Antares designated α Scorpii, is on average the fifteenth-brightest star in the night sky, the brightest object in the constellation of Scorpius. Distinctly reddish when viewed with the naked eye, Antares is a slow irregular variable star that ranges in brightness from apparent magnitude +0.6 to +1.6. Referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ Scorpii in the center of the constellation. Antares appears as a single star at naked eye, but it is a binary star with its two components called α Scorpii A and α Scorpii B. Classified as a red supergiant of spectral type M1.5Iab-Ib, Antares is the brightest, most massive, most evolved stellar member of the nearest OB association, the Scorpius–Centaurus Association. Antares is a member of the Upper Scorpius subgroup of the Scorpius–Centaurus Association, which contains thousands of stars with mean age 11 million years at a distance of 170 parsecs, its exact size remains uncertain, but if placed at the center of the Solar System it would reach to somewhere between the orbits of Mars and Jupiter.
Its mass is calculated to be around 12 times that of the Sun. Α Scorpii is the star's Bayer designation. It has the Flamsteed designation 21 Scorpii, as well as catalogue designations such as HR 6134 in the Bright Star Catalogue and HD 148478 in the Henry Draper Catalogue; as a prominent infrared source, it appears in the Two Micron All-Sky Survey catalogue as 2MASS J16292443-2625549 and the Infrared Astronomical Satellite Sky Survey Atlas catalogue as IRAS 16262-2619. It is catalogued as a double star WDS J16294-2626 and CCDM J16294-2626, its traditional name Antares derives from the Ancient Greek Ἀντάρης, meaning "rival to-Ares", due to the similarity of its reddish hue to the appearance of the planet Mars. The comparison of Antares with Mars may have originated with early Mesopotamian astronomers. However, some scholars have speculated that the star may have been named after Antar, or Antarah ibn Shaddad, the Arab warrior-hero celebrated in the pre-Islamic poems Mu'allaqat. In 2016, the International Astronomical Union organised a Working Group on Star Names to catalog and standardise proper names for stars.
The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Antares for the star α Scorpii A. It is now so entered in the IAU Catalog of Star Names. Antares and its red color have been known since antiquity. Antares is a variable star and is listed in the General Catalogue of Variable Stars but as a Bayer-designated star it does not have a separate variable star designation. Research published in 2017 demonstrated that Aboriginal people from South Australia observed the variability of Antares and incorporated it into their oral traditions as Waiyungari. Antares is visible in the sky all night around May 31 of each year, when the star is at opposition to the Sun. At this time, Antares sets at dawn as seen at the equator. For two to three weeks on either side of November 30, Antares is not visible in the night sky, because it is near conjunction with the Sun. Antares is a type LC slow irregular variable star, whose apparent magnitude varies between extremes of +0.6 and +1.6, although near magnitude +1.0.
There is no obvious periodicity, but statistical analyses have suggested periods of 1,733 days or 1650±640 days. No separate long secondary period has been detected, although it has been suggested that primary periods longer than a thousand days are analogous to long secondary periods. Antares is 4.57 degrees south of the ecliptic, one of four first magnitude stars within 6.5° of the ecliptic and so can be occulted by the Moon. On 31 July 2009, Antares was occulted by the Moon; the event was visible in much of the Middle East. Every year around December 2 the Sun passes 5° north of Antares. Lunar occultations of Antares are common, depending on the Saros cycle; the last cycle ended in 2010 and the next begins in 2023. Shown at right is a video of a reappearance event showing events for both components. Antares can be occulted by the planets, e.g. Venus, but these events are rare; the last occultation of Antares by Venus took place on September 17, 525 BC. Other planets have been calculated not to have occulted Antares over the last millennium, nor will this occur during the next millennium, as the planets following the ecliptic always pass northward of Antares due to the actual planetary node positions and inclinations.
Antares is a red supergiant star with a stellar classification of M1.5Iab-Ib, is indicated to be a spectral standard for that class. Due to the nature of the star, the derived parallax measurements have large errors, so that the true distance of Antares is 550 light-years from the Sun; the brightness of Antares at visual wavelengths is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the true bolometric luminosity is around 100,000 times that of the Sun. There is a large margin of error assigned to values for the bolometric luminosity 30% or more. There is considerable variation between values published by different authors, for example 75,900 L☉ and 97,700 L☉ published in 2012 and 2013; the mass of the star has been cal
The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, the largest among planetary satellites relative to the size of the planet that it orbits; the Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known. The Moon is thought to have formed not long after Earth; the most accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia. The Moon is in synchronous rotation with Earth, thus always shows the same side to Earth, the near side; the near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest visible celestial object in Earth's sky, its surface is dark, although compared to the night sky it appears bright, with a reflectance just higher than that of worn asphalt.
