Cancer is one of the twelve constellations of the zodiac. Its name is Latin for crab and it is commonly represented as one. Cancer is a medium-sized constellation that is bordered by Gemini to the west, Lynx to the north, Leo Minor to the northeast, Leo to the east, Hydra to the south, and Canis Minor to the southwest. In the equatorial coordinate system, the right ascension coordinates lie between 07h 55m 19.7973s and 09h 22m 35.0364s, while the declination coordinates are between 33.1415138° and 6.4700689°. Covering 506 square degrees or 0.921% of the sky, it ranks 31st of the 88 constellations in size. It can be seen at latitudes between +90° and -60° and is best visible at 9 p.m. during the month of March. The Sun appears in the constellation Cancer from July 21 to August 9. In tropical astrology, the Sun is considered to be in the sign Cancer from June 21 to July 22, and in sidereal astrology, from July 16 to August 15.
Cancer as depicted in Urania’s Mirror, a set of constellation cards published in London c.1825
The modern symbol for Cancer represents the pincers of a crab, but Cancer has been represented as many types of creatures, usually those living in the water, and always those with an exoskeleton. It was the location of the Sun’s most northerly position in the sky (the summer solstice) in ancient times, though this position now occurs in Taurus due to the precession of the equinoxes, around June 21. This is also the time that the Sun is directly overhead at 23.5°N, a parallel now known as the Tropic of Cancer.
The constellation is said to have been the place for the Akkadian Sun of the South, perhaps from its position at the summer solstice in very remote antiquity. But afterwards it was associated with the fourth month Duzu (June- July in the modern western calendar), and was known as the Northern Gate of Sun.
In the Egyptian records of about 2000 BCE it was described as Scarabaeus (Scarab), the sacred emblem of immortality.
In Babylonia the constellation was known as MUL.AL.LUL, a name which can refer to both a crab and a snapping turtle. On boundary stones, the image of a turtle or tortoise appears quite regularly and it is believed that this represents Cancer as a conventional crab has not so far been discovered on any of these monuments. There also appears to be a strong connection between the Babylonian constellation and ideas of death and a passage to the underworld, which may be the origin of these ideas in later Greek myths associated with Hercules and the Hydra.
In the 12th century, an illustrated astronomical manuscript shows it as a water beetle. The Persian astronomer Albumasar writes of this sign in Flowers of Abu Ma’shar. A 1488 Latin translation depicts cancer as a large crayfish, which also is the constellation’s name in most Germanic languages. Jakob Bartsch and Stanislaus Lubienitzki, in the 17th century, described it as a lobster.
Heracles attacked by Karkinos (bottom) and the Lernaean Hydra, under the aid of Athena. White-ground Ancient Greek Attic lekythos, ca. 500- 475 BCE. Louvre Museum, Paris.
In Greek mythology, Cancer is identified with the crab that appeared while Hercules was fighting the many-headed Hydra. The crab bit Hercules on the foot, Hercules crushed it and then the goddess Hera, a sworn enemy of Hercules, placed the crab among the stars.
In Chinese astronomy, the stars of Cancer lie within The Vermillion Bird of the South.
The constellation Cancer the crab at 9 p.m. March 3, 2014
[https://bobmoler.wordpress.com/2014/03/03/03032014-ephemeris-the-dim-zodiacal-constellation-of-cancer-the-crab/]
[http://www.space.com/16970-cancer-constellation.html]
Altarf (Beta Cancri) is the brightest star in Cancer. At approximately 290 light years from earth, it has an apparent magnitude of +3.50 and absolute magnitude of -1.25 (visual). The star is an orange K-type giant, about 48 times the radius of the Sun. The traditional name Tarf or Al Tarf can be translated from Arabic as ‘end’ or ‘edge.’
Altarf is known to have a fourteenth magnitude, red dwarf companion star. From its angular distance of 29 arcseconds, the companion’s distance from its parent star is estimated at some 2600 AU, and has an orbital period of 76,000 years.
In 2014, evidence was presented of a planet orbiting Altarf. Using radial velocity data from repeated observations of the star, the planet is estimated to have a minimum mass of approximately 7.8 times that of Jupiter, and an orbit of 605 days.
[https://en.wikipedia.org/wiki/Altarf]
Delta Cancri is an orange giant star approximately 180 light-years away in the constellation Cancer. It has the traditional name Asellus Australis which in Latin means ‘southern donkey colt.’ It also has the longest of all known star names, Arkushanangarushashutu, derived from ancient Babylonian which means ‘the southeast star in the Crab.’
