Crux, or the Southern Cross, is a constellation in the southern sky. It is one of the best known constellations in the southern hemisphere, and easily recognizable for the cross-shaped asterism, the Southern Cross, formed by its several brightest stars. The constellation is associated with a number of stories and figures prominently in different mythologies in the southern hemisphere.
Crux is not visible north of +20° in the northern hemisphere, and it is circumpolar south of 34°S, which means that it never sets below the horizon. On the celestial sphere, Crux is exactly opposite the constellation Cassiopeia. It is the smallest constellation in the sky.
Crux means “the cross” in Latin. Ancient Greeks considered Crux to be a part of the constellation Centaurus. Even though its stars were charted on most celestial globes, it was not until 1679 that it became a constellation in its own right. It was the French astronomer Augustin Royer who formally separated Crux from Centaurus. Some historians credit the Dutch astronomer Petrus Plancius for creating the constellation in 1613, as it was published by Jakob Bartsch in 1624.
FACTS, LOCATION & MAP
Crux is the smallest of the 88 constellations, occupying an area of only 68 square degrees.
It is located in the third quadrant of the southern hemisphere (SQ3) and can be seen at latitudes between +20° and -90°.
Crux has one star with known planets and contains no Messier objects.
The brightest star in the constellation is Acrux, Alpha Crucis, with an apparent visual magnitude of 0.77. Acrux is also the 12th brightest star in the sky.
There is one meteor shower associated with the constellation; the Crucids.
Crux belongs to the Hercules family of constellations, along with Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens, Triangulum Australe, and Vulpecula.
Crux is a famous constellation in many cultures. Ancient Greeks considered it to be part of the Centaurus constellation. Greeks were able to see Crux before its stars dropped below the horizon for Europe and most of the northern hemisphere. Some saw significance in this, linking the disappearance of the celestial cross from the sky to the crucifixion of Christ. The 2nd century Greek astronomer Ptolemy listed the stars of Crux as part of the constellation Centaurus in his Almagest. By the year 400 AD, the constellation was not visible from most of Europe anymore, and Europeans did not rediscover Crux until the great naval expeditions of the late 15th and early 16th century.
The Southern Cross carries cultural significance in many countries in the southern hemisphere. A stone image of Crux constellation has been found in Machu Picchu in Peru. The Inca knew the constellation as Chakana, which means “the stair.” The Maori called it Te Punga, or “the anchor.”
In Australian Aboriginal astronomy, the cross asterism and the Coalsack Nebula represent the head of the Emu in the Sky. The Southern Cross is represented on the Australian flag. Its stars are also featured on the flag of Brazil, where the asterism is known as Cruzeiro, or Cruzeiro do Sul. The Southern Cross is also mentioned in the Brazilian national anthem and used to be the name of the currency between 1942 and 1986 and again between 1990 and 1994.
Amerigo Vespucci charted the stars in 1501, but a more accurate depiction appeared in 1515, made by another Italian explorer, Andrea Corsali. Crux appeared on celestial globes by Petrus Plancius (1598) and Jodocus Hondius (1600). Plancius, a Dutch cartographer, depicted the constellation based on observations by Pieter Dirkszoon, a Dutch explorer.
MAJOR STARS IN CRUX
Acrux – α Crucis (Alpha Crucis)
Acrux, Alpha Crucis, is the brightest star in the constellation and the 12th brightest star in the sky. The star has an apparent visual magnitude of 0.77 and is approximately 320 light years distant. Its name is short for Alpha Crux. At -63° declination, Alpha Crucis is the southernmost first magnitude star. It cannot be seen north of latitude 27°N.
Acrux is a multiple star system composed of Alpha-1 Crucis, a B class subgiant, and Alpha-2 Crucis, a B class dwarf. The two are separated by four arc seconds. Both stars are very hot, almost class O, and their respective luminosities are 25,000 and 16,000 times that of the Sun.
Alpha-1 Crucis is a spectroscopic double, with 14th and 10th magnitude components orbiting each other with a period of 76 days. There is another B class subgiant star in the system, but it is believed to be more distant from the other components and merely located along the same line of sight.
Acrux has cultural signifance in the southern hemisphere. It is found on the flags of several countries – Australia, Papua New Guinea, New Zealand – along with the other four stars of the Southern Cross. It is also one of the 27 stars representing different states on the flag of Brazil. Alpha Crucis represents the State of São Paulo.
Mimosa (Becrux) – β Crucis (Beta Crucis)
Beta Crucis is the second brightest star of the Southern Cross and the 20th brightest star in the night sky. It has an apparent visual magnitude of 1.30 and is approximately 350 light years distant from the solar system. It is classified as a Beta Cephei variable. The star can only be seen from locations south of the Tropic of Cancer (23° 26′ 16″ N). Its estimated age is 10 million years.
Beta Crucis is a spectroscopic binary composed of two stars, about 8 AU apart, that orbit each other every five years. Becrux belongs to the spectral type B0.5IV and is thought to be the hottest first magnitude star. It got the name Mimosa because of its colour. The star represents the State of Rio de Janeiro on the flag of Brazil.
