Light travels at 299,792,458 meters per second (about 300,000 km/s or 186,000 miles/s) in a vacuum. In one year, light covers 9.46 × 10^12 km. For context: Earth to Moon is 1.3 light-seconds; Earth to Sun is 8.3 light-minutes; Earth to Pluto is 5.5 light-hours; Earth to Voyager 1 (2026) is about 22 light-hours. The entire Milky Way galaxy is about 100,000 light-years in diameter. The Andromeda Galaxy (M31) is 2.537 million light-years away – light we see from Andromeda left when early hominids (Australopithecus) lived on Earth. The observable universe has a radius of about 46.5 billion light-years (due to cosmic expansion, not 13.8 billion – the age of the universe). The most distant known galaxy (JADES-GS-z13-0) is about 32 billion light-years away (proper distance). Astronomers also use parsecs (1 parsec = 3.26 light-years) and astronomical units (1 AU = 150 million km). The light-year emphasizes that when we look at distant objects, we are looking back in time – the Hubble Deep Field shows galaxies as they were 13 billion years ago. The term "light-year" is commonly misused in science fiction, e.g., "light-years ahead" meaning far ahead, but it remains a distance measure.

Black holes were predicted by Einstein's general theory of relativity (1915), with Karl Schwarzschild finding the first exact solution (1916). The term "black hole" was popularized by John Wheeler (1967). Types: Stellar-mass black holes (3-100 solar masses) from supernova core collapse – Cygnus X-1 (first confirmed, 1971). Intermediate-mass black holes (100-100,000 solar masses). Supermassive black holes (millions to billions of solar masses) – Sagittarius A* (4.3 million M☉), M87* (6.5 billion M☉, first direct image by Event Horizon Telescope 2019). Primordial black holes (hypothetical, formed after Big Bang). A black hole's event horizon radius (Schwarzschild radius) is proportional to its mass: R_s = 2GM/c². For a 10 M☉ black hole, R_s ≈ 30 km. Spaghettification – tidal forces stretching objects into long thin shapes near smaller black holes. Hawking radiation (Stephen Hawking, 1974) – quantum effects cause black holes to slowly evaporate, eventually exploding. The first black hole image (M87*, April 10, 2019) showed a bright ring of light around a dark central shadow – light bent by extreme gravity. Gravitational waves (first detected by LIGO 2015) confirm black hole mergers (e.g., GW150914, 36 M☉ and 29 M☉ merging to 62 M☉, 3 solar masses converted to gravitational wave energy). Observational evidence includes: X-ray binaries (accretion disks heated to millions of degrees), orbits of stars around Sagittarius A* (Andrea Ghez and Reinhard Genzel, Nobel Prize 2020), and gravitational waves.

The Big Bang was not an explosion in space but an expansion of space itself. The universe has no center and no edge – every point is moving away from every other point. Georges Lemaître (1927) first proposed the expanding universe. Edwin Hubble measured galactic redshifts showing proportional distance-velocity relation (Hubble constant H₀ ≈ 70 km/s/Mpc). Fred Hoyle (who opposed the theory) coined "Big Bang" derisively in 1949. Cosmic Microwave Background (CMB) – leftover radiation from when the universe became transparent 380,000 years after Big Bang (recombination), temperature 2.725 K, discovered accidentally by Arno Penzias and Robert Wilson (Nobel Prize 1978). COBE satellite (1990) measured CMB blackbody spectrum perfectly; WMAP (2001-2010) and Planck (2009-2013) mapped CMB temperature fluctuations (differences 1 part in 100,000) – these fluctuations seeded galaxy formation. Inflation theory (Alan Guth, 1980) proposes a rapid exponential expansion 10^-36 to 10^-32 seconds after Big Bang, explaining flatness, horizon, and monopole problems. Timeline: Planck epoch (<10^-43 sec), Grand Unification epoch, Inflation, Electroweak epoch, Quark epoch (10^-12 sec), Hadron epoch (10^-6 sec), Lepton epoch (1 sec), Nucleosynthesis (3-20 minutes – creating 75% H, 25% He), Recombination (380,000 years – CMB released), Dark Ages (380,000-150 million years), First stars and galaxies (150 million-1 billion years), Reionization. The universe's composition: 68% dark energy, 27% dark matter, 5% ordinary matter (atoms). The ultimate fate depends on dark energy: continued expansion (heat death), or Big Rip if dark energy increases.

