Capricornus is one of the constellations of the zodiac. Its name is Latin for ‘horned goat’ or ‘goat horn,’ and it is commonly represented in the form of a sea-goat: a mythical creature that is half goat, half fish. Under its modern boundaries it is bordered by Aquila, Sagittarius, Microscopium, Piscis Austrinus, and Aquarius. Capricornus’s brighter stars are found on a triangle whose vertices are α2 Capricorni (Giedi), δ Capricorni (Deneb Algiedi), and ω Capricorni. Capricornus is usually drawn as a goat with the tail of a fish. The constellation is located in an area of sky called the Sea or the Water, consisting of many water-related constellations such as Aquarius, Pisces and Eridanus. It is the smallest constellation in the zodiac.
Due to the precession of the equinoxes the December solstice no longer takes place while the sun is in the constellation Capricornus, as it did until 130 BCE, but the astrological sign called Capricorn begins with the solstice. The solstice now takes place when the Sun is in Sagittarius. The sun’s most southerly position, which is attained at the northern hemisphere’s winter solstice, is now called the Tropic of Capricorn, a term which also applies to the line on the Earth at which the sun is directly overhead at noon on that solstice. The Sun is now in Capricorn from late January through mid-February.
Capricornus as shown on Plate XVI of the Uranographia of Johann Bode (1801). South of it lies the now-obsolete constellation of Globus Aerostaticus, the hot-air balloon.
[http://www.ianridpath.com/startales/capricornus.htm]
Despite its faintness, Capricornus has one of the oldest mythological associations, having been consistently represented as a hybrid of a goat and a fish since the Middle Bronze Age. First attested in depictions on a cylinder-seal from around the 21st century BCE, it was explicitly recorded in the Babylonian star catalogues as MULSUḪUR.MAŠ, ‘The Goat-Fish,’ before 1000 BCE. The constellation was a symbol of the god Ea and in the Early Bronze Age marked the winter solstice.
In Greek mythology, the constellation is sometimes identified as Amalthea, the goat that suckled the infant Zeus after his mother, Rhea, saved him from being devoured by his father, Cronos. The goat’s broken horn was transformed into the cornucopia or horn of plenty. Capricornus is also sometimes identified as Pan, the god with a goat’s head, who saved himself from the monster Typhon by giving himself a fish’s tail and diving into a river.
In Chinese astronomy, constellation Capricornus lies in The Black Tortoise of the North (Běi Fāng Xuán Wǔ).
The Nakh peoples called this constellation Roofing Towers (Chechen: Neģara Bjovnaš).
In the Society Islands, the figure of Capricornus was called Rua-o-Mere, ‘Cavern of parental yearnings.’
The planet Neptune was discovered in Capricornus by German astronomer Johann Galle, near Deneb Algedi (δ Capricorni) on September 23, 1846, which is appropriate as Capricornus can be seen best from Europe at 4:00 am in September.
[https://bobmoler.wordpress.com/2014/09/15/09152014-ephemeris-the-mer-goat-capricornus/]
[http://astropixels.com/constellations/charts/Cap.html]
Delta Capricorni is also traditionally named Deneb Algedi (or Deneb Algiedi) and Scheddi. The name Deneb Algedi derives from the Arabic ‘ðanab al-jady,’ meaning ‘the tail of the goat.’ It is the brightest star within the constellation, located approximately 39 light-years away.
Delta Capricorni is an eclipsing binary star system similar to Algol, with an orbital period of 1.022768 days and an inclination close to the line of sight from the Earth. The peak apparent visual magnitude of the pair is 2.81. The primary component, Delta Capricorni A, has an overall stellar classification of A7m III, indicating that it is a giant star that has exhausted the supply of hydrogen at its core. Compared to the Sun, it has double the mass and nearly twice the radius. It is rotating rapidly with a projected rotational velocity of 105 km s−1. The outer envelope of the star is radiating energy at an effective temperature of 7,301 K, giving it the white-hued glow of an A-type star. The secondary component is a type G or K star with around 90% of the mass of the Sun. The system has additionally two optical companions.
[https://en.wikipedia.org/wiki/Delta_Capricorni]
Beta Capricorni has the traditional name Dabih, which comes from the Arabic ‘al-dhābiḥ,’ meaning ‘the butcher.’ It is located 328 light years from Earth.
