Orion

Orion Constellation

[http://imgur.com/gallery/Morwq]

Orion is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous and recognizable constellations in the night sky. It was named after Orion, a hunter in Greek mythology. Its brightest stars are Rigel (Beta Orionis) and Betelgeuse (Alpha Orionis), a blue-white and a red supergiant, respectively.

Orion is bordered by Taurus to the northwest, Eridanus to the southwest, Lepus to the south, Monoceros to the east, and Gemini to the northeast. Covering 594 square degrees, Orion ranks twenty-sixth of the 88 constellations in size. In the equatorial coordinate system, the right ascension coordinates of the constellation lie between 04h 43.3m and 06h 25.5m, while the declination coordinates are between 22.87° and −10.97°.

Orion is most visible in the evening sky from January to March, winter in the Northern Hemisphere, and summer in the Southern Hemisphere. In the tropics (less than about 8° from the equator), the constellation transits at the zenith. In the period May–July (summer in the Northern Hemisphere, winter in the Southern Hemisphere), Orion is in the daytime sky and thus not visible at most latitudes. However, for much of Antarctica in the Southern Hemisphere’s winter months, the Sun is below the horizon even at midday. Stars (and thus Orion) are then visible at twilight for a few hours around local noon, low in the North. At the same time of day at the South Pole itself (Amundsen- Scott South Pole Station), Rigel is only 8° above the horizon, and the Belt sweeps just along it. In the Southern Hemisphere's summer months, when Orion is normally visible in the night sky, the constellation is actually not visible in Antarctica because the sun does not set at that time of year south of the Antarctic Circle. In countries close to the equator (e.g. Kenya, Indonesia, Colombia, Ecuador), Orion appears overhead in December around midnight and in the February evening sky.

Orion is located on the celestial equator, but it will not always be so located due to the effects of precession of the Earth’s axis. Orion lies well south of the ecliptic, and it only happens to lie on the celestial equator because the point on the ecliptic that corresponds to the June solstice is close to the border of Gemini and Taurus, to the north of Orion. Precession will eventually carry Orion further south, and by AD 14000 Orion will be far enough south that it will become invisible from the latitude of Great Britain.

Further in the future, Orion’s stars will gradually move away from the constellation due to proper motion. However, Orion’s brightest stars all lie at a large distance from the Earth on an astronomical scale- much farther away than Sirius, for example. Orion will still be recognizable long after most of the other constellations- composed of relatively nearby stars- have distorted into new configurations, with the exception of a few of its stars eventually exploding as supernovae, for example Betelgeuse, which is predicted to explode sometime in the next million years.

An engraving of Orion from Johann Bayer’s Uranometria, 1603 (US Naval Observatory Library)

[https://en.wikipedia.org/wiki/Orion_%28mythology%29]

The distinctive pattern of Orion has been recognized in numerous cultures around the world, and many myths have been associated with it. It has also been used as a symbol in the modern world.

The earliest depiction that has been linked to the constellation of Orion is a prehistoric (Aurignacian) mammoth ivory carving found in a cave in the Ach valley in West Germany in 1979. Archaeologists have estimated it to have been fashioned approximately 32,000 to 38,000 years ago.

The Babylonian star catalogues of the Late Bronze Age name Orion MULSIPA.ZI.AN.NA, ‘The Heavenly Shepherd’ or ‘True Shepherd of Anu’- Anu being the chief god of the heavenly realms. The Babylonian constellation was sacred to Papshukal and Ninshubur, both minor gods fulfilling the role of ‘messenger to the gods.’ Papshukal was closely associated with the figure of a walking bird on Babylonian boundary stones, and on the star map the figure of the Rooster was located below and behind the figure of the True Shepherd- both constellations represent the herald of the gods, in his bird and human forms respectively.

In ancient Egypt, the stars of Orion were regarded as a god, called Sah. Because Orion rises before Sirius, the star whose heliacal rising was the basis for the Solar Egyptian calendar, Sah was closely linked with Sopdet, the goddess who personified Sirius. The god Sopdu was said to be the son of Sah and Sopdet. Sah was syncretized with Osiris, while Sopdet was syncretized with Osiris’ mythological wife, Isis. In the Pyramid Texts, from the 24th and 23rd centuries BCE, Sah was one of many gods whose form the dead pharaoh was said to take in the afterlife.

The Rig Veda refers to the Orion Constellation as Mriga (The Deer). It is said that two bright stars in the front and two bright stars in the rear are The hunting dogs, the one comparatively less bright star in the middle and ahead of two front dogs is The hunter and three aligned bright stars are in the middle of all four hunting dogs is The Deer (The Mriga) and three little aligned but less brighter stars is The Baby Deer. The Mriga means Deer, locally known as Harnu in folk parlance. There are many folk songs narrating the Harnu.

Orion’s current name derives from Greek mythology, in which Orion was a gigantic, supernaturally strong hunter of ancient times, born to Euryale, a Gorgon, and Poseidon (Neptune), god of the sea in the Graeco-Roman tradition. One myth recounts Gaia’s rage at Orion, who dared to say that he would kill every animal on the planet. The angry goddess tried to dispatch Orion with a scorpion. This is given as the reason that the constellations of Scorpius and Orion are never in the sky at the same time. However, Ophiuchus, the Serpent Bearer, revived Orion with an antidote. This is said to be the reason that the constellation of Ophiuchus stands midway between the Scorpion and the Hunter in the sky.

The constellation is mentioned in Homer’s Odyssey and Iliad, Horace’s Odes, and Virgil’s Aeneid.

The Bible mentions Orion three times, naming it ‘Kesil’ (literally ‘fool’). Though, this name perhaps is etymologically connected with ‘Kislev,’ the name for the ninth month of the Hebrew calendar (i.e. November–December), which, in turn, may derive from the Hebrew root K-S-L as in the words ‘kesel, kisla’ (hope, positiveness, i.e. hope for winter rains).

In ancient Aram, the constellation was known as Nephîlā′, the Nephilim may have been Orion’s descendants.

The Armenians identified their legendary patriarch and founder Hayk with Orion. Hayk is also the name of the Orion constellation in the Armenian translation of the Bible.

In medieval Muslim astronomy, Orion was known as al-jabbar, ‘the giant.’ Orion’s sixth brightest star, Saiph, is named from the Arabic, saif al-jabbar, meaning ‘sword of the giant.’

In old Hungarian tradition, ‘Orion’ is known as (magic) Archer (Íjász), or Reaper (Kaszás). In recently rediscovered myths he is called Nimrod (Hungarian ‘Nimród’), the greatest hunter, father of the twins ‘Hunor’ and ‘Magor.’ The ‘π’ and ‘o’ stars (on upper right) form together the reflex bow or the lifted scythe. In other Hungarian traditions, ‘Orion’s belt’ is known as ‘Judge’s stick’ (Bírópálca).

