Wednesday, 4 April 2012

TYC 4948-53-1 group - The Virgo Diamond-asterism

Roger Ivester of the LVAS (Las Vegas Astronomical Society) has brought to my attention a tiny but dazzling little asterism in Virgo. Roger told me in an email that the group first made its appearance in an old Sky&Telescope magazine (May 1993) but has apparently evaded proper attention ever since. I usually don't get too excited about asterisms with such few members (they lack the visual impact at the eyepiece) but based on what I've seen so far, I'm sure this will be an exception. Not having looked at this group through a telescope yet, I'm fairly certain you, I or anybody won't be disappointed by its subtle beaty.

To be sure this was just an asterism, I contacted the big kahuna Matthias Kronberger and asked for his expert opinion on it. He used VIZIER for the proper motion data and came to the same conclusion I did using Aladin (PPMX-data). So, the chance of stars simply aligning like this in the sky might seem remote (4% according to Brosch) but none the less the group is, unfortunately, just an asterism despite having similar spectra. A shame!

Virgo Diamond courtesy of The Digitized Sky Survey
The asterism is located between Gamma (Porrima) and Eta (Zaniah) Virginis. The brightest star in the group is 10.7 magnitude TYC 4948-53-1 with other four being magnitudes 12.3, 13.2, 13.2 and 13.7 and respectively. What makes this asterism curious and stand out is the diamond or square-shape and its tiny size (50"). The separation between the stars is - on average - only 30" so good seeing conditions and higher magnification is required to resolve the four main components into individual stars. Seeing the 5th star may also require larger aperture. I'll personally try to observe this grouping over the upcoming weekend - despite the near full moon and medium low altitude of 28°.

On the night of 4th of April I went after the object like predicted. The conditions were everything but favorable: SQM-L showed 16.70 from Virgo (note that the meter most certainly had some interference from the nearby 94% moon) and 17.70 measurements from the opposite direction. Weather conditions were as follows: 70% humidity, air pressure 1022 hPa and temperature -2°C (28F). My trusty old 8" Orion DSE was selected for the task for the obvious reasons. It came as no surprise that the focuser was yet again nearly frozen solid and I'd have to use so force to get the things moving again.

TYC 4948-53-1 group with 8" Orion DSE @ 200x (6')
Anyway, finding the correct position using Uranometria was not difficult but seeing something was. I had to wait closer to midnight for Virgo to rise high enough for me to finally give the asterism a real go. At first I only noticed a fairly faint star in the correct position using Baader Hyperion Zoom @ 16mm (75x - 54'). This was no doubt the combined glow of the stars in the tiny asterism. Zooming in more, @ 8mm (150x - 27') I noted two stars very close together and then a few times now and again - with optimal averted vision - I could see the two fainter stars come out of the shadows and just as quickly vanish back there again. I had the similar effect using the 6mm Baader Ortho eyepiece providing 200x (6'). Seeing two 13.2 magnitude stars under such abysmal conditions was a great victory and the asterism reminded me of Theta 1 Orionis cluster (Trapezium) buried inside the hazy heart of Orion nebula. This object is surely worth a visit and has a calculated total magnitude of (using NOMAD data) 10.4.