Gemini is the 30th largest constellation in the sky, occupying an area of 514 square degrees. It lies in the second quadrant of the northern hemisphere (NQ2) and can be seen at latitudes between +90° and -60°. The neighboring constellations are Auriga, Cancer, Canis Minor, Lynx, Monoceros, Orion and Taurus.
[http://www.constellation-guide.com/constellation-list/gemini-constellation/]
Johannes Hevelius’ Gemini from Uranographia (1690)
[http://chandra.harvard.edu/photo/constellations/gemini.html]
In Babylonian astronomy, the stars Castor and Pollux were known as the Great Twins (MUL.MASH.TAB.BA.GAL.GAL). The Twins were regarded as minor gods and were called Meshlamtaea and Lugalirra, meaning respectively ‘The One who has arisen from the Underworld’ and the ‘Mighty King.’ Both names can be understood as titles of Nergal, the major Babylonian god of plague and pestilence, who was king of the Underworld.
In Greek mythology, Gemini was associated with the myth of Castor and Pollux, the children of Leda and Argonauts both. Pollux was the son of Zeus, who seduced Leda, while Castor was the son of Tyndareus, king of Sparta and Leda’s husband. Castor and Pollux were also mythologically associated with St. Elmo’s fire in their role as the protectors of sailors. When Castor died, because he was mortal, Pollux begged his father Zeus to give Castor immortality, and he did, by uniting them together in the heavens.
In Meteorologica, Aristotle mentions that he observed Jupiter in conjunction with and then occulting a star in Gemini. This is the earliest known observation of this nature. A study published in 1990 suggests the star involved was 1 Geminorum and the event took place on 5 December 337 BCE.
When William Herschel discovered Uranus on 13 March 1781 it was located near η Gem. In 1930 Clyde Tombaugh exposed a series of photographic plates centred on δ Gem and discovered Pluto.
In Chinese astronomy, the stars that correspond to Gemini are located in two areas: the White Tiger of the West (Xī Fāng Bái Hǔ) and the Vermillion Bird of the South (Nán Fāng Zhū Què).
[http://oneminuteastronomer.com/3010/constellation-gemini/]
Gemini is fairly easy to spot in the sky, even for amateur star gazers. It is located northeast of Orion, and between Taurus and Cancer on the elliptic. Best viewing is during February. By April and May, the constellation can be seen soon after sunset in the west.
[http://www.space.com/16816-gemini-constellation.html]
Gemini lies between Taurus to the west and Cancer to the east, with Auriga and Lynx to the north and Monoceros and Canis Minor to the south.
The Sun resides in the astrological sign of Gemini from June 20 to July 20 each year (though the zodiac dates it May 21 - June 21). By mid August, Gemini will appear along the eastern horizon in the morning sky prior to sunrise. The best time to observe Gemini at night is overhead during the months of January and February. By April and May, the constellation will be visible soon after sunset in the west.
The easiest way to locate the constellation is to find its two brightest stars Castor and Pollux eastward from the familiar ‘V’ shaped asterism of Taurus and the three stars of Orion’s belt. Another way is to mentally draw a line from the Pleiades star cluster located in Taurus and the brightest star in Leo, Regulus. In doing so, you are drawing an imaginary line that is relatively close to the ecliptic, a line which intersects Gemini roughly at the midpoint of the constellation, just below Castor and Pollux.
The constellation contains 85 stars visible to observation on Earth without a telescope.
The Sun viewed from the star Pollux (in red circle). Made with Celestia.
Pollux, also designated Beta Geminorum, is an orange-hued evolved giant star approximately 34 light-years from the Sun in the northern constellation of Gemini. It is the closest giant star to the Sun.
