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A star is a massive, luminous ball of plasma that is held together by gravity . The nearest star to Earth is the Sun , which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun.
Historically, the most prominent stars on the celestial sphere were grouped together into constellations , and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations .
For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space . Almost all elements heavier than hydrogen and helium were created by fusion processes in stars. Astronomers can determine the mass , age, chemical composition and many other properties of a star by observing its spectrum , luminosity and motion through space.
The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.
A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion.The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes.
The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun expand to become a red giant , in some cases fusing heavier elements at the core or in shells around the core.
The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.
Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits . When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy .
Almost everything about a star is determined by its initial mass, including essential characteristics such as luminosity and size, as well as the star's evolution, lifespan, and eventual fate.
Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old—the observed age of the universe . The oldest star yet discovered, HE 1523-0901 , is an estimated 13.2 billion years old.
The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.
When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium, as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age. The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.
The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun. By contrast, the super- metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet- bearing star 14 Herculis has nearly triple the iron.There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements .
Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus , with an angular diameter of only 0.057 arcseconds . The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required in order to produce images of these objects. Another technique for measuring the angular size of stars is through occultation .
By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.
Stars range in size from neutron stars , which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation , which has a diameter approximately 650 times larger than the Sun—about 0.9 billion kilometres . However, Betelgeuse has a much lower density than the Sun.
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants , an interstellar medium of gas and dust , and an important but poorly understood component tentatively dubbed dark matter . The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars,
all orbiting the galaxy's 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.
Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy ,  which has an ellipse -shaped light profile. Spiral galaxies are disk-shaped assemblages with dusty, curving arms.
Galaxies with irregular or unusual shapes are known as peculiar galaxies , and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation , producing what is called a starburst galaxy . Small galaxies that lack a coherent structure could also be referred to as irregular galaxies .
There are probably more than 100 billion galaxies in the observable universe . Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter .
The majority of galaxies are organized into a hierarchy of associations called clusters , which, in turn, can form larger groups called superclusters . These larger structures are generally arranged into sheets and filaments , which surround immense voids in the universe .
Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies.
They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.
A spiral galaxy is shaped like a disk, usually with a bulge in the center and with arms that spiral outwards as the galaxy rotates. Spiral galaxies tend to contain more middle-aged stars along with clouds of gas and dust. Elliptical galaxies contain older stars and very little gas and dust. They can be different shapes ranging from round, to flattened, elongated spheres.
TYPES OF GALAXIES
There are indeed different types of galaxies. The main types are spiral galaxies (like our own MilkyWay), elliptical galaxies and irregular galaxies. An irregular galaxy has an undefined shape and has lots of young stars, dust and gas.
In colloquial usage, a constellation is what astronomers call an asterism : a group of celestial bodies (usually stars) that appear to form a pattern in the sky or appear visibly related to each other.
Examples are Orion (which appears like a human figure with a belt, often referred to as "The Hunter"), Leo (which contains bright stars that outline the form of a lion), Scorpius (which can seem reminiscent of a scorpion), and Crux (a cross).
In astronomy, however, a constellation is an area of the sky, and contains all the stars and other celestial objects within that area. The International Astronomical Union (IAU) divides the sky into 88 official constellations  with exact boundaries, so that every direction or place in the sky is defined by one constellation.
Most of these constellations are centered on the traditional constellations of Western culture. Constellations were devised by ancient people to be able to recognize stars in the sky. The shapes of constellations resemble objects familiar to those people.
SOME CONSTELLATION FAMILIES Leo , Virgo , Libra , Scorpius , Sagittarius , Capricornus , Aquarius , Pisces , Aries , Taurus , Gemini , Cancer 12 Zodiac Ursa Major , Ursa Minor , Draco , Canes Venatici , Boötes , Coma Berenices , Corona Borealis , Camelopardalis , Lynx , Leo Minor 10 Ursa Major Constellations Total Family Hercules , Sagitta , Aquila , Lyra , Cygnus , Vulpecula , Hydra , Sextans , Crater , Corvus , Ophiuchus , Serpens , Scutum , Centaurus , Lupus , Corona Australis , Ara , Triangulum Australe , Crux Hercules 19 Cassiopeia , Cepheus , Andromeda , Perseus , Pegasus , Cetus , Auriga , Lacerta , Triangulum Perseus 09
The Solar System consists of the Sun and those celestial objects bound to it by gravity , all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the retinue of objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly-flat disc called the ecliptic plane . The four smaller inner planets, Mercury , Venus , Earth and Mars , also called the terrestrial planets , are primarily composed of rock and metal.
Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these regions, five individual objects, Ceres , Pluto , Haumea , Makemake and Eris , are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets . In addition to thousands of small bodies in those two regions, various other small body populations, such as comets , centaurs and interplanetary dust , freely travel between regions.
The four outer planets, Jupiter , Saturn , Uranus and Neptune , also called the gas giants , are composed largely of hydrogen and helium and are far more massive than the terrestrials.
The Solar System is also home to two regions populated by smaller objects. The asteroid belt , which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal.
The solar wind , a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere , which extends out to the edge of the scattered disc. The hypothetical Oort cloud , which acts as the source for long-period comets , may also exist at a distance roughly a thousand times further than the heliosphere.
Six of the planets and three of the dwarf planets are orbited by natural satellites , [b] usually termed "moons" after Earth's Moon . Each of the outer planets is encircled by planetary rings of dust and other particles.
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary . Technically, the term natural satellite could refer to a planet orbiting a star , or a dwarf galaxy orbiting a major galaxy , but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets , and minor planets . As of September 2008, 335 bodies are formally classified as moons. They include 167 orbiting six of the eight planets, 6 orbiting three of the five dwarf planets, 104 asteroid moons , and 58 satellites of Trans-Neptunian objects , some of which will likely turn out to be dwarf planets. Some 150 additional small bodies were observed within Saturn's ring system, but they were not tracked long enough to establish orbits. Planets around other stars are likely to have natural satellites as well, although none have been observed.
The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth 's moon: the four Galilean moons , Saturn 's Titan , and Neptune 's Triton . Saturn has an additional six mid-sized moons massive enough to have achieved hydrostatic equilibrium , and Uranus has five. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon, known as the Moon ; and Mars has two tiny moons, Phobos and Deimos . It has been suggested that a few moons, notably Europa , one of Jupiter's Galilean moons, may harbour life, though there is currently no direct evidence to support this claim.
Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do). Pluto has three known satellites, the rather large Charon and the smaller Nix and Hydra . Haumea has two moons , and Eris has one . The Pluto-Charon system is unusual in that the center of mass lies in open space between the two, a characteristic of a double planet system.
The first artificial satellite, Sputnik 1 , was launched by the Soviet Union in 1957. By 2009 thousands of satellites had been launched into orbit around the Earth. These originate from more than 50 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently operational, whereas thousands of unused satellites and satellite fragments orbit the Earth as space debris . A few space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Venus, Mars, Jupiter and Saturn.
In the context of spaceflight , a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon .
Satellites are used for a large number of purposes. Common types include military (spy) and civilian Earth observation satellites, communication satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit , polar orbit , and geostationary orbit .
Satellites are usually semi-independent computer controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, attitude control and orbit control.