In Scandinavian tradition, ‘Orion’s belt’ was known as Frigg’s Distaff (friggerock) or Freyja’s distaff.

The Finns call the Orion’s belt and the stars below it as Väinämöisen viikate (Väinämöinen’s scythe). Another name for the asterism of Alnilam, Alnitak and Mintaka is Väinämöisen vyö’ (Väinämöinen’s Belt) and the stars ‘hanging’ from the belt as Kalevanmiekka (Kaleva’s sword).

In Siberia, the Chukchi people see Orion as a hunter; an arrow he has shot is represented by Aldebaran (Alpha Tauri), with the same figure as other Western depictions.

The Seri people of northwestern Mexico call the three stars in the belt of Orion Hapj (a name denoting a hunter) which consists of three stars: Hap (mule deer), Haamoja (pronghorn), and Mojet (bighorn sheep). Hap is in the middle and has been shot by the hunter; its blood has dripped onto Tiburón Island.

The same three stars are known in Spain and most of Latin America as ‘Las tres Marías’ (Spanish for ‘The Three Marys’). In Puerto Rico, the three stars are known as the ‘Los Tres Reyes Magos’ (Spanish for The three Wise Men).

The Ojibwa (Chippewa) Native Americans call this constellation Kabibona'kan, the Winter Maker, as its presence in the night sky heralds winter. To the Lakota Native Americans, Tayamnicankhu (Orion’s Belt) is the spine of a bison. The great rectangle of Orion are the bison’s ribs; the Pleiades star cluster in nearby Taurus is the bison’s head; and Sirius in Canis Major, known as Tayamnisinte, is its tail.

The Malay called Orion’ Belt Bintang Tiga Beradik (the ‘Three Brother Star’).

In China, Orion was one of the 28 lunar mansions Sieu (Xiu). It is known as Shen, literally meaning ‘three,’ for the stars of Orion’s Belt. The Chinese character shēn originally meant the constellation Orion (pinyin: shēnxiù); its Shang dynasty version, over three millennia old, contains at the top a representation of the three stars of Orion’s belt atop a man’s head (the bottom portion representing the sound of the word was added later).

The imagery of the belt and sword has found its way into popular western culture, for example in the form of the shoulder insignia of the 27th Infantry Division of the United States Army during both World Wars.

Representation of the central tenet of the Orion Correlation Theory: the outline of the Giza pyramids superimposed over a photograph of the stars in Orion’s Belt.

The Orion correlation theory (or Giza- Orion correlation theory) is a hypothesis in alternative Egyptology. Its central claim is that there is a correlation between the location of the three largest pyramids of the Giza pyramid complex and Orion’s Belt of the constellation Orion, and that this correlation was intended as such by the builders of the pyramids. Depending on the version of the theory, additional pyramids can be included to complete the picture of the Orion constellation, and the Nile River can be included to match with the Milky Way galaxy.

[https://en.wikipedia.org/wiki/Orion_correlation_theory]

Using Orion to find stars in neighbor constellations

Orion is very useful as an aid to locating other stars. By extending the line of the Belt southeastward, Sirius (α CMa) can be found; northwestward, Aldebaran (α Tau). A line eastward across the two shoulders indicates the direction of Procyon (α CMi). A line from Rigel through Betelgeuse points to Castor and Pollux (α Gem and β Gem). Additionally, Rigel is part of the Winter Circle. Sirius and Procyon, which may be located from Orion by following imaginary lines, also are points in both the Winter Triangle and the Circle.

In artistic renderings, the surrounding constellations are sometimes related to Orion: he is depicted standing next to the river Eridanus with his two hunting dogs Canis Major and Canis Minor, fighting Taurus He is sometimes depicted hunting Lepus the hare. He also sometimes is depicted to have a lion’s hide in his hand.

On winter evenings, Orion dominates the sky, surrounded by numerous striking constellations, all decorated with brilliant stars.

[http://www.space.com/24476-orion-constellation-winter-night-sky.html]

Constellation of Orion

[https://www.davidmalin.com/fujii/source/Ori.html]

Orion’s seven brightest stars form a distinctive hourglass-shaped asterism, or pattern, in the night sky. Four stars- Rigel, Betelgeuse, Bellatrix and Saiph- form a large roughly rectangular shape, in the center of which lie the three stars of Orion’s Belt- Alnitak, Alnilam and Mintaka. Coincidentally, these seven stars are among the most distant that can easily be seen with the naked eye. Descending from the ‘belt’ is a smaller line of three stars (the middle of which is in fact not a star but the Orion Nebula), known as the hunter’s ‘sword.’ The ο and π stars East of Bellatrix can be seen as composing Orion’s bow.

Rigel and reflection nebula IC 2118 in Eridanus. Rigel B is not visible in the glare of the main star

[https://en.wikipedia.org/wiki/Rigel]

Rigel (Beta Orionis) is the brightest star in the constellation. With an apparent visual magnitude of 0.18, it is also the sixth brightest star in the sky. Even though it does not have the designation alpha, it is almost always brighter than Betelgeuse, Alpha Orionis. Rigel is really a star system composed of three stars. It has been a known visual binary since 1831, possibly even earlier, when F. G. Struve first measured it. Rigel is surrounded by a shell of expelled gas.

The name Rigel comes from the Arabic phrase ‘Riǧl Ǧawza al-Yusra,’ which means ‘the left foot of the central one.’ Rigel marks Orion’s left foot. Another Arabic name for the star is ‘iǧl al-ǧabbār,’ or ‘the foot of the great one.’ The star’s other two variant names, Algebar and Elgebar, are derived from this phrase.

Rigel is a blue supergiant. It belongs to the spectral type B8lab and is 772.51 light years distant. It has 85,000 times the luminosity of the Sun and 17 solar masses. It is classified as a slightly irregular variable star, with its luminosity varying from 0.03 to 0.3 magnitudes over 22- 25 days.

The primary component in the system, Rigel A, is 500 times brighter than Rigel B, which is itself a spectroscopic binary star. Rigel B has a magnitude of 6.7. It consists of a pair of B9V class main sequence stars that orbit a common centre of gravity every 9.8 days.

Rigel is associated with several nearby dust clouds which it illuminates. The most famous one is IC 2118, the Witch Head Nebula, a faint reflection nebula located about 2.5 degrees to the northwest of Rigel, in the constellation Eridanus.

Rigel is a member of the Taurus-Orion R1 Association. It was considered by some to be an outlying member of the Orion OB1 Association, a group of several dozen hot giants belonging to the spectral types O and B, located in the Orion Molecular Cloud Complex. However, the star is too close to us to be a real member of that particular stellar association.

