OpenStem

@openstem · Joined Jul 2026

7420 public items8 groups

Note~446 words · 2 min

Fixing a Position on the Sky Just as any point on Earth can be located with longitude and latitude, any point on the sky can be located with the equatorial coordinate system: right ascension and declination. The celestial equator is simply

Astronomy

Astronomy · L3 · Celestial Coordinates & Precession

@openstem

Note~316 words · 2 min

The Raw Material: Giant Molecular Clouds Stars are born inside giant molecular clouds — vast reservoirs of cold (roughly 10-30 K), dense interstellar gas, mostly molecular hydrogen, laced with dust. Cold, dense gas is exactly what gravity n

Astronomy

Astronomy · L3 · Star Formation & Molecular Clouds

@openstem

Note~411 words · 2 min

Rings: Countless Independent Orbits A planetary ring is not a solid object. It is a swarm of countless individual particles — from dust grains to boulder-sized chunks, mostly water ice with some rock — each independently orbiting the planet

Astronomy

Astronomy · L3 · Planetary Ring & Moon Systems

@openstem

Note~455 words · 2 min

Same Material, Different Light A nebula is, at its simplest, an interstellar cloud of gas and dust. What separates the different named types is not what the cloud is made of, but how — or whether — it produces visible light, which in turn d

Astronomy

Astronomy · L3 · Nebula Types

@openstem

Note~665 words · 3 min

Stars and stellar evolution: from nebula to remnant Stars are self-gravitating balls of plasma held together by gravity and powered by nuclear fusion in their cores. Their life histories depend critically on their initial mass — from sub-st

Astronomy

Astronomy · L4 · Stars and stellar evolution: from nebula to remnant

@openstem

Note~622 words · 3 min

Orbital mechanics: vis-viva, orbital energy, and Hohmann transfers Kepler's laws describe the shape and timing of an orbit; the vis-viva equation adds the missing piece — the instantaneous speed of a body anywhere along that orbit. It under

Astronomy

Astronomy · L4 · Orbital mechanics: vis-viva, orbital energy, and Hohmann transfers

@openstem

Note~648 words · 3 min

Tidal forces: differential gravity, the Roche limit, and tidal locking Tides are not caused by gravity pulling on a body — they are caused by gravity pulling unevenly across an extended body. This distinction explains everything from ocean

Astronomy

Astronomy · L4 · Tidal forces: differential gravity, the Roche limit, and tidal locking

@openstem

Note~463 words · 2 min

Blackbody radiation and the luminosity-radius-temperature relation Almost everything astronomers know about a star's physical size, energy output, and surface conditions comes from treating its light as blackbody radiation — thermal emissio

Astronomy

Astronomy · L4 · Blackbody radiation: Stefan-Boltzmann, Wien's law, and stellar L-R-T

@openstem

Note~481 words · 2 min

Stellar structure: hydrostatic equilibrium and mass continuity A star is a self-gravitating ball of hot plasma held in a delicate, long-lived balance. The basic equations of stellar structure describe that balance at the level of undergradu

Astronomy

Astronomy · L4 · Stellar structure: hydrostatic equilibrium and mass continuity

@openstem

Note~458 words · 2 min

Galactic dynamics: the rotation curve problem and dark matter One of the strongest pieces of evidence for dark matter comes from a straightforward application of Newtonian gravity to how fast stars and gas orbit within galaxies — and the su

Astronomy

Astronomy · L4 · Galactic dynamics: the rotation curve problem and dark matter

@openstem

Note~540 words · 3 min

The cosmic distance ladder: parallax, Cepheids, and Type Ia supernovae No single technique measures distance across the entire observable universe. Instead, astronomers use a chain of overlapping methods — the cosmic distance ladder — where

Astronomy

Astronomy · L4 · The cosmic distance ladder: parallax, Cepheids, and Type Ia supernovae

@openstem

Note~558 words · 3 min

General relativity: the equivalence principle and classic tests General relativity replaces Newton's picture of gravity as a force with a geometric picture: mass and energy curve spacetime, and objects simply follow the straightest paths av

Astronomy

Astronomy · L4 · General relativity: the equivalence principle and classic tests

@openstem

Note~514 words · 3 min

Exoplanet atmospheres: transmission spectroscopy and the habitable zone Two of the most active questions in exoplanet science are what a planet's atmosphere is made of, and whether the planet could plausibly host liquid water. Both question

Astronomy

Astronomy · L4 · Exoplanet atmospheres: transmission spectroscopy and the habitable zone

@openstem

Note~548 words · 3 min

The interstellar medium: gas, dust, the 21 cm line, and extinction The space between stars is not empty. It is filled with a diffuse, multi-phase mixture of gas and dust — the interstellar medium (ISM) — that shapes star formation, alters t

Astronomy

Astronomy · L4 · The interstellar medium: gas, dust, the 21 cm line, and extinction

@openstem

Note~530 words · 3 min

Active galactic nuclei: accretion disks, jets, and supermassive black holes At the centre of most large galaxies sits a supermassive black hole (SMBH), typically millions to billions of solar masses. Most of the time this SMBH is quiescent,

Astronomy

Astronomy · L4 · Active galactic nuclei: accretion disks, jets, and supermassive black holes

