Module 1 Complete Glossary

All Terms from Lectures 1-13

Comprehensive vocabulary reference for Module 1: Foundations. Terms are organized by lecture and marked by importance.

Author

Dr. Anna Rosen

How to Use This Glossary

This glossary contains all terms introduced in Module 1, organized by the lecture where they first appear.

Tier Legend:

★ Core terms — You should be able to define and use these on an exam
◇ Supporting terms — Helpful vocabulary; less likely to be tested directly

Study Strategy

Start with ★ Core terms — these are your priority
Use the lecture headers to focus on specific topics
Test yourself: cover the definition and try to explain from memory
Cross-reference with the Concept Map to see how terms connect

Lecture 1: Spoiler Alerts — The Universe Is Weird

★ Absorption: Light removed from a beam by intervening material, often at specific wavelengths where atoms absorb photons. Absorption lines appear dark against a brighter continuum.
★ Blackbody: An idealized object that absorbs all incident radiation and emits a characteristic spectrum determined only by its temperature. Stars approximate blackbodies. The blackbody spectrum peaks at $\lambda_{\text{peak}} = b/T$ (Wien's law).
◇ Cosmic Web: The large-scale structure of the universe: galaxies arranged in filaments, walls, and clusters, separated by vast voids. The cosmic web is still evolving—gravity continues to pull matter from voids into filaments and clusters.
◇ Dark Energy: The unknown cause of the accelerating expansion of the universe, comprising ~68% of the cosmic energy budget. Often modeled as a cosmological constant (Λ), but its physical origin remains one of the deepest mysteries in physics.
◇ Dark Matter: Invisible matter that does not emit, absorb, or reflect light but exerts gravitational influence. Dark matter outweighs ordinary matter ~5:1 and is essential for explaining galaxy rotation curves and cosmic structure formation.
★ Emission: Light produced and sent out by a source, often at specific wavelengths determined by atomic transitions. Emission lines appear bright against a darker background.
◇ Extinction: The dimming of light by dust through a combination of absorption and scattering. Extinction is wavelength-dependent: blue light is extinguished more than red (interstellar reddening).
★ Flux (F): The amount of light energy arriving at a detector per unit time per unit area. Measured in erg s⁻¹ cm⁻². Flux depends on both luminosity and distance: $F = L/(4\pi d^2)$.
★ Frequency (ν): The number of wave cycles passing a point per second, measured in Hertz (Hz). Higher frequency means higher energy photons: $E = h\nu$.
◇ Hubble Constant (H₀): The current expansion rate of the universe, approximately 70 km/s/Mpc. A galaxy 1 Mpc away recedes at ~70 km/s; one 10 Mpc away recedes at ~700 km/s. The inverse of H₀ gives a rough estimate of the universe's age.
★ Inference: Drawing conclusions about quantities we cannot directly access (like a star's temperature) from quantities we can measure (like its color). Inference requires a model—a physical relationship connecting observable to unobservable.
★ Inverse-square law: The principle that flux decreases with the square of distance: double the distance, quarter the brightness. $F = L/(4\pi d^2)$. This geometric spreading is why distance determination is central to astronomy.
◇ Ionized: An atom that has lost one or more electrons, leaving it with a net positive charge. Ionization requires energy (heat or UV radiation). Ionized gas emits different spectral lines than neutral gas.
★ Light-year: The distance light travels in one year—approximately $10^{18}$ cm or about 63,000 AU. A unit of distance, not time. Proxima Centauri is 4.2 light-years away.
◇ Lookback time: The time light takes to travel from a distant object to us—we see the object as it was that long ago. Looking at the Andromeda Galaxy (2.5 million light-years away), we see it as it was 2.5 million years ago.
★ Luminosity (L): The total light energy emitted by a source per unit time—its intrinsic brightness. Measured in erg/s or solar luminosities ($L_\odot$). Luminosity is what the object emits; flux is what we receive.
★ Model: A mathematical relationship encoding physical assumptions that connects what we measure to what we want to know. Models can be tested by comparing their predictions to new observations.
◇ Neutral: An atom with equal numbers of protons and electrons, having no net electric charge. Neutral hydrogen emits at 21 cm (radio); ionized hydrogen emits at 656 nm (optical).
★ Observable: A quantity that can be directly measured—brightness, position, wavelength, timing. Astronomers have remarkably few types of observables; nearly everything else is inferred.
★ Photon: A discrete packet of electromagnetic energy; the quantum of light. Each photon carries energy $E = h\nu$, where $h$ is Planck's constant and $\nu$ is frequency.
◇ Planck's constant (h): A fundamental constant that relates photon energy to frequency: $h = 6.63 \times 10^{-27}$ erg·s. The appearance of $h$ in an equation signals that quantum mechanics is involved.
◇ Redshift (Cosmological): The stretching of light wavelengths due to the expansion of space. More distant galaxies show larger redshifts because light has traveled through more expanding space to reach us.
◇ Rotation Curve: A plot of orbital speed versus distance from a galaxy's center. Flat rotation curves—where speed stays constant at large radii—are the primary evidence for dark matter halos around galaxies.
★ Spectroscopy: The technique of spreading light into its component wavelengths to analyze composition, temperature, and motion. Spectroscopy transformed astronomy from cataloging what's there to understanding what it's made of.
★ Spectrum: Brightness measured as a function of wavelength—the distribution of light across different colors/energies. A spectrum contains far more information than a single brightness measurement.
★ Speed of light (c): The speed at which light travels in vacuum: $c = 3 \times 10^{10}$ cm/s. A universal constant—the speed limit of the universe. Nothing with mass can reach this speed.
◇ Standard candle: An object whose intrinsic luminosity can be determined independently, allowing distance to be calculated from observed brightness. Examples: Cepheid variables (period → luminosity), Type Ia supernovae (consistent peak brightness).
◇ Thermal radiation: Light emitted by an object due to its temperature—all objects above absolute zero emit thermal radiation. Hotter objects emit more light at shorter wavelengths (Wien's law).
★ Wavelength (λ): The spatial period of a light wave—the distance between successive crests. Measured in nanometers (nm) for visible light, centimeters for radio. Related to frequency by $c = \lambda\nu$.
◇ ΛCDM: The standard cosmological model: Λ (cosmological constant/dark energy) + CDM (Cold Dark Matter). Fits observations remarkably well with only six free parameters, but leaves dark matter and dark energy unexplained.