Rigel is only about 10 million years old. Eventually, it will grow into a red supergiant, one very similar to Betelgeuse.

A direct-sky image of Betelgeuse, a star that is shedding its mass as it nears the end of its life.

[https://www.space.com/31693-dying-star-betelgeuse-puzzles-astronomers.html]

Betelgeuse (Alpha Orionis) is the second brightest star in Orion and the eighth brightest star in the sky. It is a red supergiant, belonging to the spectral class M2lab. The suffix -ab indicates that Betelgeuse is classified as an intermediate luminous supergiant, one not as bright as others such as Deneb in the constellation Cygnus. Some recent findings, however, suggest that the star emits more light than 100,000 Suns, which would in fact make it more luminous than most stars in its class, so the classification is likely outdated.

The star has an apparent visual magnitude of 0.42 and is approximately 643 light years distant. Betelgeuse is one of the most luminous stars known. It has an absolute magnitude of -6.05.

Betelgeuse is also one of the largest stars known, with an apparent diameter between 0.043 and 0.056 arcseconds. It is difficult to get an accurate measurement because the star appears to change shape from time to time and, as a result of a huge mass loss, it has a large envelope surrounding it.

It is classified as a semi-regular variable star. Its apparent visual magnitude varies from 0.2 to 1.2, which means that Betelgeuse occasionally outshines its bright neighbour Rigel. This, however, only happens very rarely. The star’s variation in brightness was first noted by Sir John Herschel in his Outlines of Astronomy in 1836.

Betelgeuse is believed to be about 10 million years old, which is not much for a red supergiant, but the star is thought to have evolved very rapidly because of its enormous mass. It will likely explode as a supernova in the next million years. When it does, it will be easy to find in the sky, not just at night, but also in broad daylight. At its current distance from the solar system, the supernova would shine brighter than the Moon and be the brightest ever recorded supernova in history.

The origin of the name Betelgeuse is not entirely certain. The last part, -elgeuse, is derived from the Arabic name for the constellation, al-Jauzā’, which was a feminine name from old Arabian legends and can be roughly translated as ‘the middle one.’ The most widely accepted explanation is that the name is a corruption of the Arabic phrase Yad al-Jauzā,’ or the Hand of al-Jauzā’, which is to say, the hand of Orion, which became Betelegeuse through a mistransliteration into medieval Latin, with the first Arabic letter standing for y being mistaken for the one for b, which led to the name Bait al-Jauzā’, or ‘the house of Orion’ during the Renaissance. This eventually led to the star’s modern name, Betelgeuse.

Betelgeuse is part of two prominent winter asterisms: the Winter Triangle and the Winter Hexagon. The other two stars forming the Winter Triangle, also known as the Great Southern Triangle, are Sirius and Procyon, the brightest stars in the constellations Canis Major and Canis Minor respectively. The same stars are also part of the Winter Hexagon, along with Rigel, Aldebaran in the constellation Taurus, Capella in Auriga, and Pollux and Castor in Gemini.

Bellatrix

[https://coraskywalker.wordpress.com/tag/bellatrix-star/]

Bellatrix (Gamma Orionis) sometimes also known as the Amazon Star, is the third brightest star in Orion and the 27th brightest star in the sky, only slightly dimmer than Castor in Gemini. Its name comes from the Latin word for ‘the female warrior.’ It has a mean apparent visual magnitude of 1.64 and is approximately 240 light years distant.

Bellatrix is a hot, luminous blue-white giant star, classified as an eruptive variable. Its magnitude varies between 1.59 and 1.64. The star belongs to the spectral class B2 III. It is one of the hotter stars visible to the naked eye. It emits about 6,400 times more light than the Sun and has eight or nine solar masses. Within a few million years, Bellatrix will become an orange giant and eventually a massive white dwarf.

Before its own variability was confirmed, Bellatrix was used as a standard for stellar luminosity, one against which other stars were compared and checked for variability.

Saiph in Orion

[http://www.newforestobservatory.com/2012/01/15/2513/]

Saiph (Kappa Orionis) is the southeastern star of Orion’s central quadrangle. It is the sixth brightest star in the constellation, with an apparent visual magnitude of 2.06. The star is approximately 720 light years distant.

Saiph is a blue supergiant, belonging to the spectral class B0.5. Its name is derived from the Arabic phrase ‘saif al jabbar,’ which means ‘the sword of the giant.’ Like many other bright stars in Orion, Saiph too will end its life in a supernova explosion.

A closer look at the Betelgeuse-Bellatrix-Meissa area

[https://bestdoubles.wordpress.com/2011/01/06/the-proud-head-of-orion-marching-with-meisssa-lambda-%CE%BB-orionisand-o%CF%83-111/]

Lambda Orionis is blue giant belonging to the spectral type O8III, approximately 1,100 light years distant. It has a visual magnitude of 3.39. The star’s traditional name, Meissa, comes from the Arabic Al-Maisan, which means the shining one. Lambda Orionis is also sometimes called Heka, from the Arabic Al Hakah, or a white spot, referring to the Arabic lunar mansion that includes both Lambda and Phi Orionis.

Meissa is really a double star. The companion, a hot blue-white dwarf belonging to the spectral class B0.5V, has an apparent magnitude of 5.61 and is separated from the brighter component by 4.4 arcseconds.

[https://en.wikipedia.org/wiki/Meissa]

Orion’s Belt: Alnitak, Alnilam, and Mintaka, are the bright bluish stars from east to west (left to right) along the diagonal in this gorgeous cosmic vista. Otherwise known as the Belt of Orion, these three blue supergiant stars are hotter and much more massive than the Sun. They lie about 1,500 light-years away.

Orion’s Belt is one of the best known asterisms in the night sky. It is formed by three bright stars in the constellation Orion: Mintaka (Delta Orionis), Alnilam (Epsilon Orionis), and Alnitak (Zeta Orionis).

Mintaka

[https://coraskywalker.wordpress.com/tag/mintaka-star/]

Mintaka (Delta Orionis) is the westernmost of the three stars in the Belt of Orion. It is the right-most star when observed from the Northern Hemisphere, facing south. The name Mintaka is derived from the Arabic word manţaqah, which means ‘area’ or ‘region.’

Mintaka is a multiple star, classified as an eclipsing binary variable. The primary component is a double star consisting of a class B giant and a hot class O star which orbit each other every 5.63 days and eclipse each other slightly, causing a 0.2 magnitude drop in luminosity. The system also contains a magnitude 7 star separated by about 52” from the primary component, and a very faint 14th magnitude star in between.

