Introduction
Key points
- The Sun is important for us and was even more important spiritually in the past.
- The knowledge about the processes occuring in the Sun went hand-in-hand with the progress in physics, that includes the search for the energy source
- Fundamentals: mass 2x1030 kg, radius 700 000 km, luminosity 4x1026 W, metalicity 0.02 (textbook, maybe 0.012 following the recent development,
effective temperature 5770 K. Age about 4.5x109 years, "middle age".
- Inside: code, radiative zone, convective zone. Outside: photosphere, chromosphere, corona.
- Observations: many kinds. Many spectral regions, synoptic, high spatial and temporal resolutions available. 1" = 725 km in the photosphere
- Solar activity = bunch of phenomena connected to the variable magnetic field. Sunspots, prominence, flares, CMEs,...
- Solar activity in Sun-as-a-star observations: calcium and H-alpha emissions, UV and X-ray emission, X-ray flashes.
- Mt.Wilson's H-K project gives an indication that the Sun-like activity is common among Sun-like stars.
- Starspots: Doppler tomography allows to "resolve" spots on rapidly rotating stars. They exist.
- Corona: hot because of the magnetic field (details remain to be determined). Stellar coronae around cool stars also registered (YY Gem is a prominent example).
- Solar flares: rapid releases of the magnetic energy. X-ray rapid emission, particle beams, CME ejected to the interplanetary space. Particle beam responsible also for the
emission of the lower atmospheric layers -- emission even in the broad-band white-light continuum. Very strong flares = white-light continuum more prominent.
- Stellar superflares: rapid luminosity increases registered on other Sun-like stars, observed in the visible region of the electromagnetic spectrum. T Tauri and
RS CVn stars known for a long time and well understood. Standalone G-type stars have also superflares, which was a surprise at that time. These superflares have to do
with the existence of huge starspots (the frequency correlated to the rotation rate). The frequency dependency on flare energy is a power-law with N ~ Ea.
For the solar flares and also stellar flares a ~ -2.
- Stars are dynamical, some of the undergo pulsations. Sun-like oscillations: hydromagnetic (sound) waves (p-modes) travelling throughout the outer convection zone.
For the Sun: foundation of helioseismology around 1960. A possibility to indirectly learn about the solar interior. The frequency of p-modes depends strongly on the
internal stratification. The p-modes spectrum may be measured also for other stars by means of high-cadence photometry (the frequencies are in the order of mHz). Hence,
from these measurement, we may learn about the interior structure of other stars: Foundation of asteroseismology.
- Details on many of these sub-topics will be given in the following lectures.