OpenStem

@openstem · Joined Jul 2026

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Flashcards11 cards

What distinguishes a planet from a dwarf planet under the IAU 2006 definition?1 / 11

A planet (IAU 2006) must: (1) orbit the Sun, (2) have sufficient mass for gravity to achieve a roughly spherical shape (hydrostatic equilibrium), AND (3) have cleared the neighbourhood around its orbit of other debris. A dwarf planet meets criteria 1 and 2 but NOT 3. Pluto was reclassified as a dwarf planet because it shares its orbit with Kuiper Belt objects. The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Astronomy

Astronomy · L4 · The Solar System — Planets & Their Properties

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Flashcards9 cards

State the vis-viva equation and define every symbol in it.1 / 9

v² = GM(2/r − 1/a), where v is orbital speed at radius r, G is the gravitational constant, M is the central body's mass, r is the instantaneous distance from the focus, and a is the orbit's semi-major axis. It applies to any conic orbit (ellipse, parabola, hyperbola) once the correct sign and value of a are used.

Astronomy

Astronomy · L4 · Orbital Mechanics — Vis-Viva & Transfer Orbits

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What physically causes a tidal force? Why does it stretch a body rather than just pull on it?1 / 9

A tidal force is the DIFFERENCE in a gravitational field's strength across an extended body. The near side of a body feels stronger gravity from the perturber than the centre does, and the far side feels weaker gravity than the centre. Relative to the body's own centre of mass, this produces an outward stretch on both the near and far sides — a tidal bulge on two sides, not one.

Astronomy

Astronomy · L4 · Tidal Forces, the Roche Limit & Tidal Locking

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What is a blackbody, and why are stars well approximated as blackbodies?1 / 8

A blackbody is an idealised object that absorbs all incident radiation (reflecting none) and re-emits energy purely as a function of its temperature, with a specific, universal spectral shape (the Planck spectrum). Stellar photospheres are dense, opaque plasma that absorb and re-emit radiation many times before it escapes, closely approximating this idealised thermal equilibrium — so stars' continuous spectra are well fit by blackbody curves.

Astronomy

Astronomy · L4 · Blackbody Radiation & the Luminosity-Radius-Temperature Relation

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State the equation of hydrostatic equilibrium for a star and explain what balances what.1 / 8

dP/dr = −Gm(r)ρ(r)/r², where P is pressure, ρ is density, m(r) is the mass enclosed within radius r, and G is the gravitational constant. It states that at every radius, the outward push of the pressure gradient exactly balances the inward pull of gravity on that shell of gas — without this balance, a star would either collapse or blow apart.

Astronomy

Astronomy · L4 · Stellar Structure Equations

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What is a galactic rotation curve?1 / 8

A rotation curve plots a galaxy's orbital speed v as a function of galactocentric radius r — how fast stars and gas orbit the galactic centre at different distances from it. It is measured using Doppler shifts of starlight or the 21 cm hydrogen line across a galaxy's disk.

Astronomy

Astronomy · L4 · Galactic Dynamics & the Rotation Curve Problem

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Write the parallax distance formula and define its terms.1 / 8

d (pc) = 1/p (arcsec), where p is the parallax angle in arcseconds and d is distance in parsecs. This defines the parsec itself: 1 parsec is the distance at which a star shows a parallax angle of exactly 1 arcsecond as Earth orbits the Sun.

Astronomy

Astronomy · L4 · The Cosmic Distance Ladder, Quantitatively

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State Einstein's equivalence principle.1 / 8

The effects of a uniform gravitational field are locally indistinguishable from the effects of uniform acceleration in the absence of gravity. A person in a windowless, accelerating rocket cannot tell (by any local experiment) whether they are accelerating through space or sitting stationary in a gravitational field of equivalent strength. This equivalence is the conceptual foundation of general relativity.

Astronomy

Astronomy · L4 · General Relativity Basics

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What is transmission spectroscopy, and when does it apply to exoplanets?1 / 8

Transmission spectroscopy analyses starlight that passes THROUGH a transiting exoplanet's atmosphere on its way to the observer. It applies only during a transit (when the planet crosses in front of its star as seen from Earth). Atmospheric gases absorb specific wavelengths, so the transit depth varies with wavelength — deeper at wavelengths absorbed by atmospheric constituents.

Astronomy

Astronomy · L4 · Exoplanet Atmospheres & the Habitable Zone

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What is the interstellar medium (ISM), and what are its main components by mass?1 / 8

The interstellar medium is the matter filling the space between stars within a galaxy. By mass, it is roughly 99% gas (mostly hydrogen, ~10% helium by number, trace heavier elements) and 1% dust (microscopic solid grains of carbon and silicates). Despite dust being a small mass fraction, it dominates the ISM's effect on starlight passing through it.

