The Constellation Crux – The Southern Cross

Image of the southern Milky Way showing Crux and the dark Coalsack Nebula, just right of center; the Southern Pointers Rigil Kent and Hadar, left of center, and the eta Carinae Nebula at extreme right. Credit: A. Fuji.

Image of the southern Milky Way showing Crux and the dark Coalsack Nebula, just right of center; the Southern Pointers Rigil Kent and Hadar, left of center, and the eta Carinae Nebula at extreme right. Credit: A. Fuji.

Following the southern Milky Way, through the bright constellations Orion, Canis Major, Puppis, Vela, and Carina, you finally arrive at the famous constellation Crux, the Southern Cross, arguably the most famous constellation in the night sky.  It’s also the smallest constellation by area, just 68 square degrees compared to Hydra’s 1302 square degrees.  Most new stargazers are a little startled by its tiny size.  The cross is just 6o long from top to bottom.

(This article is an excerpt of the Cosmic Pursuits course Fundamentals of Stargazing, to be released in February 2016).

Crux is visible well above the southern horizon in the late evenings in February and in the early evenings well through May and June. As you search for the constellation, don’t get fooled by the “False Cross” which is well west of Crux and much closer to Canopus, the second brightest star in the sky.  It’s made of two stars in Carina and two in Vela, and it’s slightly larger and more oblique than the true Southern Cross, and more diamond-shaped. The two arms of Crux cross at nearly right angles, and Crux is smaller and closer to two prominent stars Hadar and Rigil Kent in Centaurus.  The map below will help you tell one from the other.

Crux lies under the hind legs of the much larger constellation Centaurus.  Its four bright stars are Acrux, Mimosa, Gacrux, and δ (delta) Crucis.  The brightest star, Acrux, at the base of the cross, gets its name from a combination of “Alpha” and “Crux”.  This magnitude 0.8 star gets its blue-white color as a consequence of its high temperature of nearly 30,000 K (Kelvin). A close look with a small telescope reveals Acrux as a pleasing double star. The brighter of the two stars, which itself has another stellar companion too close to resolve, shines some 25,000x brighter than our sun. The fainter star outshines our sun by 16,000x. Like most hot, massive stars, each component of Acrux is burning furiously through its store of fuel. They will likely end their lives in supernova explosions in several million years, and since they are just 320 light years from Earth, will grow bright enough for a few weeks to cast shadows at midnight.

The thickest part of the southern Milky Way from Vela and Carina to Crux, the southern Cross.

The thickest part of the southern Milky Way from Vela and Carina to Crux, the southern Cross.

Mimosa, or beta (β) Crucis, is also a hot blue star.  It has an apparent magnitude 1.3 and lies 280 light years away. The star takes its name from the Mimosa flower found in Central and South America. Mimosa, Acrux, and delta Crucis likely share a common origin 10 million years ago with many other stars in Scorpius and Centaurus.  It comes in at magnitude 1.6.

The red-orange Gacrux, or gamma (γ) Crucis, makes a striking color contrast with the other bright blue-white stars of the constellation. The star has burned through most of its nuclear fuel and has swollen and cooled to just 3,500K. Gacrux lies fairly close to Earth– just 88 light years– so it’s intrinsically much fainter than Acrux and Mimosa. It will expire gently, casting off its outer layers as a planetary nebula and leave behind a dim white dwarf.

On a line between δ Crucis and Acrux lies the 5th-brightest star in Crux, the orange star ε (epsilon) Crucis, a star of magnitude 3.6.  The position of this star also distinguishes Crux from the False Cross, which has no fifth star in this position.  The star is bright enough to warrant inclusion, along with the four brighter stars of Crux, on the flags of Australia and Brazil.  The flag of New Zealand leaves off this star.

Crux as a whole is a splendid sight in binoculars.  There are many new stars here, and some excellent deep-sky objects, a few of which will be covered in the section on deep-sky objects.  In dark sky, an unresolved whitish band of stars along the Milky Way is obvious.  But look in the corner of the cross between Acrux and Mimosa for a dark patch of sky set strikingly amid the background stars.  This is the “Coalsack”, an immense cloud of cold gas and dust that blocks the visible light from the background stars.  More on the Coalsack in the deep-sky tours.

Crux as it might appear in binoculars.

Crux as it might appear in binoculars.

During southern sea voyages of the 16th century, European explorers assigned the constellation’s present name. Northern seafarers were unsettled by the disappearance of the North Star as they sailed south across the equator. But they saw Crux as a good omen. In the early 16th century, Amerigo Vespucci noted two of the bright stars, Acrux and Mimosa, and Andrea Corsali mapped the full constellation, which he described as ‘so fair and beautiful that no other heavenly sign may be compared to it’. Vespucci recalled Dante’s reference to these four stars in his Divine Comedy. When Dante and Beatrice finally ascended from Hell on the far side of the world, they saw four brilliant stars which they took to represent the four principal virtues, Justice, Prudence, Fortitude, and Temperance:

To the right hand I turn’d and fix’d my mind
On the other pole attentive where I saw
Four stars ne’er seen before save by the ken
Of our first parents. Heaven of their rays
Seem’d joyous. Oh thou northern site, bereft
Indeed, and widow’d, since of these deprived”

Dante may have known of Crux from historical records of classical observations. Or he may have learned of the stars from the 13th-century travelogues of his contemporary Marco Polo, who likely saw them as he sailed south around the Malay peninsula on the way to China. But of course, Crux has been known as long as humans have looked at the sky, and many indigenous cultures include these stars in their legends…

  • Australian Aborigines saw the dark nebula in Crux called the Coalsack as the head of a great, evil emu
  • In Indonesia and Malaysia, and some coastal Australian Aboriginal tribes, Crux was a stingray
  • The Maori of New Zealand see Crux as an anchor called “Te Punga”
  • The /Xam bushmen of southern Africa thought the three brightest stars of Crux were celestial female lions
  • And to the !Kung bushmen, the Coal Sack in Crux, the dark nebula which blocks out background stars, was called the “Old Bag of the Night”

Directly south of Crux you’ll find the tiny and obscure constellation Musca, the Fly, which first appeared on star charts in the 18th century. The fly’s head is marked by λ Musci, his tail by 3rd magnitude α Musci, and his wings by β and γ Musci. While the constellation is not particularly striking, it is set in a rich star field and there are many star clusters and nebulae in this area. In binoculars, they appear as fuzzy and indistinct stars.  Southeast of Musca, you will find the even fainter constellation Apus, the Bird of Paradise. It has little of interest, save for the fine pair of red-orange stars δ1 and δ2 just north of γ. The pair looks lovely in any pair of binoculars. To the west of Apus lies the small constellation Chamaeleon, but it holds little of interest.

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