Astrophysics, Cosmology & Relativity — 60 Terms Defined

Gravitational Lensing Astrophysics, Cosmology & Relativity Arcseconds (Angular Deflection) The bending of light rays from a background astronomical source around a massive foreground object acting as a gravitational lens. θ_deflection = (4 G M) / (c² b)

Gravitational Time Dilation Astrophysics, Cosmology & Relativity Seconds (Relativistic Time Difference) The phenomenon where time passes measurably slower in regions of lower gravitational potential (stronger gravity). t_0 = t_f √(1 - 2 G M / (r c²))

Gravitational Waves Astrophysics, Cosmology & Relativity Dimensionless Strain h (10⁻²¹) Ripples in the curvature of spacetime that propagate outward as transverse waves at the speed of light, generated by accelerating asymmetric mass distributions. h_ij = (2 G / (c⁴ r)) (d²I_ij / dt²), Strain h = ΔL / L ~ 10⁻²¹

Great Attractor Astrophysics, Cosmology & Relativity M_☉ (Gravitational Anomaly Mass) A massive gravitational anomaly in intergalactic space at the center of the Laniakea Supercluster pulling galaxies across hundreds of millions of light-years. v_peculiar ~ 600 km/s towards Laniakea Supercluster center

Hawking Radiation Astrophysics, Cosmology & Relativity Kelvin (Blackbody Temperature T_H) Thermal blackbody radiation predicted to be emitted by black holes due to quantum field fluctuations near the event horizon. T_H = ℏ c³ / (8 π G M k_B), P = ℏ c⁶ / (15360 π G² M²)

Hertzsprung-Russell (H-R) Diagram Astrophysics, Cosmology & Relativity Luminosity (L_☉) vs Surface Temp (K) A foundational scatter plot of stellar luminosity versus surface temperature (or spectral type) used to trace stellar evolution phases. L ∝ R² T^4 (Plot of Absolute Magnitude vs Spectral Type)

Hubble's Law Astrophysics, Cosmology & Relativity km/s/Mpc (Hubble Constant) The cosmological observation that galaxies recede from Earth at velocities directly proportional to their proper distance. v = H₀ d, H₀ ≈ 70 (km/s) / Mpc

Length Contraction Astrophysics, Cosmology & Relativity Meters (Spatial Length) The relativistic effect where an object's length in the direction of motion is measured shorter by a stationary observer than its proper length L₀. L = L₀ / γ = L₀ √(1 - v²/c²)

Light Year Astrophysics, Cosmology & Relativity Light-Years (ly) An astronomical unit of length equal to the distance light travels in a vacuum in one Julian year (365.25 days). 1 ly = c × 1 year ≈ 9.461 × 10¹⁵ m ≈ 63,241 AU

Local Group Astrophysics, Cosmology & Relativity Megaparsecs (Mpc) The small galaxy cluster group comprising the Milky Way, Andromeda (M31), Triangulum (M33), and over 80 dwarf satellite galaxies. Diameter ~ 3 Mpc (~10 million ly), Mass ~ 2 × 10¹² M_☉

Lorentz Factor Astrophysics, Cosmology & Relativity Dimensionless Factor (γ ≥ 1) The fundamental relativistic factor by which time dilation, length contraction, and relativistic mass scale for objects moving at velocity v. γ = 1 / √(1 - v²/c²) = 1 / √(1 - β²)

Main Sequence Astrophysics, Cosmology & Relativity H-R Diagram Band The continuous evolutionary diagonal band on the Hertzsprung-Russell diagram containing stars fusing hydrogen into helium in their cores. L ∝ M^3.5 (Mass-Luminosity Relation)

Mass-Energy Equivalence Astrophysics, Cosmology & Relativity Joules or MeV / GeV The fundamental relativistic principle stating that mass and energy are directly proportional and interchangeable physical quantities. E = m c² = γ m₀ c²

Minkowski Space Astrophysics, Cosmology & Relativity Flat 4D Spacetime Metric The flat four-dimensional spacetime manifold combining 3D Euclidean space and time used to formulate Special Relativity. ds² = -c² dt² + dx² + dy² + dz²

Multiverse Astrophysics, Cosmology & Relativity Cosmological Hypothesis A hypothetical set of multiple co-existing universes that together comprise all of physical reality. Cosmic Inflation Bubble Universes & Many-Worlds Everett Branching

Nebula Astrophysics, Cosmology & Relativity Light-Years (Diameter) An interstellar cloud of dust, hydrogen, helium, and ionized gases in space, serving as a site for star formation. M_Jeans = (5 k_B T / (G μ m_H))^(3/2) (3 / (4 π ρ))^(1/2)

Neutron Star Astrophysics, Cosmology & Relativity M_☉ (Mass), km (Radius) An ultradense stellar core remnant produced by the core-collapse supernova explosion of a massive star, composed almost entirely of neutrons. M ≈ 1.4–2.1 M_☉, R ≈ 10–12 km, ρ ≈ 10¹⁷ kg/m³

Olbers' Paradox Astrophysics, Cosmology & Relativity Cosmological Paradox The apparent paradox that if the universe were static, infinite, and uniformly populated with stars, the night sky should be uniformly bright as the Sun. Night sky brightness J = ∫ 4 π I r² dr / (4 π r²) → ∞ (in static infinite universe)

Parsec Astrophysics, Cosmology & Relativity Parsecs (pc, kpc, Mpc) An astronomical unit of distance equal to the distance at which a star exhibits a trigonometric parallax angle of exactly one arcsecond (1″). 1 pc = 1 AU / tan(1″) ≈ 3.2616 ly ≈ 3.0857 × 10¹⁶ m

Proper Time Astrophysics, Cosmology & Relativity Seconds (Proper Time τ) The physical time interval measured by a clock moving along a specific worldline through spacetime. dτ = dt / γ = √(dt² - dx²/c²)

Protostar Astrophysics, Cosmology & Relativity Solar Masses & Years An early evolutionary mass phase of a star formed by the gravitational contraction of an interstellar gas cloud before core hydrogen fusion ignites. t_collapse ~ t_freefall = √(3π / (32 G ρ))

Pulsar Astrophysics, Cosmology & Relativity Milliseconds to Seconds (Rotation Period) A highly magnetized, rapidly rotating neutron star that emits focused beams of electromagnetic radiation from its magnetic poles. P_spin ~ 1.4 ms to 10 s, P_dot = dP / dt

Quasar Astrophysics, Cosmology & Relativity Luminosity / Redshift z An extremely luminous Active Galactic Nucleus (AGN) powered by gas accreting onto a supermassive black hole at the center of a distant galaxy. L_quasar ~ 10⁴⁰ W (100–1000 times Milky Way Luminosity)

Red Giant Astrophysics, Cosmology & Relativity Solar Radii (R_☉) & Kelvin A large, luminous star in a late evolutionary stage that has exhausted core hydrogen fuel and expanded its outer envelope. Radius R ~ 10–100 R_☉, T_surface ~ 3,000–4,500 K