Mintaka is approximately 900 light years distant. Its brightest components are both roughly 90,000 times as luminous as our Sun and have more than 20 solar masses. They will both end their lives in violent supernova explosions.

In the order of brightness, the apparent magnitudes of the components are 2.23 (3.2/3.3), 6.85, 14.0. Mintaka is the faintest of the three stars in Orion’s Belt and the seventh brightest star in Orion. It is the closest bright star to the celestial equator: it rises and sets almost exactly east and west.

Alnilam lights up NGC 1990

[https://en.wikipedia.org/wiki/Alnilam]

Alnilam (Epsilon Orionis) is a hot, bright blue supergiant. It has an apparent magnitude of 1.70 and is approximately 1,300 light years distant. It belongs to the spectral class B0.

Alnilam is the central star in Orion’s Belt. It is the fourth brightest star in the Orion constellation and the 30th brightest star in the night sky. It radiates about 375,000 solar luminosities. The star’s Flamsteed designation is 46 Orionis.

Alnilam is surrounded by the reflection nebula NGC 1990, a molecular cloud illuminated by the light emitted by the star. The wind blowing from the star’s surface has the speed of 2,000 kilometres per second. The star’s estimated age is around four million years. It is losing mass and its internal hydrogen fusion is shutting down. Alnilam will soon evolve into a red supergiant, one much brighter than Betelgeuse, and eventually explode as a supernova.

The name Alnilam comes from the Arabic word an-niżām, which is related to the word nażm, which means ‘the string of pearls.’

Alnitak (in lower right corner) and Flame Nebula

[https://en.wikipedia.org/wiki/Alnitak]

Alnitak (Zeta Orionis) is a multiple star system in Orion, approximately 700 light years distant. It has an apparent magnitude of 1.72. The name Alnitak is derived from the Arabic word an-nitaq, which means ‘the girdle.’

The brightest component in the system, Alnitak A, is yet another hot, blue supergiant, one with an absolute magnitude of -5.25. The star has a visual magnitude of 2.04 and belongs to the spectral class O9. It is the brightest O class star known.

It is in fact a close binary star, composed of the O9.7 class supergiant, one with a mass 28 times solar, and a blue dwarf belonging to the spectral class OV, with an apparent magnitude of about 4. The dwarf was first discovered in 1998.

Alnitak is the easternmost star in Orion’s Belt. The system lies next to and lights up the bright emission nebula NGC 2024 (Flame Nebula).

[http://www.constellation-guide.com/constellation-list/orion-constellation/]

η Orionis Aa/Ab/B

[https://it.wikipedia.org/wiki/Eta_Orionis]

Eta Orionis (η Ori, η Orionis) lies a little to the west of Orion’s belt between Delta Orionis and Rigel, being closer to Delta Orionis than to Rigel. It lies at a distance of around 1,000 light years from Earth and is part of the Orion Arm.

This is a quadruple star system, of which three members can be resolved with a telescope. The primary component is an eclipsing binary star in a triple-star grouping. These stars have orbital periods of 8 days and 9.2 years. It includes a variable star with a pulsation period of around 8 hours. Three of the components are B-type main sequence stars with stellar classifications of B1 V, B3 V and B2 V.

[https://en.wikipedia.org/wiki/Eta_Orionis]

σ Orionis (lower right) and the Horsehead nebula. The brighter stars are Alnitak and Alnilam

Sigma Orionis is a multiple star system in the constellation Orion, consisting of the brightest members of a young open cluster. It is found at the eastern end of the belt, south west of Alnitak and west of the Horsehead Nebula which it partially illuminates. The total brightness of the component stars is magnitude 3.80.

The brightest member of the σ Orionis AB system appears as a late O class star, but is actually made up of three stars. All three are very young main sequence stars with masses between 11 and 18 M☉. The primary component Aa is the class O9.5 star, with a temperature of 35,000 K and a luminosity over 40,000 L☉. Lines representing a B0.5 main sequence star have been shown to belong to its close companion Ab, which has a temperature of 31,000 K and a luminosity of 18,600 L☉.

The spectrum of component B, the outer star of the triple, cannot be detected. The luminosity contribution from σ Ori B can be measured and it is likely to be a B0-2 main sequence star. Its visual magnitude of 5.31 is similar to σ Ori Ab.

[https://en.wikipedia.org/wiki/Sigma_Orionis]

From left to right, the Running Man Nebula, the Orion Nebula (M42 and M43), and Iota Orionis (brightest star on right-hand side).

[http://www.daviddarling.info/encyclopedia/I/Iota_Orionis.html]

Iota Orionis is the brightest star in an asterism known as Orion’s sword. It has the traditional names Hatsya or Hatysa, and in Arabic Na’ir al Saif, which means simply ‘the Bright One of the Sword.’ It is located at a distance of roughly 1,330 light-years (410 parsecs).

Iota Orionis is a quadruple system dominated by a massive spectroscopic binary. The two components of ι Ori A are a stellar class O9 III star (blue giant) and a class B0.8 III/IV star about 2 magnitudes fainter. The collision of the stellar winds from this pair makes the system a strong X-ray source. Oddly, the two objects of this system appear to have different ages, with the secondary being about double the age of the primary. In combination with the high eccentricity of their orbit, this suggests that the binary system was created through a capture, rather than by being formed together and undergoing a mass transfer. This capture may have occurred, for example, through an encounter between two binary systems.

ι Ori has a B8 giant companion at 11" (approximately 5,000 AU[9]) which has been shown to be variable, and likely to be a young stellar object. There is also a fainter A0 star at 49" catalogued as ι Ori C.

[https://en.wikipedia.org/wiki/Iota_Orionis]

θ1 Orionis C is seen as a double star (right inset) with VLTI near-infrared interferometry

Theta1 Orionis C is a member of the Trapezium open cluster that lies within the Orion Nebula. The star C is the most massive of the four bright stars at the heart of the cluster. It is an O class blue main sequence star with a B-type main sequence companion. Its high luminosity and large distance (about 1,500 light years) give it an apparent visible magnitude of 5.1.

Theta1 Orionis consists of multiple components, primarily the four stars of the Trapezium cluster, all within one arc-minute of each other. Theta2 Orionis is a more distant grouping of three main stars plus several fainter companions, 1-2 arc-minutes from Theta1.

Theta1 C is itself a binary of two massive stars, C1 and C2, plus a very close fainter companion apparently escaping the system.

Theta1 Orionis C1 is responsible for generating most of the ultraviolet light that is slowly ionizing (and perhaps photoevaporating) the Orion Nebula. This UV light is also the primary cause of the glow that illuminates the Orion Nebula. The star emits a powerful stellar wind that is a hundred thousand times stronger than the Sun’s, and the outpouring gas moves at 1,000 km/s.

