Capricornus, the 40th largest constellation, is the smallest of the 12 zodiac constellations, covering an area of just 414 square degrees of the southern sky between latitudes +60° and -90°. The most luminous star in Capricornus is a complex, four-star system that has a collective magnitude of +2.85. The primary star in the system, Delta Capricorni A, is a white A-class giant star that shines at least 8.5 times brighter than the Sun.
The constellation is best seen during the month of September in an area of the sky known as the “Water”, or sometimes “The Sea”, along with other watery themed constellations, including Aquarius (Water-carrier), Cetus (Whale), Delphinus (Dolphin), Eridanus (Great River), Hydra (Water serpent), Piscis Austrinus (Southern Fish), and Pisces (Fishes).
Capricornus does not contain many deep sky objects. In fact, it contains just one Messier object (M30), and a compact group of galaxies (HCG 87), as well as a few double stars. Below are some details of the most conspicuous objects to be found in the constellation.
Hickson Compact Group 87
This group of four or five galaxies is situated 400 million light-years away, and orbits around a common center once every 100-million years.
HCG 87 consists of an edge-on spiral galaxy (PGC 65415), seen to the lower left of the image, that is 175,000 light-years wide and 375 million light-years from our Sun; and an elliptical galaxy (PGC 65409), which appears as a fuzzy patch to its lower right, that is 80,000 light-years wide, and some 390 million light-years distant. Furthermore, both galaxies are believed to contain massive gas consuming black holes at their nuclei.
Also found in this compact galaxy group is PGC 65412, seen at the top of the image, which is a spiral galaxy 80,000 light-years across, and 400 million light years from Earth; and PGC 65414, visible in the center, which is a small spiral galaxy that’s 35,000 light years wide, 455 million light-years distant, and is undergoing intense star forming activity.
Messier 30 (NGC 7099)
This F3 spectral class globular cluster is located about 28,000 light years away, and was among the first deep sky objects to be discovered by Charles Messier in 1764. The most luminous stars in the cluster are mostly highly evolved, magnitude +12 red giants.
Stretching across 90 light years, M30 is an easy target even for small telescopes. However, like most other densely packed globular clusters in an around the Milky Way, M30 has undergone a process of core collapse, in which a large percentage of the clusters’ mass is concentrated in a small area around the core. In the case of M30, at least 50% of the clusters’ mass is concentrated in an area that measures less than 35 light years across. M30’s light is blue-shifted, which means that is approaching the solar system at a speed of 181.9 km per second.
Deneb Algedi (Delta Capricorni)
Located only about 39 light years away, Deneb Algedi consists of a spectroscopic eclipsing binary, and two other stars that may or may not be gravitationally bound to the primary pair. The brightest star in the system, Delta Capricorni A, is accompanied by an unresolved companion that orbits the primary star around a common centre of mass once every 1.023 days, causing a drop in the primary’s luminosity of 0.2 magnitudes during every eclipse. Deneb Algedi is also a Delta Scuti-type variable star, which are stars that vary in brightness due to both non-radial and radial pulsations that occur on their “surfaces”.
The two suspected companions are of 13th and 16th magnitude, and located one and two arc minutes away from the primary pair, respectively. An orbital period for the secondary pair of stars has not been established.
Algiedi (Alpha Capricorni)
Algiedi, aka Algedi, is an optical binary system that derives its name from the Arabic word, al-jady, which means “the billy goat”. Composed of two separate binary systems, the primary pair is designated Prima Giedi, and consists of Alpha-1 Capricorni, a magnitude 4.30 G-class yellow supergiant star, and a magnitude 8 companion that is located 0.65 seconds of arc away from the primary.
The second pair is designated Secunda Giedi, whose primary star is a magnitude 3.58 G-type yellow giant that is orbited by a somewhat dimmer companion. The two binary systems are located 690 and 109 light years away respectively, which translates into a separation of 0.11 degrees.
Yen (Zeta Capricorni)
Located about 400 light years away, Yen is another double star in Capricornus, and it consists of a G-type yellow supergiant that is orbited by a white dwarf. However, the only noteworthy aspect of this system is that the more luminous star in the system is a so-called “barium star” that contains an over-abundance of the exotic metal praseodymium.