Crux is the smallest constellations in the night sky, and is located in the southern celestial hemisphere where it remains visible throughout the year (circumpolar). Despite its small size, Crux is also one of the sky’s brightest constellations, with its famous asterism of stars known as the Southern Cross containing two first magnitude stars. As a result, Crux has proved a useful navigational guide for sailors, who would use an imaginary line running through two of its stars, Acrux and Gacrux, to point towards the south celestial pole.
Crux is a southern constellation that can be seen by observers located between +20° and -90° of latitude. It is circumpolar south of 34° S, although best viewed from April to June, while residents located a little north of the equator can also see Crux slightly above the southern horizon during late spring. Meaning “cross” in Latin, the constellation is dominated by a cross-shaped asterism found westwards of Centaurus, with Musca located to its south.
Hercules Constellation Family
Crux is a member of the Hercules family of constellations, together with Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens, Triangulum Australe, and Vulpecula
In the fourth millennium BC, Crux was visible as far north as Britain and around 150 AD the ancient Greeks astronomer Ptolemy could see Crux from his base in Egypt, but considered its stars as part of the constellation Centaurus. By 400 AD, however, the precession of the equinoxes meant Crux had dropped below the horizon for most European observers, with some linking the cross’ disappearance with the crucifixion of Christ.
In the 15th century, the constellation was rediscovered by European navigators during their expeditions around the globe, and in 1592 the Dutch cartographer Petrus Plancius recognized it as its own constellation, having distinguished its stars from those of neighboring Centaurus. These days, the stars of the Southern Cross are depicted on the flags of Australia, Brazil, New Zealand, Papua New Guinea and Samoa.
Crux has held great significance for many southern hemisphere people located across Australasia, South America and Africa.
In Australia, for instance, several Aboriginal cultures saw Crux and the dark nebula (Coalsack) it contains as the head of an evil Emu, while others living around the coast saw it as a stingray, a belief shared with indigenous tribes from Indonesia and Malaysia. The Maori of New Zealand, on the other hand, saw Crux as an anchor called “Te Punga” belonging to the canoe of a great warrior named Tamarereti.
In South America, the constellation was known to the Incas as Chakana (stair), and to the Patagonian Mapuches as Melipal (four stars); while in Brazil various cultures saw Crux as either a Great Rhea, a swarm of angry bees emerging from a beehive (Coalsack Nebula), or as forming part of a bird snare.
In Africa, the Xam bushmen saw the constellation’s three brightest stars as depicting female lions, while amongst Sotho, Tswana and Venda people its stars represented male giraffes.
– Acrux (Alpha Crucis) holds the distinctions of being the night sky’s 12th most luminous star with a visual magnitude of 0.77, as well as its southern most first-magnitude star, positioned as it is fractionally further south than Alpha Centauri. Acrux is actually a multiple star system located 321 light years away whose two main components are massive blue-white sub-giants with an orbital period believed to be around 1,500 years. They are 25,000 and 16,000 times brighter than the Sun respectively.
– Mimosa (Beta Crucis) is a blue-white giant located around 350 light years away that has a visual magnitude of 1.25, making it the night sky’s 19th brightest star overall. It is, however, a binary system whose component stars are around 8 AU apart, and have an orbital period of five years. At 28,000 Celsius, Beta Crucis, also known as Becrux, or Mimosa, is believed to be the hottest of all first-magnitude star.
– Gacrux (Gamma Crucis) is a red giant (M4III) found 88 light years from our solar system with a visual magnitude 1.59. It is the night sky’s 26th brightest star, and is the northernmost star of the Southern Cross, where its red-orange color forms a startling contrast with the asterism’s other blue-white companions.
– Delta Crucis is a blue-white subgiant (B2IV) situated 360 light years distant that shines with an apparent magnitude of 2.78. This Beta Cepheid completes the Southern Cross, and has a luminosity around 5,600 times that of our sun.
Other stars of interest in Crux includes the blue-white dwarfs Zeta Crucis and Lambda Crucis; the orange giant stars Epsilon Crucis and Iota Crucis; and the red giant NGC 4349-127.
Notable Deep Sky Objects
While Crux dos not contain any Messier objects, its location along the rich part of the Milky Way means that it does have a number of notable deep-sky objects.
– The Jewel Box (Kappa Crucis Cluster) – also known as NGC 4755 and Caldwell 94 – is one of the youngest open star cluster ever discovered with an estimated age of between 7 and 10 million years. The cluster is located is around 20 light years across, 6,440 light years distant, and shines with a visual magnitude of 4.2. It also contains around one hundred stars, most of the brightest of which are young blue supergiants, although the cluster does contains some red supergiants, including near its center. The three brightest stars in this open cluster are called the “traffic lights” on account of their different hues, with the dominant star, Kappa Crucis, a red supergiant of magnitude 5.98.
– Coalsack Nebula (Caldwell 99) is a dark nebula around 50 light years wide and 600 light years distant that also crosses over into the nearby constellations of Centaurus and Musca. It appears as a dark patch permeating the Milky Way’s bright southern region, with the effect caused by the region’s interstellar gas and dust cloud obscuring the starlit background. The Coalsack Nebula has been known about as long as our species has existed and Aboriginal people have known about it for more than 40,000 years.
Other notable deep sky objects of interest in Crux includes the open clusters NGC 4609, Harvard 5, NGC 4103 and NGC 4349.