OpenStem

@openstem · Joined Jul 2026

7420 public items8 groups

Note~467 words · 2 min

Stellar kinematics: proper motion, radial velocity, and space velocity A star's true motion through space is a three-dimensional velocity vector, but no single measurement captures all of it directly. Astrometry breaks that velocity into tw

Astronomy

Astronomy · L4 · Stellar Kinematics: Proper Motion, Radial Velocity & Space Velocity

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Note~266 words · 1 min

Energy Generation in Main-Sequence Stars Stars on the main sequence are powered by hydrogen fusion in their cores. Two pathways dominate: the proton–proton (pp) chain (dominant in stars like the Sun and below) and the CNO cycle (dominant in

Astronomy

Astronomy · L5 · Stellar Nucleosynthesis: pp Chain and CNO Cycle

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Note~342 words · 2 min

The Schwarzschild Solution In 1916, Karl Schwarzschild found the first exact solution to the Einstein field equations: the spacetime geometry outside a static, spherically symmetric, non-rotating, uncharged mass M. In Schwarzschild coordina

Astronomy

Astronomy · L5 · The Schwarzschild Black Hole: Horizon, Photon Sphere, and ISCO

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Note~418 words · 2 min

From Core Collapse to a Neutron Star When the iron core of a massive star (initial mass roughly 8–20 M☉) exceeds the Chandrasekhar mass, electron degeneracy pressure can no longer support it against gravity. The core collapses in millisecon

Astronomy

Astronomy · L5 · Neutron Star Structure, the TOV Equation, and Pulsars/Magnetars

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Note~334 words · 2 min

Radiation from an Accelerating Quadrupole General relativity predicts that accelerating mass distributions radiate energy as ripples in spacetime curvature — gravitational waves — propagating at the speed of light. Because gravitational mon

Astronomy

Astronomy · L5 · Gravitational Waves: Quadrupole Radiation, Chirp Signals, and Detection

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Note~304 words · 2 min

Two Puzzles in Standard Big Bang Cosmology Extrapolating the FLRW/Friedmann description of the universe back toward the Big Bang, without any exotic early-universe physics, leaves two well-posed puzzles that are not contradictions but natur

Astronomy

Astronomy · L5 · Cosmic Inflation: Solving the Horizon and Flatness Problems

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Note~236 words · 1 min

A Convergent, Multi-Scale Case for Dark Matter No single observation proves dark matter exists; rather, several independent lines of evidence — spanning galaxy, cluster, and cosmological scales, using entirely different physical mechanisms

Astronomy

Astronomy · L5 · Dark Matter: Evidence and Candidate Particles

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Note~375 words · 2 min

Bottom-Up Assembly in a CDM Universe In the ΛCDM paradigm, the small, nearly scale-invariant density perturbations seeded by inflation grow under gravity, with the smallest scales crossing the non-linear collapse threshold first. This produ

Astronomy

Astronomy · L5 · Hierarchical Structure Formation and Galaxy Assembly

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Note~295 words · 1 min

Radiative Transfer and Line Formation A stellar spectrum results from radiative transfer through an atmosphere with a temperature gradient: continuum photons escape from progressively shallower (cooler) depths as one moves to wavelengths wh

Astronomy

Astronomy · L5 · Spectral Line Formation, the Curve of Growth, and Non-LTE Effects

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Note~313 words · 2 min

Why Accretion Disks Need Viscosity Gas accreting onto a compact object (white dwarf, neutron star, or black hole) generically carries angular momentum and settles into a rotating disk rather than falling straight in. To actually accrete, th

Astronomy

Astronomy · L5 · Accretion Disks: The Eddington Limit and the Alpha-Disk Model

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Note~326 words · 2 min

From Merger to Heavy Elements When two neutron stars (or a neutron star and a black hole) merge, tidal forces and hydrodynamic shocks in the final milliseconds strip and eject a small but astrophysically crucial amount of extremely neutron-

Astronomy

Astronomy · L5 · Compact Object Mergers: Kilonovae and r-Process Nucleosynthesis

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Note~395 words · 2 min

Acoustic Oscillations Before Recombination Prior to recombination (z ≈ 1100), photons and baryons were tightly coupled into a single fluid by Thomson scattering. Gravitational potential wells sourced mainly by dark matter compressed this fl

Astronomy

Astronomy · L5 · The CMB Angular Power Spectrum and Cosmological Parameters

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Note~290 words · 1 min

Two Competing Formation Channels Giant planets can, in principle, form via two very different physical processes, operating on very different timescales and favouring different disk conditions: What Kepler-Era Demographics Add Large transit

Astronomy

Astronomy · L5 · Planet Formation Channels and What Exoplanet Demographics Reveal

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Note~298 words · 1 min

Two Enrichment Timescales Galactic chemical evolution is fundamentally shaped by the fact that different nucleosynthesis sources operate on very different delay timescales relative to star formation, so a stellar population's detailed abund

Astronomy

Astronomy · L5 · Galactic Chemical Evolution: Yields, Enrichment Timescales, and Gradients

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Note~298 words · 1 min

The Jeans Instability, Generalised to an Expanding Universe The classical Jeans criterion asks when self-gravity overcomes pressure support in a static medium, with the boundary set by the Jeans length λ_J = c_s√(π/Gρ). In an expanding FLRW

