Download Crux The Southern Cross

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.
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Horizon finder for Crux, and the big triangle in May at 10pm
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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.
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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.
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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
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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.
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