At an apparent visual magnitude of 1.1, Pollux is the brightest star in the constellation, brighter even than its neighbor Castor (Alpha Geminorum). The star is larger than the Sun, with about two times its mass and almost nine times its radius. Once an A-type main sequence star, Pollux has exhausted the hydrogen at its core and evolved into a giant star with a stellar classification of K0 III. The effective temperature of this star’s outer envelope is about 4666 K, which lies in the range that produces the characteristic orange hue of K-type stars. Pollux has a projected rotational velocity of 2.8 km/s.The abundance of elements other than hydrogen and helium, what astronomers term the star’s metallicity, is somewhat uncertain, with estimates ranging from 85% to 155% of the Sun’s abundance.
Evidence for a low level of magnetic activity came from the detection of weak X-ray emission using the ROSAT orbiting telescope. The X-ray emission from this star is about 10^27 erg/s, which is roughly the same as the X-ray emission from the Sun. A magnetic field with a strength below 1 Gauss has since been confirmed on the surface of Pollux, one of the weakest fields ever detected on a star. The presence of this field suggests that Pollux was once a star with a much stronger magnetic field.
Since 1993, scientists have suspected an extrasolar planet orbiting Pollux, from measured radial velocity oscillations. The existence of the planet, Pollux b, was confirmed and announced on June 16, 2006. Pollux b is calculated to have a mass at least 2.3 times that of Jupiter. The planet is orbiting Pollux with a period of about 590 days.
[https://en.wikipedia.org/wiki/Pollux_(star)]
Castor A and B
Castor, also designated Alpha Geminorum is the second-brightest star in the constellation of Gemini and one of the brightest stars in the night sky. Although it has the identifier Alpha, it is fainter than Beta Geminorum (Pollux).
Castor is a double star . The two stars have magnitudes of 1.9 and 3.0. A third star more distant from the main components was discovered, and was given the variable star designation YY Geminorum.
All three of the visual components are spectroscopic binaries and Castor is a complex multiple star system made up of six individual stars. Castor A and B both have orbits of a few days with a much fainter companion. The Castor C components orbit in less than a day. Castor C is believed to be in orbit around the bright pair, but with an extremely long period of several thousand years. The combined apparent magnitude of all six stars is +1.58.
Castor is 51 light-years away from Earth, determined from its large annual parallax. The two brightest stars are both A-class main-sequence stars, more massive and brighter than the Sun. The properties of their red dwarf companions are difficult to determine, but are both thought to have less than half the mass of the Sun. The two red dwarfs of Castor C are almost identical, with masses around a half M☉ and luminosities less than 10% of the Sun.
Castor B is a star with particularly strong spectral lines of certain metals. Castor C is a variable star, classified as a BY Dra type. BY Draconis variables are cool dwarf stars which vary as they rotate due to star spots or other variations in their photospheres. All the red dwarfs in the Castor system have emissions lines in their spectra, and all are Flare stars.
[https://en.wikipedia.org/wiki/Castor_(star)]
Alhena (center) with Mu Geminorum at upper right
[http://www.daviddarling.info/encyclopedia/A/Alhena.html]
Gamma Geminorum, also named Alhena, is the third-brightest star in the constellation of Gemini. It has an apparent visual magnitude of 1.9, making it easily visible to the naked eye even in urban regions. It is located at a distance of roughly 109 light-years (33 parsecs) from the Sun.
Alhena is an evolving star that is exhausting the supply of hydrogen at its core and has entered the subgiant stage. The spectrum matches a stellar classification of A0 IV. Compared to the Sun it has 2.8 times the mass and 3.3 times the radius. It is radiating around 123 times the luminosity of the Sun from its outer envelope at an effective temperature of 9,260 K. This gives it a white hue typical of an A-class star.
This is a spectroscopic binary system with a period of 12.6 years in a highly eccentric orbit.
The traditional name Alhena is derived from the Arabic ‘Al Han’ah,’ ‘the brand’ (on the neck of the camel).
[https://en.wikipedia.org/wiki/Gamma_Geminorum]
μ Gem is the star on the left, surrounded by the S249 nebula. η Gem is the bright star on the right, near the IC 443 supernova remnant.