Fritz Zwicky (1933) first inferred dark matter in galaxy clusters (Coma Cluster) – visible mass was insufficient to hold clusters together by gravity. Vera Rubin (1970s) measured galaxy rotation curves – stars at galaxy edges orbit as fast as those near the center, implying additional unseen mass (flat rotation curves). Without dark matter, galaxies would fly apart. Evidence: (1) Galaxy rotation curves; (2) Galaxy cluster dynamics (Bullet Cluster gravitational lensing shows dark matter separated from ordinary matter in a collision); (3) Gravitational lensing (light bending around large masses); (4) Cosmic microwave background fluctuations (WMAP, Planck); (5) Large-scale structure formation (simulations require cold dark matter). Candidate particles: WIMPs (Weakly Interacting Massive Particles) – predicted by supersymmetry, mass 10-1000 GeV/c² – experiments (LUX, XENONnT, PandaX) have not yet detected. Axions (hypothetical particles solving strong CP problem, mass 10^-6 to 10^-3 eV). Sterile neutrinos (heavier than ordinary neutrinos). MACHOs (Massive Compact Halo Objects – black holes, neutron stars) were ruled out by microlensing surveys. Dark matter does not interact via electromagnetic or strong nuclear forces – only via gravity and possibly weak nuclear force. The European Gaia spacecraft maps dark matter distribution via stellar motions. Direct detection experiments (cryogenic detectors, liquid xenon) search for rare WIMP-nucleon collisions. The Large Hadron Collider (LHC) searches for dark matter particles produced in proton collisions. Understanding dark matter is one of the biggest unsolved problems in physics.

The Milky Way is one of the two largest galaxies in the Local Group (along with Andromeda M31). Structure: Galactic bulge (old red stars, supermassive black hole Sagittarius A*), central bar (about 27,000 light-years long), four main spiral arms (Perseus, Sagittarius, Norma, Scutum-Centaurus), and the Orion Spur (our Sun's location). The galactic disk is about 1,000 light-years thick. The Milky Way is surrounded by a spherical halo of old stars and globular clusters (about 150 known). The Sun orbits the galactic center at 828,000 km/h (514,000 mph), taking about 225-250 million years per orbit (galactic year). The last galactic year was when dinosaurs (Triassic-Jurassic boundary) existed. The Milky Way and Andromeda are approaching each other at 110 km/s and will collide in about 4.5 billion years, merging into a giant elliptical galaxy ("Milkomeda" or "Milkdromeda"). Current star formation rate: about 1.65 solar masses per year. The galaxy has about 1,500 globular clusters (only 150 cataloged because dust obscures the rest). The galactic coordinate system is centered on the Sun. The Milky Way's total mass is about 1.5 trillion solar masses (including dark matter). The observable universe contains about 2 trillion galaxies (Hubble Ultra Deep Field). Gaia spacecraft (2013-present) is mapping positions, distances, and motions of over 1.8 billion stars in the Milky Way.

