structure. At the largest scales, the fundamental component of assembly is the galaxy, which are assembled out of dwarf galaxies. The galaxies are organized into groups and clusters, often within larger superclusters, that are strung along great filaments between nearly empty voids, forming a web that spans the observable universe. Galaxies and dwarf galaxies have a variety of morphologies, with the shapes determined by their formation and evolutionary histories, including interaction with other galaxies. Depending on the category, a galaxy may have one or more distinct features, such as spiral arms, a halo and a nucleus. At the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can also have satellites in the form of dwarf galaxies and globular clusters.
A planetary nebula, more correctly known as a stellar-remnant nebula, is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life. The term for this class of objects is a misnomer that originated in the 1780s with astronomer William Herschel, because when viewed through his telescope, these objects were somewhat similar in appearance to Uranus, the planet which he had recently discovered. Herschel's name for these objects was adopted by astronomers and has not been changed, even though planetary nebulae are unrelated to the planets of the solar system. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.
The mechanism for formation of most planetary nebulae is thought to be the following: at the end of the star's life, during the red giant phase, the outer layers of the star are expelled via pulsations and strong stellar winds. Without these opaque layers, the hot, luminous core emits ultravioletradiation that ionizes the ejected outer layers of the star. This energized shell radiates as a planetary nebula.
Planetary nebulae play a crucial role in the chemicalevolution of the galaxy, returning material to the interstellar medium that has been enriched in heavy elements and other products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium). In more distant galaxies, planetary nebulae may be the only objects that can be resolved to yield useful information about chemical abundances.