Jason was an ancient Greek mythological hero, the leader of the Argonauts whose quest for the Golden Fleece featured in Greek literature. He was the son of the rightful king of Iolcos, he was married to the sorceress Medea. He was the great-grandson of the messenger god Hermes, through his mother's side. Jason appeared in various literary works in the classical world of Greece and Rome, including the epic poem Argonautica and the tragedy Medea. In the modern world, Jason has emerged as a character in various adaptations of his myths, such as the 1963 film Jason and the Argonauts and the 2000 TV miniseries of the same name. Jason's father is invariably Aeson. According to various authors, she could be: Alcimede, daughter of Phylacus Polymede, or Polymele, or Polypheme, a daughter of Autolycus Amphinome Theognete, daughter of Laodicus Rhoeo Arne or ScarpheJason was said to have had a younger brother Promachus. By Medea: Alcimenes, murdered by Medea. Thessalus, twin of Alcimenes and king of Iolcus.
Tisander, murdered by Medea Mermeros killed either by the Corinthians or by Medea Pheres, as above Eriopis, their only daughter Medus or Polyxemus, otherwise son of Aegeus Argus seven sons and seven daughtersBy Hypsipyle: Euneus, King of Lemnos and his twin Nebrophonus or Deipylus or Thoas Pelias was power-hungry and sought to gain dominion over all of Thessaly. Pelias was the progeny of a union between their shared mother, the daughter of Salmoneus, the sea god Poseidon. In a bitter feud, he overthrew Aeson, he spared his half-brother for unknown reasons. Aeson's wife Alcimede I had a newborn son named Jason whom she saved from Pelias by having female attendants cluster around the infant and cry as if he were still-born. Fearing that Pelias would notice and kill her son, Alcimede sent him away to be reared by the centaur Chiron,. Pelias, fearing that his ill-gotten kingship might be challenged, consulted an oracle, who warned him to beware of a man wearing only one sandal. Many years Pelias was holding games in honor of Poseidon when the grown Jason arrived in Iolcus, having lost one of his sandals in the river Anauros while helping an old woman to cross.
She blessed him. When Jason entered Iolcus, he was announced as a man wearing only one sandal. Jason, aware Pelias. Pelias replied, "To take my throne, which you shall, you must go on a quest to find the Golden Fleece." Jason accepted this condition. Jason assembled for a number of heroes, known as the Argonauts after their ship, the Argo; the group of heroes included the Boreads who could fly, Philoctetes, Telamon, Orpheus and Pollux, Atalanta and Euphemus. The isle of Lemnos is situated off the Western coast of Asia Minor; the island was inhabited by a race of women. The women had neglected their worship of Aphrodite, as a punishment the goddess made the women so foul in stench that their husbands could not bear to be near them; the men took concubines from the Thracian mainland opposite, the spurned women, angry at Aphrodite, killed all the male inhabitants while they slept. The king, was saved by Hypsipyle, his daughter, who put him out to sea sealed in a chest from which he was rescued; the women of Lemnos lived for a while with Hypsipyle as their queen.
During the visit of the Argonauts the women mingled with the men creating a new "race" called Minyae. Jason fathered twins with the queen. Heracles pressured them to leave, he had not taken part, unusual considering the numerous affairs he had with other women. After Lemnos the Argonauts landed among the Doliones, he forgot to mention what lived there. What lived in the land beyond Bear Mountain were the Gegeines, which are a tribe of Earthborn giants with six arms and wore leather loincloths. While most of the crew went into the forest to search for supplies, the Gegeines saw that few Argonauts were guarding the ship and raided it. Heracles was among those guarding the ship at the time and managed to kill most them before Jason and the others returned. Once some of the other Gegeines were killed and the Argonauts set sail. Sometime after their fight with the Gegeines, they sent some men to find water. Among these men was Heracles' servant Hylas, gathering water while Heracles was out finding some wood to carve a new oar to replace the one that broke.
