Mississauga Centre RASC

82nd Meeting

                                                             Speakers’ Night       

Day:                Friday April 13, 2007

Chair:                Randy Attwood

Speaker:          Marshall McCall

Randy Attwood showed the NASA video “This Week at NASA” detailing space weather, solar bursts affecting the GPS system, the Marshall Space Flight centre, the STS-1 and Apollo 13 anniversaries, and an educational clip about infrared radiation. 

The Chemistry of Naked Galaxies

Professor Marshall McCall, Chair of the department of Physics and Astronomy at York University has worked at the Dominion Astrophysical Observatory in Victoria B.C., and at Mt. Stromlo and Siding Spring in Australia. His research interest is in the structure and evolution of galaxies. He spoke about the chemistry of naked galaxies.

Professor McCall first outlined the three fundamental units of astronomy:  i) the zork being a body that shines by reflected light, ii) a star which is a body shining by its own light, iii) a galaxy which is a collection of stars.  In addition, there is the interstellar medium.  He described the distance time scale in light travel time, the colours and intrinsic brightness of stars.  A galaxy contains hot and cold gas and is held together by the combined gravity of all the components. The appearance of a spiral galaxy, shaped like a pancake, is dependent on the viewing angle and has a very low motion perpendicular to the plane i.e. it is dynamically cold.  Hot stars burn quickly , are blue in colour and indicate youth whereas red stars are small or old. In the absence of blue stars, the presence of red stars alone indicates old age.   In spiral galaxies, the disc is blue with young stars. In contrast, the bulge is red with old, cool stars and dynamically hot i.e. with motion out of the plane of the galaxy.  The bulge is hence distinct from the rest of the galaxy.

The satellite galaxies of the Andromeda galaxy, M32 a dwarf elliptical, and NCG 205 a dwarf spheroidal have no gas and no disc and are dynamically hot. A giant elliptical like M87 in the Virgo cluster with 100 billion stars is also a dynamically hot system (DHS). In the Virgo cluster, hydrogen gas is located in spiral galaxies and in between galaxies as hot gas.

A naked galaxy is missing interstellar medium. DHS’s are therefore elliptical galaxies, bulges of spiral galaxies or dwarf structures.  The question is then: why no gas? Was is converted to stars or thrown away?

As heavy stars go through their evolution, they develop an iron core and surrounding shells. The core collapses into a supernova remnant (SNR) which is enriched in heavy elements. A less massive star eventually forms a white dwarf  with a surrounding planetary nebula (PN) that goes into the interstellar medium.  The big difference between the two is that the SNR material is enriched in comparison to the medium from which the star formed, whereas the PN returns the same material as that from which it formed.  Professor McCall graphically demonstrated using coloured water how supernovas but not planetary nebulae enrich the medium around them.  Planetary nebulae shine especially well with OIII light (from doubly ionized oxygen) that can be detected in distant galaxies. When many galaxies are studied, there is a direct relation between the oxygen abundance and galaxy mass implying that the closed-box model (where nothing leaves or enters the system) doesn’t hold.  Therefore DHS’s must have blown away their gas, not converted it to stars.  In the virial theorem, the kinetic energy and potential energy equal zero.  Gas loss caused by a supernova-driven winds decreases the gas in galaxies over time.  If the temperature is greater than a certain value, gas will blow away and a naked galaxy result.  A little galaxy needs only a small amount of energy to blow away the gas, so its enrichment is poor compared to a heavy galaxy.  PN allow us to examine naked galaxies prior to the wind and enable us to study whether the gas between galaxies is formed from gas of naked galaxies. 

 Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to Miss Centre. meeting reports page
Mississauga Centre RASC