Wednesday Colloquium


"Solving the puzzles of Milky Way"

Paweł Pietrukowicz (Astronomical Observatory, Warsaw University)

Our location among billions of stars of our disky Galaxy makes the studies on its structure and evolution extremely difficult. On-going large-scale optical and infrared galaxy surveys are on a good way to find answers to key questions.


"Distance to accretion discs; theory versus observation"

Jean-Pierre Lasota (Institut d’Astrophysique de Paris and NCAC, Warsaw)

Accretion discs around compact objects such as white dwarfs, neutron stars and black holes are ubiquitous and have been intensely studied in last 40 years. Their basic properties are quite well understood. In particular the instability which drives the outbursts observed in binary systems containing white dwarfs (« dwarf novae »), neutron stars and black holes (« X-ray transients ») has been clearly identified. This (thermal-viscous) instability is supposed to be present in accretion discs below some critical luminosity and for a long time this had been confirmed by observations: bright disc-containing systems never exhibit outbursts. In 1999 Hubble Space Telescope observations of the famous dwarf nova SS Cygni put this system at a distance at which according to theory it was too bright to have outbursts. I will describe the subsequent « fight » between theory and observations and its (happy for the model) outcome. Next, I will present the recent case of the outbursting black hole system known as HLX-1 which observations put in a galaxy at 95 megaparsecs (310 million light years) whereas the outburst models require it to be much closer to us, maybe even in our Galaxy. To conclude I will briefly discuss some psychological, sociological and philosophical aspects of these (apparent) conflicts between theory and observations.


"On Charles Babbage's difference engine and other old computers"

Andrzej Krasiński (NCAC, Warsaw)

The first (mechanical!) computer was designed by Charles Babbage in the first half of the 19th century. The work on the actual construction of a prototype was far advanced already in 1830, but the project was abandoned in consequence of organisational and psychological problems. In connection with a later version of this project, Ada Lovelace (daugter of George Byron) wrote her pioneering work. Today, she is considered to be the precursor of computer programming theory. In 1985, a team of engineers led by Doron Swade from the Science Museum in London began work on turning Babbage's design into a real machine. The calculator was completed and made accessible to the public in 1991, and in 2002 the construction of the printer was completed (also according to Babbage's design). Today, two complete copies of this machine exist and work (in shows for the public) in the Science Museum in London and in the Computer History Museum in Mountain View (California, USA). In the second