Mu Geminorum has the traditional name Tejat Posterior, which means back foot, because it is the foot of Castor, one of the Gemini twins.
It has an average apparent visual magnitude of 2.9, which makes it the fourth-brightest member of Gemini. The distance to this star is roughly 230 light-years (71 parsecs).
This star is a slow irregular variable of type LB. Its brightness varies between magnitude +2.75 and +3.02 over a 72-day period, along with a 2,000-day period of long term variation. It is a red giant at a stellar classification of M3 III, with a surface temperature of 3,773 K, meaning it is brighter, yet cooler, than the Sun. The star is currently on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen.
[https://en.wikipedia.org/wiki/Mu_Geminorum]
Mebsuta (ε Geminorum)
[https://it.wikipedia.org/wiki/Mebsuta]
Epsilon Geminorum, also named Mebsuta, is located on the outstretched right ‘leg’ of the twin Castor. The apparent visual magnitude of +3.06 makes it one of the brighter stars in this constellation.
The distance to this star is about 840 light-years (260 parsecs). Because Epsilon Geminorum is located near the ecliptic it can be occulted by the Moon or a planet. Just such an occultation took place on April 8, 1976 by Mars, which allowed the oblateness of the planet’s outer atmosphere to be measured. Prior to that, the star was occulted by Mercury on June 10, 1940. On September 3, 2015, Epsilon Geminorum is expected to be occulted by the asteroid Iphigenia.
The spectrum of this star matches a stellar classification of G8 Ib, where the luminosity class of Ib indicates this is a lower luminosity supergiant star. Alternatively, it may be a star that has passed through the asymptotic giant branch stage and possesses a detached shell of dust. The estimated mass of this star is over 19 times the mass of the Sun, and it has expanded to a radius measured at around 105–175 times that of the Sun.
Epsilon Geminorum is radiating around 8,500 times the luminosity of the Sun from its outer atmosphere at an effective temperature of 4,662 K. It is this temperature that gives it the yellow-hued glow of a G-type star. A surface magnetic field with a strength of –0.14 ± 0.19 G has been detected on this star. This topologically complex field is most likely generated by a dynamo formed from the deep convection zone in the star’s outer envelope.
The traditional name Mebsuta has its roots in ancient Arabic, where it and the star Mekbuda (Zeta Geminorum) were the paws of a lion. Mebsuta comes from a phrase referring to the outstretched paw.
[https://en.wikipedia.org/wiki/Epsilon_Geminorum]
η Gem is the bright star lying just outside the supernova remnant IC 443
Eta Geminorum, also named Propus, is a triple star system in the constellation of Gemini. It is a naked-eye variable star around 380 light years from the Sun.
The traditional names are Tejat Prior, Propus (meaning forward foot in Greek), Praepes and Pish Pai (meaning foreleg in Persian). This star, along with γ Gem (Alhena), μ Gem (Tejat Posterior), ν Gem and ξ Gem (Alzirr) were ‘Al Han’ah’, ‘the brand’ (on the neck of the camel). They also were associated in ‘Al Nuḥātai,’ the dual form of ‘Al Nuḥāt,’ ‘a Camel’s Hump.’
η Geminorum lies at the foot of the Castor side of Gemini, about two degrees west of μ Geminorum and two degrees southeast of the bright open cluster M35. Between the two stars are several faint areas of nebulosity. η Gem just to the west of the supernova remnant shell IC 443. Further east around μ Gem is the emission nebula S249. In between is the small faint emission nebula IC 444 around the 7th magnitude 12 Geminorum.
This is a triple system, with the luminous class M star having a close companion known only from radial velocity variations, and a more distant companion resolved visually. The close companion is 6th magnitude, it is given a G0 spectral type, and is assumed to be a giant on the basis of its brightness. The luminous main component of η Geminorum is an asymptotic giant branch star, a highly evolved cool luminous star that was originally 2-8 M☉ on the main sequence.