Its gravitational influence produces the ocean tides, body tides, the slight lengthening of the day. The Moon's average orbital distance is 1.28 light-seconds. This is about thirty times the diameter of Earth; the Moon's apparent size in the sky is the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly during a total solar eclipse; this matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is increasing. The Moon was first reached in September 1959 by an unmanned spacecraft; the United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, six manned landings between 1969 and 1972, with the first being Apollo 11. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, the Moon's history. Since the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft.
Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems and mythology; the usual English proper name for Earth's natural satellite is "the Moon", which in nonscientific texts is not capitalized. The noun moon is derived from Old English mōna, which stems from Proto-Germanic *mēnô, which comes from Proto-Indo-European *mḗh₁n̥s "moon", "month", which comes from the Proto-Indo-European root *meh₁- "to measure", the month being the ancient unit of time measured by the Moon; the name "Luna" is used. In literature science fiction, "Luna" is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of Earth's moon; the modern English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, luna. The adjective selenic is so used to refer to the Moon that this meaning is not recorded in most major dictionaries.
It is derived from the Ancient Greek word for the Moon, σελήνη, from, however derived the prefix "seleno-", as in selenography, the study of the physical features of the Moon, as well as the element name selenium. Both the Greek goddess Selene and the Roman goddess Diana were alternatively called Cynthia; the names Luna and Selene are reflected in terminology for lunar orbits in words such as apolune and selenocentric. The name Diana comes from the Proto-Indo-European *diw-yo, "heavenly", which comes from the PIE root *dyeu- "to shine," which in many derivatives means "sky and god" and is the origin of Latin dies, "day"; the Moon formed 4.51 billion years ago, some 60 million years after the origin of the Solar System. Several forming mechanisms have been proposed, including the fission of the Moon from Earth's crust through centrifugal force, the gravitational capture of a pre-formed Moon, the co-formation of Earth and the Moon together in the primordial accretion disk; these hypotheses cannot account for the high angular momentum of the Earth–Moon system.
The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body with the proto-Earth. The impact blasted material into Earth's orbit and the material accreted and formed the Moon; the Moon's far side has a crust, 30 mi thicker than that of the near side. This is thought to be; this hypothesis, although not perfect best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, the giant impact hypothesis emerged as the most consensual theory. Before the conference, there were parti
Beta Crucis called Mimosa, is a binary star system. It forms part of the prominent asterism called the Southern Cross. Β Crucis is the system's Bayer designation. Although β Crucis is at −60° declination, therefore not visible north of 30° latitude, in the time of the ancient Greeks and Romans it was visible north of 40° due to the precession of equinoxes, these civilizations regarded it as part of the constellation of Centaurus, it bore the historical name Becrux. Mimosa, derived from the Latin for'actor', may come from the flower of the same name. Becrux is a modern contraction of the Bayer designation. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars; the WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN. In Chinese, 十字架, meaning Cross, refers to an asterism consisting of α Crucis, β Crucis, γ Crucis, δ Crucis. Β Crucis itself is known as 十字架三. Based on parallax measurements, β Crucis is located at a distance of 280 ly from the Earth.
In 1957, German astronomer Wulff-Dieter Heintz discovered that it is a spectroscopic binary with components that are too close together to resolve with a telescope. The pair orbit each other every 5 years with an estimated separation that varies from 5.4 to 12.0 Astronomical Units. The system is only 8 to 11 million years old; the primary, β Crucis A, is a massive star with about 16 times the Sun's mass. The projected rotational velocity of this star is about 35 km s−1. However, the orbital plane of the pair is only about 10°, which means the inclination of the star's pole is likely to be low; this suggests that the azimuthal rotational velocity is quite high, at about 120 km s−1. With a radius of about 8.4 times the radius of the Sun, this would mean the star has a rotational period of only about 3.6 days.β Crucis A is a known β Cephei variable, although with an effective temperature of about 27,000 K it is at the hot edge of the instability strip where such stars are found. It has three different pulsation modes.
