Reports - 2012
Astronomy Benalla General Meeting Presentations - Wednesday 18th April 2012
The Constellation Virgo.
Presenter: Jeff Knight
In April, long before the sun sets, the constellation of the young maiden rises directly from the east, and by 8:00 pm, when it
is sufficiently dark, Virgo is lying just above the horizon for all to seek her out.
Even though she occupies a huge part of the night sky (second in size only to Hydra!), she is difficult to find, as most of her
defining stars are only 3rd and 4th magnitude. Her principal star (Alpha Virginis) is known from antiquity as Spika (“Ear of
Wheat”), and is a 1st magnitude star, allowing us some point of reference to ‘find’ the lady.
Currently, we are greatly assisted in finding her by the presence of Saturn just a few degrees below Spika. Virgo’s body
presently stretches from below this planet, halfway out to Mars toward the north (Virgo’s head). The constellations of Leo
and Libra lie either side of her on the ecliptic.
Our earliest understandings of Virgo depict a beautiful young woman, renowned for her purity, and holding a ‘branch’ in her
right hand and ‘ears of wheat’ in her left. She is the only ‘lady’ among the twelve zodiacal constellations, bearing also the
wings of an angel as she travels her heavenly path.
The principal stars that make up this constellation are between 36 and 262 light years from us. Spika is the furthest, but as
it is a fast-burning, Spectral B class binary star, more than 2000 times brighter than our sun, it is by far the more prominent.
Porrima (Gamma Virginis) is nearly the closest at 39 light years, and is also a binary, but the twin F class stars have an
orbiting period of 120 years, in contrast to Spika’s binary orbital period of only four days.
The Virgo Cluster
This part of the heavens is exceptionally rich in deep-sky objects, and are almost exclusively galaxies. The count is
continuing, but there is estimated up to 2000 galaxies alone lying in a cluster just west of Vindemiatrix (Epsilon Virginis).
What I personally find fascinating is that this cluster extends precisely over the area depicted in Virgo as “The Branch”.
Charles Messier, in his day, did well to find and name 11 of these objects, which he concluded were nebulae, but only now
can we appreciate the extent and the reality of what these objects really are.
Galaxies of every shape, size and description lie in this cluster. - Spiral, Barred spiral, Elliptical, Lenticular, Irregular,
Compact, and everything in between. Many contain myriads of huge star clusters, some appear to be violently ‘ripping’
each other apart, and some contain mysterious and massive bodies that emit high-powered jets of energy far out into space
for thousands of light years.
The beauty and richness that lie in this patch of space known as “The Branch” is simply incredible, and beyond the
imagination of man to conceive.
The galaxies of this cluster are tens of millions of light years distant from us, but probably the more famous M104 lies just
west of Spika, near the left elbow of Virgo, and is not part of this cluster – The ‘Sombrero Galaxy’- below:
My thanks to Wayne and Les from our viewing group for allowing me to see first-hand this amazing wonder of deep space at
a recent night out at the old Mokoan Yacht Club (Winton Wetlands). Whilst it didn’t look as majestic through an 8” or 12”
Dobsonian as it does in a ‘Hubble’ photo, it was nevertheless a thrill to be seeing it ‘live’.
Virgo is a constellation that is mysterious, and has many treasures for those who are prepared to seek them. Finding her is
the first step, so get your bearings and start looking. Both Saturn and Mars will be very helpful over the next few months as
they draw together to meet at the feet of Spika.
For us in the southern hemisphere, April and May is an excellent time to get to know the ‘lady’.
-
Jeff Knight
Pictures from the Hubble Site
