The Crux constellation is the smallest out of all the 88 modern constellations. It’s name comes from the latin translation of “Cross”, which is reflected in it’s cross shaped asterism commonly known as the Southern Cross. The Crux became a constellation in it’s own right in 1679.
The five main stars of Crux that comprise the Southern Cross asterism are Acrux, Mimosa, Gacrux, Imai and Ginan, and there are currently only two stars in Crux that have been discovered to host planets. There are no Messier objects in the constellation of Crux, however there are many interesting stars and other deep-sky objects such as Coalsack Nebula or the Jewel Box Cluster.
The Crux constellation is associated with many stories and makes an appearance in many mythologies in the southern hemisphere. It is particularly important in Australia and New Zealand, because it is circumpolar and can be seen throughout the year.
History And Mythology Of The Crux Constellation
The ancient Greeks considered Crux to be part of the constellation of Centaurus. However, the French astronomer, Augustin Royer, separated Crux from Centaurus. That being said, some historians and astronomers credit Dutch astronomer Petrus Plancius for creating the constellation in 1613, as it was published by Jakob Bartsch in 1624.
The ancient Greeks saw Crux before it’s stars dropped below the horizon for Europe and most of the northern hemisphere. Christians saw the significance in this and linked the disappearance of the cross from the sky with the crucifixion of Christ.
Location Of The Crux Constellation
As we mentioned above, the Crux constellation is the smallest out of all the 88 modern constellations. It spreads out for around 68 square degrees in the sky.
It is visible between +20° and -90° in the southern hemisphere and it is circumpolar south of 34°S, this means that it never sets below the horizon. The Crux constellation is best seen a 9pm, during May. It’s right ascension is 12.6h and it’s declination is -60°.
The neighboring constellations around Crux are Centaurus and Musca and it belongs to the Hercules family of constellations.
The Southern Cross asterism stars — Acrux, Mimosa, Gacrux, Imai and Ginan — are the brightest stars in the Crux constellation.
Acrux, also known as Alpha Crucis, is the brightest star in the constellation of Crux, and the 13th brightest star in the night sky. However, it can also be the 12th brightest star in the night sky, due to its variability in brightness.
Acrux is a multiple star system. The primary star is a blue-white subgiant star located at around 321 light-years away from the Sun, and the secondary star is a blue dwarf.
The primary star, Acrux A, is around 25,000 times brighter than our Sun and has 1780% of our Sun’s mass. Acrux B the secondary star, is 16,000 times brighter than our Sun and has 552% of our Sun’s mass.
Acrux A is also bigger than our Sun, having 780% of our Sun’s radius. The secondary star Acrux B has 540% of our Sun’s radius. Both stars are hotter than the Sun, too, with Acrux A having a surface temperature of 24,000 K and Acrux B having a surface temperature of 28,000 K.
Gacrux, also known as Gamma Crucis, is the third brightest star in the constellation of Crux, and the 25th brightest star in the night sky. It is located around 88.6 light-years away from the Sun, yet is the nearest M-class giant star to it.
Gacrux is around 820 times brighter than our Sun, and has 150% of its mass and 840% of its radius. It has an apparent magnitude of +1.64. It is also the nearest red giant to the solar system.
Imai is the fourth brightest star in the constellation of Crux. Designated as Delta Crucis, it is a blue-white subgiant star with an apparent magnitude of 2.79.
Imai is around 10,000 times brighter than our Sun, and has 890% of its mass and 800% of its radius. It has a temperature of around 22,570 K. It is also a fast-spinning star with a rotational velocity of around 210 km/130.4 miles per second.
Designated Beta Crucis, Mimosa is the second brightest star in the constellation of Crux and the 20th brightest star in the night sky. It has an apparent magnitude of 1.25 and is located around 280 light-years away from Earth.
Mimosa is around 34,000 times brighter than our Sun. It has 1600% of its mass and 840% of its radius. It is also quite a young star in comparison to others, having an estimated age between 8 to 11 million years.
Ginan, also known as Epsilon Crucis, is located around 230 light-years away from Earth. It is the fifth brightest star in the constellation of Crux and has an apparent magnitude of 3.58. It is an orange-hued star that is 302 times brighter than our Sun.
Ginan has 152% of the Sun’s mass and around 2841% of its radius. It is evolving away from the main sequence and thus it will continuously expand.
There are also many other stars in the Crux constellation. Four of them can be seen with the naked-eye.
Zeta Crucis is a binary star system that is located around 360 light-years away from us. It is visible to the naked-eye and has an an apparent magnitude of 4.04.