Neutron stars are supported against further collapse by neutron degeneracy pressure (Pauli exclusion principle). Their escape velocity is about 0.5c (150,000 km/s). Surface temperature is about 600,000 K (100x Sun) at formation, cooling over time. The density is comparable to atomic nuclei (4×10^14 g/cm³). The structure: solid crust (10^11 g/cm³, iron nuclei and electrons), outer core (liquid neutrons, protons, electrons), inner core (unknown – possibly quark matter or hyperons). Pulsars – rotating neutron stars with magnetic fields 10^8-10^15 Gauss (up to 1 quadrillion times Earth's) that emit beams of radiation from magnetic poles. If the beam points toward Earth, we observe pulses (lighthouse effect). The first pulsar (PSR B1919+21) was discovered by Jocelyn Bell Burnell (1967), initially nicknamed LGM-1 ("Little Green Men"). The Crab Pulsar (PSR B0531+21) spins 30 times/second, slowing by 36.5 ns/day (observed slowdown confirms gravitational wave emission). Magnetars are neutron stars with extreme magnetic fields (10^14-10^15 G) – cause starquakes and gamma-ray bursts. Soft gamma repeaters (SGRs) are magnetars. PSR J1748-2446ad is the fastest known pulsar (716 Hz, 43,000 rpm). Neutron star mergers produce heavy elements (gold, platinum, uranium) and gravitational waves (e.g., GW170817, first observed neutron star merger with electromagnetic counterparts). The maximum mass limit for neutron stars (Tolman–Oppenheimer–Volkoff limit) is about 2.3 solar masses – beyond that, they collapse into black holes. PSR J0348+0432 (2.01 M☉) is the most massive confirmed neutron star.

Parallax is the apparent shift of nearby stars against background stars as Earth orbits the Sun. One parsec (parallax-arcsecond) is the distance giving 1 arcsecond shift over 2 AU baseline. The Gaia spacecraft measures parallax with 10 microarcsecond precision for 1.8 billion stars, extending accurate distances to 50,000 light-years. Cepheid variable stars pulsate with periods of 1-70 days; Henrietta Leavitt discovered (1912) the period-luminosity relation (longer period = intrinsically brighter). Comparing apparent brightness to known intrinsic luminosity gives distance. Cepheids were used by Edwin Hubble to measure distances to Andromeda (1923-1924), proving it was a separate galaxy. Type Ia supernovae (carbon-oxygen white dwarfs accreting mass from companion until reaching Chandrasekhar limit 1.4 M☉, exploding in a thermonuclear runaway) have remarkably consistent peak brightness – they are "standard candles." Observations of distant Type Ia supernovae (1998, Saul Perlmutter, Brian Schmidt, Adam Riess – Nobel Prize 2011) discovered the universe's expansion is accelerating (dark energy). Hubble's law: v = H₀ × d, where v is recessional velocity (redshift), H₀ is Hubble constant (≈70 km/s/Mpc). For distant galaxies, redshift measurements provide distance. Hubble constant tension: local measurements (~73 km/s/Mpc) vs CMB-based measurements (~67 km/s/Mpc) – a significant unsolved problem in cosmology. Other distance methods: main sequence fitting (star clusters), tip of red giant branch (TRGB), water masers (megamasers). The distance ladder explains why we can measure distances from nearby planets (radar) to the edge of the observable universe (redshift z≈10-15).

Detection methods: Transit (planet passes in front of star, dimming brightness by 0.1-1%). NASA's Kepler Space Telescope (2009-2018, 2,700+ confirmed exoplanets) used this method. TESS (2018-present) continues the all-sky survey. Radial velocity (Doppler spectroscopy) measures star's wobble from planet's gravity (51 Pegasi b discovered this way). Direct imaging (blocking starlight to see planet) – HR 8799 system (4 planets directly imaged, 2008). Microlensing (planet's gravity bends light from background star) – OGLE-2005-BLG-390Lb. Astrometry (precise star position measurements). Notable exoplanets: Kepler-452b (Earth-sized in habitable zone, 1.5x Earth radius, 1,400 light-years). TRAPPIST-1 system (7 Earth-sized planets, 3 in habitable zone, 40 light-years). Proxima Centauri b (nearest exoplanet, 4.24 light-years, 1.3 Earth masses, habitable zone but subject to stellar flares). LHS 475 b (TESS, Earth-sized, 41 light-years). HD 189733b (deep blue color from silicate clouds, winds 8,700 km/h). WASP-12b (egg-shaped due to tidal forces, being consumed by its star). Exoplanet atmospheres studied by spectroscopy (JWST, Hubble): detected water vapor, carbon dioxide, methane, clouds on WASP-39b, WASP-17b, TRAPPIST-1 planets. Biosignatures search: oxygen + methane (disequilibrium) and phosphine (controversial detection in Venus's clouds 2020). The habitable zone (Goldilocks zone) is where temperatures allow liquid water (0.5-1.5 AU for Sun-like stars). Super-Earths (1-10 Earth masses) and mini-Neptunes (2-4 Earth radii) are common but not present in our solar system. The Roman Space Telescope (2027) will use microlensing to find thousands of rogue planets (planets not orbiting any star). The ultimate goal: find evidence of extraterrestrial life via atmospheric biosignatures.

