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”); It is a rare word, unrelated to the far more common Latin caelum, meaning “sky, heaven, atmosphere”. It is the eighth-smallest constellation, and subtends a solid angle of around 0.038 steradians, just less than that of Corona Australis.
|Pronunciation||//, genitive //|
|Right ascension||04h 19.5m to 05h 05.1m|
|Declination||−27.02° to −48.74°|
|Area||125 sq. deg. (81st)|
|Stars with planets||1|
|Stars brighter than 3.00m||0|
|Stars within 10.00 pc (32.62 ly)||0|
|Brightest star||α Cae (4.45m)|
|Visible at latitudes between +40° and −90°.|
Best visible at 21:00 (9 p.m.) during the month of January.
Due to its small size and location away from the plane of the Milky Way, Caelum is a rather barren constellation, with few objects of interest. The constellation's brightest star, Alpha Caeli, is only of magnitude 4.45, and only one other star, (Gamma) γ 1 Caeli, is brighter than magnitude 5 . Other notable objects in Caelum are RR Caeli, a binary star with one known planet approximately 20.13 parsecs (65.7 ly) away; X Caeli, a Delta Scuti variable that forms an optical double with γ 1 Caeli; and HE0450-2958, a Seyfert galaxy that at first appeared as just a jet, with no host galaxy visible.
Caelum was first introduced in the eighteenth century by Nicolas Louis de Lacaille, a French astronomer who introduced thirteen other southern constellations at the same time. Lacaille gave the constellation the French name Burin, which was originally Latinized to Caelum Scalptorium (“The Engravers’ Chisel”).
Francis Baily shortened this name to Caelum, as suggested by John Herschel. In Lacaille's original chart, the constellation was shown both as a burin and an échoppe, although it has come to be recognized simply as a chisel. Johann Elert Bode stated the name as plural with a singular possessor, Caela Scalptoris – in German (die ) Grabstichel (“the Engraver’s Chisels”) – but this did not stick.
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.[a] Its main asterism consists of four stars, and twenty stars in total are brighter than magnitude 6.5 .
The constellation's boundaries, as set by Eugène Delporte in 1930, are defined by a 12-sided polygon. 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°. The International Astronomical Union (IAU) adopted the three-letter abbreviation “Cae” for the constellation in 1922.
Caelum is a faint constellation: It has no star brighter than magnitude 4 and only two stars brighter than magnitude 5.
Lacaille gave six stars Bayer designations, labeling them Alpha (α ) to Zeta (ζ ) in 1756, but omitted Epsilon (ε ) and designated two adjacent stars as Gamma (γ ). Bode extended the designations to Rho (ρ ) for other stars, but most of these have fallen out of use. Caelum is too far south for any of its stars to bear Flamsteed designations.[b]
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 , 20.17 parsecs (65.8 ly) from Earth. (Beta) β Caeli, another F-type star of magnitude 5.05 , is further away, being located 28.67 parsecs (93.5 ly) 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 216 parsecs (700 ly).
(Gamma) γ 1 Caeli is a double-star with a red giant primary of magnitude 4.58 and a secondary of magnitude 8.1 . The primary is 55.59 parsecs (181.3 ly) from Earth. The two components are difficult to resolve with small amateur telescopes because of their difference in visual magnitude and their close separation. This star system forms an optical double with the unrelated X Caeli (previously named γ 2 Caeli), a Delta Scuti variable located 98.33 parsecs (320.7 ly) from Earth. 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 still occasionally referred to by their Bayer designations, although they are only on the edge of naked-eye visibility. (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. The system is approximately 52.55 parsecs (171.4 ly) away. (Lambda) λ Caeli, at magnitude 6.24, is much redder and farther away, being a red giant around 227 parsecs (740 ly) from Earth. (Zeta) ζ Caeli is even fainter, being only of magnitude 6.36 . This star, located 132 parsecs (430 ly) away, 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 20.13 parsecs (65.7 ly). 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, and thus it cannot be easily seen with amateur equipment. 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.
- While parts of the constellation technically rise above the horizon to observers between 41°N and 62°N, stars within a few degrees of the horizon are to all intents and purposes unobservable.
- Southern constellations such as Caelum have no Flamsteed designations because Flamsteed only catalogued stars that were visible from England,.
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- J. E. Bode: Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniß der geraden Aufsteigung und Abweichung von 17240 Sternen, Doppelsternen, Nebelflecken und Sternhaufen. Berlin, 1801, p.17
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- "HR 1557 – Star in double system". SIMBAD. Retrieved 15 January 2013.
- "CD-41 1593B – Star in double system". SIMBAD. Retrieved 15 January 2013.
- Ashland Astronomy Studio: Where Art and Science Converge. "Lambda Caeli (HIP 21998)". Archived from the original on 29 January 2014. Retrieved 15 January 2014.
- "HR 1518 – Star". SIMBAD. Retrieved 15 January 2013.
- "Zeta Caeli – Star". SIMBAD. Retrieved 15 January 2013.
- "V* RR Caeli – Eclipsing binary of Algol type (detached)". SIMBAD. Retrieved 15 January 2013.
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