Stars, Galaxies & Cosmology

Modern astronomy spans scales from planetary systems to the observable universe itself — 93 billion light-years across. This pack covers stellar physics, galactic structure, and cosmological principles that form the backbone of astrophysical understanding.

## Stellar Formation & Evolution

Stars form in GMC (giant molecular clouds) — cold (10-20K), dense regions of molecular hydrogen and dust spanning 15-600 light-years. Gravitational collapse begins when a region exceeds the JM (Jeans mass) — the threshold where gravity overcomes thermal pressure. Fragmentation produces multiple collapsing cores simultaneously, which is why most stars form in clusters.

A collapsing core becomes a PMS (protostar) when central temperature reaches ~2000K and deuterium fusion begins. The PMS accretes material from its surrounding ACD (accretion disk) while driving bipolar jets along the rotation axis. T Tauri stars (low-mass PMS) show strong variability and emission lines.

MS (main sequence) begins when core temperature reaches ~10 million K and hydrogen fusion via the PPR (proton-proton chain) or CNO (carbon-nitrogen-oxygen cycle) stabilizes HSE (hydrostatic equilibrium) — gravitational contraction balanced by radiation pressure. The MS lifetime depends critically on mass: a 0.5 M☉ (solar mass) star burns for ~80 Gyr; our Sun ~10 Gyr; a 10 M☉ star only ~20 Myr. The mass-luminosity relation L ∝ M^3.5 explains why: massive stars are enormously more luminous and exhaust fuel rapidly.

The HRD (Hertzsprung-Russell Diagram) plots LUM (luminosity) versus T_eff (effective temperature/spectral class). The MS runs diagonally from hot, luminous O-type stars (upper left) to cool, dim M-type stars (lower right). Post-MS evolution creates branches: RGB (red giant branch), HB (horizontal branch), AGB (asymptotic giant branch).