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Download Crux The Southern Cross
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Crux The Southern Cross Novice Observing Challenge – May Target Stars for May Star Designation Acrux Alpha Crucis Target Constellations for May The constellation of Crux reaches culmination at 10pm in May. Found high in the sky and due South. With the popular name of “Southern Cross” it takes pride of place on our Australian flag. It also features on the national flags of 4 other countries Crux is the smallest constellation in the sky. Note that the two “Pointers” don’t point quite directly at Crux. They point slightly above the top of the cross shape – just above Gacrux. (AY crux) Mimosa Beta Crucis (Mim mo sar) Gacrux Gamma Crucis (Ga crux) Finding Crux In May Crux is due south and high above the horizon (about 65°) It is an obvious cross shape and at this time of year it stands right way up. It is easy to confuse Crux with the False Cross and Diamond Cross which are both nearby. Crux is smaller and has brighter stars. The False Cross is slightly skewed out of shape, and the Diamond Cross lack the fainter fifth (epsilon) star. Only the Southern Cross has two bright pointers showing the way. As Crux is always in the South it's a good icon to get your bearings. The stars in Crux start at the bottom of the cross with Acrux then Becrux (Mimosa) then Gacrux at the top of the cross. New Astronomers' Group Version 1.0 Horizon finder for Crux, and the big triangle in May at 10pm Astronomical Society of Victoria Page 1 of 5 AIN A0002118S 2012 Target Star Details Target Star In constellation Classification Colour Coordinates Distance Nearby finder constellation Nearby finder stars Acrux Crux, (Southern Cross) Double star Acrux A - White Acrux B - White RA 12h 26m 35.9 Dec -63° 05′ 56.73” 320 ly N/A Alpha & Beta Centauri, the Pointers Observing notes: At first you may think your finder scope or small binoculars are splitting this double. No, you are mistaking a fainter nearby star. With a separation of only 3.9 arc sec you will need a good 6-8inch telescope at high power to split Acrux. While my short 4” F/5 refractor certainly can't do it, but does hint at an unresolved double. Target Star In constellation Classification Coordinates Distance Nearby finder constellation Nearby finder stars Mimosa (Becrux) Crux, Southern Cross Single Star RA 12 47 43.3 Dec -59 41 19 352 Ly N/A Alpha & Beta Centauri, the Pointers Observing notes: From a distance of just over 350 Light years Mimosa still manages to rank 20 th of brightest stars. That makes for a really big star that is about 7 magnitudes (250x) brighter than our own Sun. In your finder or Binoculars you will spot the must see Jewel box cluster just to the East (left in May) of Mimosa. Target Star In constellation Classification Coordinates Distance Nearby finder constellation Nearby finder stars Gacrux Crux, Southern Cross Single Star RA 12 31 09.0 Dec -57 06 47 88 Ly N/A Alpha & Beta Centauri, the Pointers Observing notes: The orange colour of Gacrux really stands out (more so in Binos) and tells you this is quite different to the other main stars of Crux. It is listed as a much cooler “M” class star. If you placed this star at the same distance as Acrux and Mimosa some (4x further away) it would be some 16x fainter due to the inverse-square law nature of light, and probably would not be noticed at all by us Amateur Astronomers. New Astronomers' Group Version 1.0 Astronomical Society of Victoria Page 2 of 5 AIN A0002118S 2012 Sky Chart for Crux May at 10 pm Constellation Observing notes Please help us add more – see the Contribution Guide Naked eye Inner Suburban Sky - You will struggle to see the 5th star (Epsilon) of the Southern Cross. Outer Suburban Sky - 5th star of the cross usually visible if the sky is clear of haze and cloud. Dark sky - You know it's a good night when the Coal Sack is clearly visible. Here's a tip to help you remember star names. If you start with Alpha and go around in a clockwise motion the brightness of the main five stars are in order of brightness: Alpha, Beta, Gamma, Delta, and Epsilon. Finder/Binoculars Crux will not quite fit within the field of view and many fainter stars will show up with even modest aperture (finder). The main stars making up the cross are so bright they stand out like beacons so it's pretty easy to navigate the Cross and it's surrounds without getting too lost. 102mm (4") f5 Refractor The extra bit of aperture from binoculars to small scope really helps to bring out the