The nymphs of the stream where Hylas was collecting were attracted to his good looks, pulled him into the stream. Heracles returned to his Labors. Others say that Heracles went to Colchis with the Argonauts, got the Golden Girdle of the Amazons and slew the Stymphalian Birds at that time; the Argonauts departed, landing again at the same spot that night. In the darkness, the Doliones took them for enemies and they started fighting each other; the Argonauts killed many of the Doliones, among them. Cyzicus' wife killed herself; the Argonauts realized their horrible mistake when dawn held a funeral for him. Soon Jason reached the court of Phineus of Salmydessus in Thrace. Zeus had sent the harpies to stea
In Greek mythology, Menelaus was a king of Mycenaean Sparta, the husband of Helen of Troy, the son of Atreus and Aerope. According to the Iliad, Menelaus was a central figure in the Trojan War, leading the Spartan contingent of the Greek army, under his elder brother Agamemnon, king of Mycenae. Prominent in both the Iliad and Odyssey, Menelaus was popular in Greek vase painting and Greek tragedy, the latter more as a hero of the Trojan War than as a member of the doomed House of Atreus. Although early authors, such as Aeschylus refer in passing to Menelaus’ early life, detailed sources are quite late, post-dating 5th-century BC Greek tragedy. According to these sources, Menelaus' father, had been feuding with his brother Thyestes over the throne of Mycenae. After a back-and-forth struggle that featured adultery and cannibalism, Thyestes gained the throne after his son Aegisthus murdered Atreus; as a result, Atreus’ sons and Agamemnon, went into exile. They first stayed with King Polyphides of Sicyon, with King Oeneus of Calydon.
But when they thought the time was ripe to dethrone Mycenae’s hostile ruler, they returned. Assisted by King Tyndareus of Sparta, they drove Thyestes away, Agamemnon took the throne for himself; when it was time for Tyndareus’ stepdaughter Helen to marry, many kings and princes came to seek her hand. Among the contenders were Odysseus, Ajax the Great and Idomeneus. Most offered opulent gifts. Tyndareus would accept none of the gifts, nor would he send any of the suitors away for fear of offending them and giving grounds for a quarrel. Odysseus promised to solve the problem in a satisfactory manner if Tyndareus would support him in his courting of Tyndareus’s niece Penelope, the daughter of Icarius. Tyndareus agreed, Odysseus proposed that, before the decision was made, all the suitors should swear a most solemn oath to defend the chosen husband in any quarrel, it was decreed that straws were to be drawn for Helen’s hand. The suitor who won was Menelaus; the rest of the suitors swore their oaths, Helen and Menelaus were married, Menelaus becoming a ruler of Sparta with Helen after Tyndareus and Leda abdicated the thrones.
Menelaus and Helen had a daughter Hermione as supported, for example, by Sappho, while some variations of the myth suggest they had three sons as well: Aithiolas and Pleisthenes. Their palace has been discovered in Laconia, to the north-west of modern Sparta. Other archaeologists consider that Pellana is too far away from other Mycenaean centres to have been the "capital of Menelaus". According to legend, in return for awarding her a golden apple inscribed "to the fairest," Aphrodite promised Paris the most beautiful woman in all the world. After concluding a diplomatic mission to Sparta during the latter part of which Menelaus was absent to attend the funeral of his maternal grandfather Catreus in Crete, Paris ran off to Troy with Helen despite his brother Hector's prohibition. Invoking the oath of Tyndareus and Agamemnon raised a fleet of a thousand ships and went to Troy to secure Helen's return. Homer's Iliad is the most comprehensive source for Menelaus’s exploits during the Trojan War.
