Southern Cross Constellation

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The Southern Cross constellation is eagerly sought by travellers from the North, when visiting the Southern Hemisphere.

Southern Cross Digital Images

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A short exposure digital photograph with a Nikon Coolpix just shows the brightest stars. The Southern Cross (Crux) is very bright:

A longer exposure, digital photograph shows the Southern cross and its two Pointers.
Alpha and Beta Centauri point at the head of the Southern Cross.

The background stars and the nebula glow to the left of the coal sack have been blurred, so the brighter stars are not obscured.

To see the small stars sharply, go to the 1024 pixel image

15 seconds exposure with a Canon 12D digital camera running at 400 ASA, f2.8 105mm lens, in light-polluted Auckland city

Run your mouse over the next image to toggle the nebulosity and light pollution on or off.

Most of this "haze" is background stars of the milky way, which are not individually visible on this low-power photograph, or in your binoculars.

The two views are actually the same image, but processed in different ways to bring out either high contrast stars, or low contrast gas and dust, using a method developed by the author. The digital image processing is so powerful, it has partially overcome light pollution from Auckland City, where the astro-photograph was taken. Image processing in Photoshop makes short exposure astrophotography with ordinary digital cameras more revealing than studying the sky through binoculars.

Astrophotography by John Wattie, March 5th 2006.

See below why this star, just south of Acrux (alpha crucis), is interesting.
Can you find it in the version above (which is rotated slightly from true north/south and shows less stars)?
One of the challenges of astronomical observation is using star charts, or photgraphs, which show more or less stars than you can see, at a different angle from how you are observing.
With a dark sky and looking through night vision binoculars, so many stars are visible that even the bright Southern Cross stars can be almost overwhelmed, as here.
(7x50 or 11x80 binoculars are good for showing stars and nebulae brightly.The low power and big lenses of night vision binoculars send more light to your eyes from nebulae, comets and other extended objects than a high magnification telescope. The telescope's bigger lens or mirror is useful for point objects like stars or big bright objects like the moon. Look from the New Zealand country-side, away from city lights.)

Southern Hemisphere Stellar Show-piece,

In the next photograph the stars are thrown out of focus so as to spread the light and show the colours better
The amazing star colours are easily shown on long exposure digital colour photographs , but are barely recognised by the naked eye, because most stars are too faint to stimulate colour receptors in our retinas (cones).

The cross  has four main stars marking the tips (alpha, beta, gamma and delta).
These four stars are on the New Zealand flag.

A smaller star (epsilon), separate from the cross, is included on the Australian flag.

Two bright stars, alpha and beta Centauri, are pointers to the head of the cross.

Alpha Centauri is a triple star. It has: a close double star plus a distant, faint, red star called proxima Centauri.  Proxima Centauri is the closest star to our solar system.

Kappa Crucis, to the left (east) of Beta Crucis is not a single star.
It is a star cluster, but at the scale of the main photograph the cluster cannot be resolved. On the small image you cannot even see the cluster.

Kappa Crucis is one of the more beautiful of the star clusters available for amateur telescopes, since it has a red star sitting among many blue stars. Because it is so attractive, this cluster is called "The Jewel Box." (NGC4755).

Red stars are often "Red Giants." Some giants are unstable and keep changing brightness. They may even pulsate. Epsilon Crucis is a red giant (but is not variable in brightness). Gacrux is a slightly variable red giant. You can easily find more red giants in the Southern Cross picture. By watching carefully through a telescope over the nights, you will find some are variables.
(Amateurs check variable stars by comparing stellar brightness (magnitude) with adjacent, stable stars. More advanced workers measure with photometers.)

• Coal Sack

• The coal sack is a dust cloud, obscuring stars of the milky way beyond it.
  The coal sack is the "head of the emu" in Australian Aboriginal astronomy. The emu is the only dark sky feature identified by early humans. All the other constellations and asterisms are bright. The emu and its head is actually dust in the central plane of the milky way galaxy.
  Southern Cross stars are so bright, they can be seen despite the glare of street lights.
• This Nikon Coolpix digital photograph only reveals bright stars and is more like what you see from a city. The coal sack is lost:
  

The long exposure Canon 5D digital photograph is so sensitive, the coal sack has begun to fill in with stars. Despite light pollution, image processing brings the coal sack back:

Map of the coal sac

Do not be confused by three crosses in the Southern sky.

• Only the smallest and brightest, with two pointers aimed at the head of the cross, is the true cross. The brightest star (alpha Crucis) is at the foot of the true cross. The pointers lie to the east.
  

• The diamond cross has no pointers and is fainter than the other two. The head of the diamond cross is just below (south of) a very bright part of the milky way, which includes numerous star clusters and the amazing eta carina nebula. Many clusters and the nebula are visible in binoculars. The head of the diamond cross (theta carina) is actually in a star cluster, which makes the head look fuzzy to the naked eye. In binoculars you can see many of the cluster stars around theta carina. (Carina star field image)
  

• The false cross has two pointers, but they point to the brightest star, at the foot of the cross. Also they point from the west, not the east.
   

• Remember - the crosses can be any way up as they circle the south pole. The photographs and diagrams show the crosses as seen in June / July, when they are highest in the sky during the evening.

Direction finding in the Southern Hemisphere

There is no "Pole Star" in the southern sky.

1. A line through the head of the Southern Cross (Gacrux), carried on through the bottom of the cross (Acrux) points at the South Pole of the Heavens.
2. The line runs on, between the greater and lesser Magellanic Clouds, until it hits a bright star called Achernar.
3. Now point your hands, one at Achernar and and the other at the closest of the pointer stars to the Southern Cross (Hadar).
4. Bring your hands steadily together until they meet. They are pointing at the Celestial South Pole.
5. Now drop your hands straight down to the horizon. That marks due South from where you are.

Extra-Solar Planets

Are there other intelligent life forms out there?

They cannot live on a star and so the first step for anwering this fascinating question is finding a planet suitable for life. Nearly 200 planets circling other stars have been found so far.

HD108147 is a star in Crux with a planet circling it. ESO

HD 108147. Star field.

This image, reduced in size, but still much more magnified than the Canon 5D Southern Cross photograph, is from Aladin sky atlas.
Central coordinates of the field (J2000):
RA: 12 25 46.2686
Dec: -64 01 19.516
According to the Aladin database, this must be the same star as TYC8983-384-1:
RA: 12 25 46.2
Dec: -64 01 19

HD 108147 has shown variations in its radial velocity to and away from earth. The velocity is measured by Doppler Shift in the star's spectral lines. The planet causing this wobble is much bigger than earth. The star, with its planet, lies just south of Acrux.
It is visible in binoculars and on our digital camera Southern Cross image:

Some Southern Cross Stars.
Positions are Epoch 2006

Links

Southern cross: from Hawaiian Astronomical Society map

Possum Observatory photograph of Jewel Box Cluster: NGC4755   LuizEGiorgi

Details about Crux constellation

For other stars in Crux, try Preoria Astronomical Society

For a very high detail view of Crux from a dark sky site in Africa. (Long exposure on film).
This shows so many small stars, you may be better off with
Alec Kennedy version: Guided Olympus Camera from a New Zealand dark sky site

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