Other Minutes: All Previous Next
Location: Chemistry/Physics Building, MUN
Time: 8:00 pm
Present: 20
1. Randy's Introduction. Randy brought us up to speed on solar activity over the past month or so, including visiting http://www.spaceweather.com . Our December meeting will be early, on December 10. Randy also passed around a list of astronomically-minded Christmas gifts that can be had from merchants around town. He and Gary Case worked on this. This will be discussed after the talk by Dr. Lewis.
2. Observing.
3. Doug Grouchy. The Moon Minute. Doug explained some of the recent goings-on with the moon, starting with an explanation of the phenomenon of libration. This allows us to view some features on the lunar surface ordinarily hidden to us. Doug concentrated on Mare Humboldtianum and three nearby craters: Endymion, Atlas, and Hercules. Doug also explained what all the red and black dots in Sky and Telescope and our Handbook maps were all about (areas where libration will occur).
4. Fred Smith introduced Dr. Lewis. The talk is on High Energy Astronomical Sources. You can find out more about Dr. Lewis at: http://www.physics.mun.ca/~court/ .
High Energy sources are those that are extremely luminous for their size. Usually, they are detected through large numbers of high energy particles, such as X and Gamma-rays. For the non-explosive case, the energy is believed to come from matter in an accretion disk hitting a compact object. This can vary in size from a dwarf star (dwarf nova), a neutron star, a stellar mass black hole or an Active Galactic Nuclei (AGN). The brightest sources can output upwards of 10,000 times the Sun's total luminosity, just in the X-ray part of the spectrum alone.
With AGN, there are several kinds that we can be concerned with. A Seyfert galaxy gives off power over a wide frequency range, continuing into the X and Gamma-ray region. The emission lines produced show both broad and narrow components. There are many lines from highly ionized states. Radio galaxies are powerful in the radio part of the spectrum. Normally, there is a two-lobed pattern, corresponding with the jets from the central compact object. Quasars can be radio loud or quiet. All AGN produce broad spectral lines, compared with stellar lines (narrow).
Dr. Lewis then went over some simple calculations to show that the energy released by matter falling on a compact object is much greater than that which can be realised through nuclear fusion. This explains how all these objects with accretion disks can have such prodigious energy outputs. In reality, any such calculation of accretion infall energy would have to take into account the spiral inflow of the matter, losses to friction and heat, and often, how excess matter is "squirted out" through jets, before it can fall on the compact object.
Next, we are taken on a tour of a number of web pages, mostly from Bill Keel of the University of Alabama. We have a look at what a source of each type may look like: http://www.physics.mun.ca/~court/rasc/Talk.html . There followed a brief question and answer session. One interesting question from Gary Oakley was whether a neutron star could accumulate enough mass to turn into a black hole. Dr. Lewis noted that it could, but you would need a lot more mass and there is no known case. Something perhaps more likely would be a white dwarf accreting until it reached Chandrasekhar's limit (1.4 solar masses), producing a type I supernova, leaving a remnant neutron star or black hole.
5. Coffee.
6. Randy and Gary Case went over their Christmas shopping list. Randy noted that Calendars, BOGs, and Observer's Guides would be for sale.
Other Minutes: All Previous Next
Please send any additions or corrections to:
This document was generated using AFT v5.0793 and Vim on: Thu Nov 27 13:40:40 NST 2003