η Geminorum is near the ecliptic, so it can be occulted by the Moon and, very rarely, by planets. The last occultation by a planet took place on July 27, 1910, by Venus, and the next to last on July 11, 1837, by Mercury.
[https://en.wikipedia.org/wiki/Eta_Geminorum]
[http://www.starobserver.eu/multiplestars/south529.html]
Xi Geminorum (ξ Gem) has the traditional name Alzirr, meaning ‘the button’ in Arabic. It forms one of the four feet of the outline demarcating the Gemini twins. The star has an apparent visual magnitude of 3.35, which is bright enough for it to be seen with the naked eye. Its distance from the Earth is estimated as 58.7 light-years (18.0 parsecs).
Alzirr has a stellar classification of F5 IV-V, which is subgiant star that is in the process of evolving away from the main sequence of stars like the Sun. It has about 162% of the Sun’s mass and is radiating more than 11 times the luminosity of the Sun. This energy is being emitted from the outer envelope of the star at an effective temperature of 6,464 K. This causes the star to take on the yellow-white hue common to F-type stars.
X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.06×10^29 erg/s. It has the spectroscopic signature of a rapidly rotating star, with a projected rotational velocity of about 66 km/s. Although generally considered a single star, there is some evidence that it may instead be a spectroscopic binary system consisting of two component stars of equal mass.
[https://en.wikipedia.org/wiki/Xi_Geminorum]
Wasat (Delta Geminorum) is the bright star next to Jupiter. Jupiter is ~280x brighter.
Delta Geminorum, also named Wasat, is a triple star system in the constellation of Gemini. The traditional name derives from the Arabic word for ‘middle.’
Delta Geminorum is a subgiant star with the stellar classification F0 IV. It is about 60 light-years (18.5 parsecs) distant. This star has 1.57 times the mass of the Sun and is rotating rapidly with a projected rotational velocity of 129.7 km/s. The estimated age is 1.6 billion years.
It has an apparent visual magnitude of +3.53, allowing it to be seen with the naked eye. It is only two-tenths of a degree south of the ecliptic, and therefore is occasionally occulted by the Moon and, more rarely, by a planet. The last occultation by a planet was by Saturn on June 30, 1857, and the next will be by Venus on August 12, 2420. In 1930, the dwarf planet Pluto was discovered about 0.5° to the east of this star by American astronomer Clyde Tombaugh.
Delta Geminorum is a triple star system. The inner components form a spectroscopic binary with a period of 6.1 years and an orbital eccentricity of 0.3530. A cooler class K companion is not apparent to the naked eye but clearly visible in a small telescope. It orbits the inner pair with a period of 1,200 years and an eccentricity of 0.11. Although according to its radial velocity is away from the Sun, it is actually approaching the Solar System. In about 1.1 million years, it will make its closest approach at a separation of roughly 6.7 ly (2.1 pc).
[https://en.wikipedia.org/wiki/Delta_Geminorum]
Geminga
Geminga is a neutron star approximately 250 parsecs (around 800 light years) from the Sun in the constellation Gemini. Its name is both a contraction of ‘Gemini gamma-ray source’, and a transcription of the words ‘gh'è minga,’ ‘it’s not there’ in the Lombard dialect of Milan.
The nature of Geminga was quite unknown for 20 years after its discovery by NASA’s Second Small Astronomy Satellite (SAS-2). Finally, in March 1991 the ROSAT satellite detected a periodicity of 0.237 seconds in soft x-ray emission. Thus, it is supposed that Geminga is a sort of neutron star- the decaying core of a massive star that exploded as a supernova about 300,000 years ago:
[https://en.wikipedia.org/wiki/Geminga]
Geminga and B0355+54: Chandra Images Show That Geometry Solves a Pulsar Puzzle
X-ray images from Chandra have shown distinctly different shapes for the structures around two pulsars. Pulsars are rapidly rotating, highly magnetized, neutron stars born in supernova explosions triggered by the collapse of massive stars. In certain cases, pulsars generate extensive clouds of high-energy particles called pulsar wind nebulas. By studying the shape and orientation of these structures, astronomers may be able to explain the presence or absence of radio and gamma-ray pulses from these systems.