Since it is near the ecliptic, Delta Cancri can be occulted by the Moon and very rarely by planets. It was involved in the first recorded occultation by Jupiter. Delta Cancri also marks the famous open star cluster Praesepe (or the Beehive Cluster, also known as Messier 44). In ancient times M44 was used as a weather gauge. Aratos in his Prognostica reveals that if Asellus Borealis or Gamma Cancris is hidden by clouds, the wind will be from the south and that situation will be reversed if Asellus Australis is obscured. Delta Cancri is also a reliable signpost for finding the vividly red star X Cancri. Delta Cancri also marks the radiant of the Delta Cancrids meteor shower.
[https://en.wikipedia.org/wiki/Delta_Cancri]
Iota Cancri
[http://www.starobserver.eu/multiplestars/iotacancri.html]
Iota Cancri is a double star in Cancer. The brighter component is approximately 298 light years from Earth, and it is a yellow G-type bright giant with an apparent magnitude of +4.02. The companion is a white A-type main sequence dwarf with an apparent magnitude of +6.57. The two stars are separated by 30.6 arcseconds on the sky, and are resolvable through a small telescope.
[https://en.wikipedia.org/wiki/Iota_Cancri]
Alpha Cancri has the traditional name Acubens, from the Arabic ‘zubanāh,’ ‘the claws.’ Acubens is a fourth-magnitude star with an apparent magnitude of 4.20, making it barely visible to the naked eye under good lighting conditions. Nevertheless, it is 23 times more luminous than the Sun. Its stellar classification is A5m. The distance to Acubens is estimated to be roughly 53 parsecs from Earth, or approximately 174 light years away. Since it is near the ecliptic, it can be occulted by the Moon and very rarely by planets.
The primary component, α Cancri A, is a white A-type main sequence dwarf with an apparent magnitude of +4.26. Its companion, α Cancri B, is an eleventh magnitude star. From studying its light curve during occultation, it is thought that α Cancri A may itself be a close binary, consisting of two stars with similar brightness and a separation of 0.1 arcseconds.
[https://en.wikipedia.org/wiki/Alpha_Cancri]
Gamma Cancri has the traditional name Asellus Borealis (Latin for ‘northern donkey colt’). It is a white A-type subgiant with an apparent magnitude of +4.67. Located around 181 light-years distant, it shines with a luminosity approximately 35 times that of the Sun and has a surface temperature of 9108 K. Since it is near the ecliptic, it can be occulted by the Moon and, very rarely, by planets. It will die in about 510 million years.
[https://en.wikipedia.org/wiki/Gamma_Cancri]
Zeta Cancri (Struve 1196)
[http://www.perezmedia.net/beltofvenus/archives/000617.html]
Zeta Cancri is a star system containing at least four stars. It has the traditional name Tegmine (Tegmen) ‘the shell (of the crab).’ The star system is approximately 83.4 light years from Earth, and has a combined apparent magnitude of +4.67. Since ζ Cancri is near the ecliptic, it can be occulted by the Moon and, very rarely, by planets.
The ζ Cancri system contains two binary pairs, ζ¹ Cancri and ζ² Cancri, which are 5.06 arcseconds apart. These two binary star systems orbit around their common centre of mass once every 1100 years.
The components of ζ¹ Cancri are denoted ζ Cancri A and ζ Cancri B. They are both yellow-white main sequence dwarfs of spectral class F. The apparent magnitude the two stars are +5.58 and +5.99, respectively. The two stars are separated, as of 2008, by 1 arcsecond, requiring a large telescope to resolve them, but this separation will increase until the year 2020. They complete one orbit every 59.6 years. The estimated masses for the pair are 1.28 and 1.18 solar masses, respectively.
The components of ζ² Cancri are denoted ζ Cancri C and ζ Cancri D. ζ Cancri C is the brighter of the pair, having an apparent magnitude of +6.12. It appears to be a yellow G-type star, often reported as G5V, but now thought to be earlier, probably G0V. This star has around 1.15 solar masses. The tenth magnitude ζ Cancri D has the color of a red dwarf, and may in fact be a close pair of two red dwarfs. The separation between C and D is approximately 0.3 arcseconds, and their orbital period is 17 years.
