Simultaneous photographic observations in Hα, and photoelectric observations in Hel γ 10830 line are analyzed for two flares of importance sf and 1B. For the estimation of physical conditions in the chromosphere of flares the non-LTE calculations of the Hα and γ 10830 line profiles were made for a number of model atmospheres. Comparing the observed values of Hα and γ 10830 line intensities vvith calculated ones, we derived the temperature
changes during the development of active formations for two levels of chromosphere (Ihe levels of Hα and γ 10830 line formation).
Theories of the origin of the Oort ctoud are examined in the light of recent observations of comets and of star-forming environments, and some popular hypotheses are found to meet with difficulties. In particular chemical and experimental evidence that comets grow in an extremely cold, quiescent environment is proving difficult to reconcile with recent CO and IR observations showing that the environment of a star-forming region is characterised by turbulent,
high-velocity flows and that young stars are prone to recurrent, violent outbursts. The aggregation around young stars of
planetesimals pre-existing in molecular ciouds avoids these problems. However formed, the Oort cloud is disturbed through
interactions with its galactic environment. A record of these past disturbances, episodic or regular, is in principle recoverable through impact cratering and other geological signatures, and these terrestrial records therefore provide a further new constraint on the structure and evolution of the Oort cloud. The concepts of
r 'dense inner cloud' or a 'solar companion star' are difficult to reconcile with the impact cratering history. The debris from the very largest comets are expected to play a dominant role in producing galactic modulations of such fundamental phenomena as the rise and fali of oceans, ice ages, geomagnetic reversals and the origin of life. A ~15 Myr galactic cycle in particular is predicted. Power spectrum analyses applied to cratering, vulcanism and geomagnetic reversal records for the last -200 Myr reveal the presence of a 16 ± 2 Myr cycle.
About 200 observations from AD66 to 1910 of the tail length of Comet Halley have been used to derive the mean tail length of the comet as visible to the naked eye under very good observing conditions, The curve, covering an interval of -45-^ (t-T)^ 80 days, is skewed and peaks at -SS million km for (t-T) secular decrease of the tail length.
We present the preliminary results of a study of the solar prominences observed at Lomnický Štít coronal station with the 20 cm coronagraph during the ascending phase of cycle 22. The observational material was processed and a catalogue of prominences was prepared. On the basis of this catalogue some statistical results were obtained. They concern mainly the latitudinal, longitudinal and time distributions of the prominences and their basic morphological characteristics.
After some general remarks, a few observed time-scale in stars discussed in this colloquium are presented. Characteristic times in stellar models for the internal structure and the outerlayers are
reviewed. Examples of mode Identification are discussed. In the low frequency domain, the situation is quite simple. But the high frequency region is more difficult to decipher as many phenomena occupy the same domain: orbital motion, rotation of a non-uniform body, spheroidal g modes and Rossby waves. The linear framework is not always relevant and the nonlinear dynamic is more difficult to characterise. In general timescales alone cannot permit a mode idenfitication. Additional informations on energy content, amplitudes, coherence, multiplicity, stationarity are necessary to discriminate among the different possibilities.
High resolution UV spectroscopic observations of the binary symbiotic star AG Peg obtained with the IUE satellite in the periode 1978-81 are analyzed. The variability of the fluxes and the radial velocities of emission lines accordingto orbital phase are presented, The observaions support a binary model with mass transfer from a hot rotationaly unstable subdvarf to a cool M giant.
Cliromospheric activity is found in ten types of stars, both single and binary. Variability at many time scales is reviewed, from minutes to centuries. The emphasis is on what is not understood, incorrecty understood, and understood but not appreciated. Observational results in hand include photometry, spectroscopy, spectrophotometry, speckle interferometry, magnetic flelds, and orbltal period variations. Certain physical mechanisms responsible for variability are discussed, making the distinction between the well-established, tlie suspected, and the posslble. Specific topics
include flares, rotation, differential rotation, synchronization, circularization, starspots, spot models, migratlon periods, spot cycles, magnetic activity cycles, sector structure, Maunder minima,
pulsation, light curve solutions, convective envelope changes, mass transfer, mass loss, and orbital period variations. Specific directions for future research are identified.