In Book 3, Menelaus challenges Paris to a duel for Helen’s return. Menelaus soundly beats Paris, but before he can kill him and claim victory, Aphrodite spirits Paris away inside the walls of Troy. In Book 4, while the Greeks and Trojans squabble over the duel’s winner, Athena inspires the Trojan Pandarus to shoot Menelaus with his bow and arrow. However, Athena never intended for Menelaus to die and she protects him from the arrow of Pandarus. Menelaus is wounded in the abdomen, the fighting resumes. In Book 17, Homer gives Menelaus an extended aristeia as the hero retrieves the corpse of Patroclus from the battlefield. According to Hyginus, Menelaus killed eight men in the war, was one of the Greeks hidden inside the Trojan Horse. During the sack of Troy, Menelaus killed Deiphobus. There are four versions of Menelaus’ and Helen’s reunion on the night of the sack of Troy: Menelaus sought out Helen in the conquered city. Raging at her infidelity, he raised his sword to kill her, but as he saw her weeping at his feet, begging for her life, Menelaus' wrath left him.
He decided to take her back as his wife. Menelaus resolved to kill Helen, but her irresistible beauty prompted him to drop his sword and take her back to his ship “to punish her at Sparta”, as he claimed. According to the Bibliotheca, Menelaus raised his sword in front of the temple in the central square of Troy to kill her, but his wrath went away when he saw her rending her clothes in anguish, revealing her naked breasts. A similar version by Stesichorus in “Ilion’s Conquest” narrated that Menelaus surrendered her to his soldiers to stone her to death, but when she ripped the front of her robes, the Achaean warriors were stunned by her beauty and the stones fell harmlessly from their hands. Book 4 of the Odyssey provides an account of his homelife in Sparta; when visited by Odysseus’ son Telemachus, Menelaus recounts his voyage home. As happened to many Greeks, Menelaus' homebound fleet was blown by storms to Crete and Egypt where they were becalmed, unable to sail away, they forced him to reveal how to make the voyage home.
After their homecoming and Helen
Pictor is a constellation in the Southern Celestial Hemisphere, located between the star Canopus and the Large Magellanic Cloud. Its name is Latin for painter, is an abbreviation of the older name Equuleus Pictoris. Represented as an easel, Pictor was named by Abbé Nicolas-Louis de Lacaille in the 18th century; the constellation's brightest star is Alpha Pictoris, a white main-sequence star around 97 light-years away from Earth. Pictor hosts RR Pictoris, a cataclysmic variable star system that flared up as a nova, reaching apparent magnitude 1.2 in 1925 before fading into obscurity. Pictor has attracted attention because of its second-brightest star Beta Pictoris, 63.4 light-years distant from Earth, surrounded by an unusual dust disk rich in carbon, as well as an exoplanet. Another five stars in the constellation have been observed to have planets. Among them is HD 40307, an orange dwarf that has six planets orbiting it, one of which—HD 40307 g—is a potential super-Earth in the circumstellar habitable zone.
Kapteyn's Star, the nearest star in Pictor to Earth, is a red dwarf located 12.76 light-years away, found to have two super-Earths in orbit in 2014. Pictor A is a radio galaxy, shooting an 800,000 light-year long jet of plasma from a supermassive black hole at its centre. In 2006, a gamma-ray burst—GRB 060729—was observed in Pictor, its long X-ray afterglow detectable for nearly two years; the French astronomer Abbé Nicolas-Louis de Lacaille first described Pictor as le Chevalet et la Palette in 1756, after observing and cataloguing 10,000 southern stars during a two-year stay at the Cape of Good Hope. He devised 14 new constellations in uncharted regions of the Southern Celestial Hemisphere not visible from Europe. All but one honored instruments that symbolised the Age of Enlightenment, he gave these constellations Bayer designations, including ten stars in Pictor now named Alpha to Nu Pictoris. He labelled the constellation Equuleus Pictorius on his 1763 chart, the word "Equuleus" meaning small horse, or easel—perhaps from an old custom among artists of carrying a canvas on a donkey.
The German astronomer Johann Bode called it Pluteum Pictoris. The name was shortened to its current form in 1845 by the English astronomer Francis Baily on the suggestion of his countryman Sir John Herschel. Pictor is a small constellation bordered by Columba to the north and Carina to the east, Caelum to the northwest, Dorado to the southwest and Volans to the south; the three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "Pic". The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 18 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 04h 32.5m and 06h 52.0m, while the declination coordinates are between −42.79° and −64.15°. Pictor culminates each year at 9 p.m. on 17 March. Its position in the far Southern Celestial Hemisphere means that the whole constellation is visible to observers south of latitude 26°N, parts become circumpolar south of latitude 35°S.