@openstem

Note~511 words · 3 min

The cosmic microwave background: recombination, decoupling, and anisotropies The cosmic microwave background (CMB) is the oldest light astronomers can observe directly — a snapshot of the universe as it existed roughly 380,000 years after t

Astronomy

Astronomy · L4 · The cosmic microwave background: recombination, decoupling, and anisotropies

@openstem

Note~551 words · 3 min

Binary star systems: the mass-period relation and observational classes Stellar mass cannot be inferred from light alone — a star's luminosity and temperature reveal mass only indirectly, via theoretical stellar models. Binary star systems

Astronomy

Astronomy · L4 · Binary star systems: the mass-period relation and observational classes

@openstem

Note~586 words · 3 min

Variable stars: Cepheids, RR Lyrae, and asteroseismology Some stars do not shine at constant brightness — they pulsate, rhythmically expanding and contracting. Two classes of these pulsating variables, Cepheids and RR Lyrae stars, are among

Astronomy

Astronomy · L4 · Variable stars: Cepheids, RR Lyrae, and asteroseismology

@openstem

Note~613 words · 3 min

Interplanetary trajectory design: patched conics, delta-v, and gravity assists Getting a spacecraft from Earth to another planet is, in full generality, an unsolvable few-body gravitational problem. Mission designers make it tractable by ap

Astronomy

Astronomy · L4 · Interplanetary trajectory design: patched conics, delta-v, and gravity assists

@openstem

Note~487 words · 2 min

Stellar populations: the initial mass function and cluster dynamical evolution Stars are not born one at a time — they form together, hundreds to millions at once, out of a single fragmenting molecular cloud. The statistical distribution of

Astronomy

Astronomy · L4 · Stellar populations: the initial mass function and cluster dynamical evolution

@openstem

Note~464 words · 2 min

Radio interferometry: from single dishes to the Event Horizon Telescope Radio astronomy faces a resolution problem optical astronomy does not: radio wavelengths are so much longer than visible light that a single dish would need to be impra

Astronomy

Astronomy · L4 · Radio interferometry: from single dishes to the Event Horizon Telescope

@openstem

Note~397 words · 2 min

Comparative planetary atmospheres and the energy-balance climate model A remarkably simple energy-balance argument — absorbed sunlight equals emitted thermal radiation — predicts a 'bare' equilibrium temperature for any planet. Comparing th

Astronomy

Astronomy · L4 · Comparative planetary atmospheres and the energy-balance climate model

@openstem

Note~488 words · 2 min

Asteroseismology and helioseismology: reading stellar interiors from oscillations Stars ring like enormous, imperfect bells. Turbulent convection near the surface stochastically excites a rich spectrum of resonant oscillation modes, and the

Astronomy

Astronomy · L4 · Asteroseismology and helioseismology: reading stellar interiors from oscillations

@openstem

Note~520 words · 3 min

Weighing Sagittarius A*: stellar orbits at the Galactic Center The strongest evidence that a supermassive black hole sits at the center of the Milky Way comes not from imaging it directly, but from patiently tracking the orbits of ordinary

Astronomy

Astronomy · L4 · Weighing Sagittarius A*: stellar orbits at the Galactic Center

@openstem

Note~596 words · 3 min

Biosignature gases, the Fermi paradox, and habitability beyond the classical HZ Astrobiology sits at the intersection of chemistry, planetary science, and statistics: what atmospheric signatures would convincingly indicate life, why has no

Astronomy

Astronomy · L4 · Biosignature gases, the Fermi paradox, and habitability beyond the classical HZ

@openstem

Note~488 words · 2 min

Gravitational lensing in practice: cluster mass mapping and time-delay cosmography Beyond the qualitative fact that mass bends light, gravitational lensing is a precise quantitative tool: it maps the total mass distribution of galaxy cluste

Astronomy

Astronomy · L4 · Gravitational lensing in practice: cluster mass mapping and time-delay cosmography

@openstem

Note~471 words · 2 min

White dwarfs: electron degeneracy pressure and the Chandrasekhar limit A white dwarf is the exposed core left behind after a Sun-like star sheds its outer layers late in life. With fusion permanently extinguished, nothing thermal remains to

Astronomy

Astronomy · L4 · White Dwarfs: Electron Degeneracy Pressure & the Chandrasekhar Limit

@openstem

Note~493 words · 2 min

Protostellar evolution: from collapse to the main sequence Between the onset of a molecular cloud's gravitational collapse and a star's arrival on the stable main sequence lies a sequence of distinct observational stages, classified by how

Astronomy

Astronomy · L4 · Protostellar Evolution: From Collapse to the Main Sequence

@openstem

Note~401 words · 2 min

Supernova classification: light curves and spectral types Supernovae are classified observationally, by spectrum and light-curve shape, using a scheme that predates a full understanding of the underlying physics. As a result, the classifica

Astronomy

Astronomy · L4 · Supernova Classification: Light Curves & Spectral Types

@openstem

Note~530 words · 3 min

Orbital resonances, Lagrange points, and the restricted three-body problem Beyond simple two-body orbits, gravitational interactions between three or more bodies produce two of the Solar System's most important recurring structures: mean-mo

Astronomy

Astronomy · L4 · Orbital Resonances, Lagrange Points & the Restricted Three-Body Problem

@openstem