Stars of constellations Orion and Eridanus are photographed above Sinai seashores near Dahab, northeast Egypt. At this latitude of 28 degrees north even the southern most part of the constellation Eridanus, the River, is visible. The bright star at the southern end of the river is Achernar, Alpha star in Eridanus; means the end of the river in Arabic. Achernar is the 10th brightest star in the night sky with +0.46 magnitude. Eridanus is the 6th largest constellation in the sky and its unique figure take it from the celestial equator near Orion Belt to declination -57 at Achernar (57 degrees below the celestial equator).
[http://twanight.org/newTWAN/photos.asp?ID=3003327]
Eridanus is the sixth largest constellation in the night sky, occupying an area of 1138 square degrees. It is located in the first quadrant of the southern hemisphere (SQ1) and can be seen at latitudes between +32° and -90°. The neighboring constellations are Caelum, Cetus, Fornax, Horologium, Hydrus, Lepus, Orion, Phoenix, Taurus and Tucana.
[http://www.constellation-guide.com/constellation-list/eridanus-constellation/]
Johannes Hevelius’ Eridanus from Uranographia (1690)
[http://chandra.harvard.edu/photo/constellations/eridanus.html]
The name Eridanus refers to the Po River, the main river of northern Italy. Its association with a river comes from the way its stars trace a tortuous path. In some star maps, Eridanus is depicted as a river flowing from the waters poured by Aquarius. In such maps, Aquarius is visualized as facing Eridanus, requiring a different perspective and a redesign of how the stars of Aquarius connect, so the water pours onto the same side as Eridanus.
According to one theory, the Greek constellation takes its name from the Babylonian constellation known as the Star of Eridu (MUL.NUN.KI). Eridu was an ancient city in the extreme south of Babylonia; situated in the marshy regions it was held sacred to the god Enki-Ea who ruled the cosmic domain of the Abyss- a mythical conception of the fresh-water reservoir below the Earth’s surface.
Eridanus is connected to the myth of Phaethon, who took over the reins of his father Helios’ sky chariot (i.e., the Sun), but didn’t have the strength to control it and so veered wildly in different directions, scorching both Earth and heaven. Zeus intervened by striking Phaethon dead with a thunderbolt and casting him to Earth. The constellation was supposed to be the path Phaethon drove along; in later times, it was considered a path of souls. Since Eridanos was also a Greek name for the Po (Latin Padus), in which the burning body of Phaethon is said by Ovid to have extinguished, the mythic geography of the celestial and earthly Eridanus is complex.
Another association with Eridanus is a series of rivers all around the world. First conflated with the Nile River in Egypt, the constellation was also identified with the Po River in Italy. The stars of the modern constellation Fornax were formerly a part of Eridanus.
The stars of Eridanus are also depicted as a river in Indian astronomy starting close to the head of Orion just below Auriga. Eridanus is called Srotaswini in Sanskrit, srótas meaning the course of a river or stream. Specifically, it is depicted as the Ganges on the head of Dakshinamoorthy or Nataraja, a Hindu incarnation of Shiva. Dakshinamoorthy himself is represented by the constellation Orion.
The stars that correspond to Eridanus cannot be fully seen from China. In Chinese astronomy, the northern part is located within the White Tiger of the West (Xī Fāng Bái Hǔ). The unseen southern part was classified among the Southern Asterisms (Jìnnánjíxīngōu) by Xu Guangqi, based on knowledge of western star charts.
[https://bobmoler.wordpress.com/2011/12/27/122711-ephemeris-the-constellation-of-eridanus-the-river/]
[http://www.pa.msu.edu/people/horvatin/Astronomy_Facts/constellation_pages/eridanus.htm]
The position of Achernar (lower right)
Achernar is the name of the brightest component (by visual brightness) of the binary system designated Alpha Eridani, which is the brightest ‘star’ or point of light in, and lying at the southern tip of, the constellation of Eridanus, and the tenth-brightest in the night sky. The system bears the traditional name of Achernar, derived from the Arabic ‘ākhir an-nahr,’ meaning ‘The End of the River.’ The two components are designated Alpha Eridani A and Alpha Eridani B (known informally as Achernar B). As determined by the Hipparcos astrometry satellite, it is approximately 139 light-years (43 pc) from the Sun.
Achernar is in the deep southern sky and never rises above the horizon beyond 33°N, roughly the latitude of Dallas, Texas. It is best seen from the southern hemisphere in November; it is circumpolar above (i.e. south of) 33°S, roughly the latitude of Santiago. On this latitude, e.g. the south coast of South Africa (Cape Town to Port Elizabeth) when in lower culmination it is barely visible to the naked eye as it is only 1 degree above the horizon, but still circumpolar. Further south, it is well visible at all times during night.