NASA’s Chandra X-ray Observatory has taken deep exposures of two nearby energetic pulsars flying through the Milky Way galaxy. The shape of their X-ray emission suggests there is a geometrical explanation for puzzling differences in behavior shown by some pulsars.
Pulsars- rapidly rotating, highly magnetized, neutron stars born in supernova explosions triggered by the collapse of massive stars- were discovered 50 years ago via their pulsed, highly regular, radio emission. Pulsars produce a lighthouse-like beam of radiation that astronomers detect as pulses as the pulsar’s rotation sweeps the beam across the sky.
Since their discovery, thousands of pulsars have been discovered, many of which produce beams of radio waves and gamma rays. Some pulsars show only radio pulses and others show only gamma-ray pulses. Chandra observations have revealed steady X-ray emission from extensive clouds of high-energy particles, called pulsar wind nebulas, associated with both types of pulsars. New Chandra data on pulsar wind nebulas may explain the presence or absence of radio and gamma-ray pulses.
This four-panel graphic shows the two pulsars observed by Chandra. Geminga is in the upper left and B0355+54 is in the upper right. In both of these images, Chandra’s X-rays, colored blue and purple, are combined with infrared data from NASA’s Spitzer Space Telescope that shows stars in the field of view. Below each data image, an artist’s illustration depicts more details of what astronomers think the structure of each pulsar wind nebula looks like.
For Geminga, a deep Chandra observation totaling nearly eight days over several years was analyzed to show sweeping, arced trails spanning half a light year and a narrow structure directly behind the pulsar. A five-day Chandra observation of the second pulsar, B0355+54, showed a cap of emission followed by a narrow double trail extending almost five light years.
The underlying pulsars are quite similar, both rotating about five times per second and both aged about half a million years. However, Geminga shows gamma-ray pulses with no bright radio emission, while B0355+54 is one of the brightest radio pulsars known yet not seen in gamma rays.
A likely interpretation of the Chandra images is that the long narrow trails to the side of Geminga and the double tail of B0355+54 represent narrow jets emanating from the pulsar’s spin poles. Both pulsars also contain a torus, a disk-shaped region of emission spreading from the pulsar’s spin equator. These donut-shaped structures and jets are crushed and swept back as the pulsars fly through the Galaxy at supersonic speeds.
In the case of Geminga, the view of the torus is close to edge-on, while the jets point out to the sides. B0355+54 has a similar structure, but with the torus viewed nearly face-on and the jets pointing nearly directly towards and away from Earth. In B0355+54, the swept-back jets appear to lie almost on top of each other, giving a doubled tail.
Both pulsars have magnetic poles quite close to their spin poles, as is the case for the Earth’s magnetic field. These magnetic poles are the site of pulsar radio emission so astronomers expect the radio beams to point in a similar direction as the jets. By contrast the gamma-ray emission is mainly produced along the spin equator and so aligns with the torus.
For Geminga, astronomers view the bright gamma-ray pulses along the edge of the torus, but the radio beams near the jets point off to the sides and remain unseen. For B0355+54, a jet points almost along our line of sight towards the pulsar. This means astronomers see the bright radio pulses, while the torus and its associated gamma-ray emission are directed in a perpendicular direction to our line of sight, missing the Earth.
These two deep Chandra images have, therefore, exposed the spin orientation of these pulsars, helping to explain the presence, and absence, of the radio and gamma-ray pulses.