Astronomy

Astronomy · L4 · The Interstellar Medium: Gas, Dust & the 21 cm Line

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What is an active galactic nucleus (AGN)?1 / 8

An active galactic nucleus is the compact central region of a galaxy that outshines the rest of the galaxy's stars combined, powered not by starlight but by gravitational energy released as material falls onto a supermassive black hole (SMBH) at the galaxy's centre. AGN luminosity can vary dramatically on timescales of days, indicating an extremely compact emitting region.

Astronomy

Astronomy · L4 · Active Galactic Nuclei, Quasars & Supermassive Black Holes

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What is recombination, and why is the name slightly misleading?1 / 8

Recombination is the epoch (~380,000 years after the Big Bang, at temperature ~3,000 K) when the universe cooled enough for free electrons and protons to combine into neutral hydrogen atoms. The name is slightly misleading because electrons and protons had never been 'combined' before — the early universe was always a hot, fully ionised plasma prior to this event.

Astronomy

Astronomy · L4 · The Cosmic Microwave Background: Recombination & Anisotropies

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Why are binary star systems so important for measuring stellar masses?1 / 8

Mass cannot be measured directly from a star's light — only its effects on motion reveal it. In a binary system, the two stars' mutual gravitational orbit obeys Kepler's laws in a form that depends on both masses, so measuring the orbit (period and separation) directly yields the total system mass — binaries are essentially the only way to measure stellar masses independently of stellar models.

Astronomy

Astronomy · L4 · Binary Star Systems & the Mass-Period Relation

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What physically causes a Cepheid variable to pulsate?1 / 8

Cepheids pulsate due to the kappa (κ) mechanism: in a particular temperature zone within the star (where helium is partially ionised), the gas's opacity increases as it is compressed, rather than decreasing as it normally would. This traps radiation, driving further expansion, which then decreases opacity and lets radiation escape, causing the star to contract again — a self-sustaining pulsation cycle rather than a one-off event.

Astronomy

Astronomy · L4 · Variable Stars: Cepheids, RR Lyrae & Asteroseismology

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What is the patched conic approximation, and why do mission designers use it?1 / 8

The patched conic approximation splits an interplanetary trajectory into separate two-body segments — departure (dominated by Earth's gravity), heliocentric cruise (dominated by the Sun), and arrival (dominated by the target body) — solving each segment as a simple Keplerian conic and 'patching' them together where one body's gravitational influence hands off to the next. It is far less computationally demanding than integrating the true many-body gravitational field, and gives an accurate enough first-pass trajectory for mission planning.

Astronomy

Astronomy · L4 · Interplanetary Trajectory Design

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What is the initial mass function (IMF), in one sentence?1 / 9

The initial mass function is the distribution describing how many stars of each mass are born together out of a single star-forming episode — a statistical statement about the relative numbers of low-mass and high-mass stars produced, not a description of any single star's life.

Astronomy

Astronomy · L4 · Stellar Populations, the Initial Mass Function & Cluster Evolution

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State the diffraction-limited angular resolution of a single dish, and explain why this is a much harder problem for radio telescopes than optical ones.1 / 9

The diffraction limit is θ ≈ 1.22λ/D, where λ is the observing wavelength and D is the dish diameter. Radio wavelengths (millimetres to metres) are 10⁴–10⁷ times longer than optical wavelengths (hundreds of nanometres), so for the SAME angular resolution, a radio telescope needs a correspondingly larger diameter than an optical one — reaching optical-telescope-like resolution with a single radio dish would require an impractically enormous structure.

Astronomy

Astronomy · L4 · Radio Astronomy: Single Dishes, Interferometry & the EHT

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Flashcards8 cards

Set up the energy balance that defines a planet's equilibrium temperature.1 / 8

At equilibrium, the power a planet absorbs from its star equals the power it radiates away as a blackbody. Absorbed power is the intercepted stellar flux times the planet's cross-sectional area times (1 − A) (A = albedo, the reflected fraction): P_abs = (1−A)·S·πR². Emitted power, radiating from the whole spherical surface, is P_emit = 4πR²σT⁴. Setting these equal and cancelling πR² gives the equilibrium temperature.

Astronomy

Astronomy · L4 · Comparative Planetary Atmospheres & Energy-Balance Climate

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Flashcards9 cards

What are the two main classes of stellar oscillation modes, and what restoring force distinguishes each?1 / 9

Pressure modes (p-modes) are acoustic waves restored by pressure gradients — essentially trapped sound waves. Gravity modes (g-modes) are restored by buoyancy in a stably (radiatively) stratified region, oscillating as displaced fluid parcels are pushed back toward equilibrium by their density contrast with the surroundings.

