| Constellations (7) | |
|---|---|
| Aquarius, Capricornus, Cetus, Delphinus, Eridanus, Phoca, Pisces | |
| Nomenclature | |
| Abbreviation | Hip |
| Genitive | Hippocampi |
| Pronunciation | Name: /'hi•pō•kam•pis/ Genitive: /'hi•pō•kam•pē/ |
| Symbolism | the seahorse |
| Geometry | |
| Midpoint right ascension | 00h 43.00m |
| Midpoint declination | −11° 17.94' |
| Northernmost border | +33° 40.91' |
| Southernmost border | −54° 54.97' |
| Westernmost border | 20h 06.77m |
| Easternmost border | 05h 11.22m |
| Quadrant | SQ1 |
| Crossed by | Ecliptic Equator Spring meridian |
| Bordering caelregios | Araneus (NE) Selachimorphus (SE) Solarium (S) Tarandus (W) Testudo (N) |
| Segments | 96 |
| Area | 5086.472 sq. deg. (1st) |
| Proportion of the sky | 123.300‰ |
| Average constellation area | 726.639 sq. deg. (2nd) |
| Stars | |
| Named stars | 40 |
| Stars in the figure | 4 |
| Naked eye stars (m < 6.50) |
492 |
| BF stars | 523 |
| Bright stars (m < 3.00) |
9 |
| Brightest star | Achernar (0.46m) |
| Nearby stars (D < 10.00 pc, 32.62 ly) |
45 |
| Nearest star | UV Ceti (2.68 pc, 8.73 ly) |
| Messier objects | 6 |
| Visibility | |
| Full visibility range | 35°N–56°S |
| Partial visibility range | 90°N–35°N 56°S–90°S |
| Midnight culmination date | October 3 |
| Zodiac | |
| Astrological sign | 12/23–3/20 |
| Solar sign | 1/19–4/18 |
Hippocampus is a caelregio located in the first quadrant of the southern hemisphere at its midpoint, but it extends into the northern hemisphere and into the fourth quadrant. Hippocampus is the largest caelregio with an area of 5086 square degrees, covering about 123‰ of the sky. Hippocampus is divided into seven constellations (listed in the infobox).
Hippocampus contains couple of notable bright stars such as Fomalhaut and Achernor as well as several notable planetary systems including Epsilon Eridani and Gliese 876.
Name and symbolism[]
Hippocampus is named after the Latin word for seahorse, which is a sea creature like Pisces the fish and Cetus the whale. So it is imagined that whales (Cetus) eat seahorses and fishes (Pisces) in the river (Eridanus) created by the water-bearer (Aquarius). Also the seal (Phoca) swims in that river.
Aqua the water (Latin name) and Chtapodi the octopus (Greek name) were former names for Hippocampus until December 2010 and July 2012, respectively.
Notable stars[]
Bright stars[]
A B-type main sequence Achernar is the Hippocampus's brightest star at a magnitude of 0.46, located in Eridanus.
The other notable bright stars in Hippocampus are Fomalhaut (a 1.16m A-type main sequence located in Phoca), Beta Ceti (a 2.04m G-type giant located in Cetus), Alpha Ceti (a 2.56m M-type giant located in Cetus), and Deneb Algiedi (a 2.87m F-type giant located in Capricornus).
Nearby stars[]
A flare star UV Ceti is the Hippocampus's nearest star at a distance of 8.73 light-years (2.68 parsecs). UV Ceti is also a binary star.
A more notable nearby star is Tau Ceti, which is a solar-type star, but it is smaller, dimmer, and less massive than our Sun. Tau Ceti is located just 11.91 light-years (3.65 parsecs) away. Tau Ceti has been the subject of a scientific interest because it is a nearby solar-type star and may contain planets. Astronomers used radial velocity and astrometric methods and found no evidence for planets until December 2012, more to be mentioned.
Located in Pisces, the white dwarf Van Maanen's Star was the second known white dwarf, discovered in 1917 by the Dutch-American astronomer Adriaan van Maanen. It is the closest known solitary white dwarf at a distance of 14.07 ly (4.31 pc) and the third closest overall after Sirius B and Procyon B.