Gacrux – γ Crucis (Gamma Crucis)
Gamma Crucis, or Gacrux, is a red giant belonging to the spectral class M4III. It has an apparent visual magnitude of 1.59 and is approximately 88 light years distant from Earth.
Gamma Crucis is the third brightest star in Crux and the 26th brightest star in the sky. It is a binary star, with a primary component belonging to the spectral class M4 III and a white, A3 class star with a visual magnitude of 6.4 for an optical companion. The companion is about 400 light years distant from Earth.
δ Crucis (Delta Crucis)
Delta Crucis is a subgiant belonging to the spectral class B2IV. It has an apparent magnitude of 2.775 and is about 360 light years distant. It is classified as a Beta Cephei type variable.
ζ Crucis (Zeta Crucis)
Zeta Crucis is another double star in Crux constellation. It is composed of a blue-white dwarf belonging to the spectral class B2.5 V, and a faint (12.49m) companion.
Zeta Crucis has an apparent visual magnitude of 4.04 and is approximately 360 light years distant.
ε Crucis (Epsilon Crucis)
Epsilon Crucis is an orange giant, a class K3III star with a visual magnitude of 3.56, approximately 228 light years distant from Earth. The star is also represented on the flag of Brazil: it represents the State of Espírito Santo.
θ Crucis (Theta Crucis)
Theta Crucis is a designation shared by two distinct star systems. Theta-1 Crucis is a spectroscopic double belonging to the spectral class Am, approximately 230 light years distant. It has a visual magnitude of 4.30.
Theta-2 Crucis is also a spectroscopic binary star, composed of a pair of stars that orbit each other with a period of 3.4280 days. The star system has a stellar classification of B2 IV. It is about 750 light years distant from the solar system and has an apparent magnitude of 4.72. One of the stars is a Beta Cephei variable and its luminosity varies between 4.70 and 4.74.
λ Crucis (Lambda Crucis)
Lambda Crucis is a blue-white dwarf with an apparent magnitude of 4.6. It is about 360 light years away from the solar system. Its spectral classification is B4Vne, and the star is also classified as a Beta Cephei variable, the type of variable star that exhibits change in brightness as a result of the pulsation of its surface. The star’s luminosity varies by 0.02 magnitudes over a period of 0.3951 days.
ι Crucis (Iota Crucis)
Iota Crucis is an orange giant, belonging to the spectral class K0 III, approximately 125 light years distant. It has a visual magnitude of 4.69. The star has an optical companion, a class G8 star with a magnitude of 10.8.
BZ Crucis is a Be star classified as a B1IVe class star, a B class subgiant showing emission lines in its spectrum. It is a Gamma Cassiopeiae type variable star, which is to say a shell star that has a circumstellar gas disk around the equator. BZ Crucis is an X-ray source. It has an apparent magnitude of 5.316 and is approximately 1,000 light years distant. It is located behind the Coalsack Nebula.
NGC 4349-127 is a red giant star almost 20 times as massive as Jupiter. It is 7,097 light years distant. It has an apparent visual magnitude of 7.4 and an absolute magnitude of -4.3. A companion star, believed to be a brown dwarf, was discovered in 2007.
DEEP SKY OBJECTS IN CRUX
Coalsack Nebula (Caldwell 99)
The Coalsack Nebula is a famous dark nebula, easily seen as a large dark patch in the southern region of the Milky Way.
It is between 30 and 35 light years in radius. The nebula is about 600 light years distant from Earth.
As a result of its size, the Coalsack Nebula was known in pre-historic times in the southern hemisphere. It was observed by the Spanish navigator and conquistador Vicente Yáñez Pinzón in 1499. Amerigo Vespucci named it il Canopo fosco, which means “the dark Canopus.”
In Aboriginal astronomy in Australia, the nebula represents the head of the Emu, the bird associated with several local creation myths.
The Jewel Box – Kappa Crucis Cluster (NGC 4755, Caldwell 94)
The Kappa Crucis Cluster, also known as the Jewel Box (or Herschel’s Jewel Box), is an open star cluster in Crux. It is one of the youngest clusters ever discovered, with an estimated age of only 14 million years.
The cluster has a visual magnitude of 4.2 and contains about a hundred stars. It is approximately 6,440 light years distant from the solar system.
The cluster can only be observed from the southern hemisphere. It was discovered by the French astronomer Nicolas Louis de Lacaille on his trip to South Africa in 1751-52.
19th century English astronomer Sir John Herschel described the cluster as “a casket of variously coloured precious stones,” which is how the cluster appears in a telescope and how it subsequently got the name the Jewel Box.
To the naked eye, the cluster appears like a star near Beta Crucis, one of the stars that form the Southern Cross.
The brightest stars in the Kappa Crucis Cluster are supergiants.
The three brightest stars got the nickname “traffic lights” because of their different colours.
The dominant star in the cluster is Kappa Crucis (HD 111973), a red, M-type supergiant with a visual magnitude of 5.98.
The star stands in stark contrast with other stars in the cluster, which are mostly blue.
Kappa Crucis is approximately 1,700 light years distant. It has an absolute magnitude of -6.0.