Astronomy

Astronomy · L4 · Asteroseismology: Probing Stellar Interiors

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Why must astronomers observe stars near the Galactic Center in the near-infrared rather than visible light?1 / 8

Roughly 30 magnitudes of visual extinction from intervening interstellar dust in the Galactic plane make the Galactic Center essentially invisible at optical wavelengths. Near-infrared light is scattered and absorbed far less by dust, so adaptive-optics-corrected infrared imaging (e.g. at Keck and the VLT) is what makes it possible to resolve and track individual stars there at all.

Astronomy

Astronomy · L4 · The Galactic Center and Sagittarius A*

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Flashcards9 cards

Define a biosignature, and give two examples of candidate gas-phase biosignatures.1 / 9

A biosignature is a substance, feature, or pattern whose presence provides evidence of past or present biological activity. Gas-phase examples include O₂/O₃ (a byproduct of oxygenic photosynthesis on Earth) and CH₄ in persistent coexistence with an oxidizing atmosphere (a byproduct of methanogenesis).

Astronomy

Astronomy · L4 · Astrobiology: Biosignatures, the Fermi Paradox & Habitability

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Flashcards8 cards

Write the (scalar, single-lens-plane) gravitational lens equation and explain each term.1 / 8

β = θ − (D_ls/D_s) α̂(θ), where β is the source's true angular position (had it not been lensed), θ is the observed image position, α̂(θ) is the deflection angle produced by the lens mass at that impact parameter, and D_ls, D_s (and implicitly D_l) are angular-diameter distances between lens, source, and observer that rescale the raw deflection into an apparent angular shift.

Astronomy

Astronomy · L4 · Gravitational Lensing as a Quantitative Astrophysical Tool

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Flashcards8 cards

What holds a white dwarf up against gravitational collapse, and why is that mechanism unusual?1 / 8

Electron degeneracy pressure — a quantum-mechanical effect from the Pauli exclusion principle, which forbids any two electrons from occupying the same quantum state. Packed to extreme density, electrons are forced into ever-higher momentum states purely to satisfy this exclusion, producing a pressure that exists even at zero temperature — unlike ordinary thermal (gas) pressure, which depends on temperature.

Astronomy

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

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Flashcards8 cards

What observational property is used to classify young stellar objects into Class 0, I, II, and III?1 / 8

The shape of the object's spectral energy distribution (SED) — how its brightness is distributed across infrared and longer wavelengths. Progressively more of the total luminosity appearing at shorter wavelengths (and less obscured by cool dust) as the classification moves from Class 0 through III traces the young star gradually clearing away its surrounding envelope and disk.

Astronomy

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

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Flashcards8 cards

What single spectral feature first splits all supernovae into Type I versus Type II?1 / 8

The presence or absence of hydrogen lines in the supernova's spectrum near peak brightness. A Type I supernova shows no hydrogen lines; a Type II supernova does. This purely observational split was defined before the underlying physical mechanisms were fully understood, and it still does not map onto a single explosion mechanism.

Astronomy

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

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Flashcards8 cards

What is a mean-motion resonance between two orbiting bodies?1 / 8

A situation where two bodies' orbital periods form a ratio of small whole numbers (for example 2:1 or 3:2), so their relative alignment repeats in a fixed, predictable pattern on every cycle — producing a small gravitational tug that repeats at the same orbital phase every time, rather than averaging away as it would at a random, non-resonant period ratio.

Astronomy

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

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Flashcards8 cards

What is proper motion, and what does it directly measure?1 / 8

A star's observed angular rate of motion across the sky, measured in arcseconds per year. It captures only the component of the star's true velocity that is perpendicular to the line of sight — its plane-of-sky, or 'tangential,' motion — and says nothing on its own about motion toward or away from the observer.

Astronomy

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

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Flashcards5 cards

How does the transit method detect exoplanets?1 / 5

When a planet passes in front of (transits) its host star, it blocks a small fraction of the star's light. The fractional dip in flux is:

Astronomy

Astronomy · L5 · Exoplanet Detection: Transit and Radial-Velocity Methods

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Flashcards4 cards

Write the FLRW line element and identify each component.1 / 4

The Friedmann–Lemaître–Robertson–Walker metric for a homogeneous, isotropic universe is:

Astronomy

Astronomy · L5 · FLRW Cosmology & Friedmann Dynamics

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Flashcards8 cards

Write the Schwarzschild metric and identify its terms.1 / 8

The Schwarzschild solution describes spacetime around a static, spherically symmetric, uncharged mass M:

Astronomy

Astronomy · L5 · Schwarzschild Black Holes

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