[https://en.wikipedia.org/wiki/Theta1_Orionis_C]

In stellar evolution, an FU Orionis star (also FU Orionis object, or FUor) is a pre–main-sequence star which displays an extreme change in magnitude and spectral type. One example is the star V1057 Cyg, which became 6 magnitudes brighter and went from spectral type dKe to F-type supergiant.

The prototypes of this class are: FU Orionis, V1057 Cygni, V1515 Cygni, and the embedded protostar V1647 Orionis, which erupted in January 2004:

[https://en.wikipedia.org/wiki/FU_Orionis_star]

The protostar V1647 Orionis resides at the tip of a conical glow called McNeil’s Nebula, which was discovered in January 2004 near the peak of an outburst. This image, from the Frederick C. Gillett Gemini Telescope in Hawaii, shows the nebula as it appeared on Feb. 14, 2004.

Using combined data from a trio of orbiting X-ray telescopes, including NASA’s Chandra X-ray Observatory and the Japan-led Suzaku satellite, astronomers have obtained a rare glimpse of the powerful phenomena that accompany a still-forming star. A new study based on these observations indicates that intense magnetic fields drive torrents of gas into the stellar surface, where they heat large areas to millions of degrees. X-rays emitted by these hot spots betray the newborn star's rapid rotation.

Astronomers quickly determined that V1647 Ori was a protostar, a stellar infant still partly swaddled in its birth cloud. “Based on infrared studies, we suspect that this protostar is no more than a million years old, and probably much younger,” said astrophysicist Kenji Hamaguchi, lead author of the study.

Protostars have not yet developed the energy-generating capabilities of a normal star such as the sun, which fuses hydrogen into helium in its core. For V1647 Ori, that stage lies millions of years in the future. Until then, the protostar shines from the heat energy released by the gas that continues to fall onto it, much of which originates in a rotating circumstellar disk.

The mass of V1647 Ori is likely only about 80 percent of the sun's, but its low density bloats it to nearly five times the sun's size. Infrared measurements show that most of the star's surface has a temperature around 6,400 degrees Fahrenheit (3,500 C), or about a third cooler than the sun's.

Yet during the 2003 outburst, the protostar’s X-ray brightness increased by 100 times and the temperature of its X-ray-emitting regions reached about 90 million F (50 million C). A new eruption began in 2008 and continues today.

During the outbursts, the brightness variations at optical and infrared wavelengths could be accounted for by changes in the protostar’s main energy source, the inflow of matter onto the star. Because changes in X-ray brightness closely followed those in the optical and infrared, the higher-energy emission must also be linked to accretion.

“V1647 Ori gave us the first direct evidence that a protostar surges in X-ray activity as its rate of mass accretion rises,” said co-author Nicolas Grosso. This connection since has been underscored by a few other young stars whose outbursts included elevated X-rays.

To explore the emission process in detail and identify where on the star or disk the X-rays arise, the scientists re-analyzed all observations of V1647 Ori from three premier X-ray satellites- Chandra, Suzaku and the European Space Agency’s XMM-Newton. Their goal was to find patterns that might provide clues to the sites of and mechanisms for producing the high-energy emission.

Writing in the July 20 issue of The Astrophysical Journal, the team reports that strong similarities among 11 separate X-ray light curves allowed them to identify cyclic X-ray variations. Remarkably, these periodic signals establish that the star is spinning once each day. V1647 Ori is among the youngest stars whose spin rates have been determined using an X-ray-based technique.

“Considering that V1647 Ori is about five times the size of the sun, the rapid spin confirms that we’re watching a young stellar object that is in the process of pulling itself together,” said co-author Joel Kastner.

The cyclic X-ray changes represent the appearance and disappearance of hot regions on the star that rotate in and out of view. The model that best agrees with the observations, say the researchers, involves two hot spots of unequal brightness located on opposite sides of the star. Both spots are thought to be pancake-shaped areas about the size of the sun, but the more southerly spot is about five times brighter.

The hot spots represent the footprints of magnetically driven accretion flows from the disk to the surface of the young star. To reach the high temperatures associated with X-ray emission, matter must be hitting the protostar at a speed of about 4.5 million mph (2,000 km/s). As a result, the hot spots reach temperatures some 13,000 times hotter than anywhere else on the star.

“One attractive possibility for driving such high-speed matter involves magnetic fields that are undergoing a continual cycle of shearing and reconnection in mass accretion,” said David Weintraub, a member of the study team.

Both the star and its circumstellar disk possess magnetic fields. Because the star rotates faster than the disk, these fields become twisted and sheared, storing up energy much like a wound-up rubber band. When the tangled field eventually rearranges into a more stable state, it suddenly unleashes its stored energy in a powerful blast. This process, called magnetic reconnection, also powers X-ray flares on the sun.

But while the physical processes may be similar, their time scales are vastly different. The peak X-ray output of a solar flare lasts less only minutes. The outbursts of V1647 Ori persist for years.

For comparison, consider the most powerful solar flare on record, the X28 eruption of Nov. 4, 2003. Hamaguchi calculates that the steady X-ray brightness of V1647 Ori’s current outburst is a few thousand times stronger than the peak luminosity of the solar flare. What actually causes the star’s outbursts? Astronomers don't really know. They suspect that gas from the outer portion of the disk makes its way inward, gradually building up the inner disk closer to the star. The strong magnetic activity may only turn on after some threshold is reached, but once it does the gas rapidly flows onto the hotspots and produces X-rays.

Thanks to Chandra, Suzaku and XMM-Newton, the outbursts of V1647 Ori are giving astronomers a glimpse of the extreme childhood of a sun-like star.

[https://www.nasa.gov/topics/universe/features/xray-flaunt.html]

Orion: Head to Toe

Cradled in cosmic dust and glowing hydrogen, stellar nurseries in Orion the Hunter lie at the edge of a giant molecular cloud some 1,500 light-years away. Spanning nearly 25 degrees, this breath-taking vista stretches across the well-known constellation from head to toe (left to right). The Great Orion Nebula, the closest large star forming region, is right of center. To its left are the Horsehead Nebula, M78, and Orion’s belt stars. You will also find red giant Betelgeuse at the hunter’s shoulder, bright blue Rigel at his foot, and the glowing Lambda Orionis (Meissa) nebula at the far left, near Orion’s head. Of course, the Orion Nebula and bright stars are easy to see with the unaided eye, but dust clouds and emission from the extensive interstellar gas in this nebula-rich complex, are too faint and much harder to record. In this mosaic of broadband telescopic images, additional image data acquired with a narrow hydrogen alpha filter was used to bring out the pervasive tendrils of energized atomic hydrogen gas and the arc of the giant Barnard’s Loop.