[http://chandra.harvard.edu/photo/2017/geminga/index.html]
AAVSO light curve of U Geminorum’s outburst behavior
U Geminorum is an archetypal example of a dwarf nova. The binary star system consists of a white dwarf closely orbiting a red dwarf. Roughly every 100 days it undergoes an outburst that greatly increases its brightness. The star has been monitored by amateur and professional astronomers ever since, though its location near the zodiac means that some outbursts are undoubtedly missed due to the seasonal gap.
The U Geminorum binary has a very short orbital period of 4 hours and 11 minutes; this orbit alone makes the system variable, as the components transit and eclipse each other with each revolution. Normally, the combined apparent magnitude varies between 14.0 and 15.1; during an outburst however, the star can brighten a hundredfold, to 9th magnitude. Though the average interval is 100 days, the period is in fact highly irregular, varying from as little as 62 days to as long as 257. As is the case with dwarf novae, the outbursts are theorized results of a periodic surge of influx from the white dwarf’s accretion disk, caused by instability in the disk itself.
Distance estimates for U Geminorum have varied from 52 parsecs (170 light-years) to 112 parsecs (370 light-years), with a best estimate of 82 parsecs (270 light-years).
[https://en.wikipedia.org/wiki/U_Geminorum]
Artist’s illustration of the giant star Tau Geminorum (left) and its brown dwarf companion- the dark disk at right.
Tau Geminorum is a star in the northern zodiac constellation of Gemini. It has the apparent visual magnitude of +4.42, making it visible to the naked eye under suitably good seeing conditions. This star is close enough to the Earth that its distance can be measured using the parallax technique, which yields a value of roughly 321 light-years (98 parsecs).
It is an evolved giant star of the spectral type K2 III, meaning it is probably fusing carbon and oxygen in its core to form iron and nickel. It has double the mass of the Sun and has expanded to 27 times the Sun’s radius. Tau Geminorum is radiating 224 as much luminosity as the Sun from its expanded outer atmosphere at an effective temperature of 4,528 K, giving it the characteristic orange-hued glow of a K-type star. It appears to be rotating slowly with a projected rotational velocity of 5.8 km/s.
This star has a brown dwarf companion Tau Geminorum b, whose mass is 18.1 Jupiter masses. This brown dwarf takes 305 days or 0.84 years or 26.4 megaseconds to revolve around Tau Gem.
[https://en.wikipedia.org/wiki/Tau_Geminorum]
Messier 35
Messier 35 (also known as M35, or NGC 2168) is an open cluster in the constellation Gemini. The cluster is scattered over an area of the sky almost the size of the full moon and is located 850 parsecs (2,800 light-years) from Earth.
The mass of M35 has been computed using a statistical technique based on proper motion velocities of its stars. The mass within the central 3.75 parsecs was found to be between 1600 and 3200 solar masses (95 percent confidence), consistent with the mass of a realistic stellar population within the same radius.
The compact open cluster NGC 2158 lies directly southwest of M35.
[https://en.wikipedia.org/wiki/Messier_35]
The Eskimo Nebula (NGC 2392), also known as the Clownface Nebula or Caldwell 39, is a bipolar double-shell planetary nebula (PN). The formation resembles a person’s head surrounded by a parka hood. It is surrounded by gas that composed the outer layers of a Sun-like star. The visible inner filaments are ejected by a strong wind of particles from the central star. The outer disk contains unusual light-year-long filaments:
[https://en.wikipedia.org/wiki/Eskimo_Nebula]
Eskimo Nebula
This stellar relic, first spied by William Herschel in 1787, is nicknamed the Eskimo Nebula (NGC 2392) because, when viewed through ground-based telescopes, it resembles a face surrounded by a fur parka. In this Hubble telescope image, the ‘parka’ is really a disk of material embellished with a ring of comet-shaped objects, with their tails streaming away from the central, dying star. Although the Eskimo’s ‘face’ resembles a ball of twine, it is, in reality, a bubble of material being blown into space by the central star’s intense ‘wind’ of high-speed material. This object is an example of a planetary nebula, so named because many of them have a round appearance resembling that of a planet when viewed through a small telescope. A planetary nebula forms when dying sun-like stars eject their outer gaseous layers, which then become bright nebulae with amazing and confounding shapes. The Eskimo Nebula is about 5,000 light-years from Earth in the constellation Gemini and began forming about 10,000 years ago.
