Caelum

Caelum /ˈsiːləm/ is a faint constellation in the southern sky, introduced in the 1750s by Nicolas Louis de Lacaille. Its name means "chisel" in Latin, and it was formerly known as Caelum Scalptorium ("the engravers' chisel"); a rare word, unconnected with the far more common Latin caelum, meaning "sky, heaven, atmosphere".^[3] It is the eighth-smallest constellation, and subtends a solid angle of around 0.038 steradians, just less than that of Corona Australis.

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 (Gamma¹ 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 Gamma¹ Caeli; and HE0450-2958, a Seyfert galaxy that at first appeared as just a jet with no host galaxy visible.

History

Seen as "Cela Sculptoris" in the lower right of this 1825 star chart from Urania's Mirror

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.^[4] The French name Lacaille gave the constellation, Burin, was originally latinized to Caelum Scalptorium ("The Engravers' Chisel"), and Francis Baily shortened this name to Caelum after a suggestion by John Herschel.^[5] 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.^[5] Johann Elert Bode stated the name as plural with a singular possessor, Caela Scalptoris ("the Engraver's Chisels") but this did not stick.^[6]

Characteristics

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 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.^{[1][lower-alpha 1]} Its main asterism consists of four stars, and twenty stars in total are brighter than magnitude 6.5.^[1] 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 04^h 19.5^m and 05^h 05.1^m, while the declination coordinates are between −27.02° and −48.74°.^[2] The International Astronomical Union (IAU) adopted the three-letter abbreviation "Cae" for the constellation in 1922.^[7]

Notable features

The constellation Caelum as it can be seen by the naked eye.

Stars

Caelum is a faint constellation, having no star brighter than fourth magnitude and only two brighter than fifth magnitude. Lacaille gave six stars Bayer designations, labeling them Alpha to Zeta in 1756, but omitted Epsilon and designated two nearby stars as Gamma. Bode extended the designations to Rho for other stars but most have fallen out of use.^[6] Caelum is too far south for any of its stars to bear Flamsteed designations.^{[lower-alpha 2]}

The brightest star, Alpha 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.^[9][10] Beta Caeli, another F-type star of magnitude 5.05, is further away, being located 28.67 parsecs (93.5 ly) from Earth. Unlike Alpha, Beta Caeli is a subgiant star, slightly evolved from the main sequence.^[11] Delta Caeli, also of magnitude 5.05, is a B-type subgiant and is much farther from Earth, at 216 parsecs (700 ly).^[12]

Gamma¹ 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.^[13] This star system forms an optical double with the unrelated X Caeli (Gamma² Caeli), a Delta Scuti variable located 98.33 parsecs (320.7 ly) from Earth.^[14] 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.^[15] X Caeli itself is also a binary star, specifically a contact binary,^[16] 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,^[17] which varies between magnitudes 6.44 and 6.56.^[18]

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 Caeli^[19] is another double star, containing a white giant of magnitude 6.07^[20] and a star of magnitude 10.66 with unknown spectral type.^[21] The system is approximately 52.55 parsecs (171.4 ly) away.^[20] Lambda Caeli,^[22] at magnitude 6.24, is much redder and farther away, being a red giant around 227 parsecs (740 ly) from Earth.^[23] Zeta 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.^[24] The other twelve naked-eye stars in Caelum are not referred to by Bode's Bayer designations anymore, including RV Caeli.

The Seyfert galaxy HE0450-2958, an unusual active galaxy in Caelum

One of the nearest stars in Caelum is the eclipsing binary star RR Caeli, at a distance of 20.13 parsecs (65.7 ly).^[25] This star system consists of a dim red dwarf and a white dwarf.^[26] Despite its closeness to the Earth, the system's apparent magnitude is only 14.40^[25] 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.^[27] 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.^[28]

Deep-sky objects

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.^[29] Although it has been suggested that the object is an ejected supermassive black hole,^[30] 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.^[31]

Notes

References

External links

Wikimedia Commons has media related to Caelum.

• Starry Night Photography – Caelum Constellation
• Star Tales – Caelum
• Caelum Constellation at Constellation Guide

Coordinates: 05^h 00^m 00^s, −40° 00′ 00″