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).

## Post-Main Sequence

When core hydrogen is exhausted, the core contracts and heats while hydrogen fusion continues in a surrounding shell. The envelope expands and cools — the star becomes a RG (red giant). For stars <8 M☉: helium flash ignites triple-alpha process (3 He4 → C12) on the HB. After helium exhaustion, thermal pulses on the AGB eject the envelope as a PN (planetary nebula), leaving a WD (white dwarf) — a degenerate carbon-oxygen core supported by electron degeneracy pressure. The Chandrasekhar limit (1.4 M☉) is the maximum WD mass.

For massive stars (>8 M☉): successive fusion shells build an onion structure — H, He, C, Ne, O, Si — until an iron core forms. Iron fusion is endothermic — no energy release. When the core exceeds the Chandrasekhar limit, electron degeneracy fails, and the core collapses in milliseconds. The resulting bounce and neutrino energy deposition drives a CCS (core-collapse supernova — Type II, Ib, Ic).

The remnant depends on mass: NS (neutron star) for cores 1.4-3 M☉ (supported by neutron degeneracy, ~10 km radius, ~10^14 g/cm³), or BH (black hole) for cores >3 M☉ (Tolman-Oppenheimer-Volkoff limit exceeded, singularity forms behind EH — event horizon at Schwarzschild radius r_s = 2GM/c²).

PSR (pulsars) are rapidly rotating NS with strong magnetic fields (~10^12 G). Misaligned magnetic and rotation axes produce beamed radiation — lighthouse effect. MSP (millisecond pulsars) are old NS spun up by accretion from a binary companion.

Type Ia SNe: thermonuclear detonation of a WD exceeding the Chandrasekhar limit via accretion from a companion. Uniform peak luminosity makes them STD (standard candles) for cosmological distance measurement — crucial for discovering accelerating expansion.

## Galactic Structure

The Milky Way (MW) is a barred spiral galaxy: central BAR (~27,000 ly long), surrounding DSK (disk) with SPA (spiral arms — logarithmic, driven by density wave theory), thin disk (~1,000 ly thick, young stars, gas, dust), thick disk (~3,000 ly, older metal-poor stars), central BLG (bulge — old, metal-rich stars), and HAL (halo — globular clusters, metal-poor stars, DM).

DM (dark matter) dominates galactic mass. RTC (rotation curves) — orbital velocity vs. distance from center — remain flat at large radii instead of declining as Keplerian motion predicts. This implies a massive DM HAL extending far beyond the visible galaxy. MW total mass: ~1.5 trillion M☉; visible matter is only ~5%. DM candidates: WIMPs (weakly interacting massive particles) or axions — neither detected directly yet.

SMBH (supermassive black holes) occupy the centers of most galaxies. Sgr A* (the MW's SMBH): ~4 million M☉, imaged by the Event Horizon Telescope. The M-sigma relation — SMBH mass correlates with host galaxy bulge velocity dispersion — suggests co-evolution between galaxies and their central BH.

Galaxy classification (Hubble sequence): E (elliptical, E0-E7), S0 (lenticular), S (spiral, Sa-Sd), SB (barred spiral), Irr (irregular). Ellipticals contain old, red stellar populations with little gas; spirals have active SF (star formation) in arms. Galaxy mergers drive morphological transformation — major mergers can convert spirals to ellipticals.

## Cosmology

The Big Bang model rests on three observational pillars: Hubble expansion (redshift-distance relation), CMB (cosmic microwave background — 2.725K blackbody radiation from recombination at z~1100, ~380,000 years post-BB), and BBN (Big Bang nucleosynthesis — predicted primordial abundances of H, He, Li match observations: ~75% H, ~25% He by mass).

The Friedmann equations (derived from GR — general relativity) describe the expansion dynamics. Key parameters: H0 (Hubble constant — current expansion rate, ~67-73 km/s/Mpc depending on measurement method — the "Hubble tension"), Ω_m (matter density parameter), Ω_Λ (dark energy density parameter), Ω_k (curvature parameter).

The current concordance model (ΛCDM): Ω_m ≈ 0.31, Ω_Λ ≈ 0.69, Ω_k ≈ 0. The universe is spatially flat, 13.8 Gyr old, and expanding at an accelerating rate. The composition: ~68% DE (dark energy), ~27% DM, ~5% baryonic (ordinary) matter.

DE drives accelerating expansion, discovered 1998 via Type Ia SNe at high redshift being dimmer than expected (further away → universe expansion accelerated). The COS (cosmological constant) Λ is the simplest DE model — vacuum energy density constant in space and time. The "cosmological constant problem": quantum field theory predicts vacuum energy 10^120 times larger than observed.

CMB anisotropies (~10^-5 fluctuations) encode the seeds of all cosmic structure. The angular power spectrum reveals acoustic oscillations — sound waves in the primordial plasma. The first peak (~1°) constrains spatial curvature (flat). The second and third peaks constrain baryon density and DM density. WMAP and Planck missions measured these with extraordinary precision.

INF (cosmic inflation): a period of exponential expansion (~10^-36 to 10^-32 seconds post-BB) driven by a scalar field. INF solves the horizon problem (why is the CMB uniform across causally disconnected regions?), the flatness problem (why is Ω_k ≈ 0?), and the monopole problem. INF also generates the primordial density fluctuations (quantum fluctuations stretched to cosmic scales) that seeded LSS (large-scale structure).

LSS: galaxies are not randomly distributed. They form a cosmic web — filaments, walls, and nodes (galaxy clusters) surrounding vast voids. N-body simulations (e.g., Millennium, IllustrisTNG) reproduce observed LSS from initial CMB fluctuations evolved through gravitational collapse + dark energy expansion. Galaxy clusters (10^14-10^15 M☉) are the largest gravitationally bound structures.

## Distance Ladder

Measuring cosmic distances requires a chain of methods: parallax (up to ~10 kpc with Gaia), Cepheid period-luminosity relation (PLR — Leavitt's law, up to ~30 Mpc), Type Ia SNe STD candles (up to ~1 Gpc), and Hubble's law (v = H0 × d) for the most distant objects. Each rung calibrates the next. Systematic errors compound — contributing to the H0 tension between local (SH0ES: 73.0) and early-universe (Planck CMB: 67.4) measurements.