CRUX, The Southern Cross. This constellation is familiar to most Australians because of its presence on the Australian flag. Found high in our southern sky it is the smallest of the constellations. Some of the most stunning objects to observe here in the southern hemisphere are in this region.
Crux was part of Centaurus until late in the 16th century when navigators of the southern seas mapped it separately.
Alpha and Beta Centauri the front legs of Centaurus the Centaur (which surrounds Crux on three sides) point the way to the Southern Cross.
The Southern Cross played an important role in navigation for early explorers. It can be used to get a rough position of the South Celestial Pole by drawing a line joining Gamma Crucis (γ) to Acrux (alpha crucis) (α) and extending it 4 ½ times. You will end up in a position near the South Celestial Pole. Another method is to find the bright star Achernar in Eridanus, the South Celestial Pole is approximately half way between there and Crux.
The World’s first astronomers the Australian Aborigines included these stars in their legends, which gave them a level of confidence about the place of human beings in the universe. They used the stars to predict and explain natural occurrences in our universe. By watching the nightly movement of the stars and the gradual annual shift of the constellations they created a calendar based on the position of constellations in the sky.
Crux appears as a cross or kite shape in the southern sky and is one of the easiest constellations to find having 6 main stars.
Acrux ( a), the brightest (0.8mag) at the foot of the cross is a bluish white pair mags 1.3 and 1.7, 4.4 arc seconds apart. It is easily separated with a 75mm telescope at high magnification. I can split them without difficulty at 86 x magnification with my 5 inch (128 mm) refractor. The brightest of the pair is also a double but difficult to separate in small telescopes.
Beta Cru (b), Mimosa mag 1.2 is a brilliant blue white giant marking the end of the eastern limb of Crux. A crimson-red carbon star, EsB 365 mag 8.6 lies 2’.4 in pa 260° in the same field. Centre beta in your field of view and the carbon star is easily located at approximately 9 0’clock as a bright red pinpoint star. If you have difficulty seeing this, look slightly away so your vision is averted as your peripheral vision is more sensitive to faint objects. In your telescope, 86 x magnification should distinguish it clearly.
Gamma Cru (g) marks the head of the cross, this 1.6mag red giant has an unrelated 6.5mag companion.
Delta Cru (d) mag 2.8 a fainter blue white giant marks the western arm of the Cross.
Epsilon southeast of delta when Crux is upright in the sky, is a mag 3.6 orange giant star.
Mu Crucis (m) 5° anti-clockwise from gamma Cru, is a double star easily located with binoculars being a wide bright-white pair of mag 4.3 and 5.5
The pointers in the constellation Centaurus: alpha Centauri known as Rigel Kentarus (furthest from the cross) is the 3rd brightest star in the sky. –0.27 mag. Observed in small scopes as a yellow pair 1.4 and 1.0 mag that orbit each other every 80 years, 21 seconds apart. This star lies in the star system closest to our solar system and is one of the brightest stars in the sky.
Beta Centaurus, Hadar mag 0.6, a blue giant lies 100 times further away but emits 10,000 times more light than alpha
The Coal Sack Nebula is a dark irregular shaped nebula, approximately 7° by 4°, 400 light years away, covers approximately 30° of sky. One of the largest and densest dark nebulae, it is located in the south eastern corner of Crux just east of Alpha Crucis. Made up of dust and gas it hides thousands of stars beyond it.
This breathtaking patch of sky would be above you were you to stand at the South Pole of the Earth. On the upper left of this image are the four stars that mark the boundaries of the famous Southern Cross. At the top of this constellation, also known as The Crux, is the orange star Gamma Crucis. The band of stars, dust, and gas crossing the middle of the photograph is part our Milky Way Galaxy. Just below the Southern Cross on the far left is the dark Coal Sack Nebula, and the bright nebula on the far right is the Carina Nebula. The Southern Cross is such a famous constellation that it is depicted on the national flag of Australia. (Extract from http://apod.nasa.gov/apod/ap080707.html)
The Jewel Box, NGC 4755 is one of the most beautiful open clusters in the night sky. It is a hazy 4th mag object visible to the naked eye. In binoculars it appears as a small tight cluster of stars located between beta Crucis and the Coal Sack making it easy to find. Multicoloured with more than 50 stars, Kappa Crucis an orange-red giant nestles near its centre. Binoculars should resolve at least nine of its stars. In small scopes this cluster is spectacular in colour and brightness. The brighter stars forming an “A” with the smaller red, orange and blue stars settled around it. At 86 x magnificaton using a 12 mm eyepiece in my 5 inch refractor, the central bar of 3 stars forming the “A” appeared from centre outwards red, yellow and blue. With a sprinkling of bright pinpoint stars surrounding the brighter 6 to 7 mag stars, it is brilliant and certainly a box of scattered jewels in the sky.