With binoculars or a small telescope, β Capricorni can be resolved into a double star. The brighter of these two components, β¹ Capricorni or Dabih Major, has an apparent magnitude of +3.05, while the dimmer one, β² Capricorni or Dabih Minor, has an apparent magnitude of +6.09. The two components are separated by 3.5 arcminutes on the sky, putting them at least 21,000 AU (0.34 light years) apart. They take approximately 700,000 years to complete one orbit.
Both of these components are themselves made up of multiple stars. The brighter component, β¹ Capricorni, is the more complex of the pair. It has at least three components, and its spectrum is difficult to interpret. It is dominated by a pair of stars, the orange K-type bright giant β Capricorni Aa, with an apparent magnitude of +3.08, and the blue-white B-Type main sequence dwarf, β Capricorni Ab with an apparent magnitude of +7.20. The Aa component has a surface temperature of 4900 kelvins, a diameter of 35 times that of the Sun, and a luminosity 600 times that of the Sun. These two components are separated by 0.05 arcseconds (5 AU) and have an orbital period of 3.77 years. The Ab component has another, unseen companion, β Capricorni Ac, which orbits Ab with an orbital period of 8.7 days. It is thought that the Aa component is itself multiple.
The dimmer of the visual double star, β¹ Capricorni, is simpler and more studied. It is a binary star, whose brighter component, β Capricorni B, has a magnitude of 6.1. It is an A0-giant with 40 times the luminosity of the Sun. The companion, β Capricorni C, is approximately 3 arcseconds from B. β Capricorni B is unusual for having large amounts of mercury and manganese in its atmosphere.
Two other nearby stars were discovered by John Herschel. These lie 112 arcseconds away from β¹ Capricorni, and it is unclear whether they are simply optical doubles or part of the β Capricorni system. They are sometimes referred to as β Capricorni D and E.
[https://en.wikipedia.org/wiki/Beta_Capricorni]
α¹ Cap and α² Cap by the Digitized Sky Survey (DSS2)
Alpha Capricorni has the traditional name Algedi which is derived from the Arabic word ‘al-jady,’ meaning ‘the billy goat’ or ‘kid,’ and also refers to the entire constellation of Capricornus. It is an optical double star, with two components separated by 0.11° on the sky, and resolvable with the naked eye. α¹ Capricorni is a yellow G-type supergiant with an apparent magnitude of +4.30. It has 5.3 times the mass of the Sun and is radiating around 1,047 times the Sun’s luminosity. It is approximately 690 light years from Earth. α² Capricorni is a yellow G-type giant with an apparent magnitude of +3.58. This star is approximately 109 light years from Earth.
[https://en.wikipedia.org/wiki/Alpha_Capricorni]
[https://en.wikipedia.org/wiki/Alpha1_Capricorni]
[https://en.wikipedia.org/wiki/Alpha2_Capricorni]
M30 by Hubble Space Telescope; 3.5′ view
M30 (NGC 7099) is a globular cluster of stars in the southern constellation of Capricornus. It was discovered by the French astronomer Charles Messier in 1764. This cluster can be easily viewed with a pair of 10×50 binoculars, forming a patch of hazy light some 4 arcminutes wide that is slightly elongated along the east-west axis. With a larger instrument, individual stars can be resolved and the cluster will cover an angle of up to 12 arcminutes across with a compressed core one arcminute wide. It is best observed around August.
M30 is located at a distance of about 27,100 light-years from Earth, and is about 93 light-years across. The estimated age is roughly 12.93 billion years and it has a combined mass of about 160,000 times the mass of the Sun. The cluster is following a retrograde orbit through the inner galactic halo, suggesting that it was acquired from a satellite galaxy rather than forming within the Milky Way. It is currently located at a distance of about 22.2 kly (6.8 kpc) from the center of the galaxy, compared to an estimated 26 kly (8.0 kpc) for the Sun.
The M30 cluster has passed through a dynamic process called core collapse and now has a concentration of mass at its core of about a million times the Sun’s mass per cubic parsec. This makes it one of the highest density regions in the Milky Way galaxy. Stars in such close proximity will experience a high rate of interactions that can create binary star systems, as well as a type of star called a blue straggler that is formed by mass transfer. A process of mass segregation may have caused the central region to gain a greater proportion of higher mass stars, creating a color gradient with increasing blueness toward the middle of the cluster.