Pictor is a faint constellation. Within the constellation's borders, there are 49 stars brighter than or equal to apparent magnitude 6.5. Located about 97 light-years away from Earth, Alpha Pictoris is the brightest star in the constellation. A spinning star with a projected rotational velocity estimated at 206 km/s, it has a shell of circumstellar gas. Beta Pictoris is another white main sequence star of spectral type A6V and apparent magnitude 3.86. Located around 63.4 light-years distant from Earth, it is a member of the Beta Pictoris moving group—a group of 17 star systems around 12 million years old moving through space together. In 1984 Beta Pictoris was the first star discovered to have a debris disk. Since an exoplanet about eight times the mass of Jupiter has been discovered orbiting 8 astronomical units away from the star—a similar distance as that between our Sun and Saturn; the European Southern Observatory confirmed its presence through the use of direct imagery with the Very Large Telescope in late 2009.
Gamma Pictoris is an orange giant of spectral type K1III that has swollen to 1.4 times the diameter of the Sun. Shining with an apparent magnitude of 4.5, it lies 174 light-years distant from Earth. HD 42540, called 47 Pictoris by American astronomer Benjamin Apthorp Gould, is a cooler orange giant, with a spectral type of K2.5III and average magnitude 5.04. It has been suspected of being a variable star. Lacaille mistakenly named this star Mu Doradus, but had recorded its Right Ascension one hour too low. Lacaille named two neighbouring stars Eta Pictoris. Eta2 Pictoris known as HR 1663, is an orange giant of spectral type K5III and apparent magnitude 5.05. 474 light-years distant, it has a diameter 5.6 times that of the Sun. Eta1 Pictoris known as HR 1649, is 85 light-years distant and is a main sequence star of spectral type F5V and visual magnitude 5.38. A double star, it has a companion of magnitude 13. Located about 1298 light-years from Earth, Delta Pictoris is an eclipsing binary of the Beta Lyrae type.
Composed of two blue stars of spectral types B3III and O9V, the system has a period of 1.67 days, is observed to dip from apparent magnitude 4.65 to 4.9. The stars are oval-shaped. TV Pictoris is a spectroscopic binary system composed of an A-type star and an F-type star which rota
A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called multiple star systems; these systems when more distant appear to the unaided eye as a single point of light, are revealed as multiple by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems; the term double star is used synonymously with binary star. Optical doubles are so called because the two stars appear close together in the sky as seen from the Earth, their "doubleness" depends only on this optical effect. A double star can be revealed as optical by means of differences in their parallax measurements, proper motions, or radial velocities. Most known double stars have not been studied adequately to determine whether they are optical doubles or doubles physically bound through gravitation into a multiple star system. Binary star systems are important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated.
This determines an empirical mass-luminosity relationship from which the masses of single stars can be estimated. Binary stars are detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known, they may be detected by indirect techniques, such as spectroscopy or astrometry. If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other. If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Sirius, Cygnus X-1. Binary stars are common as the nuclei of many planetary nebulae, are the progenitors of both novae and type Ia supernovae; the term binary was first used in this context by Sir William Herschel in 1802, when he wrote: If, on the contrary, two stars should be situated near each other, at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will compose a separate system, remain united by the bond of their own mutual gravitation towards each other.
This should be called a real double star. By the modern definition, the term binary star is restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries. For most of the known visual binary stars one whole revolution has not been observed yet, they are observed to have travelled along a curved path or a partial arc; the more general term double star is used for pairs of stars which are seen to be close together in the sky. This distinction is made in languages other than English. Double stars may be binary systems or may be two stars that appear to be close together in the sky but have vastly different true distances from the Sun; the latter are termed optical optical pairs. Since the invention of the telescope, many pairs of double stars have been found. Early examples include Acrux. Mizar, in the Big Dipper, was observed to be double by Giovanni Battista Riccioli in 1650; the bright southern star Acrux, in the Southern Cross, was discovered to be double by Father Fontenay in 1685.