[https://en.wikipedia.org/wiki/Zeta_Cancri]
DX Cancri
[https://jumk.de/astronomie/near-stars/dx-cancri.shtml]
DX Cancri is the variable star identifier for a small star in Cancer. With an apparent visual magnitude of 14.81, it is much too faint to be seen with the naked eye. Visually viewing this star requires a telescope with a minimum aperture of 16 in (41 cm). Based upon parallax measurements, DX Cancri is located at a distance of 11.8 light-years (3.6 parsecs) from Earth. This makes it the 18th closest star (or star system) to the Sun.
The star has a stellar classification of M6.5V, identifying it as a type of main sequence star known as a red dwarf. It has about 9% of the mass of the Sun and 11% of the Sun’s radius. The outer envelope of the star has an effective temperature of 2,840 K, giving it the cool red-orange glow of an M-type star. This is a flare star that has random, intermittent changes in brightness by up to a fivefold increase.
This star has been examined for excess emission of infrared radiation caused by cold circumstellar dust, but none was found. It is a proposed member of the Castor Moving Group of stars that share a common trajectory through space. This group has an estimated age of 200 million years.
[https://en.wikipedia.org/wiki/DX_Cancri]
55 Cancri
[http://stars.astro.illinois.edu/sow/55cnc.html]
55 Cancri is a binary star approximately 41 light-years away from Earth in the constellation of Cancer. The system consists of a G-type star (designated 55 Cancri A, also named Copernicus) and a smaller red dwarf (55 Cancri B). 55 Cancri A has an apparent magnitude of 5.95, making it just visible to the naked eye under very dark skies. The red dwarf 55 Cancri B is of the 13th magnitude and only visible through a telescope. The two components are separated by an estimated distance of 1065 AU (one thousand times the distance from the Earth to the Sun). Despite their wide separation, the two stars appear to be gravitationally bound, as they share a common proper motion.
The primary star, 55 Cancri A, is a yellow dwarf star of main sequence spectral type G8V. It is smaller in radius and slightly less massive than the Sun, and so is cooler and less luminous. The star has only low emission from its chromosphere, and is not variable in the visible spectrum; but it is variable in X-rays. Age estimates for 55 Cancri A include 7.4-8.7 billion years and 10.2 ± 2.5 billion years. Observations of 55 Cancri A have thus far failed to detect any associated dust. The upper limit of fine dust around the star is less than 0.01% of the Earth’s mass. However, this does not exclude the presence of an asteroid belt or a Kuiper belt equivalent. The secondary, 55 Cancri B, is a red dwarf star much less massive and luminous than the Sun. There are indications that component B may itself be a double star, though this is uncertain.
[http://www.leviathanastronomy.com/extrasolar-planets.html]
As of 2015, five extrasolar planets (designated 55 Cancri b, c, d, e and f; named Galileo, Brahe, Lipperhey, Janssen and Harriot, respectively) are believed to orbit 55 Cancri A.
55 Cancri e (abbreviated 55 Cnc e) is an exoplanet closely orbiting its Sun-like host star 55 Cancri A. The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice that of the Earth, thus classifying it as the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes fewer than 18 hours to complete an orbit and is the innermost known planet in its planetary system.
55 Cancri e was discovered on 30 August 2004. In October 2012, it was announced that 55 Cancri e could be a carbon planet. In February 2016, it was announced that NASA’s Hubble Space Telescope had detected hydrogen and helium (and suggestions of hydrogen cyanide), but no water vapor, in the atmosphere of 55 Cancri e, the first time the atmosphere of a super-earth exoplanet was analyzed successfully.
[https://en.wikipedia.org/wiki/55_Cancri_e]
Artist’s impression of 55 Cancri b
55 Cancri b, also named Galileo, is an extrasolar planet orbiting the Sun-like star 55 Cancri A every 14.65 days. It is the second planet in order of distance from its star, and is an example of a hot Jupiter, or possibly rather ‘warm Jupiter.’
In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the winning name was Galileo for this planet. The winning name honors early-17th century astronomer and physicist Galileo Galilei.
55 Cancri b was discovered in 1996 by Geoffrey Marcy and R. Paul Butler. It was the fourth known extrasolar planet, excluding pulsar planets. Like the majority of known extrasolar planets, it was discovered by detecting variations in its star’s radial velocity caused by the planet’s gravity. By making sensitive measurements of the Doppler shift of the spectrum of 55 Cancri A, a 15-day periodicity was detected. The planet was announced in 1996, together with the planet of Tau Boötis and the innermost planet of Upsilon Andromedae.
