Crux


Transit Date of principal star:
29 March


Crux, the Southern Cross, is the most familiar constellation in the southern hemisphere. This tiny constellation (the smallest in the entire sky) was once part of Centaurus, but the sight of such a brilliant cross in the sky was so compelling that it became a constellation of its own in the sixteenth century.

Besides the cross itself, the constellation contains a unique dark nebula, a famous star cluster, and a remarkable binary.

Apart from the four bright stars that form the cross, the constellation's stars are generally fourth-magnitude. Note that while gammaA and gammaB are labelled as binary components, these stars only form an optical double. The two theta stars are also not gravitationally bound to each other; on the other hand mu1 and mu2 do form a binary system (see below).


Thousands of years ago these four stars were an object of reverence in the countries of the Near East. In the Biblical days, two thousand years ago, they were just visible at the horizon. Some might find a religious connotation, linking their disappearance with the Crucifixion of Christ. Over the millennia precession has brought the cross far to the south; it is no longer visible at latitudes north of 25 degrees.

It was the European explorers of the early sixteenth century who "rediscovered" the Southern Cross. For these adventurers the constellation was an important clock, for when it passed the meridian it was (more or less) straight up and down. Thus, by studying the constellation's inclination from the perpendicular, navigators could calculate their present time.

The principal star of note in the constellation is Acrux (alpha Crucis), a splendid binary (see below). The combined visual magnitude of both stars results in a magnitude of 0.72. The stars are 320 light years away, and each is approximately one and a half to twice the size of our Sun.

Alpha Crucis has an apparent proper motion of 236. (That is, from our viewpoint, it seems to be moving very slowly in this direction.) Others in this constellation with similar motions, and therefore part of a moving star cluster, are beta, delta, zeta, lambda, and mu Crucis. The group as a whole is quite large, forming what is called the "Scorpio-Centaurus Association". See Burnham for a discussion on this cluster.

Beta Crucis (Mimosa) is the brightest star of the group, a blue-white giant (nearly five times the Sun's size) with a visual magnitude of 1.25. The star is an estimated 580 light years away, and has a luminosity of nearly 8000. The star is a variable (see below)

Gamma Crucis (Gacrux) forms the top of the cross. The reported distance may be erroneous; it's been calculated from the visual and absolute magnitudes. The resulting parallax is so large that it should be measurable.

Although gammaA and gammaB have been so named because of a suspected duplicity (that is, that they form a binary system) the facts are different. The stars are moving in different directions (174 degrees, 129 degrees) and are therefore not held together gravitationally.

Delta Crucis is the western arm, very similar in size and distance to alpha Crucis, and part of the star cluster mentioned above. The star is a beta-CMa type variable (see below).


Double stars in Crux:

Alpha Crucis is by far the best of the group: a splendid binary of equal blue-white stars: 1.58, 2.09; PA 115, separation 4.4".

Beta Crucis has a very faint (11m) companion: PA 322, separation 44.3".

Eta Crucis has a distance companion, rather faint: 3.6, 10; PA 299, separation 44".

Iota Crucis is an easy binary to resolve: 4.7, 7.5; PA 22, 26.9".

Mu1 and Mu2 Crucis form a fixed binary, also an easy one for small telescopes: 4, 5.2; PA 17 and separation 35".


Variable stars in Crux:

Crux has four beta CMa type variables (also called beta Cephei stars). These are very hot giant stars which pulsate for some inexplicable reason. Their variation is extremely small (from less than 0.01 to 0.25 magnitudes). Below are listed the beta CMa stars in Crux and their range.

Beta Crucis: 1.23 to 1.31 every 5h40m34s.

Delta Crucis: 2.78 to 2.84 every 3h37m30s.

Theta2 Crucis: 4.7 to 4.74 every 2h8m1s.

Lambda Crucis: 4.62 to 4.64 every 9h28m57s.

Mu2 is a gamma Cas variable, with a range from 4.99 to 5.18.

Finally, R Crucis isn't (as one might think) a Mira-type long-period variable, but rather a cepheid, ranging from 6.4 to 7.23 every 5d19h49m5s.


Deep Sky Objects in Crux:

"Brilliant" is the word usually used to describe The Jewel Box (NGC 4755). Also called the Kappa Crucis star cluster, this open cluster is composed of over a hundred stars, about fifty of which are a mixture of colourful supergiants: reds and blues intermingled with yellows and whites in a profusion of sparkling light.

The cluster is just a baby, perhaps no older than ten million years. Many of the stars have very high luminosities, approaching 100,000 Suns. The central star is kappa Crucis, a blue sixth-magnitude supergiant. The cluster is considered to be from 6800 to 7800 light years away.

To locate The Jewel Box, find beta Crucis and drop down to the southeast one and a half degrees.

The Coal Sack is a large dark nebula only 550 light years away, just to the south of the Jewel Box, visible to the naked eye.

Dark nebulae are massive clouds of interstellar gases and dust, dense enough to block out most of the light from stars behind it. The Coal Sack and Horsehead Nebulae (in Orion) are the two best known dark nebulae; of all dark nebulae, the Coal Sack is the largest one visible to the unaided eye.


For a closer appreciation of Crux, visit the Binocular Section.


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