[https://www.nasa.gov/mission_pages/hubble/hubble_anniversary/eskimo_nebula.html]
The Medusa Nebula is a large planetary nebula in the constellation of Gemini on the Canis Minor border. It also known as Abell 21 and Sharpless 2-274. The braided serpentine filaments of glowing gas suggests the serpent hair of Medusa found in ancient Greek mythology:
[https://en.wikipedia.org/wiki/Medusa_Nebula]
The Dreadful Beauty of Medusa
Astronomers using ESO’s Very Large Telescope in Chile have captured the most detailed image ever taken of the Medusa Nebula. As the star at the heart of this nebula made its transition into retirement, it shed its outer layers into space, forming this colorful cloud. The image foreshadows the final fate of the Sun, which will eventually also become an object of this kind.
This beautiful planetary nebula is named after a dreadful creature from Greek mythology- the Gorgon Medusa. It is also known as Sharpless 2-274 and is located in the constellation of Gemini (The Twins). The Medusa Nebula spans approximately four light-years and lies at a distance of about 1500 light-years. Despite its size it is extremely dim and hard to observe.
Medusa was a hideous creature with snakes in place of hair. These snakes are represented by the serpentine filaments of glowing gas in this nebula. The red glow from hydrogen and the fainter green emission from oxygen gas extends well beyond this frame, forming a crescent shape in the sky. The ejection of mass from stars at this stage of their evolution is often intermittent, which can result in fascinating structures within planetary nebulae.
For tens of thousands of years the stellar cores of planetary nebulae are surrounded by these spectacularly colorful clouds of gas. Over a further few thousand years the gas slowly disperses into its surroundings. This is the last phase in the transformation of stars like the Sun before ending their active lives as white dwarfs. The planetary nebula stage in the life of a star is a tiny fraction of its total life span- just as the time a child takes to blow a soap bubble and see it drift away is a brief instant compared to a full human life span.
Harsh ultraviolet radiation from the very hot star at the core of the nebula causes atoms in the outward-moving gas to lose their electrons, leaving behind ionized gas. The characteristic colors of this glowing gas can be used to identify objects. In particular, the presence of the green glow from doubly ionized oxygen ([O III]) is used as a tool for spotting planetary nebulae. By applying appropriate filters, astronomers can isolate the radiation from the glowing gas and make the dim nebulae appear more pronounced against a darker background.
When the green [O III] emission from nebulae was first observed, astronomers thought they had discovered a new element that they dubbed nebulium. They later realized that it was simply a rare wavelength of radiation from an ionized form of the familiar element oxygen.
The nebula is also referred to as Abell 21 (more formally PN A66 21), after the American astronomer George O. Abell, who discovered this object in 1955. For some time scientists debated whether the cloud could be the remnant of a supernova explosion. In the 1970s, however, researchers were able to measure the movement and other properties of the material in the cloud and clearly identify it as a planetary nebula.
[http://www.eso.org/public/news/eso1520/]
IC 443 (also known as the Jellyfish Nebula and Sharpless 248 (Sh2-248)) may be the remains of a supernova that occurred 3,000 - 30,000 years ago, and it is one of the best-studied cases of supernova remnants interacting with surrounding molecular clouds:
[https://en.wikipedia.org/wiki/IC_443]
IC 443: What Spawned the Jellyfish Nebula?
The Jellyfish Nebula (officially known as IC 443) is a supernova remnant about 5,000 light years from Earth. Astronomers have been looking for the spinning neutron star, or pulsar, from the explosion that created the remnant. New Chandra observations have likely spotted a pulsar on the southern edge the Jellyfish Nebula. The X-ray data also provide new details about the structure and properties of this pulsar.