Even when this inner planet, with a mass at least 78% times that of Jupiter was accounted for, the star still showed a drift in its radial velocity. This eventually led to the discovery of the outer planet 55 Cancri d in 2002.
55 Cancri b is in a short-period orbit, though not so extreme as that of the previously detected hot Jupiter 51 Pegasi b. The orbital period indicates that the planet is located close to a 1:3 mean motion resonance with 55 Cancri c.
In 2012, b’s upper atmosphere was observed transiting the star; so its inclination is about 85 degrees, coplanar with 55 Cancri e. This helped to constrain the mass of the planet but the inclination was too low to constrain its radius. The mass is about .85 that of Jupiter.
55 Cancri b is a gas giant with no solid surface. The atmospheric transit has demonstrated hydrogen in the upper atmosphere. That transit is so tangential, that properties such as its radius, density, and temperature are unknown. Assuming a composition similar to that of Jupiter and that its environment is close to chemical equilibrium, 55 Cancri b’s upper atmosphere is predicted to be cloudless with a spectrum dominated by alkali metal absorption.
The atmosphere’s transit indicates that it is slowly evaporating under the sun’s heat. The evaporation is slower than that for previously studied (hotter) hot Jupiters. The planet is unlikely to have large moons, since tidal forces would either eject them from orbit or destroy them on short timescales relative to the age of the system.
[https://en.wikipedia.org/wiki/55_Cancri_b]
55 Cancri c, also named Brahe, is an extrasolar planet in an eccentric orbit around the Sun-like star 55 Cancri A, making one revolution every 44.34 days. It is the third known planet in order of distance from its star. 55 Cancri c was discovered on June 13, 2002 and has a mass roughly half of Saturn. Its name honors the astronomer Tycho Brahe.
In the 5-planet solution for the 55 Cancri system, the orbit of 55 Cancri c is mildly eccentric: at apoastron the planet is about 19% further from the star than it is at periastron. It is located closer to 55 Cancri A than Mercury is to our sun, though it has a longer orbital period than the hot Jupiters. The planet is located close to a 3:1 resonance with the inner planet 55 Cancri b; however, simulations indicate that the two planets are not actually in this resonance.
A limitation of the radial velocity method used to discover the planet is that only a lower limit on the mass can be obtained. Since the planet was detected indirectly through observations of its star, properties such as its radius, composition, and temperature are unknown. With a mass similar to that of Saturn, 55 Cancri c is likely to be a gas giant with no solid surface.
[https://en.wikipedia.org/wiki/55_Cancri_c]
An artist’s impression of 55 Cancri f. The three bright dots near its star are the three innermost planets.
55 Cancri f, also named Harriot, is the fourth known planet (in order of distance) from the star 55 Cancri and the first planet to have been given the designation of ‘f.’ Its name honors the astronomer Thomas Harriot. It is the first known planet outside our solar system to spend its entire orbit within what astronomers call the ‘habitable zone.’ Furthermore, its discovery made 55 Cancri the first star other than the Sun known to have at least five planets.
55 Cnc f’s orbit compared to the orbit of Venus (0.72AU)
55 Cancri f is located about 0.781 AU away from the star and takes 260 days to complete a full orbit. A limitation of the radial velocity method used to detect 55 Cancri f is that only a minimum mass can be obtained, in this case around 0.144 times that of Jupiter, or half the mass of Saturn. The radial velocity data of 55 Cancri A indicates that the orbit is consistent with being almost circular, with an eccentricity 0.0002.
Since the planet was detected indirectly through observations of its star, properties such as its radius, composition and temperature are unknown. With a mass half that of Saturn, 55 Cancri f is likely to be a gas giant with no solid surface. It is not known if the composition and appearance is more like Saturn or Neptune. Based on its temperature, it should be a planet covered in water clouds. Since it orbits the ‘habitable zone,’ liquid water could exist on the surface of a possible moon.
[https://en.wikipedia.org/wiki/55_Cancri_f]
55 Cancri d, also named Lipperhey, is an extrasolar planet in a long-period orbit around the Sun-like star 55 Cancri A, located at a similar distance from its star as Jupiter is from our Sun. It is the fifth and outermost known planet in its planetary system. 55 Cancri d was discovered on June 13, 2002, and its name honors the spectacle maker and telescope pioneer Hans Lippershey.
At the time of discovery, 55 Cancri A was already known to possess one planet (55 Cancri b), however there was still a drift in the radial velocity measurements which was unaccounted-for. In 2002, further measurements revealed the presence of a long-period planet in an orbit at around 5 AU from the star. The same measurements also indicated the presence of another inner planet, designated 55 Cancri c.
