Crux is the smallest of the 88 constellations, occupying an area of only 68 square degrees. It is located in the third quadrant of the southern hemisphere (SQ3) and can be seen at latitudes between +20° and -90°. Crux belongs to the Hercules family of constellations, along with Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens, Triangulum Australe, and Vulpecula. The neighboring constellations are Centaurus and Musca.
[http://www.constellation-guide.com/constellation-list/crux-constellation/]
Crux is easily visible from the southern hemisphere at practically any time of year. It is also visible near the horizon from tropical latitudes of the northern hemisphere for a few hours every night during the northern winter and spring. For instance, it is visible from Cancun (southeastern Mexico) or any other place at latitude 25° N or less at around 10 pm at the end of April.
In tropical regions Crux can be seen in the sky from April to June. Crux is exactly opposite to Cassiopeia on the celestial sphere, and therefore it cannot appear in the sky with the latter at the same time. For locations south of 34°S, Crux is circumpolar and thus always visible in the night sky.
Due to precession, Crux will move closer to the South Pole in the next millennia, up to 67 degrees south declination for the middle of the constellation. But in AD 18000 or BCE 8000 Crux will be/was less than 30 degrees south declination making it visible in Northern Europe. Even in AD 14000 it will be visible for most parts of Europe and the whole United States.
Crux lies under the hind legs of Centaurus. It contains a dark cloud of dust known to modern astronomers as the Coalsack, but named Macula Magellanica on this illustration from the Uranographia of Johann Bode (1801).
[http://www.ianridpath.com/startales/crux.htm]
Crux was known to the Ancient Greeks due to the fact that it can be seen from southern Egypt; Ptolemy regarded it as part of the constellation Centaurus. It was entirely visible as far north as Britain in the fourth millennium BCE. However, the precession of the equinoxes gradually lowered its stars below the European horizon, and they were eventually forgotten by the inhabitants of northern latitudes. By AD 400, most of the constellation never rose above the horizon for Athenians.
The 15th-century Venetian navigator Alvise Cadamosto made note of what was probably the Southern Cross on exiting the Gambia River in 1455, calling it ‘carro dell' ostro (‘southern chariot’). However, Cadamosto’s accompanying diagram was inaccurate. Historians generally credit João Faras- astronomer and physician of King Manuel I of Portugal who accompanied Pedro Álvares Cabral in the discovery of Brazil in 1500- for being the first European to depict it correctly. Faras sketched and described the constellation (calling it ‘Las Guardas,’ ‘The Guards’) in a letter written on the beaches of Brazil on May 1, 1500, to the Portuguese monarch.
Explorer Amerigo Vespucci seems to have observed not only the Southern Cross but also the neighboring Coalsack Nebula on his second voyage in 1501-02.
Another early modern description clearly describing Crux as a separate constellation is attributed to Andreas Corsali, an Italian navigator who from 1515 to 1517 sailed to China and the East Indies in an expedition sponsored by King Manuel I. In 1516, Corsali wrote a letter to the monarch describing his observations of the southern sky, which included a rather crude map of the stars around the south celestial pole including the Southern Cross and the two Magellanic Clouds seen in an external orientation, as on a globe.
Emery Molyneux and Petrus Plancius have also been cited as the first uranographers to distinguish Crux as a separate constellation; their representations date from 1592, the former depicting it on his celestial globe and the latter in one of the small celestial maps on his large wall map. Both authors, however, depended on unreliable sources and placed Crux in the wrong position. Crux was first shown in its correct position on the celestial globes of Petrus Plancius and Jodocus Hondius in 1598 and 1600. Its stars were first catalogued separately from Centaurus by Frederick de Houtman in 1603. Later adopters of the constellation included Jakob Bartsch in 1624 and Augustin Royer in 1679. Royer is sometimes wrongly cited as initially distinguishing Crux.
In Australian Aboriginal astronomy, Crux and the Coalsack mark the head of the ‘Emu in the Sky’ in several Aboriginal cultures, while Crux itself is said to be a possum sitting in a tree (Boorong people of the Wimmera region of northwestern Victoria), a representation of the sky deity Mirrabooka (Quandamooka people of Stradbroke Island), a stingray (Yolngu people of Arnhem Land), or an eagle (Kaurna people of the Adelaide Plains). Two Pacific constellations also included Gamma Centauri. Torres Strait Islanders in modern-day Australia saw Gamma Centauri as the handle and the four stars as the trident of Tagai’s Fishing Spear. The Aranda people of central Australia saw the four Cross stars as the talon of an eagle and Gamma Centauri as its leg.
