Horologium (constellation)

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Horologium
Constellation
Horologium
AbbreviationHor
GenitiveHorologii
Pronunciation/ˌhɒrəˈliəm, -ˈlɒ-/,[1]
genitive /ˌhɒrəˈliˌ, -ˈlɒ-/
Symbolismthe Pendulum Clock
Right ascension 3h
Declination−60°
QuadrantSQ1
Area249 sq. deg. (58th)
Main stars6
Bayer/Flamsteed
stars
10
Stars with planets4
Stars brighter than 3.00m0
Stars within 10.00 pc (32.62 ly)1
Brightest starα Hor (3.85m)
Messier objects0
Meteor showers0
Bordering
constellations
Eridanus
Hydrus
Reticulum
Dorado
Caelum
Visible at latitudes between +30° and −90°.
Best visible at 21:00 (9 p.m.) during the month of December.

Horologium is a faint constellation in the southern celestial hemisphere, one of twelve described in the 18th century by French astronomer Nicolas-Louis de Lacaille and one of several depicting scientific instruments.[further explanation needed] It is one of the eighty-eight constellations designated by the International Astronomical Union (IAU). The constellation's brightest star – and only star brighter than apparent (visual) magnitude 4 – is Alpha Horologii, an ageing orange giant star that has swollen to around 11 times the diameter of the Sun. R Horologii[further explanation needed] is a long-period variable star with one of the widest ranges in brightness known. Four star systems in the constellation have planets, with one – Gliese 1061 – containing one in its circumstellar habitable zone.

History[edit]

The French astronomer Nicolas Louis de Lacaille first described the constellation as l'Horloge à pendule & à secondes (clock with pendulum and seconds hand) in 1752,[2][3] after he had observed and catalogued almost 10,000 southern stars during a two-year stay at the Cape of Good Hope. He devised fourteen new constellations in uncharted regions of the Southern Celestial Hemisphere not visible from Europe. All but one honoured instruments that symbolised the Age of Enlightenment.[a] Lacaille Latinised the name to Horologium on his 1763 chart.[4]

Characteristics[edit]

Covering a total of 248.9 square degrees or 0.603% of the sky, Horologium ranks 58th in area out of the 88 modern constellations.[5] Its position in the Southern Celestial Hemisphere means the whole constellation is visible to observers south of 23°N.[5][b] Horologium is bordered by five different constellations: Eridanus (the Po River), Caelum (the chisel), Reticulum (the reticle), Dorado (the dolphinfish/swordfish), and Hydrus (the male water snake). The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is 'Hor'.[6] The official constellation boundaries are defined by a twenty-two sided polygon (illustrated in infobox). In the equatorial coordinate system, the right ascension coordinates of these borders lie between  02h 12.5m and  04h 20.2m, while the declination coordinates are between −39.63° and −67.04°.[7]

Features[edit]

The constellation Horologium as it can be seen by the naked eye

Stars[edit]

Horologium has only one star brighter than apparent magnitude 4,[8] and 41 stars brighter than or equal to magnitude 6.5.[c][5] Lacaille charted and designated 11 stars with the Bayer designations Alpha through to Lambda Horologii in 1756. Francis Baily removed the designations of Epsilon and Theta Horologii as he held they were too faint to warrant naming. He was unable to find a star that corresponded to the coordinates Lacaille's Beta Horologii. Determining that the coordinates were wrong, he assigned the designation to another star. Kappa Horologii, too, was unable to be verified, yet was most likely the star HD 18292. Gould assigned designations to what became Mu and Nu Horologii as he felt they were bright enough to warrant them.[4]

At magnitude 3.9, Alpha Horologii is the brightest star in the constellation, located 115.0 (± 0.5) light-years distant from Earth.[10] Bode depicted it as the clock's pendulum, while Lacaille made it one of the weights.[11] It is an orange giant star of spectral type K2III that has swollen to around 11 times the diameter of the Sun, having spent much of its life as a white main sequence star.[12] With an estimated 1.55 times the mass of the Sun,[13] it is radiating 38 times the Sun's luminosity from its photosphere at an effective (surface) temperature of 5,028 K.[14] At magnitude 4.93, Delta Horologii is the second brightest star in the constellation,[15] and forms a wide optical double with Alpha.[16] Delta itself is actually a binary system composed of a white main sequence star of spectral type A5V that is 1.41 times as massive as the Sun with a magnitude of 5.15 and its fainter companion of magnitude 7.29.[17] The system is located 179 (± 4) light-years from the earth.[10]

