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Location: Chemistry/Physics Building, MUN
Time: 8:00 pm
Present: 19
Our last meeting for the summer will be the third Wednesday of June. Star-B-Que will be on August 12-14 this summer.
Robert reported on an observing session of about four people at Butterpot. There was bad seeing, but they managed to view Jupiter and Saturn and a blue-green fireball. In addition, they viewed about 15-20 deep-sky objects.
For the sky next month, Jupiter and Saturn will not be in their best positions, slowly disappearing from view. Mars is an early morning object. Uranus, Mars, and Neptune will be close to one another. Gamma Virgo, an interesting binary, will be near Jupiter. The Virgo Galactic Cluster will be visible this month, as well. Robert hopes to have another observing session sometime around June 6, near the new moon.
Chris Stevenson gave us a talk on Nebula, including information on how they relate to the different nebular filters available. Chris noted there are three different types of nebula: emission, reflection, and planetary. We also commonly use a number of different filters at the eyepiece: O-III, H-Beta, and Cyanogen.
Chris quickly reviewed the history of Astronomical Spectroscopy, including Dr. Wollaston, Fraunhofer, Sir W. Huggins ([OIII] Cat's Eye Nebula), Henry Draper, and W.H. Wright. We were also treated to the poem "Forbidden Emissions".
Forbidden lines arise because of the low pressure in the interstellar medium. At earth pressures, the atoms involved would be quickly collisionally de-excited (time between collisions is very short in a dense, high-pressure environment). Because of the long time between potential collisions, this allows a low-probability transition to occur that ordinarily would never occur at high pressures and densities due to a lack of time.
The forbidden lines, through ratios of intensities and densities, can tell us things about the chemical enrichment and the temperature of the nebula in question.
[O-III] confusingly enough really refers to oxygen that is only doubly ionised.
The [O-III] transitions cool nebular gases to 5 - 15000 K. Most of the energy in emission nebula is dissipated in the O-lines. Ironically, this leaves these lines fairly faint.
Reflection nebula are blue, due to the scattering of starlight. Sometimes you can get a blue glow due to synchrotron radiation in Supernova remnants such as the Crab Nebula.
So, generally, an O-III filter is useful for most emission nebula. An H-Beta filter is useful on a few emission nebula that have low-ionisation conditions. In these cases, the hydrogen Balmer lines will dominate (eg. Horsehead Nebula).
A question and answer session followed.
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