[http://apod.nasa.gov/apod/ap101023.html]

The Orion Nebula (also known as M42, or NGC 1976) is a diffuse nebula situated in the Milky Way, being south of Orion’s Belt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of about 1,350 light years and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light years across. It has a mass of about 2000 times the mass of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.

The Orion Nebula is one of the most scrutinized and photographed objects in the night sky, and is among the most intensely studied celestial features. The nebula has revealed much about the process of how stars and planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed protoplanetary disks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula:

[https://en.wikipedia.org/wiki/Orion_Nebula]

The Great Nebula in Orion

The Great Nebula in Orion is a colorful place. Visible to the unaided eye, it appears as a small fuzzy patch in the constellation of Orion. Long exposure, digitally sharpened images like this, however, show the Orion Nebula to be a busy neighborhood of young stars, hot gas, and dark dust. The power behind much of the Orion Nebula (M42) is the Trapezium- four of the brightest stars in the nebula. Many of the filamentary structures visible are actually shock waves- fronts where fast moving material encounters slow moving gas. The Orion Nebula spans about 40 light years and is located about 1500 light years away in the same spiral arm of our Galaxy as the Sun.

[https://www.nasa.gov/multimedia/imagegallery/image_feature_693.html]

The Trapezium or Orion Trapezium Cluster, also known as Theta1 Orionis, is a tight open cluster of stars in the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo Galilei. It is a relatively young cluster that has formed directly out of the parent nebula. The five brightest stars are on the order of 15-30 solar masses in size. They are within a diameter of 1.5 light-years of each other and are responsible for much of the illumination of the surrounding nebula:

[https://en.wikipedia.org/wiki/Trapezium_Cluster]

In the center of the Trapezium

In Orion’s Great Nebula is a bright star cluster known as the Trapezium, shown above. New stellar systems are forming there in gigantic globs of gas and dust known as Proplyds. Looking closely at the above picture also reveals that gas and dust surrounding some of the dimmer stars appears to form structures that point away from the brighter stars. The above false color image was made by combining several exposures from the orbiting Hubble Space Telescope.

[http://apod.nasa.gov/apod/ap030302.html]

M78 (NGC 2068) is another nebula in Orion. With an overall magnitude of 8.0, it is significantly dimmer than the Great Orion Nebula that lies to its south; however, it is at approximately the same distance, at 1600 light-years from Earth. It can easily be mistaken for a comet in the eyepiece of a telescope. M78 is associated with the variable star V351 Orionis, whose magnitude changes are visible in very short periods of time:

M78 and Reflecting Dust Clouds

An eerie blue glow and ominous columns of dark dust highlight M78 and other bright reflection nebula in the constellation of Orion. The dark filamentary dust not only absorbs light, but also reflects the light of several bright blue stars that formed recently in the nebula. Of the two reflection nebulas pictured above, the more famous nebula is M78, in the image center, while NGC 2071 can be seen to its lower left. The same type of scattering that colors the daytime sky further enhances the blue color. M78 is about five light-years across and visible through a small telescope. M78 appears above only as it was 1600 years ago, however, because that is how long it takes light to go from there to here. M78 belongs to the larger Orion Molecular Cloud Complex that contains the Great Nebula in Orion and the Horsehead Nebula.

[http://apod.nasa.gov/apod/ap140326.html]

NGC 2169 is an open star cluster, approximately 3,600 light years away from the solar system. It has an apparent magnitude of 5.9. The cluster was originally discovered by the Italian astronomer Giovanni Batista Hodierna in the mid-17th century and then later independently spotted by William Herschel on October 15, 1784. It is sometimes called The 37 Cluster because it resembles the number 37. NGC 2169 is just under seven arc minutes in diameter and consists of about 30 stars, which are only eight million years old. The brightest one has an apparent magnitude of 6.94:

[http://www.constellation-guide.com/constellation-list/orion-constellation/]

The 37 Cluster

For the mostly harmless denizens of planet Earth, the brighter stars of open cluster NGC 2169 seem to form a cosmic 37. (Did you expect 42?.) Of course, the improbable numerical asterism appears solely by chance and lies at an estimated distance of 3,600 light-years toward the constellation Orion. As far as galactic or open star clusters go, NGC 2169 is a small one, spanning about 7 light-years. Formed at the same time from the same cloud of dust and gas, the stars of NGC 2169 are only about 8 million years old. Such clusters are expected to disperse over time as they encounter other stars, interstellar clouds, and experience gravitational tides while traveling through the galaxy. Over four billion years ago, our own Sun was likely formed in a similar open cluster of stars.

[https://apod.nasa.gov/apod/ap051118.html]

NGC 2023 is a reflection nebula in Orion constellation. The nebula is notable for being one of the brightest sources of fluorescent molecular hydrogen. It is lit by the B star HD 37903, the most luminous star lighting the surface of the molecular cloud Lynds 1630 (Horsehead Nebula, or Barnard 33), and one of the largest reflection nebulae in the sky. It is four light years wide. NGC 2023 can be found a third of a degree from the Horsehead Nebula. It is 1467.7 light years distant from Earth:

[http://www.constellation-guide.com/constellation-list/orion-constellation/]

Sunset glow in Orion

The magnificent reflection nebula NGC 2023 lies nearly 1500 light-years from Earth. It is located within the constellation of Orion (The Hunter), in a prestigious area of the sky close to the well-known Flame and Horsehead Nebulae. The entire structure of NGC 2023 is vast, at four light-years across. This NASA/ESA Hubble Space Telescope picture just takes in the southern part, with the subtle shades of colour closely resembling those of a sunset on Earth.

NGC 2023 surrounds a massive young B-type star. These stars are large, bright and blue-white in colour, and have a high surface temperature, being several times hotter than the Sun. The energy emitted from NGC2023’s B-type star illuminates the nebula, resulting in its high surface brightness: good news for astronomers who wish to study it. The star itself lies outside the field of view, at the upper left, and its brilliant light is scattered by Hubble’s optical system, creating the bright flare across the left side of the picture, which is not a real feature of the nebula.

Stars are forming from the material comprising NGC 2023. This Hubble image captures the billowing waves of gas, 5000 times denser than the interstellar medium. The unusual greenish clumps are thought to be Herbig–Haro objects. These peculiar features of star-forming regions are created when gas ejected at hundreds of kilometres per second from newly formed stars impacts the surrounding material. These shockwaves cause the gas to glow and result in the strange shapes seen here. Herbig–Haro objects typically only last for a few thousand years, which is the blink of eye in astronomical terms.

[https://www.spacetelescope.org/images/potw1130a/]

NGC 1999 is a dust-filled bright nebula with a vast hole of empty space represented by a black patch of sky, as can be seen in the photograph. It is a reflection nebula, and shines from the light of the variable star V380 Orionis. It is located 1,500 light-years away from Earth in the constellation Orion.