The great variety of star colors in this open cluster underlies its name: The Jewel Box. One of the bright central stars is a red supergiant, in contrast to the many blue stars that surround it. The cluster, also known as Kappa Crucis contains just over 100 stars, and is about 10 million years old. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 6,400 light-years away, so the light that we see today was emitted from the cluster before even the Great Pyramids in Egypt were built. The Jewel Box, pictured above, spans about 20 light-years. (Extract from: http://apod.nasa.gov/apod/ap100817.html)
There is also a multitude of objects within Centaurus to explore here are a few to locate (See Centaurus for more information).
Omega Centauri, NGC 5139 is the largest, brightest and one of the nearest (at 17,000 ly) globular clusters in the sky. At mag 3.7 approximately 20’ across it can be seen as a fuzzy ball to the naked eye. With more than a million stars to light it, it is spectacular in view through a telescope. Sometimes described as “like a ball of fire”, this cluster is more condensed near the centre with a powdering of stars as it spreads out. A great object for small and large telescopes. It is best observed with a magnification low enough so the complete cluster fits into your field of view. It can be located by visualizing a triangle (see star map above) using beta Centaurus (Hadar) and beta Crucis (Mimosa) to form the base and Omega Centaurus sits on the top of the triangle, (approximately 3 times distance between Acrux and beta Crucis).
This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, Omega Centauri is the largest, containing over ten million stars. Omega Centauri is also the brightest globular cluster, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye. Cataloged as NGC 5139, Omega Centauri is about 18,000 light-years away and 150 light-years in diameter. Unlike many other globular clusters, the stars in Omega Centauri show several different ages and trace chemical abundances, indicating that the globular star cluster has a complex history over its 12 billion year age. (Extract from: http://apod.nasa.gov/apod/ap130501.html)
Centaurus A, NGC 5128: Now that you have found omega (ω) Centaurus, use it as a marker to locate this 7th mag. giant elliptical galaxy, a famous southern radio source. In good skies it should be visible with binoculars with 75mm scopes showing it clearly. You will need at least 100mm is needed to outline the galaxy and observe the dark bisecting dust lane. In my 128 mm refractor, at 80 to 100 x magnification, I could see a distinct dark bar across the bright round luminous haze. Centaurus A should be found above Omega Centaurus at the top of the triangle approximately 1 degree away from alpha and beta Crux.
The closest active galaxy to Earth, Centaurus A is only 11 million light-years distant. Spanning over 60,000 light-years, the peculiar elliptical galaxy is also known as NGC 5128. Forged in a collision of two otherwise normal galaxies, Centaurus A’s fantastic jumble of young blue star clusters, pinkish star forming regions, and imposing dark dust lanes are seen here in remarkable detail. The colorful galaxy portrait was recorded under clear Chilean skies at the Cerro Tololo Inter-American Observatory. Near the galaxy’s center, left over cosmic debris is steadily being consumed by a central black hole with a billion times the mass of the Sun. As in other active galaxies, that process likely generates the radio, X-ray, and gamma-ray energy radiated by Centaurus A. (Extract from: http://apod.nasa.gov/apod/ap120404.html)
NGC 3918 “The Blue Planetary” is a bright pale blue magnitude 8 planetary nebula near the western border of the Southern Cross in Centaurus. It is bright enough it should be visible in good seeing with a 50 mm telescope. With the Celestron 8 inch at 75 x magnification it appears as a small bluish disk of approximately 12˝. Burnham describes it as “a tiny blue disc very like Uranus”.
NGC 4945 a near edge on mag. 9 spiral galaxy 15´ x 1.5´ is worth a look. Visible as a faint streak in a small telescope. It formed a narrow luminous haze in the C8 at 75 x magnification. It is located southwest of Omega Centauri.
Large spiral galaxy NGC 4945 is seen edge-on near the center of this cosmic galaxy portrait. In fact, NGC 4945 is almost the size of our own Milky Way Galaxy. Its own dusty disk, young blue star clusters, and pink star forming regions standout in the sharp, colorful telescopic image. About 13 million light-years distant toward the expansive southern constellation Centaurus, NGC 4945 is only about six times farther away than Andromeda, the nearest large spiral galaxy to the Milky Way. Though the galaxy’s central region is largely hidden from view for optical telescopes, X-ray and infrared observations indicate significant high energy emission and star formation in the core of NGC 4945. Its obscured but active nucleus qualifies the gorgeous island universe as a Seyfert galaxy and likely home to a central supermassive black hole. (Extract from: http://apod.nasa.gov/apod/ap130123.html)
NGC 5286 is a small 7.6 mag., bright globular cluster southeast of Omega Centauri approximately 2.5´ across. In the C8 it is a nice object with a condensed centre and scattered outlying stars.
Sources used: Hartung’s, D. Malin & D. Frew 1995; The Southern Sky Guide, D Ellyard & W. Tirion, 2001; Collins Pocket Guide – Stars & Planets 1993; Sky Watching, D. Levy 1996, Burnham’s Celestial Handbook 1966.
All information on aboriginal legends from “Explorers of the Southern Sky – a history of Australian Astronomy – Raymond Haynes et al 1996.