Various peoples in the East Indies viewed the four main stars as the body of a ray. In both Indonesia and Malaysia, it is known as Bintang Pari and Buruj Pari respectively (‘ray stars’). The Javanese people of Indonesia called this constellation Gubug pèncèng (‘raking hut’) or lumbung (‘the granary’), because the shape of the constellation was like a raking hut.
The Māori name for the Southern Cross is Te Punga (‘the anchor’). It is thought of as the anchor of Tama-rereti’s waka (the Milky Way), while the Pointers are its rope. In Tonga it is known as Toloa (‘duck’); it is depicted as a duck flying south, with one of his wings (δ Crucis) wounded because Ongo tangata (‘two men,’ α and β Centauri) threw a stone at it. The Coalsack is known as Humu (the ‘triggerfish’), because of its shape. In Samoa the constellation is called Sumu (‘triggerfish’) because of its rhomboid shape, while α and β Centauri are called Luatagata (‘Two Men’), just as they are in Tonga. The peoples of the Solomon Islands saw several figures in the Southern Cross. These included a knee protector and a net used to catch Palolo worms. Neighboring peoples in the Marshall Islands saw these stars as a fish.
In Mapudungun, the language of Patagonian Mapuches, the name of the Southern Cross is Melipal, which means ‘four stars.’ In Quechua, the language of the Inca civilization, Crux is known as ‘Chakana,’ which means literally ‘stair’ (chaka, bridge, link; hanan, high, above), but carries a deep symbolism within Quechua mysticism. Acrux and Mimosa make up one foot of the Great Rhea, a constellation encompassing Centaurus and Circinus along with the two bright stars. The Great Rhea was a constellation of the Bororo people of Brazil. The Mocoví people of Argentina also saw a rhea including the stars of Crux. Their rhea is attacked by two dogs, represented by bright stars in Centaurus and Circinus. The dogs’ heads are marked by Alpha and Beta Centauri. The rhea’s body is marked by the four main stars of Crux, while its head is Gamma Centauri and its feet are the bright stars of Musca. The Bakairi people of Brazil had a sprawling constellation representing a bird snare. It included the bright stars of Crux, the southern part of Centaurus, Circinus, at least one star in Lupus, the bright stars of Musca, Beta and Delta Chamaeleonis, Volans, and Mensa. The Kalapalo people of Mato Grosso state in Brazil saw the stars of Crux as Aganagi angry bees having emerged from the Coalsack, which they saw as the beehive.
Among Tuaregs, the four most visible stars of Crux are considered iggaren, i.e. four Maerua crassifolia trees. The Tswana people of Botswana saw the constellation as Dithutlwa, two giraffes- Acrux and Mimosa forming a male, and Gacrux and Delta Crucis forming the female.
The modern constellation Crux is not included in the Three Enclosures and Twenty-Eight Mansions system of traditional Chinese uranography because its stars are too far south for observers in China to know about them prior to the introduction of Western star charts. Based on the work of Xu Guangqi and the German Jesuit missionary Johann Adam Schall von Bell in the late Ming Dynasty, this constellation has been classified as one of the 23 Southern Asterisms (Jìnnánjíxīngōu) under the name Cross (Shízìjià). Possibly Acrux (Alpha Crucis), Mimosa (Beta Crucis) and Gacrux (Gamma Crucis) are bright stars in this constellation never seen in Chinese sky. The name of the western constellation in modern Chinese is (nán shí zì zuò), meaning ‘the southern cross-shaped constellation.’
[https://en.wikipedia.org/wiki/Crux_%28Chinese_astronomy%29]
[http://www.derekscope.co.uk/constellation-20th/crux/]
Locating the south celestial pole
In the Southern Hemisphere, the Southern Cross is frequently used for navigation in much the same way that the Polaris is used in the Northern Hemisphere. Alpha and Gamma (known as Acrux and Gacrux respectively) are commonly used to mark south. Tracing a line from Gacrux to Acrux leads to a point close to the Southern Celestial Pole. Alternatively, if a line is constructed perpendicularly between Alpha Centauri and Beta Centauri, the point where the above-mentioned line and this line intersect marks the Southern Celestial Pole. Another way to find south, strike line through Gacrux and Acrux, 3 1/2 times the distance between Gacrux and Acrux, directly below that point is south. The two stars of Alpha and Beta Centauri are often referred to as the ‘Southern Pointers’ or just ‘The Pointers,’ allowing people to easily find the asterism of the Southern Cross or the constellation of Crux. Very few bright stars of importance lie between Crux and the pole itself, although the constellation Musca is fairly easily recognized immediately beneath Crux.