It was previously believed that the black patch was a dense cloud of dust and gas which blocked light that would normally pass through, called a dark nebula. Analysis of this patch determined that the patch looks black not because it is an extremely dense pocket of gas, but because it is truly empty. The exact cause of this phenomenon is still being investigated, although it has been hypothesized that narrow jets of gas from some of the young stars in the region punctured the sheet of dust and gas, as well as, powerful radiation from a nearby mature star may have helped to create the hole. Researchers believe this discovery should lead to a better understanding of the entire star forming process:

[https://en.wikipedia.org/wiki/NGC_1999]

NGC 1999: South of Orion

South of the large star-forming region known as the Orion Nebula, lies bright blue reflection nebula NGC 1999. At the edge of the Orion molecular cloud complex some 1,500 light-years distant, NGC 1999's illumination is provided by the embedded variable star V380 Orionis. That nebula is marked with a dark sideways T-shape near center in this cosmic vista that spans about 10 light-years. The dark shape was once assumed to be an obscuring dust cloud seen in silhouette against the bright reflection nebula. But recent infrared images indicate the shape is likely a hole blown through the nebula itself by energetic young stars. In fact, this region abounds with energetic young stars producing jets and outflows with luminous shock waves. Cataloged as Herbig-Haro (HH) objects, named for astronomers George Herbig and Guillermo Haro, the shocks look like red gashes in this scene that includes HH1 and HH2 just below NGC 1999. The stellar jets push through the surrounding material at speeds of hundreds of kilometers per second.

[https://apod.nasa.gov/apod/ap131128.html]

The Horsehead Nebula (also known as Barnard 33) is a dark nebula located just to the south of the star Alnitak, which is farthest east on Orion’s Belt, and is part of the much larger Orion Molecular Cloud Complex. The Horsehead Nebula is approximately 1500 light years from Earth. It is one of the most identifiable nebulae because of the shape of its swirling cloud of dark dust and gases, which bears some resemblance to a horse's head when viewed from Earth:

[https://en.wikipedia.org/wiki/Horsehead_Nebula]

Orion’s Horsehead Nebula

The Horsehead Nebula is one of the most famous nebulae on the sky. It is visible as the dark indentation to the red emission nebula seen above and to the right of center in the above photograph. The bright star on the left is located in the belt of the familiar constellation of Orion. The horse-head feature is dark because it is really an opaque dust cloud which lies in front of the bright red emission nebula. Like clouds in Earth’s atmosphere, this cosmic cloud has assumed a recognizable shape by chance. After many thousands of years, the internal motions of the cloud will alter its appearance. The emission nebula’s red color is caused by electrons recombining with protons to form hydrogen atoms. Also visible in the picture are blue reflection nebulae, which preferentially reflect the blue light from nearby stars.

[http://apod.nasa.gov/apod/ap030129.html]

NGC 2174 (also known as Monkey Head Nebula) is an H II emission nebula located in the constellation Orion and is associated with the open star cluster NGC 2175. It is thought to be located about 6,400 light-years away from Earth. The nebula may have formed through hierarchical collapse:

[https://en.wikipedia.org/wiki/NGC_2174]

At the Edge of NGC 2174

This fantastic skyscape lies near the edge of NGC 2174 a star forming region about 6,400 light-years away in the nebula-rich constellation of Orion. It follows mountainous clouds of gas and dust carved by winds and radiation from the region's newborn stars, now found scattered in open star clusters embedded around the center of NGC 2174, off the top of the frame. Though star formation continues within these dusty cosmic clouds they will likely be dispersed by the energetic newborn stars within a few million years. Recorded at infrared wavelengths by the Hubble Space Telescope, the interstellar scene spans about 6 light-years. The image celebrates the upcoming 24th anniversary of Hubble’s launch onboard the space shuttle orbiter Discovery on April 24, 1990.

[https://apod.nasa.gov/apod/ap140403.html]

The Flame Nebula, designated as NGC 2024 and Sh2-277, is an emission nebula about 900 to 1,500 light-years away.

The bright star Alnitak (ζ Ori), the easternmost star in the Belt of Orion, shines energetic ultraviolet light into the Flame and this knocks electrons away from the great clouds of hydrogen gas that reside there. Much of the glow results when the electrons and ionized hydrogen recombine. Additional dark gas and dust lies in front of the bright part of the nebula and this is what causes the dark network that appears in the center of the glowing gas. The Flame Nebula is part of the Orion Molecular Cloud Complex, a star-forming region that includes the famous Horsehead Nebula.

At the center of the Flame Nebula is a cluster of newly formed stars, 86% of which have circumstellar disks. X-ray observations by the Chandra X-ray Observatory show several hundred young stars, out of an estimated population of 800 stars. X-ray and infrared images indicate that the youngest stars are concentrated near the center of the cluster:

[https://en.wikipedia.org/wiki/Flame_Nebula]

Inside the Flame Nebula

Stars are often born in clusters, in giant clouds of gas and dust. Astronomers have studied two star clusters using NASA’s Chandra X-ray Observatory and infrared telescopes and the results show that the simplest ideas for the birth of these clusters cannot work, as described in our latest press release.

This composite image shows one of the clusters, NGC 2024, which is found in the center of the so-called Flame Nebula about 1,400 light years from Earth. In this image, X-rays from Chandra are seen as purple, while infrared data from NASA’s Spitzer Space Telescope are colored red, green, and blue.

A study of NGC 2024 and the Orion Nebula Cluster, another region where many stars are forming, suggest that the stars on the outskirts of these clusters are older than those in the central regions. This is different from what the simplest idea of star formation predicts, where stars are born first in the center of a collapsing cloud of gas and dust when the density is large enough.

The research team developed a two-step process to make this discovery. First, they used Chandra data on the brightness of the stars in X-rays to determine their masses. Next, they found out how bright these stars were in infrared light using data from Spitzer, the 2MASS telescope, and the United Kingdom Infrared Telescope. By combining this information with theoretical models, the ages of the stars throughout the two clusters could be estimated.

According to the new results, the stars at the center of NGC 2024 were about 200,000 years old while those on the outskirts were about 1.5 million years in age. In Orion, the age spread went from 1.2 million years in the middle of the cluster to nearly 2 million years for the stars toward the edges.

Explanations for the new findings can be grouped into three broad categories. The first is that star formation is continuing to occur in the inner regions. This could have happened because the gas in the outer regions of a star-forming cloud is thinner and more diffuse than in the inner regions. Over time, if the density falls below a threshold value where it can no longer collapse to form stars, star formation will cease in the outer regions, whereas stars will continue to form in the inner regions, leading to a concentration of younger stars there.