John Michell was the first to suggest that double stars might be physically attached to each other when he argued in 1767 that the probability that a double star was due to a chance alignment was small. William Herschel began observing double stars in 1779 and soon thereafter published catalogs of about 700 double stars. By 1803, he had observed changes in the relative positions in a number of double stars over the course of 25 years, concluded that they must be binary systems. Since this time, many more double stars have been measured; the Washington Double Star Catalog, a database of visual double stars compiled by the United States Naval Observatory, contains over 100,000 pairs of double stars, including optical doubles as well as binary stars. Orbits are known for only a few thousand of these double stars, most have not been ascertained to be either true binaries or optical double stars; this can be determined by observing the relative motion of the pairs. If the motion is part of an orbit, or if the stars have similar radial velocities and the difference in their proper motions is small compared to their common proper motion, the pair is physical.
One of the tasks that remains for visual observers of double stars is to obtain sufficient observations to prove or disprove gravitational connection. Binary stars are classified into four types accordi
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes.
The brightest stars in the night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x
A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so all of them disintegrate and never hit the Earth's surface. Intense or unusual meteor showers are known as meteor outbursts and meteor storms, which produce at least 1,000 meteors an hour, most notably from the Leonids; the Meteor Data Centre lists over 900 suspected meteor showers of which about 100 are well established. Several organizations point to viewing opportunities on the Internet; the first great meteor storm in the modern era was the Leonids of November 1833. One estimate is a peak rate of over one hundred thousand meteors an hour, but another, done as the storm abated, estimated in excess of two hundred thousand meteors during the 9 hours of storm, over the entire region of North America east of the Rocky Mountains.
American Denison Olmsted explained the event most accurately. After spending the last weeks of 1833 collecting information, he presented his findings in January 1834 to the American Journal of Science and Arts, published in January–April 1834, January 1836, he noted the shower was of short duration and was not seen in Europe, that the meteors radiated from a point in the constellation of Leo and he speculated the meteors had originated from a cloud of particles in space. Work continued, yet coming to understand the annual nature of showers though the occurrences of storms perplexed researchers; the actual nature of meteors was still debated during the XIX century. Meteors were conceived as an atmospheric phenomenon by many scientists until the Italian astronomer Giovanni Schiaparelli ascertained the relation between meteors and comets in his work "Notes upon the astronomical theory of the falling stars". In the 1890s, Irish astronomer George Johnstone Stoney and British astronomer Arthur Matthew Weld Downing, were the first to attempt to calculate the position of the dust at Earth's orbit.
They studied the dust ejected in 1866 by comet 55P/Tempel-Tuttle in advance of the anticipated Leonid shower return of 1898 and 1899. Meteor storms were anticipated, but the final calculations showed that most of the dust would be far inside of Earth's orbit; the same results were independently arrived at by Adolf Berberich of the Königliches Astronomisches Rechen Institut in Berlin, Germany. Although the absence of meteor storms that season confirmed the calculations, the advance of much better computing tools was needed to arrive at reliable predictions. In 1981 Donald K. Yeomans of the Jet Propulsion Laboratory reviewed the history of meteor showers for the Leonids and the history of the dynamic orbit of Comet Tempel-Tuttle. A graph from it was re-published in Sky and Telescope, it showed relative positions of the Earth and Tempel-Tuttle and marks where Earth encountered dense dust. This showed that the meteoroids are behind and outside the path of the comet, but paths of the Earth through the cloud of particles resulting in powerful storms were near paths of nearly no activity.