The primary star is a B-type main sequence star. It is 737 times brighter than our Sun and has 640% of its mass. It is also cooler than our Sun.
Theta Crucis is located around 850 light-years away from us. It has an apparent magnitude of 4.7. Theta Crucis is a spectroscopic binary star and it’s primary star is 809 times brighter than our Sun. It also has system consisting of a B-type main-sequence blueish-white star, as well as a Cephei variable star.
Lambda Crucis is located around 384 light-years away from us and has an apparent magnitude of 4.62. It is near the constellation border with Centaurus and is a suspected Beta-Cephei variable star.
It is 790 times brighter than our Sun and has 300% of its radius and around 500% of its mass.
Iota Crucis is located at around 125 light-years away from Earth as has an apparent magnitude of 4.69. It is a wide double star that appears with an orange hue.
It is around 24 times brighter than our Sun and has 726% of its radius. Iota Crucis is also an ageing star with a surface temperature of around 4,824 K.
BZ Crucis is a Be star classified as a B1IVe class star, a B class subgiant showing emission lines in its spectrum. It has an apparent magnitude of 5.316 and is approximately 1,000 light years away from us.
BZ Crucis is an X-ray source and aa Gamma Cassiopeiae type variable star, which means it is a shell star that has a circumstellar gas disk around the equator.
NGC 4349-127 is a red giant star almost 20 times as massive as Jupiter. It is located around 7,097 light years from Earth and has an apparent magnitude of 7.4 and an absolute magnitude of -4.3. It was first discovered in 2007.
Deep Sky Objects
There are two main Deep Sky Objects in Crux Constellation — Coalsack Nebula and The Jewel Block Cluster.
The Coalsack Nebula, also known as the Southern Coalsack or Caldwell 99, is a famous dark nebula, easily seen as a large dark patch in the southern region of the Milky Way. It obscures the stars as they cross their southernmost region of the sky, east of Acrux.
It stretches nearly 7° by 5° and cross over into the neighbouring constellations, Centaurus and Musca. The nebula is about 600 light years distance from Earth and has a radius of between 30 and 35 light years.
The Jewel Block Cluster
The Jewel Box Cluster, also known as Kappa Crucis Cluster, NGC 4755, or Caldwell 94 is estimated to be about 14 million years old although it is one of the youngest clusters ever discovered. It was first discovered by French astronomer Nicolaus Louis de Lacaille on his trip to South Africa in 1751.
It has around 100 stars and the total integrated magnitude of the cluster is 4.2. The cluster is located around 1.0° southeast of the star Mimosa and is around 6,440 light years away from us. It can only be seen by the southern hemisphere.
- Acrux, Gacrux and Mimosa are known as the Southern Pointers as they are used together find the South Celestial Pole.
- Within the Crux constellation, there are around 49 stars that are brighter or equal to apparent magnitude 6.5.
- A total of 15 of the 23 brightest stars in Crux are spectrally blue-white B-type stars.
- The Southern Cross asterism is represented on the Australian flag. Its stars are featured on the flag of Brazil.
- The constellation of Crux was invisible for most of Europe in 400 AD. Europeans did not rediscover Crux until the great naval expeditions of the late 15th and early 16th centuries.
- The Coalsack Nebula represented the head of the emu in the sky in Australian aboriginal astronomy. Among the Wardaman people, it is said to be the head and shoulders of a law-man watching the people to ensure they do not break traditional law.
- Crux 1 – http://www.astronomytrek.com/star-constellation-facts-crux/
- Crux 2 – https://in-the-sky.org/data/constellation.php?id=31
- Acrux – https://www.constellation-guide.com/the-southern-cross/
- Gacrux – https://www.star-facts.com/gacrux/
- Imai – https://www.star-facts.com/imai/
- Ginan – https://www.star-facts.com/ginan/
- Theta Crucis – https://theskylive.com/sky/stars/theta1-crucis-star
- Lambda Crucis – https://theskylive.com/sky/stars/lambda-crucis-star
- NGC 4349-127 – https://en.wikipedia.org/wiki/NGC_4349
- Coalsack Nebula – https://astronomynow.com/2015/10/14/zooming-into-the-coalsack-nebula/
- The Jewel Block Cluster – https://en.wikipedia.org/wiki/Jewel_Box_(star_cluster)
- Some Images created with the NightVision app – https://www.nvastro.com/nvj.html
- Some Images created with the Stelvision Sky Map https://www.stelvision.com/en/sky-map/