[http://voices.nationalgeographic.com/files/2014/08/Screen-Shot-2014-08-22-at-1.00.54-PM.png]
[http://oneminuteastronomer.com/1985/constellation-crux-southern-cross/]
Predominating the asterism is the most southerly first-magnitude star and brightest star in the constellation, the blue-white Alpha Crucis or Acrux, followed by four other stars, descending in clockwise order by magnitude: Beta, Gamma (one of the closest red giants to Earth), Delta and Epsilon Crucis. Many of these brighter stars are members of the Scorpius–Centaurus Association, a large but loose group of hot blue-white stars that appear to share common origins and motion across the southern Milky Way.
The four main stars that form the asterism are Alpha, Beta, Gamma, and Delta Crucis. Also known as Acrux, Alpha Crucis is a triple star 321 light-years from Earth. Blue-tinged and magnitude 0.8 to the unaided eye, it has two close components of magnitude 1.3 and 1.8, as well as a wide component of magnitude 5. The two close components are divisible in a small amateur telescope and the wide component is divisible in a pair of binoculars.
Beta Crucis, called Mimosa, is a blue-hued giant of magnitude 1.3, 353 light-years from Earth. It is a Beta Cephei-type Cepheid variable with a variation of less than 0.1 magnitudes.
Gamma Crucis, called Gacrux, is an optical double star. The primary is a red-hued giant star of magnitude 1.6, 88 light-years from Earth. The secondary is of magnitude 6.5, 264 light-years from Earth.
Delta Crucis is a blue-white hued star of magnitude 2.8, 364 light-years from Earth. It is the dimmest of the Southern Cross stars.
Epsilon Crucis is an orange-hued giant star of magnitude 3.6, 228 light-years from Earth:
Dark Reflections in the Southern Cross
NASA’s Wide-field Infrared Survey Explorer, or WISE, captured this colorful image of the reflection nebula IRAS 12116-6001. This cloud of interstellar dust cannot be seen directly in visible light, but WISE’s detectors observed the nebula at infrared wavelengths.
In images of reflection nebulae taken with visible light, clouds of dust reflect the light of nearby stars. The dust is warmed to relatively cool temperatures by the starlight and glows with infrared light, which WISE can detect. Reflection nebulae are of interest to astronomers because they are often the sites of new star formation.
The bright blue star on the right side of the image is the variable star Epsilon Crucis. In the Bayer system of stellar nomenclature, stars are given names based on their relative brightness within a constellation. The Greek alphabet is used to designate the star’s apparent brightness compared to other stars in the same constellation. ‘Alpha’ is the brightest star in the constellation, ‘beta’ the second brightest, and so on. In this case, ‘epsilon’ is the fifth letter of the Greek alphabet, so Epsilon Crucis is the fifth brightest star in the constellation Crux.
Crux is a well-known constellation that can be easily seen by observers in the Southern Hemisphere and from low northern latitudes. Also known as the Southern Cross, Crux is featured in many country's flags, including Australia, Brazil and New Zealand (although New Zealand’s flag does not include Epsilon Crucis).
The colors used in this image represent specific wavelengths of infrared light. The blue color of Epsilon Crucis represents light emitted at 3.4 and 4.6 microns. The green-colored star seen beside Epsilon Crucis is emitting light at 12 microns. This star is IRAS 12194-6007, a carbon star that is near the end of its lifecycle. Since the infrared wavelengths emitted by this star are longer than those from Epsilon Crucis, it is cooler. The green and red colors seen in the reflection nebula represent 12- and 22-micron light coming from the nebula’s dust grains warmed by nearby stars.
[http://www.nasa.gov/mission_pages/WISE/multimedia/gallery/pia13442.html]
NGC 4349-127 is a probable red giant approximately 7,097 light-years away in the constellation of Crux. As a member of the open cluster NGC 4349 (hence the name NGC 4349-127), it is located about 2000 parsecs (about 6500 light years) from the Sun. Its mass is estimated at 3.9 times Solar, and its age is about 200 million years.