In 2008, after a complete orbit of this planet had been observed, the true orbit was revealed, indicating that planet’s 14 year orbit was in fact near-circular, located about 5.77 AU from the star. A limitation of the radial velocity method used to discover 55 Cancri d is that only a lower limit on the planet’s mass can be obtained. In the case of 55 Cancri d, this lower limit was around 3.835 times the mass of Jupiter. In 2004, astrometric measurements with the Fine Guidance Sensors on the Hubble Space Telescope suggest that the planet’s orbit is inclined by around 53° with respect to the plane of the sky. If this measurement is confirmed, it implies that the planet’s true mass is 25% greater than the lower limit, at around 4.8 Jupiter masses; and it will not be coplanar with the innermost planets e and b (both 85°).
Given the planet’s high mass, the planet is a gas giant with no solid surface. Since the planet has only been detected indirectly, parameters such as its radius, composition, and temperature are unknown. Assuming a composition similar to that of Jupiter and that the planet’s atmosphere is close to chemical equilibrium, it is predicted that 55 Cancri d is covered in a layer of water clouds: the planet’s internal heat probably keeps it too warm to form the ammonia-based clouds that are typical of Jupiter. Its surface gravity is likely to be about 4 to 5 times stronger than Jupiter, or about 10 to 15 times that of Earth which is because the radius of the planet is unlikely to be much more than Jupiter’s and is probably slightly smaller than Jupiter due to the high metal content in the parent star.
[https://en.wikipedia.org/wiki/55_Cancri_d]
A METI (Messaging to Extra-Terrestrial Intelligence) message was sent to 55 Cancri. It was transmitted from Eurasia’s largest radar, the 70- meter (230-foot) Eupatoria Planetary Radar. The message was named Cosmic Call 2; it was sent on July 6, 2003, and it will arrive at 55 Cancri in May 2044.
[https://en.wikipedia.org/wiki/55_Cancri]
RX J0806.3+1527: Orbiting Stars Flooding Space with Gravitational Waves
Chandra data (above, graph) from observations of RX J0806.3+1527 (or J0806), show that its X-ray intensity varies with a period of 321.5 seconds. This implies that J0806 is a binary star system where two white dwarf stars are orbiting each other (above, illustration) approximately every 5 minutes.
The short orbital period implies that the stars are only about 50,000 miles apart, a fifth of the distance from the Earth to the Moon, and are moving in excess of a million miles per hour. According to Einstein’s General Theory of Relativity, such a system should produce gravitational waves- ripples in space-time- that carry energy away from the system at the speed of light.
Energy loss by gravitational waves will cause the stars to move closer together. X-ray and optical observations indicate that the orbital period of this system is decreasing by 1.2 milliseconds every year, which means that the stars are moving closer together at a rate of about 2 feet per day.
The distance estimate of the system is about 1,600 light years from Earth.
[http://chandra.harvard.edu/photo/2005/j0806/]
Cancer is best known among stargazers as the home of M44 or Praesepe (Latin for ‘manger’), an open cluster also called the Beehive Cluster, located right in the center of the constellation:
M44: The Beehive Cluster
A mere 600 light-years away, M44 is one of the closest star clusters to our solar system. Also known as the Praesepe or the Beehive cluster its stars are young though, about 600 million years old compared to our Sun’s 4.5 billion years. Based on similar ages and motion through space, M44 and the even closer Hyades star cluster in Taurus are thought to have been born together in the same large molecular cloud. An open cluster spanning some 15 light-years, M44 holds 1,000 stars or so and covers about 3 full moons (1.5 degrees) on the sky in the constellation Cancer. Visible to the unaided eye, M44 has been recognized since antiquity. Described as a faint cloud or celestial mist long before being included as the 44th entry in Charles Messier’s 18th century catalog, the cluster was not resolved into its individual stars until telescopes were available. A popular target for modern, binocular-equiped sky gazers, the cluster's few yellowish tinted, cool, red giants are scattered through the field of its brighter hot blue main sequence stars in this colorful stellar group snapshot.