The periods of all three modes are in the range of 4.03–4.59 hours. The star has a stellar classification of B0.5 III, with the luminosity class of'III' indicating that this is a giant star that has exhausted the supply of hydrogen at its core. The high temperature of the star's outer envelope is what gives the star the blue-white hue, characteristic of B-type stars, it is generating a strong stellar wind and is losing about 10−8 M☉ per year, or the equivalent of the mass of the Sun every 100 million years. The wind is leaving the system with a velocity of 2,000 km; the secondary, β Crucis B, may be a main sequence star with a stellar class of B2. In 2007, a third companion was announced, which may be a low pre-main sequence star; the X-ray emission from this star was detected using the Chandra X-ray Observatory. Two other stars, located at angular separations of 44 and 370 arcseconds, are optical companions that are not physically associated with the system; the β Crucis system may be a member of the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus Association.
This is a stellar association of stars. Β Crucis is represented in the flags of Australia, New Zealand and Papua New Guinea as one of five stars making up the Southern Cross. It is featured in the flag of Brazil, along with 26 other stars, each of which represents a state. Mimosa represents the State of Rio de Janeiro. A vessel named. An episode dedicated to the vessel features in the television documentary series Mighty Ships. Http://jumk.de/astronomie/big-stars/becrux.shtml
Betelgeuse designated α Orionis, is on average the ninth-brightest star in the night sky and second-brightest in the constellation of Orion. It is a distinctly reddish, semiregular variable star whose apparent magnitude varies between 0.0 and 1.3, the widest range of any first-magnitude star. Betelgeuse is one of three stars that make up the Winter Triangle asterism, it marks the center of the Winter Hexagon. If the human eye could view all wavelengths of radiation, Betelgeuse would be the brightest star in the night sky. Classified as a red supergiant of spectral type M1-2, the star is one of the largest stars visible to the naked eye. If Betelgeuse were at the center of the Solar System, its surface would extend past the asteroid belt engulfing the orbits of Mercury, Earth and Jupiter. However, there are several other red supergiants in the Milky Way that could be larger, such as Mu Cephei and VY Canis Majoris. Calculations of its mass range from under ten to a little over twenty times that of the Sun.
It is calculated to be 640 light-years away, yielding an absolute magnitude of about −6. Less than 10 million years old, Betelgeuse has evolved because of its high mass. Having been ejected from its birthplace in the Orion OB1 Association—which includes the stars in Orion's Belt—this runaway star has been observed moving through the interstellar medium at a speed of 30 km/s, creating a bow shock over four light-years wide. Betelgeuse is in a late stage of stellar evolution, it is expected to explode as a supernova within the next million years. In 1920, Betelgeuse became the first extrasolar star to have the angular size of its photosphere measured. Subsequent studies have reported an angular diameter ranging from 0.042 to 0.056 arcseconds, with the differences ascribed to the non-sphericity, limb darkening and varying appearance at different wavelengths. It is surrounded by a complex, asymmetric envelope 250 times the size of the star, caused by mass loss from the star itself; the angular diameter of Betelgeuse is only exceeded by the Sun.
Α Orionis is the star's designation given by Johann Bayer in 1603. The traditional name Betelgeuse is derived from the Arabic إبط الجوزاء Ibṭ al-Jauzā’, meaning "the underarm of Orion", or يد الجوزاء Yad al-Jauzā’, meaning "the hand of Orion". In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars; the WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Betelgeuse for this star. It is now so entered in the IAU Catalog of Star Names. Betelgeuse and its red coloration have been noted since antiquity. In the nineteenth century, before modern systems of stellar classification, Angelo Secchi included Betelgeuse as one of the prototypes for his Class III stars. By contrast, three centuries before Ptolemy, Chinese astronomers observed Betelgeuse as having a yellow coloration; the variation in Betelgeuse's brightness was first described in 1836 by Sir John Herschel, when he published his observations in Outlines of Astronomy.
From 1836 to 1840, he noticed significant changes in magnitude when Betelgeuse outshone Rigel in October 1837 and again in November 1839. A 10-year quiescent period followed. Observers recorded unusually high maxima with an interval of years, but only small variations from 1957 to 1967; the records of the American Association of Variable Star Observers show a maximum brightness of 0.2 in 1933 and 1942, a minimum of 1.2, observed in 1927 and 1941. This variability in brightness may explain why Johann Bayer, with the publication of his Uranometria in 1603, designated the star alpha as it rivaled the brighter Rigel. From Arctic latitudes, Betelgeuse's red colour and higher location in the sky than Rigel meant the Inuit regarded it as brighter, one local name was Ulluriajjuaq "large star". In 1920, Albert Michelson and Francis Pease mounted a 6-meter interferometer on the front of the 2.5-meter telescope at Mount Wilson Observatory. Helped by John Anderson, the trio measured the angular diameter of Betelgeuse at 0.047", a figure which resulted in a diameter of 3.84 × 108 km based on the parallax value of 0.018".