In 2007, this star was found to have a substellar companion, probably a brown dwarf, with nearly 20 times the mass of Jupiter. Within an eccentricity of about 0.19, its orbit is moderately elliptical, about the same as Mercury in the Solar System. It orbits its host star at a distance of 2.38 AU in a period of 677.8 days.
This object was discovered by Christophe Lovis and Michel Mayor of the Geneva Observatory using the radial velocity technique.
[https://en.wikipedia.org/wiki/NGC_4349-127]
Iota Crucis is a binary star 125 light-years from Earth. The primary is an orange-hued giant of magnitude 4.6 and the secondary is of magnitude 9.5.
Mu Crucis is a double star where the unrelated components are about 370 light-years from Earth. The primary is a blue-white hued star of magnitude 4.0 and the secondary is a blue-white hued star of magnitude 5.1. Mu Crucis is divisible in small amateur telescopes or large binoculars:
Image showing mu Cru and other double stars in this area
Mu Crucis (μ Cru) is simply one of the most beautiful doubles in the sky and is very suitable for viewing with binoculars or a small telescope.
The pair, whose components are of visual magnitude 3.9 and 5.0, share common proper motions and distances and Hipparcos places both stars at about 412 light years from Earth. If this is so, then the two stars are separated by some 0.0192pc or 0.063ly suggesting the period maybe as long 30, 000 years. The second star, μ2 Crucis, is a Gamma Cassiopeia-type (GCAS) variable, changing by 0.19 mag in an unknown period.
This double was discovered by Dunlop in 1826 and listed as Δ126 in his catalogue . The spectral types are both B and Hartung record them as both white whereas Richard Jaworski sees a tinge of yellow in the fainter star. The current separation and position angle are 35″ and 17deg and the brightness of the stars almost make this a southern equivalent of Albireo but without the colors.
The bright components and wide separation of 35” make this system easily to split with modest binoculars and in a small telescope they are a fine sight. So when you are next outside with your binoculars or your telescope, and finished marveling at the Jewel Box, have a look at this fine double star that is just 4 degrees north of the Jewel Box in the constellation of Crux.
[http://assa.saao.ac.za/sections/deep-sky/double-stars/news-and-articles/may-double-star-of-the-month/]
BG Crucis is a star in the constellation Crux. A Cepheid variable, its apparent magnitude ranges from 5.34 to 5.58 over 3.3428 days. A yellow-white supergiant that pulsates between spectral types F5Ib and G0p, it is around 4.3 times as massive and 2,000 times as luminous as the Sun.
[https://en.wikipedia.org/wiki/BG_Crucis]
PSR B1259-63 is a pulsar and member of an eclipsing binary star system with the blue B2e-class star SS 2883. The pair has an eccentric orbit that is inclined to the line of sight from Earth by about 36°, leading to a 40-day-long eclipse each time the pulsar passes behind the star. The pulsar has a period of about 48 ms and a luminosity of 8.3 × 1035 erg/s. It emits very high energy gamma rays that vary on a time scale of several days.
The star SS 2883 has about 10 solar masses and is 6 solar radii in size. The rate of rotation is about 280 km/s at the equator, or 70% of the breakup velocity.
[https://en.wikipedia.org/wiki/PSR_B1259-63/LS_2883]
BH Crucis, also known as Welch’s Red Variable, is a Mira variable that ranges from magnitude 6.6 to 9.8 over 530 days. Discovered in October 1969, it has become redder and brighter (mean magnitude changing from 8.047 to 7.762) and its period lengthened by 25% in the first thirty years since its discovery.
The star HD 106906 and the planet HD 106906 b, with Neptune’s orbit for comparison
HD 106906 is a main sequence star system in the constellation Crux of spectral type F5V, and it has a planet- HD 106906 b- that has a larger orbit than any other exoplanet discovered to date, at about 300 light-years from Earth. It is estimated to be about eleven times the mass of Jupiter and is located about 650 AU, or nearly 97 billion kilometers (60 billion miles), away from its host star. HD 106906 b is unique to science; while its mass estimate is nominally consistent with identifying it as an exoplanet, it appears at a much wider separation from its parent star than thought possible for in-situ formation from a protoplanetary disk.