[http://apod.nasa.gov/apod/ap140222.html]
Epsilon Cancri is the brightest member at magnitude 6.3. Praesepe is also one of the larger open clusters visible; it is most easily observed when Cancer is high in the sky. North of the Equator, this period stretches from February to May. Ptolemy described the Beehive Cluster as ‘the nebulous mass in the breast of Cancer.’ It was one of the first objects Galileo observed with his telescope in 1609, spotting 40 stars in the cluster. The Greeks and Romans identified the nebulous object as a manger from which two donkeys, represented by the neighboring stars, Asellus Borealis and Asellus Australis, were eating. The stars represent the donkeys that the god Dionysus and his tutor Silenus rode in the war against the Titans. The ancient Chinese interpreted the object as a ghost or demon riding in a carriage, calling it a ‘cloud of pollen blown from under willow catkins.’
An alignment of Saturn and Mars with the Beehive Cluster took place in 2006:
Planets, Bees, and a Donkey
The heralded alignment of wandering planets Saturn and Mars with the well-known Beehive Cluster took place last weekend on Saturday, June 17 2006. Recorded in dark Arizona skies on that date, this view finds Mars above and right of Saturn- the brightest celestial beacons in the scene- with the Beehive cluster of stars (M44) at the lower right. The two planets appear in conjunction separated by just over half a degree. But about another half a degree along a line joining the two and continuing towards the lower left lies the third brightest object in the image, giant star Asellus Australis. Asellus Australis is also known as Delta Cancri, a middling bright star 136 light-years away in the constellation Cancer, the Crab. Of course, this star’s Latin name translates to ‘Southern Donkey.’
[http://apod.nasa.gov/apod/ap060622.html]
The smaller, denser open cluster Messier 67 can also be found in Cancer:
King Cobra Cluster (Messier 67)
Messier 67 (M67), nicknamed the King Cobra Cluster, is an open cluster located in the northern constellation Cancer, the Crab.
The cluster has an apparent magnitude of 6.1 and lies at an approximate distance of 2,610 to 2,930 light years from Earth. It has the designation NGC 2682 in the New General Catalogue. Messier 67 can be found roughly halfway and slightly above the imaginary line connecting the bright stars Regulus in Leo and Procyon in Canis Minor. The cluster lies only 1.75 degrees west of Acubens, Alpha Cancri, a multiple star system with a visual magnitude of 4.20. Acubens is located about 9 degrees southeast of Messier 44, the famous Beehive Cluster.
With an apparent diameter of 30 arc minutes, M67 appears roughly the same size as the full Moon. Small 10×50 binoculars show the cluster as an elongated patch of light, while small telescopes reveal its brightest stars. 6-inch and 8-inch instruments resolve dozens of stars, while 12-inch telescopes reveal about 100 individual stars in the cluster. The best time of year to observe M67 from northern latitudes is during the late winter and early spring.
Messier 67 is one of the oldest known open clusters and the single oldest open cluster listed by Messier in his catalogue. The estimated age of M67 is in the range from 3.2 to 5 billion years. The few open clusters that are known to be older are not as close to Earth as M67.
The average age of the stars in the cluster is around 4 billion years, which means that they are roughly the same age as the Sun and have similar elemental abundancies. Open clusters are typically younger and the stars tend to disperse over time, usually before they reach this age. For example, the Beehive Cluster (M44), another Messier cluster in the Cancer constellation, is only 600 million years old. The stars of M67, however, are expected to stay together for another 5 billion years before dissociating.
Messier 67 and Praesepe (M44)
Messier 67 contains more than 500 stars and is often studied for the insight it provides into stellar evolution. The cluster contains 11 bright orange K-type giant stars and about 200 white dwarfs. With the exception of about 30 blue stragglers, the stars in M67 are all roughly the same age and lie at the same distance. The origins of the blue stragglers have not been explained yet. The brightest of these stars have the spectral classification B8 or B9 and a visual magnitude of 10, which makes them about 50 times more luminous than the Sun. Not counting the blue stragglers, the cluster does not have any main sequence stars bluer than spectral class F.
Messier 67 has undergone mass segregation, which means that its heavier stars have moved toward the cluster’s centre and less massive ones have migrated toward the outer region over time. As a result, the cluster’s stars are much more gravitationally bound than those in younger open clusters.
The cluster’s mass, while uncertain, is estimated to be between 1,080 and 1,400 times that of the Sun and the distance is believed to be in the range between 800 and 900 parsecs. The cluster contains more than 100 Sun-like stars and a great many red giants. The high number of stars similar to the Sun had led scientists to consider M67 as the Sun’s possible parent cluster, but simulations have indicated that this was very unlikely.
