The Caelum Constellation.
Caelum is a faint constellation in the southern sky, introduced in the 1750s by Nicolas Louis de Lacaille and counted among the 88 modern constellations. Its name means “chisel” in Latin, and it was formerly known as Caelum Scalptorium, “the engravers’ chisel” (bottom right in the diagram above). Caelum is bordered by Dorado and Pictor to the south, Horologium, and Eridanus to the east, Lepus to the north, and Columba to the west. Covering only 125 square degrees, it ranks 81st of the 88 modern constellations in size. It appears prominently in the southern sky during the Southern Hemisphere’s summer, and the whole constellation is visible for at least part of the year to observers south of latitude 41°N. Its main asterism consists of four stars, and twenty stars in total are brighter than magnitude 6.5 . In the equatorial coordinate system, the right ascension coordinates of these borders lie between 04h 19.5m and 05h 05.1m, while the declination coordinates are between −27.02° and −48.74°, and its best viewing month is January.
The brightest star, (Alpha) α Caeli, is a double star, containing an F-type main-sequence star of magnitude 4.45 and a red dwarf of magnitude 12.5 , 65.8 light years from Earth. (Beta) β Caeli, another F-type star of magnitude 5.05 , is further away, being located 93.5 light years from Earth. Unlike α, β Caeli is a subgiant star, slightly evolved from the main sequence. (Delta) δ Caeli, also of magnitude 5.05 , is a B-type subgiant and is much farther from Earth, at 700 light years.
(Gamma) γ 1 Caeli is a double-star with a red giant primary of magnitude 4.58 and a secondary of magnitude 8.1. This star system forms an optical double with the unrelated X Caeli (previously named γ 2 Caeli), a Delta Scuti variable. These are a class of short-period (six hours at most) pulsating stars that have been used as standard candles and as subjects to study astroseismology. X Caeli itself is also a binary star, specifically a contact binary, meaning that the stars are so close that they share envelopes. The only other variable star in Caelum visible to the naked eye is RV Caeli, a pulsating red giant of spectral type M1III, which varies between magnitudes 6.44 and 6.56 .
Three other stars in Caelum are (Nu) ν Caeli is another double star, containing a white giant of magnitude 6.07 and a star of magnitude 10.66, with unknown spectral type. (Lambda) λ Caeli, at magnitude 6.24, is much redder and farther away, being a red giant. (Zeta) ζ Caeli is even fainter, being only of magnitude 6.36, is a K-type subgiant of spectral type K1. The other twelve naked-eye stars in Caelum are not referred to by Bode’s Bayer designations anymore, including RV Caeli.
One of the nearest stars in Caelum is the eclipsing binary star RR Caeli, at a distance of 65.7 light years This star system consists of a dim red dwarf and a white dwarf. Despite its closeness to the Earth, the system’s apparent magnitude is only 14.40 due to the faintness of its components. In 2012, the system was found to contain a giant planet, and there is evidence for a second substellar body. The system is a post-common-envelope binary and is losing angular momentum over time, which will eventually cause mass transfer from the red dwarf to the white dwarf. In approximately 9–20 billion years, this will cause the system to become a cataclysmic variable.
Due to its small size and location away from the plane of the Milky Way, Caelum is rather devoid of deep-sky objects, and contains no Messier objects. The only deep-sky object in Caelum to receive much attention is HE0450-2958, an unusual Seyfert galaxy. Originally, the jet’s host galaxy proved elusive to find, and this jet appeared to be emanating from nothing. Although it has been suggested that the object is an ejected supermassive black hole, the host is now agreed to be a small galaxy that is difficult to see due to light from the jet and a nearby starburst galaxy.
Credits: Go Astronomy, Wikipedia.