Crux the Southern Cross

“...we saw the southern cross clearly for the first time. It was steeply inclined and appeared from time to time between the clouds, the centre of which in the flickering sheet lightning shone with a silvery radiance. If a traveller be permitted to mention his personal feelings, I may remark that on this night one of the dreams of my earliest youth was fulfilled.”

Alexander von Humboldt (sailing the Atlantic 1799)

Both of these stars are of spectral class B and are rich in helium. The brighter star, alpha¹ shines with the light of 25000 Suns while the other, alpha² has a luminosity around 20000. Alpha¹ is a spectroscopic binary with possible masses of 14 and 10 times that of our Sun and an orbit around each other of 75.78 days.

There is a further possible member of this stellar family, a 5th magnitude white star lying 91” distant and it completes a delightful field. However, it may not be gravitationally bound, lying further away.

The eastern arm of the Cross is defined by beta Crucis. This B-type giant has a luminosity around 5,800 times the Sun. Most observers use this glittering beacon as a signpost to nearby NGC 4755, the ‘Jewel Box’. However, most observers therefore miss out on one of the loveliest visions in the heavens. Imagine a dazzling diamond on a backdrop of black satin. There is a rich, deep red incandescent ruby of the 9th magnitude within its glow. Truly a breathtaking surprise. The larger the mirror, the better the effect. Nicknamed 'Ruby Crucis', this carbon star fluctuates in brightness and is at it's most impressive when at minimum, thereby enhancing the deep ruby colour. See image by John Glossop.

Gamma Crucis or Gacrux lies at the apex of the ‘Cross’, an orange giant of magnitude 1.63. With a spectral classification of M3 and a distance of 220 light years, it has a luminosity of 900 Suns. Gamma also has an optical companion called SAO 240022 of magnitude 6.4. An A-type star, its white colour is in lovely contrast to that of orange gamma.

The fourth bright star of Crux is delta, a white magnitude 2.82 diamond. It lies on the western arm of the Cross but distance values differ between 250 and 570 light years. If it is this last figure of 570, then delta shines with the brightness of 1,900 Suns. Between delta and alpha lies little epsilon, overshadowed by its apparent near neighbours. A class K yellow star of 3.59 magnitude, it lies about 59 light years away.

Many city dwellers have never seen the dark obscuring nebula the ‘Coal Sack’. This pear-shaped darkness, 7º x 5º in size, lies only 400 light years away and absorbs the light of the background stars, appearing like a hole in the heavens. Until the 1930’s, this was thought to be the case. It was referred to by early European voyagers as “Magellan’s Spot” and “Black Magellanic Cloud”. In Peru it was a heavenly doe suckling its young.

The Aborigines of Australia saw it as an evil spirit in the shape of an emu who has cornered a frightened opossum in the branches of a tree, represented by the Southern Cross. The ‘Coal Sack’ is the head of the Emu complete with beak, while a long slender neck arches gracefully past alpha and beta Centauri, the ‘Pointers’. It then opens out into a large body among the star fields of Norma and Ara and to complete the illusion, it even has two long legs. The Emu needs a nice dark sky to see well and once you have seen it you will never forget it.

A good test of sky clarity is to detect with the naked eye a magnitude 6.5 star CPD 3376 that shines within the borders of the Coal Sack. There is a fainter star also nearby but it needs a really good sky to detect.

There are a number of open clusters in Crux, many of them smallish and loosely concentrated but worth the hunt. I will, however, have to describe one of these clusters without reservation. The beautiful ‘Jewel Box’, NGC 4755, lies 1.5º south-east of beta Crucis. It shelters within its borders around 70 stars including the 6th magnitude family member, orange-red kappa. Sir John Herschel thought it “a gorgeous piece of fancy jewellery”.

To show the colours of these stars that inspired Sir John, a large aperture is needed. Then the observer may see red, orange, white and possibly greenish gems inlaid amongst a field of faint stars.

A small telescope gives a different form to this group. The brighter stars only are seen with the 6 brightest stars forming the shape of a figure A, orange-red kappa on the central bar. Most of these suns are of spectral class B and have a luminosity in the order of thousands of times our Sun. The ‘Jewel Box’ is thought to lay 7,700 light years distant and may only be a few million years old. A nearby star HD 111613 lies 10’ from NGC 4755 and may be a member. If this is so, then this A-type supergiant of 5.74 magnitude has an incredible luminosity of over 80,000x our Sun!

Good luck and good hunting.

Credits:

NGC 4755 'Jewel Box' ~ Credit & Copyright Michael Bessell (RSAA, ANU), MSO