As the stars in M67 are all roughly the same age and have a similar composition to the Sun, the cluster is often studied by scientists looking to determine how many planets form in such environments and if they mostly form around more massive or less massive stars.
This artist’s impression shows one of the three newly discovered planets in the star cluster Messier 67.
In 2014, an international team of astronomers discovered three extrasolar planets in the cluster using the HARPS (High Accuracy Radial velocity Planet Searcher) instrument on ESO’s 3.6m telescope at the La Silla Observatory in Chile. All three planets are closer to their hosts than the habitable zone, where liquid water could exist. Two of the exoplanets were detected circling the dwarf stars YBP1194 and YBP1514, while one was found orbiting a K3-type giant star, designated S364. Two of these planets, designated YBP1194b and YBP1514b, are the first exoplanets ever discovered orbiting Sun-like stars in an open cluster.
YBP1194b, orbiting the star YBP1194, has a period of 6.9 days and a mass of at least 0.34 Jupiter masses. The star itself is one of the closest solar twins ever found. Its properties are almost identical to those of the Sun. The planet YBP1514b has an orbital period of 5.1 days and a mass 0.40 times that of Jupiter, while S364b takes 121.7 days to orbit its parent star and has a mass of at least 1.54 Jupiter masses.
The discovery of these planets showed that planets are about as common in open clusters as they are around isolated stars. However, they are not very easy to detect and very few of them have been discovered inside open clusters. The first ever planet to be found in an open cluster was detected orbiting the star Ain, Epsilon Tauri, one of the members of the famous Hyades cluster in Taurus. Scientists also recently discovered two planets in the Beehive Cluster (M44) and NGC 6811, an open cluster in Cygnus.
[http://www.messier-objects.com/messier-67-king-cobra-cluster/]
Abell 30: X-rays from a Reborn Planetary Nebula
This composite image shows a planetary nebula, Abell 30, located about 5500 light years from Earth. A planetary nebula is formed in the late stage of the evolution of a sun-like star. The evolution of A30 stalled and then started up again, so the planetary nebula was reborn, a special, rarely-seen phase of evolution. These images of the planetary nebula Abell 30, (a.k.a. A30), show one of the clearest views ever obtained of a special phase of evolution for these objects. The inset image on the right is a close-up view of A30 showing X-ray data from NASA’s Chandra X-ray Observatory in purple and Hubble Space Telescope (HST) data showing optical emission from oxygen ions in orange. On the left is a larger view showing optical and X-ray data from the Kitt Peak National Observatory and ESA’s XMM-Newton, respectively. In this image the optical data show emission from oxygen (orange) and hydrogen (green and blue), and X-ray emission is colored purple.
A planetary nebula- so called because it looks like a planet when viewed with a small telescope- is formed in the late stage of the evolution of a sun-like star. After having steadily produced energy for several billion years through the nuclear fusion of hydrogen into helium in its central region, or core, the star undergoes a series of energy crises related to the depletion of hydrogen and subsequent contraction of the core. These crises culminate in the star expanding a hundred-fold to become a red giant.
Eventually the outer envelope of the red giant is ejected and moves away from the star at a relatively sedate speed of less than 100,000 miles per hour. The star meanwhile is transformed from a cool giant into a hot, compact star that produces intense ultraviolet (UV) radiation and a fast wind of particles moving at about 6 million miles per hour. The interaction of the UV radiation and the fast wind with the ejected red giant envelope creates the planetary nebula, shown by the large spherical shell in the bigger image.
In rare cases, nuclear fusion reactions in the region surrounding the star’s core heat the outer envelope of the star so much that it temporarily becomes a red giant again. The sequence of events- envelope ejection followed by a fast stellar wind- is repeated on a much faster scale than before, and a small-scale planetary nebula is created inside the original one. In a sense, the planetary nebula is reborn.
The large nebula seen in the larger image has an observed age of about 12,500 years and was formed by the initial interaction of the fast and slow winds. The cloverleaf pattern of knots seen in both images, correspond to the recently ejected material. These knots were produced much more recently, as they have an observed age of about 850 years, based on observations of their expansion using HST. The diffuse X-ray emission seen in the larger image and in the region around the central source in the inset is caused by interactions between wind from the star and the knots of the ejected material. The knots are heated and eroded by this interaction, producing X-ray emission. The cause of the point-like X-ray emission from the central star is unknown.