In 1985, E. D. Kondrat'eva and E. A. Reznikov of Kazan State University first identified the years when dust was released, responsible for several past Leonid meteor storms. In 1995, Peter Jenniskens predicted the 1995 Alpha Monocerotids outburst from dust trails. In anticipation of the 1999 Leonid storm, Robert H. McNaught, David Asher, Finland's Esko Lyytinen were the first to apply this method in the West. In 2006 Jenniskens published predictions for future dust trail encounters covering the next 50 years. Jérémie Vaubaillon continues to update predictions based on observations each year for the Institut de Mécanique Céleste et de Calcul des Éphémérides; because meteor shower particles are all traveling in parallel paths, at the same velocity, they will all appear to an observer below to radiate away from a single point in the sky. This radiant point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon when viewed from the middle of the tracks.
Meteor showers are always named after the constellation from which the meteors appear to originate. This "fixed point" moves across the sky during the night due to the Earth turning on its axis, the same reason the stars appear to march across the sky; the radiant moves from night to night against the background stars due to the Earth moving in its orbit around the sun. See IMO Meteor Shower Calendar 2017 for maps of drifting "fixed points." When the moving radiant is at the highest point it will reach in the observer's sky that night, the sun will be just clearing the eastern horizon. For this reason, the best viewing time for a meteor shower is slightly before dawn — a compromise between the maximum number of meteors available for viewing, the lightening sky which makes them harder to see. Meteor showers are named after the nearest constellation or bright star with a Greek or Roman letter assigned, close to the radiant position at the peak of the shower, whereby the grammatical declension of the Latin possessive form is replaced by "id" or "ids".
Hence, meteors radiating from near the star delta Aquarii are called delta Aquariids. The International Astronomical Union's Task Group on Meteor Shower Nomenclature and the IAU's Meteor Data Center keep track of meteor shower nomenclature and which showers are e
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower; the appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but class S stars and most carbon stars. The most common red giants are stars on the red-giant branch that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process. Red giants are stars that have exhausted the supply of hydrogen in their cores and have begun thermonuclear fusion of hydrogen in a shell surrounding the core, they have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a reddish-orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size.
Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun, spectral types of K or M, have surface temperatures of 3,000–4,000 K, radii up to about 200 times the Sun. Stars on the horizontal branch are hotter, with only a small range of luminosities around 75 L☉. Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red giant branch, up to several times more luminous at the end of the thermal pulsing phase. Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to the surface in what is called a dredge-up; the first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, sometimes third, dredge up occurs during helium shell burning on the asymptotic-giant branch and convects carbon to the surface in sufficiently massive stars; the stellar limb of a red giant is not defined, contrary to their depiction in many illustrations.
Rather, due to the low mass density of the envelope, such stars lack a well-defined photosphere, the body of the star transitions into a'corona'. The coolest red giants have complex spectra, with molecular lines, emission features, sometimes masers from thermally pulsing AGB stars. Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells, red-giant photospheres, as well as those of red supergiants, have just a few large cells, the features of which cause the variations of brightness so common on both types of stars. Red giants are evolved from main-sequence stars with masses in the range from about 0.3 M☉ to around 8 M☉. When a star forms from a collapsing molecular cloud in the interstellar medium, it contains hydrogen and helium, with trace amounts of "metals"; these elements are all uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen and establishes hydrostatic equilibrium.
Over its main sequence life, the star converts the hydrogen in the core into helium. For the Sun, the main-sequence lifetime is 10 billion years. More-massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars; when the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and so the core begins to contract due to its own gravity. This brings additional hydrogen into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core; the outer layers of the star expand thus beginning the red-giant phase of the star's life. As the star expands, the energy produced in the burning shell of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star's visible light output towards the red—hence it becomes a red giant. At this time, the star is said to be ascending the red-giant branch of the Hertzsprung–Russell diagram; the evolutionary path the star takes as it moves along the red-giant branch, which ends with the complete collapse of the core, depends on the mass of the star.
For the Sun and stars of less than about 2 M☉ the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate, it will continue to heat until it reaches a temperature of 108 K, hot enough to begin fusing helium to carbon via the triple-alpha process. Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly in a so-called helium flash. In more-massive stars, the collapsing core will reach 108 K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, produce no helium flash; the core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram. An analogous process oc