Comets leave dust trails along their orbits. When Earth crosses these trails, tiny particles (sand to pea-sized) enter the atmosphere at 11-72 km/s (25,000-160,000 mph). Friction with air molecules heats them to incandescence (1,600-3,000°C), creating visible streaks. Most meteors burn up at 75-100 km altitude. If any fragment survives to the ground, it's called a meteorite. Major meteor showers: Quadrantids (January, 120 meteors/hour at peak, parent body 2003 EH1 – possibly an extinct comet). Lyrids (April, comet C/1861 G1 Thatcher, 20/hour). Eta Aquariids (May, Halley's Comet, 50/hour). Perseids (August 11-13, comet Swift-Tuttle, 60-100/hour, known for fireballs). Orionids (October, Halley's Comet, 20/hour). Leonids (November 17-18, comet Tempel-Tuttle, 10-15/hour but spectacular storms (e.g., 1833, 1966) every 33 years – next storm predicted 2034-2035). Geminids (December 13-14, asteroid 3200 Phaethon, 120-150/hour – best of the year, unusual because parent is an asteroid). The radiant is the point in the sky from which meteors appear to originate (named for the constellation: Perseus for Perseids, Gemini for Geminids). Moonlight affects visibility (new moon best). Meteor showers are best viewed after midnight when Earth's rotation faces into the debris stream. Cameras and radio reflection (meteor scatter) are used for monitoring. Some meteor showers are associated with spacecraft (e.g., Mars meteor showers from comet debris). Large fireballs (bolides) – brighter than Venus – sometimes produce sonic booms. The Chelyabinsk event (2013) was a 20m asteroid (not a shower) that injured 1,500 people.

Types: Type II (core-collapse) – massive stars (>8 solar masses) exhaust nuclear fuel, iron core collapses (in free-fall in 0.1 seconds), rebounds, producing a shockwave that blows off outer layers. The core becomes a neutron star or black hole. Neutrinos (detected by Super-Kamiokande and other detectors) carry 99% of energy; only 1% is light. Example: Supernova 1987A (Large Magellanic Cloud, 168,000 light-years, first naked-eye supernova since Kepler 1604, neutrinos detected, progenitor star Sanduleak -69° 202). Type Ia – carbon-oxygen white dwarf accreting mass from a companion star (or merging with another white dwarf) until reaching Chandrasekhar limit (1.4 M☉), triggering thermonuclear runaway that completely destroys the star. No neutron star remains. Type Ia supernovae are standard candles (consistent peak luminosity) used to measure cosmic distances (discovering dark energy, 1998, Nobel Prize 2011). Historical supernovae: SN 1054 (Crab Nebula, recorded by Chinese and Arab astronomers, visible in daylight for 23 days). Tycho's Supernova (1572, Tycho Brahe used it to challenge Aristotle). Kepler's Supernova (1604, last naked-eye Milky Way supernova). Supernova remnants: Crab Nebula (pulsar at center), Vela, Cassiopeia A (300 years old, X-ray imaging). Supernovae produce heavy elements: iron, nickel, and all elements heavier than iron (gold, platinum, uranium) via rapid neutron capture (r-process) in the explosion – "we are stardust." Rates: about 2-3 supernovae per century per galaxy. Milky Way is overdue – last seen was Kepler 1604 (420 years ago). Supernova 2023ixf (M101 galaxy, 21 million light-years) was brightest in a decade. Neutrino detectors (IceCube, Super-K) and gravitational wave observatories (LIGO, Virgo) now provide early warnings for supernova explosions. A supernova within 30-50 light-years could cause mass extinctions (ozone layer damage, cosmic rays). Betelgeuse (640 light-years, red supergiant) could go supernova in next 100,000 years – will be bright as a full moon.