Studies of A30 and other planetary nebulas help improve our understanding of the evolution of sun-like stars as they near the end of their lifetime. The X-ray emission reveals how the material lost by the stars at different evolutionary stages interact with each another. These observations of A30, located about 5,500 light years away, provide a picture of the harsh environment that the solar system will evolve towards in several billion years, when the sun's strong stellar wind and energetic radiation will blast those planets that survived the previous, red giant phase of stellar evolution.
The structures seen in A30 originally inspired the idea of reborn planetary nebulas, and only three other examples of this phenomenon are known. A new study of A30, using the observatories mentioned above, has been reported by an international team of astronomers in the August 20th, 2012 issue of The Astrophysical Journal.
[http://chandra.harvard.edu/photo/2012/a30/index.html]
4C+29.30: Black Hole Powered Jets Plow Into Galaxy
This composite image of a galaxy illustrates how the intense gravity of a supermassive black hole can be tapped to generate immense power. The image contains X-ray data from NASA’s Chandra X-ray Observatory (blue), optical light obtained with the Hubble Space Telescope (gold) and radio waves from the NSF’s Very Large Array (pink).
This multi-wavelength view shows 4C+29.30, a galaxy located some 850 million light years from Earth. The radio emission comes from two jets of particles that are speeding at millions of miles per hour away from a supermassive black hole at the center of the galaxy. The estimated mass of the black hole is about 100 million times the mass of our Sun. The ends of the jets show larger areas of radio emission located outside the galaxy.
The X-ray data show a different aspect of this galaxy, tracing the location of hot gas. The bright X-rays in the center of the image mark a pool of million-degree gas around the black hole. Some of this material may eventually be consumed by the black hole, and the magnetized, whirlpool of gas near the black hole could in turn, trigger more output to the radio jet.
Most of the low-energy X-rays from the vicinity of the black hole are absorbed by dust and gas, probably in the shape of a giant doughnut around the black hole. This doughnut, or torus blocks all the optical light produced near the black hole, so astronomers refer to this type of source as a hidden or buried black hole. The optical light seen in the image is from the stars in the galaxy.
The bright spots in X-ray and radio emission on the outer edges of the galaxy, near the ends of the jets, are caused by extremely high energy electrons following curved paths around magnetic field lines. They show where a jet generated by the black hole has plowed into clumps of material in the galaxy (mouse over the image for the location of these bright spots). Much of the energy of the jet goes into heating the gas in these clumps, and some of it goes into dragging cool gas along the direction of the jet. Both the heating and the dragging can limit the fuel supply for the supermassive black hole, leading to temporary starvation and stopping its growth. This feedback process is thought to cause the observed correlation between the mass of the supermassive black hole and the combined mass of the stars in the central region or bulge of a galaxy.
[http://chandra.harvard.edu/photo/2013/4c2930/index.html]
Cancer is also home to the quasars QSO J0842+1835 and OJ 287. QSO J0842+1835 is located 8.4 billion light-years from Earth, and it was used to measure the speed of gravity in VLBI experiment conducted by Edward Fomalont and Sergei Kopeikin in September 2002. OJ 287 is located 3.5 billion light years away that has produced quasi-periodic optical outbursts going back approximately 120 years, as first apparent on photographic plates from 1891. It was first detected at radio wavelengths during the course of the Ohio Sky Survey. Its central supermassive black hole of about 18 billion solar masses is among the largest known, more than six times the value calculated for the previous largest object:
[http://quasar.square7.ch/fqm/0851+202.html]
OJ287 and a confirmation of General Relativity
An international group of astronomers lead by Mauri Valtonen (Tuorla Observatory) has verified General Relativity as the correct theory of gravitation in strong gravitational field using results from a distant quasar called OJ287. Continuous monitoring of OJ287 over the last few years has proven correct a decade old model of this quasar as a precessing binary black hole system which produces two major outburst peaks per 12 year orbital period. The occurrence of the latest outburst on 13 September 2007, within a day of the predicted time, finally clinched the model.
[http://www.astro.utu.fi/news/080419.shtml]
The Delta Cancrids is a medium strength meteor shower lasting from December 14 to February 14, the main shower from January 1 to January 24. The radiant is located in the constellation of Cancer, near Delta Cancri. It peaks on January 17 each year, with only four meteors per hour. It was first discovered in 1872, but the first solid evidence of this phenomenon came in 1971. The source of this meteor shower is unknown, it has been suggested that it is similar to the orbit of asteroid 2001 YB5.
[https://en.wikipedia.org/wiki/Delta_Cancrids]
[https://en.wikipedia.org/wiki/Cancer_%28constellation%29]
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