The Jellyfish Nebula, also known by its official name IC 443, is the remnant of a supernova lying 5,000 light years from Earth. New Chandra observations show that the explosion that created the Jellyfish Nebula may have also formed a peculiar object located on the southern edge of the remnant, called CXOU J061705.3+222127, or J0617 for short. The object is likely a rapidly spinning neutron star, or pulsar.
When a massive star runs out of thermonuclear fuel, it implodes, forming a dense stellar core called a neutron star. The outer layers of the star collapse toward the neutron star then bounce outward in a supernova explosion. A spinning neutron star that produces a beam of radiation is called a pulsar. The radiation sweeps by like a beacon of light from a lighthouse and can be detected as pulses of radio waves and other types of radiation.
This new composite image includes a wide-field view from an astrophotographer that shows the spectacular filamentary structure of IC 443. Within the inset box, another optical image from the Digitized Sky Survey (red, green, orange, and cyan) has been combined with X-ray data from Chandra (blue). The inset shows a close-up view of the region around J0617.
The Chandra image reveals a small, circular structure (or ring) surrounding the pulsar and a jet-like feature pointing roughly in an up-down direction that passes through the pulsar. It is unclear if the long, pink wisp of optical emission is related to the pulsar, as similar wisps found in IC 443 are unrelated to X-ray features from the pulsar. The ring may show a region where a high speed wind of particles flowing away from the pulsar, is slowing down abruptly. Alternately, the ring may represent a shock wave, similar to a sonic boom, ahead of the pulsar wind. The jet could be particles that are being fired away from the pulsar in a narrow beam at high speed.
The X-ray brightness of J0617 and its X-ray spectrum, or the amount of X-rays at different wavelengths, are consistent with the profiles from known pulsars. The spectrum and shape of the diffuse, or spread out, X-ray emission surrounding J0617 and extending well beyond the ring also match with expectations for a wind flowing from a pulsar.
X-ray close-up
The comet-like shape of the diffuse X-ray emission suggests motion towards the lower right of the image. As pointed out in previous studies, this orientation is about 50 degrees away from the direction expected if the pulsar was moving away from the center of the supernova remnant in a straight line. This misalignment has cast some doubt on the association of the pulsar with the supernova remnant. However, this misalignment could also be explained by movement towards the left of material in the supernova remnant pushing J0617’s cometary tail aside.
This latest research points to an estimate for the age of the supernova remnant to be tens of thousands of years. This agrees with previous work that pegged IC 443’s age to be about 30,000 years. However, other scientists have inferred much younger ages of about 3,000 years for this supernova remnant, so its true age remains in question.
[http://chandra.harvard.edu/photo/2015/ic443/index.html]
Gemini is also home to the Geminids meteor shower:
When Gemini Sends Stars to Paranal
From a radiant point in the constellation of the Twins, the annual Geminid meteor shower rain down on planet Earth. Tonight (2015 December 13), the Geminds reach their peak and could be quite spectacular. The featured blended image, however, captured the shower’s impressive peak in the year 2012. The beautiful skyscape collected Gemini’s lovely shooting stars in a careful composite of 30 exposures, each 20 seconds long, from the dark of the Chilean Atacama Desert over ESO’s Paranal Observatory. In the foreground Paranal’s four Very Large Telescopes, four Auxillary Telescopes, and the VLT Survey telescope are all open and observing. The skies above are shared with bright Jupiter (left), Orion, (top left), and the faint light of the Milky Way. Dust swept up from the orbit of active asteroid 3200 Phaethon, Gemini’s meteors enter Earth’s atmosphere traveling at about 22 kilometers per second.
[http://apod.nasa.gov/apod/ap151213.html]
[https://en.wikipedia.org/wiki/Gemini_%28constellation%29]
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