However, limb darkening and measurement errors resulted in uncertainty about the accuracy of these measurements. The 1950s and 1960s saw two developments that would affect stellar convection theory in red supergiants: the Stratoscope projects and the 1958 publication of Structure and Evolution of the Stars, principally the work of Martin Schwarzschild and his colleague at Princeton University, Richard Härm; this book disseminated ideas on how to apply computer technologies to create stellar models, while the Stratoscope projects, by taking balloon-borne telescopes above the Earth's turbulence, produced some of the finest images of solar granules and sunspots seen, thus confirming the existence of convection in the solar atmosphere. Astronomers in the 1970s saw some major advances in astronomical imaging technology beginning with Antoine Labeyrie's invention of speckle interferometry, a pr
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines; each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary due to the temperature of the photosphere, although in some cases there are true abundance differences; the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are classified under the Morgan-Keenan system using the letters O, B, A, F, G, K, M, a sequence from the hottest to the coolest; each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, class D for white dwarfs; the full spectral class for the Sun is G2V, indicating a main-sequence star with a temperature around 5,800 K. The conventional color description takes into account only the peak of the stellar spectrum. In actuality, stars radiate in all parts of the spectrum; because all spectral colors combined appear white, the actual apparent colors the human eye would observe are far lighter than the conventional color descriptions would suggest. This characteristic of'lightness' indicates that the simplified assignment of colors within the spectrum can be misleading.
Excluding color-contrast illusions in dim light, there are indigo, or violet stars. Red dwarfs are a deep shade of orange, brown dwarfs do not appear brown, but hypothetically would appear dim grey to a nearby observer; the modern classification system is known as the Morgan–Keenan classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type. Other modern stellar classification systems, such as the UBV system, are based on color indexes—the measured differences in three or more color magnitudes; those numbers are given labels such as "U-V" or "B-V", which represent the colors passed by two standard filters. The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system. Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions.
Main-sequence stars vary in surface temperature from 2,000 to 50,000 K, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are listed from hottest to coldest; the spectral classes O through M, as well as other more specialized classes discussed are subdivided by Arabic numerals, where 0 denotes the hottest stars of a given class. For example, A0 denotes A9 denotes the coolest ones. Fractional numbers are allowed; the Sun is classified as G2. Conventional color descriptions are traditional in astronomy, represent colors relative to the mean color of an A class star, considered to be white; the apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, stars with particular spectral features such as carbon stars may be far redder than any black body.
The fact that the Harvard classification of a star indicated its surface or photospheric temperature was not understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated, this was suspected to be true. In the 1920s, the Indian physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to the solar chromosphere to stellar spectra. Harvard astronomer Cecilia Payne demonstrated that the O-B-A-F-G-K-M spectral sequence is a sequence in temperature; because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals; the Yerkes spectral classification called the MKK system from the authors' initial
Fomalhaut designated Alpha Piscis Austrini is the brightest star in the constellation of Piscis Austrinus and one of the brightest stars in the sky. It is a class A star on the main sequence 25 light-years from the Sun as measured by the Hipparcos astrometry satellite. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified, it is classified as a Vega-like star that emits excess infrared radiation, indicating it is surrounded by a circumstellar disk. Fomalhaut, K-type main-sequence star TW Piscis Austrini, M-type, red dwarf star LP 876-10 constitute a triple system though the companions are separated by several degrees. Fomalhaut holds a special significance in extrasolar planet research, as it is the center of the first stellar system with an extrasolar planet candidate imaged at visible wavelengths; the image was published in Science in November 2008. Fomalhaut is the third-brightest star known to have a planetary system, after the Pollux.
Α Piscis Austrini is the system's Bayer designation. It bears the Flamsteed designation of 24 Piscis Austrini; the classical astronomer Ptolemy put it in Aquarius, as well as Piscis Austrinus. In the 1600s Johann Bayer planted it in the primary position of Piscis Austrinus. Following Ptolemy, John Flamsteed in 1725 additionally denoted it 79 Aquarii; the current designation reflects modern consensus on Bayer's decision, that the star belongs in Piscis Austrinus. Under the rules for naming objects in multiple star systems, the three components – Fomalhaut, TW Piscis Austrini and LP 876-10 – are designated A, B and C, respectively. On its discovery, the planet was designated Fomalhaut b; the star's traditional name derives from Fom al-Haut from scientific Arabic فم الحوت fam al-ḥūt "the mouth of the Fish", a translation of how Ptolemy labeled it. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars; the WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included the name Fomalhaut for this star.