Astronomy

Astronomy · L5 · Cosmological Perturbation Theory: From Jeans Instability to Structure Growth

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Note~325 words · 2 min

Millisecond Pulsars as Galactic Clocks A pulsar timing array turns a network of millisecond pulsars into a single, galaxy-sized gravitational-wave detector. Each pulsar's rotation is modelled with extreme precision (spin frequency and spin-

Astronomy

Astronomy · L5 · Pulsar Timing Arrays and the Nanohertz Gravitational-Wave Background

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Note~364 words · 2 min

Collapse at Horizon Re-Entry Unlike an ordinary stellar-collapse black hole, a primordial black hole (PBH) forms directly from a sufficiently large density perturbation in the radiation-dominated early universe, at the instant that perturba

Astronomy

Astronomy · L5 · Primordial Black Holes: Formation and Dark-Matter Constraints

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Note~383 words · 2 min

The Baryon Asymmetry Big bang nucleosynthesis and the CMB independently constrain the baryon-to-photon ratio to a small, positive value: A hot, symmetric early universe should have produced equal numbers of baryons and antibaryons, which wo

Astronomy

Astronomy · L5 · Baryogenesis and the Sakharov Conditions

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Note~335 words · 2 min

From the Overdensity Field to the Correlation Function Large-scale structure is described statistically through the overdensity field δ(x) = [ρ(x) − ρ̄]/ρ̄. Its two-point correlation function ξ(r) is the excess probability, above random Poi

Astronomy

Astronomy · L5 · Large-Scale Structure: The Two-Point Correlation Function and the Cosmic Web

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Note~304 words · 2 min

From Cosmic Dawn to Reionization After recombination (z ≈ 1100) leaves the universe neutral, the 'dark ages' persist until the first stars and galaxies (Population III and early Population II) form around z ~ 20-30. Their ionizing UV photon

Astronomy

Astronomy · L5 · The Epoch of Reionization and the Redshifted 21 cm Line

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Note~305 words · 2 min

The Tidal Radius and the Hills Mass A star is tidally disrupted once its pericenter distance falls inside the tidal radius, where the black hole's differential gravity across the star overwhelms the star's self-gravity: Because r_t grows on

Astronomy

Astronomy · L5 · Tidal Disruption Events: From Stellar Shredding to Observed Flares

@openstem

Note~377 words · 2 min

Magnetically Powered Neutron Stars A magnetar's defining feature is an extreme surface magnetic field, B ~ 10¹⁴–10¹⁵ G, roughly two to three orders of magnitude above a typical pulsar's. The Thompson & Duncan magnetar model argues this fiel

Astronomy

Astronomy · L5 · Magnetars: Extreme Fields, Giant Flares, and the Fast Radio Burst Connection

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Note~477 words · 2 min

Four Messengers, One Universe Multi-messenger astronomy combines fundamentally different carriers of astrophysical information, each sensitive to different physical conditions and each escaping its source under different circumstances: GW17

Astronomy

Astronomy · L5 · Multi-Messenger Astronomy: Gravitational Waves, Neutrinos, and Light from One Event

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Note~491 words · 2 min

Relativistic Beaming A source moving at a bulk Lorentz factor Γ close to the speed of light does not radiate isotropically as seen by a stationary observer, even if it does so in its own rest frame. Relativistic aberration sweeps that emiss

Astronomy

Astronomy · L5 · Relativistic Jets: Doppler Beaming and Apparent Superluminal Motion

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Note~461 words · 2 min

Beyond Iron: Why Fusion Stops Iron-56 sits at the peak of the nuclear binding-energy-per-nucleon curve. Fusion releases energy building nuclei up to iron, but building anything heavier by fusion would require a net input of energy rather th

Astronomy

Astronomy · L5 · Neutron-Capture Nucleosynthesis: the s-process and r-process

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Note~476 words · 2 min

The Highest-Energy Particles Known Ultra-high-energy cosmic rays (UHECRs) — almost always protons or atomic nuclei — carry energies above roughly 10^18-10^19 eV, with the most extreme detected events exceeding 10^20 eV concentrated into a s

Astronomy

Astronomy · L5 · Ultra-High-Energy Cosmic Rays & the GZK Cutoff

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Note~494 words · 2 min

Strong-lensing time-delay cosmography and the Hubble tension When a distant quasar's light passes close to an intervening massive galaxy, gravitational lensing can split it into multiple images. Because each image's light travels a differen

Astronomy

Astronomy · L5 · Strong-Lensing Time-Delay Cosmography & the Hubble Tension

@openstem

Note~616 words · 3 min

The Kerr metric: frame dragging, the ergosphere, and the Penrose process A rotating black hole is described by the Kerr metric, characterised entirely by its mass M and spin angular momentum J (or the dimensionless spin parameter a = J/Mc,

Astronomy

Astronomy · L5 · The Kerr Metric: Frame Dragging, the Ergosphere & the Penrose Process

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Astronomy

Astronomy · L3 · Hubble's Law: Recession Velocity vs Distance

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Astronomy

Astronomy · L3 · Exoplanet Transit Light Curve

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