At magnitude 5.0, Beta Horologii is a white giant that is radiating 63 times the Sun's luminosity from its photosphere at an effective temperature of 8,303 K.[14] It is 312 (± 4) light-years from Earth.[18] It has been little-studied.[15] Lambda Horologii is an ageing yellow-white giant star of spectral type F2III that spins around at 140 km/second, and is hence mildly flattened at its poles (oblate).[19] It is 161 (± 1) light-years from Earth.[10]

With a magnitude of 5.24,[20] Nu Horologii is a white main sequence star of spectral type A2V located 169 (± 1) light-years from Earth,[21] that is around 1.9 times as massive as the Sun. Estimated to be around 540 million years old, it has a debris disk that appears to have two components: an inner disk is orbiting at a distance of 96+9
−37
 AU
, while an outer disk lies 410+24
−96
 AU
from the star. The estimated mass of the disks is (1.3±0.7)×10−3 times the mass of the Earth.[20]

However, Horologium does have several variable stars. R Horologii is a red giant Mira variable with one of the widest ranges in brightness known,[22] around 1000 light-years from Earth.[23] It has a minimum magnitude of 14.3 and a maximum magnitude of 4.7; its period is approximately 13 months.[24] T and U Horologii are also Mira variables.[16] The Astronomical Society of Southern Africa in 2003 reported that observations of these two stars were urgently needed as data on their light curves was incomplete.[25] TW Horologii is a semiregular variable red giant star that is classified as a carbon star.[24] It is 1370 (± 70) light-years distant from Earth.[26]

Iota Horologii is a yellow-white dwarf star 1.23 (± 0.12) times as massive and 1.16 (± 0.04) times as wide as the Sun with a spectral type of F8V,[27] 56.51 (± 0.05) light-years distant from Earth.[28] Its chemical profile, movement and age indicate it formed within the Hyades cluster but has drifted around 130 light-years away from other members.[29] It was found to have a planet at least 2.5 times as massive as Jupiter orbiting it every 307 days.[30] HD 27631 is a Sun-like star located 164.3 (± 0.3) light-years from Earth.[31] that was found to have a planet at least 1.45 times as massive as Jupiter that takes 2208 (± 66) days (six years) to complete an orbit.[32] WASP-120 is a yellow-white main sequence star around 1.4 times as massive as the Sun with a spectral type of F5V that is estimated to be 2.6 (± 0.5) billion years old. It has a massive planet around 4.85 times as massive as Jupiter that completes its orbit every 3.6 days, and has an estimated surface temperature of 1880 (± 70) K.[33]

With an apparent magnitude of 13.06, Gliese 1061 is a red dwarf of spectral type M5.5V that has 12% of the mass and 15% of the diameter of the Sun, and shines with only 0.17% of its luminosity. Located 12 light-years away from Earth, it is the 20th closest star to the Sun. In August 2019, it was announced to have three planets, one of which lay in its circumstellar habitable zone.[34]

Deep-sky objects[edit]

Composite image of NGC 1512 (left) and the dwarf galaxy NGC 1510

Horologium is also home to many deep-sky objects; there are several globular clusters in the constellation. NGC 1261 is a globular cluster of magnitude 8, located 53,000 light-years from Earth.[24] It lies 4.7 degrees north-northeast of Mu Horologii.[35] The globular cluster Arp-Madore 1 is found in the constellation, the most remotely known globular cluster in the Milky Way at a distance of 123.3 kiloparsecs (402,000 light-years) from Earth.[36]

NGC 1512 is a barred spiral galaxy 2.1 degrees west-southwest of Alpha Horologii with an apparent magnitude of 10.2.[35] About five arcmin (13.8 kpc) away is the dwarf lenticular galaxy NGC 1510. The two are in the process of a lengthy merger which has been going on for 400 million years.[37]

The Horologium-Reticulum Supercluster is a galaxy supercluster, second in size only to the Shapley Supercluster in the local universe within 200 mpc of Earth. It contains over 20 Abell galaxy clusters and covers more than 100 deg2 of the sky, centered roughly at α = 03h19m, δ = 50°2'.[38]

Notes[edit]

  1. ^ The exception is Mensa, named for the Table Mountain. The other twelve (alongside Horologium) are Antlia, Caelum, Circinus, Fornax, Microscopium, Norma, Octans, Pictor, Pyxis, Reticulum, Sculptor and Telescopium.[4]
  2. ^ While parts of the constellation technically rise above the horizon to observers between the latitudes of 23°N and 50°N, those stars within a few degrees of the horizon are difficult to see.[5]
  3. ^ Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[9]

References[edit]

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  3. ^ Lacaille, Nicolas Louis (1756). "Relation abrégée du Voyage fait par ordre du Roi au cap de Bonne-espérance". Mémoires de l'Académie Royale des Sciences (in French): 519–92 [588].
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External links[edit]

Coordinates: Sky map 03h 00m 00s, −60° 00′ 00″