Hippocampus

Hippocampus is a caelregio located in the first quadrant of the 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 s (listed in the infobox).

Hippocampus contains couple of notable bright stars such as and  as well as several notable planetary systems including  and.

Name and symbolism
Hippocampus is named after the Latin word for, which is a sea creature like the  and  the. 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 (Latin name) and Chtapodi the  (Greek name) were former names for Hippocampus until December 2010 and July 2012, respectively.

Bright stars
A   is the Hippocampus's brightest star at a  of 0.46, located in.

The other notable bright stars in Hippocampus are (a 1.16$st$  main sequence located in ),  (a 2.04$nd$   located in ),  (a 2.56$undefined$  located in Cetus), and  (a 2.87$m$  giant located in ).

Nearby stars
A  is the Hippocampus's nearest star at a  of 8.73 s (2.68 s). UV Ceti is also a.

A more notable nearby star is, which is a 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 and  methods and found no evidence for planets until December 2012, more to be mentioned.

Located in, the  was the second known white dwarf, discovered in 1917 by the Dutch-American astronomer. It is the closest known solitary white dwarf at a distance of 14.07 ly (4.31 pc) and the third closest overall after and.

Variable stars
In Cetus, the   can be found. Mira was the first (non-) 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.

is the 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
is a comprising of two   stars. It takes 484 years to orbit about the at an  of 63.

There is a  comprising of an  40 Eri A, a  40 Eri B, and a  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 are separated by 0.11° which is resolvable with the. α$m$ is nearly five and a half times further away from the observer than α$m$, 174 vs. 32 light-years.

Planetary systems
As of 2015, there are nearly 120 s known around more than 80 stars in Hippocampus. The first planetary system known in this caelregio is (P1 Hip). Gliese 876 now has four planets (and one speculative planet) including the outer three in a 1:2:4. 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 star receiving the  IL Aquarii.

(P6 Hip) has three planets (,, and one speculative) and s. Epsilon Eridani is the nearest known exoplanetary system, located 10.5 light-years distant. Fomalhaut (P69 Hip) had a 2.1 planet in an 872-year orbit detected by  in visible light from  until infrared observations using the  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 (Bonadea, P494) is the densest and most massive exoplanet known, which orbits the star  (P46 Hip) at an  of 2.37  and 40% eccentric. The planet has mass 12.99 M$m$ and only 63% the diameter of Jupiter, making a density 70 g/cm$1$, which is wholly 13 times denser than Earth, the 's densest planet, and 3 times denser than the densest known element.

(P9 Hip) contains two planets, Porus and Amechania. Porus (, 93) is a cloudless, -mass planet that spend most of its time closer to its sun than is to our Sun. Amechania (, 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.

(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 (P34 Hip), which has two confirmed planets, two unconfirmed planets, and three hypothetical planets.

(P28 Hip) is a of  and  orbited by two s detected by : Chimera (, P312) and Porewit, P512). Chimera has mass 5.9 M$2$ and orbits 3.6 AU from the star. Porewit has mass 4.5 M$J$ and orbits 5.4 AU from the star.

Another notable planetary system around a BF star is (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, (P63 Hip) contains two oldest known planets at 12.8 billion years. The host star's is only about 1.14% that of the Sun based on its abundance of, making it the second lowest metallicity planetary host star known. Planets orbiting around s, including HIP 11952, challenges the theory of as they typically don't have enough heavy materials (heavier than  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 ). Four of them are denser than Earth. Also I speculate that this star contains 13 planets (seven s and six s), five more planets than our Sun, including five detected terrestrial planets.

Notable deep sky objects
Hippocampus contains some interesting deep sky objects. The ( 2118), which is a faint  located in Eridanus, and the  ( 7293, 63), which is a  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 (NGC 7009, C55).

In Eridanus, the  can be found using a. This galaxy features a "" spiral structure that is 3300 light-years long. Another galaxy in Eridanus,, can be found. The galaxy type is unknown, probably an. This galaxy contains enormous amounts of, which astronomers call it .

In Cetus, there is the  (also known as Cetus A) (NGC 1068). This galaxy is the so-called, because it gives off radiation when the eats stars. In Pisces, there is the. 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.

, which is a, can be found using a telescope in Cetus near the star. This galaxy is approaching us at 234 km/s (145 mi/s). In Pisces, (NGC 628), which is a  similar in size to the, can be seen using a small telescope.

In Capricornus, there is the globular cluster (NGC 7099).

Notable meteor showers
Every year on May 6, the (also called May Hippocampids) peak, caused by the. It can produce about 60 meteors per hour.

Every year on July 28, the (also called July Hippocampids) peak, caused by the breakup of what are now the Marsden and KraHip Sungrazing s. 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 appears to cross Hippocampus from February 16 till April 18: it crosses the constellations Aquarius (February 16 till March 11) and then  (March 12 till April 18). This al caelregio is located between Tarandus to the west and Araneus to the east.

Hippocampus was formerly in the from January 21 till March 20.