colour differences between the main stars of Crux. Make a point of viewing all five of the main Crux stars. Note the colours, which are double, brightness differences, and anything else interesting you find. New Astronomers' Group Version 1.0 Astronomical Society of Victoria Page 3 of 5 AIN A0002118S 2012 Cloudy Night Reading Double Stars There are many stars in the sky that when viewed through a telescope appear as two dots. It is common for two stars to be locked together gravitationally to form a binary star system. Sometimes double stars may only appear close together from our vantage point on earth. If in reality they do not interact then we call this an optical double. It is possible, but rarer for 4 or 6 stars to be grouped into Computer simulation of Algol multiple star systems. In this case the stars are usually grouped into close binary systems that orbit each other. A system made up of three stars that all orbit each other at about the same distance is not stable. One star will be ejected from this sort of system given time. In 3 star multiple systems it will consist of a binary star orbited by a star that is much further out. Alpha Centauri is an example of this sort of 3 star system. There is a minimum distance between stars that any telescope will be able to split. This resolution limit can be calculated from the telescope objective size as 116/objective (mm). For example my 102mm (4”) telescope can theoretically resolve stars with a minimum separation of 1.137 arc seconds. But that is only under ideal seeing conditions that Telescope view of Castor you might get one or two nights per year. The atmosphere regularly plays havoc so it important to keep watching for 5-10 to witness those rare steady moments of steady air and catch a glimpse of double / triple magic. Here are some lists of southern sky double stars: http://www.skyandtelescope.com/observing/objects/doublestars/3073201.html Double star observing guide: http://assa.saao.ac.za/sections/doublestars/DoubleStarIntro_ASSA_1.0.pdf New Astronomers' Group Version 1.0 Astronomical Society of Victoria Page 4 of 5 AIN A0002118S 2012 Ruby Crucis Very close to Mimosa is a tiny but deep red star called Ruby Crucis. It is a type of star called a carbon star. These stars are cool, old giant stars that have collected a large amount of carbon in their outer atmosphere. This abundance of free carbon is responsible for their deep red colour. Binoculars do not give enough magnification to resolve it from a very bright neighbour in Mimosa. You can see this star in a small Mimosa with Ruby Crucis selected telescope. However you need a 200 - 300mm reflector before this star truly shows off its remarkable colour. It may look redder at the lower power as this will concentrate the light more and better excite the colour receptors in your eye. Over some 80years the two main stars of Alpha Centauri orbit each other once and their separation changes over this time. In 1958 the separation was only around 1-2arc seconds. When I first saw this at age four it was an easy split in a 2inch refractor at about 12arc seconds, and in 1980 the gap opened to its widest around 20arc seconds. Currently it’s under 5arc seconds and will complete the cycle sometime around 2038 when it will again be a challenging 1-2arc seconds. We only know this because of Astronomers have taken careful measurements of positional angle and separation over hundreds of years of observations through the telescope. What is an arc second? It is an measurement of angle on the sky. A circle = 360degress x 60=21,600arc minutes x 60=1,296,000arc seconds in a circle. How big is an arc second? Your finder scope shows about 5 degrees wide or some 18,000arc seconds, the moon about 3,600 with the biggest planet Jupiter at about 40. So a single arc second is a very, very, small angle indeed. Tip of the month (TOTM) How dark is the sky? The area around Crux has been used by the University of NSW to generate some star maps that allow you work out how dark your sky is. The faintest star you can see with the naked eye is called the limiting visual magnitude. Have a go at this yourself using the charts at: www.phys.unsw.edu.au/outreach/ssn_maps/StarCounts.pdf You can then enter your results into the University outreach web site at: http://www.phys.unsw.edu.au/outreach/ Select SSN and then enter results. Try this at various observing sites under different conditions – no moon, full moon, no cloud, light cloud etc. New Astronomers' Group Version 1.0 Astronomical Society of Victoria Page 5 of 5 AIN A0002118S 2012