Achernar is a bright, blue star with about seven times the mass of the Sun. It is a main sequence star with a stellar classification of B6 Vep, but is about 3,150 times more luminous than the Sun. Infrared observations of the star using an adaptive optics system on the Very Large Telescope show that it has a companion star in a close orbit. This appears to be an A-type star in the stellar classification range A0V–A3V, which suggests a stellar mass of about double that of the Sun. The separation of the two stars is roughly 12.3 AU and their orbital period is at least 14–15 years.
As of 2003, Achernar is the least spherical star in the Milky Way studied to date. It spins so rapidly that it has assumed the shape of an oblate spheroid with an equatorial diameter 56% greater than its polar diameter. The polar axis is inclined about 65° to the line of sight from the Earth. Since it is actually a binary star, its highly distorted shape may cause non-negligible departures of the companion’s orbital trajectory with respect to a Keplerian ellipse. A similar situation occurs for the star Regulus.
Extreme rotation speed has flattened Achernar
Because of the distorted shape of this star, there is a significant temperature variation by latitude. At the pole, the temperature may be above 20,000 K, while the equator is at or below 10,000 K. The average temperature of the star is about 15,000 K. The high polar temperatures are generating a fast polar wind that is ejecting matter from the star, creating a polar envelope of hot gas and plasma. The entire star is surrounded by an extended envelope that can be detected by its excess infrared emission, or by its polarization. The presence of a circumstellar disk of ionized gas is a common feature of Be stars such as this. The disk is not stable and periodically decretes back into the star. The maximum polarization for Achernar’s disk was observed in September 2014, and it is now decreasing.
Due to precession, Achernar lay much further south in ancient times than at present, being 7.5 degrees of the south pole around 3400 BCE (decl 82º40') and still lying at declination -76 by around 1500 BCE. Hence the Ancient Egyptians could not have known it. Even in 100 CE its declination was around -67, meaning Ptolemy could not possibly have seen it from Alexandria - whereas Theta Eridani was visible as far north as Crete. So Ptolemy’s ‘end of the river’ was certainly Theta Eridani. Alpha Eridani was not visible from Alexandria until about 1600 CE.
Alpha Eridani will continue to move north in the next few millennia, rising from Crete about 500 years hence before reaching its maximum northern declination between the 8th and 11th millennia, when it will be visible as far north as Germany and southern England.
[https://en.wikipedia.org/wiki/Achernar]
Beta Eridani is a giant star belonging to the spectral class A3 III. It is the second brightest star in the constellation, with an apparent magnitude of 2.796. It lies approximately 89 light years from the solar system and can be found near the border with Orion. The star has a visual companion with an apparent magnitude of 10.90, located 120 arc seconds away.
Like Achernar, Beta Eridani is a fast spinner, with a projected rotational velocity of 196 km/s. The star is classified as a variable. Its visual magnitude changes from magnitude 2.72 to 2.80.
Beta Eridani’s traditional name, Cursa, comes from the Arabic phrase Al Kursiyy al Jauzah, which means ‘the chair of the central one,’ and refers to a star association that also includes Lambda Eridani, Psi Eridani, and Tau Orionis in Orion constellation.
Acamar, Theta Eridani, is a binary star that is possibly a part of a multiple star system. The primary component in the system belongs to the spectral class A4 and has a visual magnitude of 3.2. The secondary component is a class A1 star with an apparent magnitude of 4.3. The two stars are separated by 8.3 seconds of arc. Acamar has an apparent visual magnitude of 3.2 and is about 161 light years distant from the Sun.
The star’s traditional name, Acamar, comes from the Arabic ‘Ākhir an-nahr,’ which means ‘the end of the river.’ Acamar used to represent the end of the celestial river, Eridanus. Now it is the brighter star Achernar that holds that distinction. The stars’ names share the same etymology. Since Achernar is not visible from Greece, Acamar was chosen to mark the river’s end by Hipparchus and Ptolemy.
Gamma Eridani has the stellar classification M1IIIb. It has an apparent visual magnitude of 2.95 and is approximately 150 light years distant from the solar system. The star’s proper name, Zaurak, means’the boat’ in Arabic.
Delta Eridani is a subgiant star of the spectral type K0 IV. It has a visual magnitude of 3.54 and is only 29.49 light years distant. Its traditional name, Rana, means ‘the frog’ in Latin.