[https://en.wikipedia.org/wiki/Messier_30]
HCG 87: A Small Group of Galaxies
Sometimes galaxies form groups. For example, our own Milky Way Galaxy is part of the Local Group of Galaxies. Small, compact groups, like Hickson Compact Group 87 (HCG 87) shown above, are interesting partly because they slowly self-destruct. Indeed, the galaxies of HCG 87 are gravitationally stretching each other during their 100-million year orbits around a common center. The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers. HCG 87 is composed of a large edge-on spiral galaxy visible near the image center, an elliptical galaxy visible to its right, and a spiral galaxy visible near the top. The small spiral near the center might be far in the distance. Several stars from our Galaxy are also visible in the foreground. Studying groups like HCG 87 allows insight into how all galaxies form and evolve.
[http://apod.nasa.gov/apod/ap150809.html]
HCG 87 is located 400 million light-years from Earth. The face-on spiral galaxy is experiencing abnormally high rates of star formation, indicating that it is interacting with one or both members of the group. Furthermore, the large elliptical galaxy and the edge-on spiral galaxy, both of which have active nuclei, are connected by a stream of stars and dust, indicating that they too are interacting. Astronomers predict that the three galaxies may merge millions of years in the future to form a giant elliptical galaxy.
Galaxy Clusters and Dark Energy: Chandra Opens New Line of Investigation on Dark Energy
Chandra images of multimillion degree Celsius gas in galaxy clusters have provided astronomers with a powerful new method to probe the mass and energy content of the universe. A recent study of 26 clusters of galaxies confirms that the expansion of the universe stopped slowing down about 6 billion years ago, and began to accelerate.
The clusters were carefully chosen for their dynamically relaxed state and because they span a large range of distances, from one billion to eight billion light years from Earth. These Chandra images show 3 of the clusters used in the study - from left to right Abell 2029, MS 2137.3-2353, and MS 1137.5+6624, seen as they looked 1 billion, 3.5 billion, and 6.7 billion years ago, respectively.
A galaxy cluster is comprised on hundreds of galaxies embedded in a cloud of extremely hot gas and dark matter. Dark matter, an invisible and unknown type of material, is postulated to hold clusters together. X-ray observations have the unique ability to determine the ratio of the mass of the hot gas and the mass of the dark matter in a cluster. The observed values of the gas fraction depend on the assumed distance to the cluster.
Because galaxy clusters are the largest bound structures in the Universe, they are thought to represent a fair sample of the matter content in the universe. If so, the ratio of hot gas and dark matter should be the same for every cluster. Using this assumption, the distance scale can be adjusted to determine which one fits the data best. These distances show that the expansion of the Universe was first decelerating and then began to accelerate about six billion years ago.
Many scientists attribute the driving force behind cosmic acceleration to dark energy a strange form of energy that acts like repulsive gravity. It could be due to extra dimensions of space, or possibly it is an indication that modifications of Einstein’s theory are needed.
Assuming that dark energy is responsible for the acceleration, combining the Chandra results with observations of the cosmic microwave background radiation indicates that dark energy makes up about 75% of the Universe, dark matter about 21%, and visible matter about 4%. The Chandra observations agree with results from the Hubble Space Telescope (HST) and other optical telescopes, which first showed evidence for an accelerating expansion of the Universe. Chandra's independent verification helps to strengthen the case for cosmic acceleration.
The new Chandra results suggest that the dark energy density may be constant. If so, the Universe would continue expanding forever, with galaxy groups and clusters spreading further and further apart. The Chandra data also allow for the possibility that the dark energy may be increasing slowly with time. In this case, the cosmic acceleration would increase until, at a far distant time, galaxies, stars, planets and even atoms will eventually be torn apart in what has been termed The Big Rip.
[http://chandra.harvard.edu/photo/2004/darkenergy/index.html]
Alpha Capricornids is a meteor shower that takes place as early as 15 July and continues until around 10 August. The shower has infrequent but relatively bright meteors, with some fireballs. It was discovered by Hungarian astronomer Miklos von Konkoly-Thege in 1871. Peter Jenniskens and Jeremie Vaubaillon identified the parent body, comet 169P/NEAT. According to them, the meteor shower was created about 3,500 to 5,000 years ago, when about half of the parent body disintegrated and fell into dust. The dust cloud evolved into Earth’s orbit recently, causing a shower with peak rates of 2-5/h, sometimes having outbursts of bright flaring meteors with rates up to 5-9/h. The bulk of the dust will not be in Earth’s path until the 24th century. The Alpha Capricornids are expected to become a major annual storm in 2220–2420 A.D., stronger than any current annual shower.
[https://en.wikipedia.org/wiki/Alpha_Capricornids]
[https://en.wikipedia.org/wiki/Capricornus]
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