Variable stars[]
In Cetus, the prototype variable star Mira can be found. Mira was the first (non-nova) variable star discovered. Mira oscillates variably for 332 days and varies from third magnitude (sometimes second) all the way down to the tenth magnitude and their spectral class varies from M5 to M9.
Achernar is the Lambda Eridani-type variable plus a binary star that show small, irregular variability on short timescales. Achernar spins so rapidly that its equatorial diameter is 56% greater than its polar diameter, making it the most ellipsoidal star known.
Multiple stars[]
p Eridani is a binary star comprising of two K-type main sequence stars. It takes 484 years to orbit about the center of mass at an average distance of 63 AU.
There is a triple star system 40 Eridani comprising of an orange dwarf 40 Eri A, a white dwarf 40 Eri B, and a red dwarf 40 Eri C. The BC binary pair are separated from A by 418 AU and have an orbital period of about 8000 years. The BC pair are separated by 35 AU and have an orbital period of 252 years. 40 Eridani A is speculated to have four rocky planets with the outermost planet orbiting just 1.6% the distance to the BC pair.
Double stars[]
The two stars of Alpha Capricorni are separated by 0.11° which is resolvable with the naked eye. α1 is nearly five and a half times further away from the observer than α2, 174 vs. 32 light-years.
Planetary systems[]
As of 2015, there are nearly 120 exoplanets known around more than 80 stars in Hippocampus. The first planetary system known in this caelregio is Gliese 876 (P1 Hip). Gliese 876 now has four planets (and one speculative planet) including the outer three in a 1:2:4 laplacian resonance. Gliese 876 was the first planetary system known around a red dwarf star, located 15.3 light-years from Earth. Gliese 876 is also a BY Draconis variable star receiving the variable star designation IL Aquarii.
Epsilon Eridani (P6 Hip) has three planets (one confirmed, one unconfirmed, and one speculative) and circumstellar disks. Epsilon Eridani is the nearest known exoplanetary system, located 10.5 light-years distant. Fomalhaut (P69 Hip) had a 2.1 MJ planet in an 872-year orbit detected by direct imaging in visible light from Hubble until infrared observations using the Spitzer Space Telescope discovered that it is likely a spherical cloud of dust instead of a planet. Then, the "zombie planet" was recovered with the same name (Illion) but a different planet number (old: P297; new: P828) after reanalyzing the original Hubble data using new, more powerful algorithms for separating planet light from starlight.
Located in Pisces, the planet HD 217786 b (Bonadea, P494) is the densest and most massive exoplanet known, which orbits the star HD 217786 (P46 Hip) at an average distance of 2.37 AU and 40% eccentric. The planet has mass 12.99 MJ and only 63% the diameter of Jupiter, making a density 70 g/cm3, which is wholly 13 times denser than Earth, the Solar System's densest planet, and 3 times denser than the densest known element osmium.
54 Piscium (P9 Hip) contains two planets, Porus and Amechania. Porus (54 Piscium b, 93) is a cloudless, Saturn-mass planet that spend most of its time closer to its sun than Mercury is to our Sun. Amechania (54 Piscium c, P903) has a mass about one-third of that of Porus that orbits closer than Porus for most of the time, but Amechania's orbital distance varies only slightly. The orbital periods are two months for Porus and one month for Amechania.
HD 11964 (P16 Hip) is a notable planetary system located in Cetus. It has two known planets: Scylla (HD 11964 c, P152) and Deino (HD 11964 b, P153), and two speculative planets. A third planet was claimed and subsequently retracted. Another example of a planetary system in Cetus is HD 1461 (P34 Hip), which has two confirmed planets, two unconfirmed planets, and three hypothetical planets.
HU Aquarii (P28 Hip) is a cataclysmic variable star of AM Her type and eclipsing binary orbited by two giant planets detected by eclipsing binary timing: Chimera (HU Aquarii b, P312) and Porewit HU Aquarii c, P512). Chimera has mass 5.9 MJ and orbits 3.6 AU from the star. Porewit has mass 4.5 MJ and orbits 5.4 AU from the star.
Another notable planetary system around a BF star is 82 Eridani (P49 Hip), which contains three planets, all are super-Earths orbiting within 0.35 AU, which is within the Mercury's distance from our Sun. This planetary system also contains seven speculative planets: two mid-Earths, three midplanets, and two mid-Jupiters, extending this planetary system to within 22 AU.