In July 2014, the International Astronomical Union launched a process for giving proper names to certain exoplanets. The process involved public voting for the new names. In December 2015, the IAU announced; the winning name was proposed by Todd Vaccaro and forwarded by the St. Cloud State University Planetarium of St. Cloud, United States of America, to the IAU for consideration. Dagon was a Semitic deity represented as half-man, half-fish. At a declination of −29.6°, Fomalhaut is located south of the celestial equator, hence is best viewed from the Southern Hemisphere. However, its southerly declination is not as great as that of stars such as Acrux, Alpha Centauri and Canopus, meaning that, unlike them, Fomalhaut is visible from a large part of the Northern Hemisphere as well, its declination is similar to that of Antares. At 40°N, Fomalhaut rises above the horizon for eight hours and reaches only 20° above the horizon, while Capella, which rises at the same time, will stay above the horizon for twenty hours.
Fomalhaut can be located in northern latitudes by the fact that the western side of the Square of Pegasus points to it. Continuing the line from Beta to Alpha Pegasi towards the southern horizon, Fomalhaut is about 45˚ south of Alpha Pegasi, with no bright stars in between. Fomalhaut is a young star, for many years thought to be only 100 to 300 million years old, with a potential lifespan of a billion years. A 2012 study gave a higher age of 440±40 million years; the surface temperature of the star is around 8,590 K. Fomalhaut's mass is about 1.92 times that of the Sun, its luminosity is about 16.6 times greater, its diameter is 1.84 times as large. Fomalhaut is metal-deficient compared to the Sun, which means it is composed of a smaller percentage of elements other than hydrogen and helium; the metallicity is determined by measuring the abundance of iron in the photosphere relative to the abundance of hydrogen. A 1997 spectroscopic study measured a value equal to 93% of the Sun's abundance of iron.
A second 1997 study deduced a value of 78%, by assuming Fomalhaut has the same metallicity as the neighboring star TW Piscis Austrini, which has since been argued to be a physical companion. In 2004, a stellar evolutionary model of Fomalhaut yielded a metallicity of 79%. In 2008, a spectroscopic measurement gave a lower value of 46%. Fomalhaut has been claimed to be one of 16 stars belonging to the Castor Moving Group; this is an association of stars which share a common motion through space, have been claimed to be physically associated. Other members of this group include Vega; the moving group has an estimated age of 200±100 million years and originated from the same location. More recent work has found that purported members of the Castor Moving Group appear to not only have a wide range of ages, but their velocities are too different to have been associated with one another in the distant past. Hence, "membership" to this dynamical group has no bearing on the age of the Fomalhaut system. Fomalhaut is surrounded by several debris disks.
The inner disk is a high-carbon small-grain ash disk, clustering at 0.1 AU from the star. Next is a disk of larger particles, with inner edge 0.4-1 AU of the star. The innermost dis
The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury; the Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter; the four smaller inner planets, Venus and Mars, are terrestrial planets, being composed of rock and metal. The four outer planets are giant planets, being more massive than the terrestrials; the two largest and Saturn, are gas giants, being composed of hydrogen and helium. All eight planets have circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed of ices, beyond them a newly discovered population of sednoids. Within these populations are several dozen to tens of thousands of objects large enough that they have been rounded by their own gravity; such objects are categorized as dwarf planets. Identified dwarf planets include the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets and interplanetary dust clouds travel between regions. Six of the planets, at least four of the dwarf planets, many of the smaller bodies are orbited by natural satellites termed "moons" after the Moon; each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; the Oort cloud, thought to be the source for long-period comets, may exist at a distance a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy. For most of history, humanity did not understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.
In the 17th century, Galileo discovered that the Sun was marked with sunspots, that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning once every 75–76 years; this was the first evidence that anything other than the planets orbited the Sun. Around this time, the term "Solar System" first appeared in English. In 1838, Friedrich Bessel measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of unmanned spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun; the principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.
The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System together comprise less than 0.002% of the Solar System's total mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic; the planets are close to the ecliptic, whereas comets and Kuiper belt objects are at greater angles to it. All the planets, most other objects, orbit the Sun in the same direction that the Sun is rotating. There are exceptions, such as Halley's Comet; the overall structure of the charted regions of the Solar System consists of the Sun, four small inner planets surrounded by a belt of rocky asteroids, four giant planets surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions; the inner Solar System includes the asteroid belt. The outer Solar System is including the four giant planets.
Since the discovery of the Kuiper belt, the outermost parts of the Solar Sys