Astronomical object

An astronomical object is something natural that exists in space. These objects can be single pieces, like planets or stars, or they can be groups of many pieces, like star clusters or galaxies. In simple words, an astronomical object is anything you can find floating out in the universe.

Some examples of astronomical objects include planets, stars, moons, asteroids, comets, and even beautiful clouds of gas and dust called nebulae. These objects can be alone or part of bigger groups, like planets orbiting a star or many stars grouped together in a galaxy.

Astronomers, who are scientists who study space, use telescopes to look at these objects. They learn how they formed, how they move, and what they are made of. Understanding astronomical objects helps us know more about our place in the universe and how it all works together.

History

Further information: History of astronomy

See also: Scientific Revolution and Copernican Revolution

According to NASA astrophysicists, early space objects began to form about 13.6 billion years ago, just 200 million years after the Big Bang. Over time, gravity pulled light together to form the first stars and galaxies.

People have watched stars, planets, nebulae, asteroids, and comets for thousands of years. Ancient cultures saw these as deities. They used the movements of these objects to travel far distances, know the seasons, and decide when to plant crops. During the Middle Ages, scientists in the Middle East recorded details about stars and nebulae and created better calendars. In Europe, scientists built tools to study space and taught others in universities.

During the Scientific Revolution, Nicolaus Copernicus's heliocentric model showed that Earth and the planets orbit the Sun in the center of the Solar System. Johannes Kepler found rules about how planets move. Galileo Galilei used telescopes to see the moons of Jupiter, the phases of Venus, craters on the Moon, and spots on the Sun. Later, scientists used new technology to learn more about stars and other objects in space.

Galaxy and larger

The universe is organized in a special way. The biggest parts of this organization are called galaxies. Galaxies group together into groups and clusters, and these clusters are part of even bigger structures called superclusters. These superclusters form lines called filaments that stretch across space, with big empty areas called voids in between.

Galaxies come in many shapes. They can be irregular, elliptical, or disk-like. Their shapes depend on how they formed and changed over time, especially if they interacted or merged with other galaxies. Disk galaxies include lenticular and spiral galaxies, which have features like spiral arms and a bright area called a halo. Most galaxies have a very large black hole at their center. Smaller galaxies called dwarf galaxies and groups of stars known as globular clusters can orbit around larger galaxies.

Within a galaxy

Stars are bright objects that form from clouds of gas pulled together by gravity. They often group together in clusters and can have different shapes and sizes depending on their mass, what they are made of, and how they change over time. Some stars orbit each other in systems, and planets, asteroids, and comets can form around new stars.

Stars change in predictable ways, which we can see in a special chart called the Hertzsprung–Russell diagram. Some stars can become unstable and change in brightness. When stars end their lives, they can leave behind beautiful glowing clouds of gas or, in big explosions, leave behind dense objects like white dwarfs, neutron stars, or black holes.

Shape

Further information: Spherical Earth § Cause

See also: Equatorial bulge and Hydrostatic equilibrium § Planetary geology

The IAU definitions of planet and dwarf planet say that a body orbiting the Sun must be round, which happens because of its own gravity. This round shape is called hydrostatic equilibrium. We see this round shape on small rocky planets like Mars and big gas planets like Jupiter.

Smaller bodies that orbit the Sun and are not round are called small Solar System bodies (SSSBs). These can look very lumpy because they are made from dust and rocks that stuck together without enough heat to make them round. Some SSSBs are just groups of small rocks held together by gravity, while others are almost round but not quite. One example is the small Solar System body 4 Vesta, which is big enough to have changed a little inside.

Stars like our Sun are also round because of gravity acting on their hot, flowing plasma, which is like a fluid. The heat from stellar fusion in stars is much stronger than the heat from when they first formed.