[http://www.constellation-guide.com/constellation-list/eridanus-constellation/]
Artist’s impression of a Jupiter-mass planet orbiting the nearby star Epsilon Eridani
Epsilon Eridani is a star with one extrasolar planet similar to Jupiter. It is an orange-hued main-sequence star of magnitude 3.7, 10.5 light-years from Earth. Its one planet, with an approximate mass of one Jupiter mass, has a period of 7 years.
NGC 1535 using the 0.8m Schulman Telescope from the Mount Lemmon Sky Center
[https://en.wikipedia.org/wiki/NGC_1535]
NGC 1535 is a planetary nebula in the constellation of Eridanus, discovered by William Herschel on February 1, 1785. It is sometimes referred to as Cleopatra’s Eye Nebula.
[https://en.wikipedia.org/wiki/NGC_1535]
IC 2118 is a faint reflection nebula believed to be an ancient supernova remnant or gas cloud illuminated by nearby supergiant star Rigel in Orion:
IC 2118: The Witch Head Nebula
Double, double toil and trouble; Fire burn, and cauldron bubble- maybe Macbeth should have consulted the Witch Head Nebula. This suggestively shaped reflection nebula is associated with the bright star Rigel in the constellation Orion. More formally known as IC 2118, the Witch Head Nebula glows primarily by light reflected from Rigel, located just outside the top right corner of the above image. Fine dust in the nebula reflects the light. The blue color is caused not only by Rigel’s blue color but because the dust grains reflect blue light more efficiently than red. The same physical process causes Earth’s daytime sky to appear blue, although the scatterers in Earth’s atmosphere are molecules of nitrogen and oxygen. The nebula lies about 1000 light-years away.
[http://apod.nasa.gov/apod/ap051227.html]
NGC 1300 is a face-on barred spiral galaxy. The center of the bar shows an unusual structure: within the overall spiral structure, a grand design spiral exists. Its spiral arms are tightly wound:
Barred Spiral Galaxy NGC 1300
Big, beautiful, barred spiral galaxy NGC 1300 lies some 70 million light-years away on the banks of the constellation Eridanus. This Hubble Space Telescope composite view of the gorgeous island universe is one of the largest Hubble images ever made of a complete galaxy. NGC 1300 spans over 100,000 light-years and the Hubble image reveals striking details of the galaxy’s dominant central bar and majestic spiral arms. In fact, on close inspection the nucleus of this classic barred spiral itself shows a remarkable region of spiral structure about 3,000 light-years across. Like other spiral galaxies, including our own Milky Way, NGC 1300 is thought to have a supermassive central black hole.
[https://apod.nasa.gov/apod/ap160109.html]
NGC 1427A is an irregular galaxy in the constellation Eridanus. Its distance is about 52 Mly. It is the brightest dwarf irregular member of the Fornax cluster and is in the foreground of the cluster’s central galaxy NGC 1399:
[https://en.wikipedia.org/wiki/NGC_1427A]
NGC 1427A: Galaxy in Motion
In this tantalizing image, young blue star clusters and pink star-forming regions abound in NGC 1427A, a galaxy in motion. The small irregular galaxy’s swept back outline points toward the top of this picture from the Hubble Space Telescope - and that is indeed the direction NGC 1427A is moving as it travels toward the center of the Fornax cluster of galaxies, some 62 million light-years away. Over 20,000 light-years long and similar to the nearby Large Magellanic Cloud, NGC 1427A is speeding through the Fornax cluster's intergalactic gas at around 600 kilometers per second. The resulting pressure is giving the galaxy its arrowhead outline and triggering the beautiful but violent episodes of star formation. Still, it is understood that interactions with cluster gas and the other cluster galaxies during its headlong flight will ultimately disrupt galaxy NGC 1427A. Many unrelated background galaxies are visible in the sharp Hubble image, including a striking face-on spiral galaxy at the upper left.
[https://apod.nasa.gov/apod/ap050304.html]
NGC 1531 is a dwarf galaxy in the constellation Eridanus that is interacting with the larger spiral galaxy NGC 1532. Although technically classified as a peculiar lenticular galaxy, the galaxy's structure is better described as amorphous:
NGC 1531/2: Interacting Galaxies
This dramatic image of an interacting pair of galaxies was made using 8-meter Gemini South telescope at Cerro Pachon, Chile. NGC 1531 is the background galaxy with a bright core just above center and NGC 1532 is the foreground spiral galaxy laced with dust lanes. The pair is about 55 million light-years away in the southern constellation Eridanus. These galaxies lie close enough together so that each feels the influence of the other’s gravity. The gravitational tug-of-war has triggered star formation in the foreground spiral as evidenced by the young, bright blue star clusters along the upper edge of the front spiral arm. Though the spiral galaxy in this pair is viewed nearly edge-on, astronomers believe the system is similar to the face-on spiral and companion known as M51, the Whirlpool Galaxy.