In Cetus, HIP 11952 (P63 Hip) contains two oldest known planets at 12.8 billion years. The host star's metallicity is only about 1.14% that of the Sun based on its abundance of iron, making it the second lowest metallicity planetary host star known. Planets orbiting around Population II stars, including HIP 11952, challenges the theory of planetary formation as they typically don't have enough heavy materials (heavier than helium in this sense) to form planetary systems.
Tau Ceti (P73 Hip) likely contains five super-Earths. The masses range from two to seven times that of Earth and orbital distances range from 0.1 AU to 1.34 AU (15 Gm to 201 Gm). Four of them are denser than Earth. Also I speculate that this star contains 13 planets (seven terrestrial planets and six gas giants), five more planets than our Sun, including five detected terrestrial planets.
Notable deep sky objects[]
Hippocampus contains some interesting deep sky objects. The Witch Head Nebula (IC 2118), which is a faint supernova remnant located in Eridanus, and the Helix Nebula (NGC 7293, C63), which is a planetary nebula located in Aquarius. The Helix Nebula is one of the nearest planetary nebulae to the Solar System at a distance of 215 parsecs or 700 light-years. This nebula contains thousands of comet-like knots of gas flowing inwards toward the central star caused by the interplay between hot fast gases in the inner region overtaking the cool slow gases in the outer region ejected earlier. Also in Aquarius, there is another planetary nebula: the Saturn Nebula (NGC 7009, C55).
In Eridanus, the barred spiral galaxy NGC 1300 can be found using a telescope. This galaxy features a "grand design" spiral structure that is 3300 light-years long. Another galaxy in Eridanus, NGC 1132, can be found. The galaxy type is unknown, probably an elliptical galaxy. This galaxy contains enormous amounts of dark matter, which astronomers call it fossil group.
In Cetus, there is the spiral galaxy M77 (also known as Cetus A) (NGC 1068). This galaxy is the so-called Seyfert galaxy, because it gives off radiation when the supermassive black hole eats stars. In Pisces, there is the polar ring galaxy NGC 660. This rare galaxy has outer rings of stars and gas rotating over the poles, hence the type. This feature may have resulted when NGC 660 captured material from a passing galaxy.
IC 1613, which is a dwarf irregular galaxy, can be found using a telescope in Cetus near the star 26 Ceti. This galaxy is approaching us at 234 km/s (145 mi/s). In Pisces, M74 (NGC 628), which is a spiral galaxy similar in size to the Milky Way Galaxy, can be seen using a small telescope.
In Capricornus, there is the globular cluster M30 (NGC 7099).
Gallery[]
 The Helix Nebula (NGC 7293, C63) |
 The Saturn Nebula (NGC 7009, C55) |
 The Witch Head Nebula (IC 2118) |
  The barred spiral galaxy NGC 1300 |
  M77 (also known as Cetus A) (NGC 1068) |
  The spiral galaxy M74 (NGC 628) |
  The mysterious galaxy NGC 1132 (probably an elliptical galaxy) |
  The polar ring galaxy NGC 660 |
Notable meteor showers[]
Every year on May 6, the Eta Aquariids (also called May Hippocampids) peak, caused by the Halley's comet. It can produce about 60 meteors per hour.
Every year on July 28, the Delta Aquariids (also called July Hippocampids) peak, caused by the breakup of what are now the Marsden and KraHip Sungrazing comets. The Delta Aquariids are divided into Southern and Northern. The Southern half is stronger that it produces 15–20 meteors per hour while the Northern produces just 10.
Visibility[]
In the northern hemisphere, Hippocampus can be visible from mid summer till late fall, although extreme southern Eridanus which contains the caelregio's brightest star Achernar can only be visible south of 30°N in places like Florida, Central America, all of Africa except for the northernmost countries like Egypt and Libya, southernmost Asia such as in India, Australia, South America, and Antarctica.
Zodiac[]
The Sun appears to cross Hippocampus from February 16 till April 18: it crosses the constellations Aquarius (February 16 till March 11) and then Pisces (March 12 till April 18). This zodiacal caelregio is located between Tarandus to the west and Araneus to the east.
Hippocampus was formerly in the sign of zodiac from January 21 till March 20.