[https://en.wikipedia.org/wiki/HD_106906]
[https://en.wikipedia.org/wiki/HD_106906_b]
The Southern Cross in a Southern Sky
Explanation: Have you ever seen the Southern Cross? This famous constellation is best seen from Earth’s Southern Hemisphere. Captured from Rio de Janeiro, Brazil, the four bright stars that mark the Southern Cross are visible just above the horizon in the featured image. On the left of this constellation, also known as The Crux, is the orange star Gamma Crucis. The band of stars, dust, and gas rising through the middle of the image mosaic is part our Milky Way Galaxy. Just to the right of the Southern Cross is the dark Coal Sack Nebula, and the bright nebula at the top of the image is the Carina Nebula. The Southern Cross is such a famous constellation that it is depicted on the national flags of Australia and New Zealand.
[http://apod.nasa.gov/apod/ap151019.html]
The Coalsack Nebula is the most prominent dark nebula in the skies, easily visible to the naked eye as a prominent dark patch in the southern Milky Way. It is large, five degrees by seven degrees, and is 600 light-years from Earth. Not all of the nebula is in the borders of Crux; some of it is technically in Musca and Centaurus:
A Cosmic Sackful of Black Coal
Dark smudges almost block out a rich star field in this new image captured by the Wide Field Imager camera, installed on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. The inky areas are small parts of a huge dark nebula known as the Coalsack, one of the most prominent objects of its kind visible to the unaided eye. Millions of years from now, chunks of the Coalsack will ignite, rather like its fossil fuel namesake, with the glow of many young stars.
The Coalsack Nebula is located about 600 light-years away in the constellation of Crux (The Southern Cross). This huge, dusky object forms a conspicuous silhouette against the bright, starry band of the Milky Way and for this reason the nebula has been known to people in the southern hemisphere for as long as our species has existed.
The Spanish explorer Vicente Yáñez Pinzón first reported the existence of the Coalsack Nebula to Europe in 1499. The Coalsack later garnered the nickname of the Black Magellanic Cloud, a play on its dark appearance compared to the bright glow of the two Magellanic Clouds, which are in fact satellite galaxies of the Milky Way. These two bright galaxies are clearly visible in the southern sky and came to the attention of Europeans during Ferdinand Magellan’s explorations in the 16th century. However, the Coalsack is not a galaxy. Like other dark nebulae, it is actually an interstellar cloud of dust so thick that it prevents most of the background starlight from reaching observers.
A significant number of the dust particles in dark nebulae have coats of frozen water, nitrogen, carbon monoxide and other simple organic molecules. The resulting grains largely prevent visible light from passing through the cosmic cloud. To get a sense of how truly dark the Coalsack is, back in 1970, the Finnish astronomer Kalevi Mattila published a study estimating that the Coalsack has only about 10 percent of the brightness of the encompassing Milky Way. A little bit of background starlight, however, still manages to get through the Coalsack, as is evident in the new ESO image and in other observations made by modern telescopes.
The little light that does make it through the nebula does not come out the other side unchanged. The light we see in this image looks redder than it ordinarily would. This is because the dust in dark nebulae absorbs and scatters blue light from stars more than red light, tinting the stars several shades more crimson than they would otherwise be.
Millions of years in the future the Coalsack’s dark days will come to an end. Thick interstellar clouds like the Coalsack contain lots of dust and gas- the fuel for new stars. As the stray material in the Coalsack coalesces under the mutual attraction of gravity, stars will eventually light up, and the coal ‘nuggets’ in the Coalsack will ‘combust,’ almost as if touched by a flame.
[https://www.eso.org/public/news/eso1539/]
The open cluster NGC 4755, better known as the Jewel Box or Crucis Cluster, has an overall magnitude of 4.2- to the naked eye it appears to be a fuzzy star- and was given its name by John Herschel. It is one of the youngest open clusters in the Milky Way and appears to have the shape of a letter A. The Jewel Box cluster is a very rich, centrally-concentrated cluster detached from the surrounding star field. It has more than 100 stars that range significantly in brightness. The brightest stars are mostly blue supergiants, though the cluster contains a few bright red supergiants. Kappa Crucis is a true member of the cluster that bears its name, and is one of the brighter stars at magnitude 5.9:
NGC 4755: A Jewel Box of Stars
Explanation: The great variety of star colors in this open cluster underlies its name: The Jewel Box. One of the bright central stars is a red supergiant, in contrast to the many blue stars that surround it. The cluster, also known as Kappa Crucis contains just over 100 stars, and is about 10 million years old. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 6,400 light-years away, so the light that we see today was emitted from the cluster before even the Great Pyramids in Egypt were built. The Jewel Box, pictured above, spans about 20 light-years, and can be seen with binoculars towards the southern constellation of the cross (Crux).
[http://apod.nasa.gov/apod/ap100817.html]
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