[https://apod.nasa.gov/apod/ap050301.html]
MACS J0416.1-2403 is a galaxy cluster at a redshift of z=0.397 with a mass 160 trillion times the mass of the Sun inside 200 kpc (650 kly). Its mass out to a radius of 950 kpc (3,100 kly) was measured as 1.15×10^15 solar masses. The system was discovered during the Massive Cluster Survey, MACS. This cluster causes gravitational lensing of distant galaxies producing multiple images. In 2015, the galaxy cluster was announced as gravitationally lensing the most distant galaxy (z = 12). Based on the distribution of the multiple image copies, scientists have been able to deduce and map the distribution of dark matter:
[https://en.wikipedia.org/wiki/MACS_J0416.1-2403]
MACS J0416
Galaxy clusters are enormous collections of hundreds or even thousands of galaxies and vast reservoirs of hot gas embedded in massive clouds of dark matter, invisible material that does not emit or absorb light but can be detected through its gravitational effects. These cosmic giants are not merely novelties of size or girth - rather they represent pathways to understanding how our entire universe evolved in the past and where it may be heading in the future.
To learn more about clusters, including how they grow via collisions, astronomers have used some of the world’s most powerful telescopes, looking at different types of light. They have focused long observations with these telescopes on a half dozen galaxy clusters. The name for this galaxy cluster project is the ‘Frontier Fields.’
One of these Frontier Fields galaxy clusters, MACS J0416.1-2403 (abbreviated MACS J0416) is featured here in a multi-wavelength image.
Located about 4.3 billion light years from Earth, MACS J0416 is a pair of colliding galaxy clusters that will eventually combine to form an even bigger cluster.
The new images of MACS J0416 contain data from three different telescopes: NASA’s Chandra X-ray Observatory (diffuse emission in blue), Hubble Space Telescope (red, green, and blue), and the NSF’s Jansky Very Large Array (diffuse emission in pink). Where the X-ray and radio emission overlap the image appears purple. Astronomers also used data from the Giant Metrewave Radio Telescope in India in studying the properties of MACS J0416.
The Chandra data shows gas in the merging clusters with temperatures of millions of degrees. The optical data shows galaxies in the clusters and other, more distant, galaxies lying behind the clusters. Some of these background galaxies are highly distorted because of gravitational lensing, the bending of light by massive objects. This effect can also magnify the light from these objects, enabling astronomers to study background galaxies that would otherwise be too faint to detect. Finally, the structures in the radio data trace enormous shock waves and turbulence. The shocks are similar to sonic booms, generated by the mergers of the clusters.
An open question for astronomers about MACS J0416 has been: are we seeing a collision in these clusters that is about to happen or one that has already taken place? Until recently, scientists have been unable to distinguish between these two explanations. Now, the combined data from these various telescopes is providing new answers.
In MACS J0416 the dark matter (which leaves its gravitational imprint in the optical data) and the hot gas (detected by Chandra) line up well with each other. This suggests that the clusters have been caught before colliding. If the clusters were being observed after colliding the dark matter and hot gas should separate from each other, as seen in the famous colliding cluster system known as the Bullet Cluster.
The cluster in the upper left contains a compact core of hot gas, most easily seen in a specially processed image, and also shows evidence of a nearby cavity, or hole in the X-ray emitting gas. The presence of these structures also suggests that a major collision has not occurred recently, otherwise these features would likely have been disrupted. Finally, the lack of sharp structures in the radio image provides more evidence that a collision has not yet occurred.
In the cluster located in the lower right, the observers have noted a sharp change in density on the southern edge of the cluster. This change in density is most likely caused by a collision between this cluster and a less massive structure located further to the lower right.
[http://chandra.harvard.edu/photo/2016/frontier/index.html]
The Eridanus Supervoid is a large supervoid (an area of the universe devoid of galaxies) discovered as of 2007. At a diameter of about one billion light years it is the second largest known void, superseded only by the Giant Void in Canes Venatici. It was discovered by linking a ‘cold spot’ in the cosmic microwave background to an absence of radio galaxies in data of the United States National Radio Astronomy Observatory’s Very Large Array Sky Survey. There is some speculation that the void may be due to quantum entanglement between our universe and another.
The Nu Eridanids, a recently discovered meteor shower, radiate from the constellation between August 30 and September 12 every year; the shower’s parent body is an unidentified Oort cloud object. Another meteor shower in Eridanus is the Omicron Eridanids, which peak between November 1 and 10.
[https://en.wikipedia.org/wiki/Eridanus_%28constellation%29]
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