Categories by location

See also: Lists of astronomical objects

See also: List of Solar System objects by size

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

Solar bodies	Extrasolar			Observable universe
	Simple bodies	Compound objects	Extended objects
Solar System Terrestrial planet Giant planet Gas giant Ice giant Heliosphere Oort cloud Hills Cloud Meteoroid Micrometeoroid Meteor Bolide Moons Moonlets Subsatellites (hypothet.) Minor planets (see below) Asteroids Dwarf planets Moons Binaries Synestia (hypothet.) Planets (see below) Ring system Trans-Neptunian objects Small Solar System body Comets Planetesimal Contact binary Sun Planets Mercury Venus Earth – Moon Mars – moons Jupiter – moons Saturn – moons Uranus – moons Neptune – moons Dwarf planets Pluto – moons Eris – Dysnomia Ceres Makemake – moon Haumea – moons Quaoar – Weywot Orcus – Vanth Gonggong – Xiangliu Sedna Others Minor planets Vulcanoids (hypothet.) ꞌAylóꞌchaxnims Atiras Near-Earth objects PHO Arjunas Atens Apollos Amors Mars-crossers Asteroid belt (families) Alindas Cybeles Eos Floras Hildas Hungarias Hygieas Koronis Marias Nysas Pallas Phocaeas Themis Vesta Trojans Earth Mars Jupiter Uranus Neptune Centaurs Damocloids Kuiper belt objects Classical KBOs Resonant TNOs Plutinos (2:3) Twotinos (1:2) Scattered disc objects Detached objects Sednoids	Exoplanets Chthonian (theoret.) Earth analog Eccentric Jupiter Exomoon Tidally detached exomoon Exocomet Hot Jupiter Hot Neptune Rogue planet Ocean (theoret.) Pulsar planet Super-Earth Tidally locked planet Eyeball planet (theoret.) Toroidal planet (theoret.) Trojan (theoret.) USP Brown dwarfs Types M  · L  · T  · Y Sub-brown dwarfs Stars (see sections below) Stellar classification Stellar population III, II, I Peculiar star Stellar evolution Variable star Compact star By luminosity / evolution Protostar Young stellar object Pre-main-sequence Main sequence Subdwarfs Subgiants Giants Red / Blue Bright giants Supergiants Red / Blue Hypergiants Ultra-cool dwarf Quasi-star (hypothet.) Compact stars (see below) Compact stars Black hole Stellar Intermediate-mass Supermassive GRBs BBHs Exotic star (hypothet.) Iron star (hypothet.) Neutron star Blitzar (hypothet.) Magnetar Pulsar Thorne–Żytkow object (hypothet.) Planck star (hypothet.) Preon star (hypothet.) Quark star (hypothet.) Strange star (hypothet.) White dwarf Black dwarf (theoret.) By peculiar stars A-type Peculiar  · Metallic Barium Blue straggler Carbon P Cygni S-type Shell Wolf–Rayet Variables – Extrinsic Rotating Alpha2 CVn Ellipsoidal Eclipsing binaries Algol Beta Lyrae W Ursae Majoris Variables – Intrinsic Pulsating Cepheids W Virginis Delta Scuti RR Lyrae Mira Semiregular Irregular Beta Cephei Alpha Cygni RV Tauri Eruptive variables Flare stars T Tauri FU Orionis RCr Borealis Luminous blue Cataclysmic Symbiotics Micronova Dwarf nova Nova Supernova Type: Ia  · Ib/c  · II Hypernova GRBs Failed supernova By spectral types O (blue) B (blue-white) A (white) F (yellow-white) G (yellow) K (orange) M (red)	Systems Planetary Star Stars in general Binary (see below) Triples Higher order Binary stars By observation Optical Visual Astrometric Spectroscopic Eclipsing Close binaries Detached Semidetached Contact X-ray Burster Stellar groupings Star cluster Stellar association Open Globular Hypercompact Constellation Asterism Galaxies Galaxies in general Group and cluster Satellite galaxy Supercluster By component Bulge Spiral arm Thin disk Thick disk Halo Corona Tidal tail Stellar stream By morphology Spiral Barred spiral Lenticular Elliptical Ring Irregular By size Brightest cluster Giant elliptical Dwarf By type Protogalaxy Starburst Dark Active Radio Seyfert Quasar Microquasar Blazar BL Lac FSRQ Red nugget Void galaxy	Discs and media Interplanetary Dust cloud Medium Magnetic field Stellar disc Accretion Circumstellar Protoplanetary Debris Interstellar Cloud Medium ORCs Intergalactic Dust Medium ORCs Nebulae Emission Planetary Supernova remnant Plerion H II region Reflection Dark nebulae Molecular cloud Bok globule Proplyd H I region Cosmic scale CMB Cosmic string (hypothet.) Dark matter MACHO WIMP Domain wall (hypothet.) Dust Filament LQG Void Supervoid	Logarithmic representation of the observable universe with the notable astronomical objects known today. From down to up the celestial bodies are arranged according to their proximity to the Earth. Infographic listing 210 notable astronomical objects marked on a central logarithmic map of the observable universe. A small view and some distinguishing features for each astronomical object are included.