Astronomical object

"Celestial object" and "Celestial body" redirect here. For other uses, see Celestial (disambiguation).

This article is about naturally occurring objects. For artificial objects, see Satellite.

Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe.^[1] 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.^[2] 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.^[3] Galaxies and dwarf galaxies have a variety of morphologies, with the shapes determined by their formation and evolutionary histories, including interaction with other galaxies.^[4] 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.

The constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the resulting fundamental components are the stars, which are typically assembled in clusters from the various condensing nebulae.^[5] The great variety of stellar forms are determined almost entirely by the mass, composition and evolutionary state of these stars. Stars may be found in multi-star systems that orbit about each other in a hierarchical organization. A planetary system and various minor objects such as asteroids, comets and debris, can form in a hierarchical process of accretion from the protoplanetary disks that surrounds newly created stars.

The various distinctive types of stars are shown by the Hertzsprung-Russell diagram (H-R diagram), which is a plot of absolute stellar luminosity versus surface temperature. Each star follows an evolutionary track across this diagram. If this track takes the star through a region containing an intrinsic variable type, then its physical properties can cause it to become a variable star. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae and Cepheid variables.^[6] Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as a compact object; either a white dwarf, neutron star or black hole.

Location or structure

The table below lists the general categories of objects by their location or structure.

Solar System	Extrasolar objects
	Simple objects	Compound objects	Extended objects
Sun Planetary system Planets Mercury Venus Earth Moon Mars satellites Jupiter satellites Saturn satellites Uranus satellites Neptune satellites Dwarf planets Pluto satellites Eris Dysnomia Ceres Makemake Haumea satellites Asteroids "Vulcanoids" "Apoheles" Near-Earth asteroids "Arjunas" Atens Apollos Amors Mars-crossers Asteroid belt Hungarias Phocaeas Nysas Alindas Hildas Pallas Marias Koronis Eos Themis Griquas Cybeles Thule Vesta Trojan asteroids Earth trojans Mars trojans Jupiter trojans Neptune trojans Outer-planet crossers Damocloids Centaurs Trans-Neptunian objects Kuiper belt Classical Kuiper-belt objects (cubewanos) Resonant trans-Neptunian objects Plutinos (2:3) Twotinos (1:2) Scattered-disc objects Detached objects Comets Oort cloud Meteoroids Meteors Meteor showers Solar wind	Exoplanets Hot Jupiters Eccentric Jupiters Pulsar planets Hot Neptunes / Super-Earths Transiting planets Rogue / Interstellar planets Hypothetical planet types Chthonian planets Ocean planets Trojan planets Brown dwarfs L-type stars T-type stars Sub-brown dwarfs Stars by spectral type O-type (blue) stars B-type (blue-white) stars A-type (white) stars F-type (yellow-white) stars G-type (yellow) stars K-type (orange) stars M-type (red) stars Peculiar stars Carbon stars S-type stars Shell stars Wolf-Rayet stars Peculiar A-type stars Metallic A-type stars Barium stars P Cygni stars Blue stragglers Stars by luminosity class Subdwarf stars Dwarf (Main sequence) stars Subgiant stars Giant stars Bright giant stars Supergiant stars Hypergiant stars Stars by population Population III stars Population II stars Halo stars Thick disk stars Population I stars Stars by stellar evolution Protostars Young stellar objects Main sequence stars Red giant stars Red supergiant stars Blue supergiant stars Wolf-Rayet stars White dwarf stars Neutron stars Variable stars Intrinsic variables Pulsating variables Cepheid variables W Virginis variables Delta Scuti variables RR Lyrae variables Mira variables Semiregular variables Irregular variables Beta Cephei variables Alpha Cygni variables RV Tauri variables Eruptive variables Flare stars T Tauri variables FU Orionis variables R Coronae Borealis variables Luminous blue variables Cataclysmic variables Symbiotic variables Dwarf novae Novae Supernovae Type I supernovae Type II supernovae Hypothetical Collapsars or Hypernovae Extrinsic variables Rotating variables Alpha2 CVn stars Rotating ellipsoidal variables Eclipsing binaries Algol stars, Algol Beta Lyrae stars W Ursae Majoris stars Compact stars White dwarfs Black dwarfs Neutron stars Magnetars Pulsars Hypothetical stars Quark stars Preon stars Black holes Stellar black hole Intermediate-mass black holes Supermassive black holes Gamma ray bursts	Planetary systems Star systems Single star systems Solar system Multiple star systems Binary stars Observation method: Optical binaries Visual binaries Astrometric binaries Spectroscopic binaries Eclipsing binaries Close binaries Detached binaries Semidetached binaries Contact binaries Unresolved binaries X-ray binaries X-ray bursters Triple star systems Stellar groupings Star clusters Stellar associations Open clusters Globular clusters Hypercompact stellar system Constellations Asterisms Galaxy components Galactic bulges Galactic bars Galactic rings Spiral arms Thin disks Thick disks Galactic halos Galactic coronae Galaxies Galaxies by morphology Spiral galaxies Barred spiral galaxies Lenticular galaxies Elliptical galaxies Ring galaxies Irregular galaxies Galaxies by size Brightest cluster galaxies Giant ellipticals Dwarf galaxies Active galaxies Quasars Blazars Radio galaxies Seyfert galaxies Starburst galaxies Dark galaxies Galaxy groups Galaxy clusters Superclusters Galaxy filaments and Voids	Accretion disc Circumstellar matter Debris disks Interplanetary medium Interplanetary magnetic field Circumstellar disk Protoplanetary disks Interstellar medium Interstellar cloud Nebulae Emission nebulae Planetary nebulae Supernova remnants Plerions H II regions Reflection nebulae Dark nebulae Molecular clouds Bok globules Proplyds H I regions Intergalactic medium Cosmic microwave background radiation Dark matter MACHOs WIMPs Hypothetical Cosmic string Domain wall

See also

• International Astronomical Naming Commission
• List of solar system objects
• Lists of astronomical objects
• List of light sources

References

1. ^ Task Group on Astronomical Designations from IAU Commission 5 (April 2008). "Naming Astronomical Objects". International Astronomical Union (IAU). http://www.iau.org/public/naming/. Retrieved 4 July 2010. 
2. ^ Narlikar, Jayant V. (1996). Elements of Cosmology. Universities Press. ISBN 8173710430. http://books.google.com/books?id=uZgbMUypq_oC&pg=PA4. 
3. ^ Smolin, Lee (1998). The life of the cosmos. Oxford University Press US. p. 35. ISBN 0195126645. 
4. ^ Buta, Ronald James; Corwin, Harold G.; Odewahn, Stephen C. (2007). The de Vaucouleurs atlas of galaxies. Cambridge University Press. p. 301. ISBN 0521820480. 
5. ^ Elmegreen, Bruce G. (January 2010). "The nature and nurture of star clusters". Star clusters: basic galactic building blocks throughout time and space, Proceedings of the International Astronomical Union, IAU Symposium. 266. pp. 3–13. Bibcode 2010IAUS..266....3E. doi:10.1017/S1743921309990809. 
6. ^ Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia (2004). Stellar interiors: physical principles, structure, and evolution. Astronomy and astrophysics library (2nd ed.). Springer. p. 86. ISBN 0387200894. 

External links

• SkyandTelescope.com SkyChart
• Monthly skymaps for every location on Earth