Another suggestion is that old stars have had more time to drift away from the center of the cluster, or be kicked outward by interactions with other stars. Finally, the observations could be explained if young stars are formed in massive filaments of gas that fall toward the center of the cluster.

The combination of X-rays from Chandra and infrared data is very powerful for studying populations of young stars in this way. With telescopes that detect visible light, many stars are obscured by dust and gas in these star-forming regions, as shown in this optical image of the region.

[https://www.nasa.gov/mission_pages/chandra/multimedia/flame-nebula.html]

Barnard’s Loop is an emission nebula in the constellation of Orion. It is part of the Orion Molecular Cloud Complex which also contains the dark Horsehead and bright Orion nebulae. The loop takes the form of a large arc centered approximately on the Orion Nebula. The stars within the Orion Nebula are believed to be responsible for ionizing the loop.

The loop extends over about 600 arcminutes as seen from Earth, covering much of Orion. It is well seen in long-exposure photographs, although observers under very dark skies may be able to see it with the naked eye.

Recent estimates place it at a distance of either 159 pc (518 light years) or 440 pc (1434 ly), giving it dimensions of either about 100 or 300 ly across respectively. It is thought to have originated in a supernova explosion about 2 million years ago, which may have also created several known runaway stars, including AE Aurigae, Mu Columbae and 53 Arietis, which are believed to have been part of a multiple star system in which one component exploded as a supernova.

Although this faint nebula was certainly observed by earlier astronomers, it is named after the pioneering astrophotographer E. E. Barnard who photographed it and published a description in 1894:

[https://en.wikipedia.org/wiki/Barnard%27s_Loop]

Barnard’s Loop Around Orion

Why is the belt of Orion surrounded by a bubble? Although glowing like an emission nebula, the origin of the bubble, known as Barnard’s Loop, is currently unknown. Progenitor hypotheses include the winds from bright Orion stars and the supernovas of stars long gone. Barnard’s Loop is too faint to be identified with the unaided eye. The nebula was discovered only in 1895 by E. E. Barnard on long duration film exposures. Orion’s belt is seen as the three bright stars across the center of the image, the upper two noticeably blue. Just to the right of the lowest star in Orion’s belt is a slight indentation in an emission nebula that, when seen at higher magnification, resolves into the Horsehead Nebula. To the right of the belt stars is the bright, famous, and photogenic Orion Nebula.

[https://apod.nasa.gov/apod/ap050420.html]

The Abell 520 galaxy cluster possesses an unusual substructure resulting from a major merger. It has been popularly nicknamed The Train Wreck Cluster, due to its chaotic structure, and is classified as a Bautz-Morgan type III cluster. Analysis of the motions of 293 galaxies in the cluster field suggested that Abell 520 was a cluster forming at the crossing of three filaments of the large scale structure.

The surprising substructure of Abell 520 was reported in 2007 from a weak gravitational lensing study. It was surprising because the study found the ‘dark core’ with a significant amount of mass in the region, where there is no concentration of bright cluster galaxies. No conventional understanding of dark matter can explain this peculiar concentration of dark matter. One interesting interpretation is that the substructure may arise from non-gravitational interaction of dark matter:

[https://en.wikipedia.org/wiki/Abell_520]

Dark matter mystery deepens in cosmic ‘Train Wreck’

This multi-wavelength image of Abell 520 shows the aftermath of a complicated collision of galaxy clusters, some of the most massive objects in the Universe. In this image, the hot gas as detected by Chandra is colored red. Optical data from the Canada-France-Hawaii and Subaru telescopes shows the starlight from the individual galaxies (yellow and orange). The location of most of the matter in the cluster (blue) was also found using these telescopes, by tracing the subtle light-bending effects on distant galaxies. This material is dominated by dark matter.

Abell 520 has similarities to the so-called Bullet Cluster (also known as 1E0657-56). As with the Bullet Cluster, it appears that the galaxies flew past one another when the clusters collided, as expected. Another parallel is that there are large separations between the regions where the galaxies are most common and where most of the hot gas lies.

There are significant differences, however, between Abell 520 and the Bullet Cluster. For example, a concentration of dark matter is found near the bulk of the hot gas, where very few galaxies are found. In addition, there is an area where there are several galaxies but very little dark matter. Both of these features are in contrast to popular theory that says dark matter and galaxies should stay together, even during a violent collision.

While the components of Abell 520- galaxies, hot gas, and dark matter- are found in unexpected places, the overall amount of these components totals what scientists expect. Therefore, these results lead to two possible explanations: one involving how galaxy clusters interact, and the other about the nature of dark matter itself. Both of these explanations would pose problems for current theories.

The first option is that the galaxies were separated from the dark matter through a complex set of gravitational ‘slingshots.’ The second option is that dark matter is affected not only by gravity, but also by an as-yet-unknown interaction between dark matter particles. This second option- self-interaction by dark matter- would require new physics and could be difficult to reconcile with observations of other objects like the Bullet Cluster. However, independent workers had earlier proposed self-interaction as a solution to problems with models of galaxies and clusters that include dark matter. The strength of the self-interaction calculated by these authors agrees with that estimated for Abell 520, but such estimates are highly uncertain.

Distance estimate: about 2.4 billion light years.

[http://chandra.harvard.edu/photo/2007/a520/index.html]

The Orionid meteor shower, usually shortened to the Orionids, is the most prolific meteor shower associated with Halley’s Comet. The Orionids are so-called because the point they appear to come from, called the radiant, lies in the constellation Orion, but they can be seen over a large area of the sky. Orionids are an annual meteor shower which last approximately one week in late October. In some years, meteors may occur at rates of 50–70 per hour:

[https://en.wikipedia.org/wiki/Orionids]

Orionid Meteors over Turkey

Meteors have been flowing out from the constellation Orion. This was expected, as mid-October is the time of year for the Orionids Meteor Shower. Pictured above, over a dozen meteors were caught in successively added exposures over three hours taken this past weekend (2006 October 23) from a town near Bursa, Turkey. The above image shows brilliant multiple meteor streaks that can all be connected to a single point in the sky just above the belt of Orion, called the radiant. The Orionids meteors started as sand sized bits expelled from Comet Halley during one of its trips to the inner Solar System. Comet Halley is actually responsible for two known meteor showers, the other known as the Eta Aquarids and visible every May. Next month, the Leonids Meteor Shower from Comet Tempel-Tuttle might show an even more impressive shower from some locations.

[http://apod.nasa.gov/apod/ap061023.html]

[https://en.wikipedia.org/wiki/Orion_%28constellation%29]

[Return to the 88 Modern Constellations]