Lecture 2: Tools of the Trade — Math Survival Kit

★ Astronomical Unit (AU): The average distance from Earth to the Sun, approximately $1.5 \times 10^{11}$ m. A convenient unit for solar system distances. Neptune is ~30 AU from the Sun.
★ Parsec (pc): The distance at which 1 AU subtends an angle of 1 arcsecond; approximately 3.26 light-years or $3.1 \times 10^{16}$ m. Derived from 'parallax arcsecond.' The nearest star is about 1.3 pc away.
★ Ratio method: A problem-solving technique that compares two situations using the same physical law, canceling constants and simplifying calculations. Avoids big-number pain: instead of computing absolute values, compare ratios.
★ Scientific notation: A way of writing numbers as a coefficient times a power of ten, e.g., $3.0 \times 10^8$ m/s. Essential for handling the extreme scales in astronomy without drowning in zeros.
◇ SI prefix: Prefixes that denote powers of ten: kilo (10³), mega (10⁶), giga (10⁹), micro (10⁻⁶), nano (10⁻⁹), etc. Allows compact notation: 1 km = 1000 m; 1 nm = 10⁻⁹ m.

Lecture 3: The Sky Is a Map

★ Angular size: The angle an object subtends as seen by an observer, measured in degrees, arcminutes, or arcseconds. Angular size depends on physical size and distance; for small angles, θ (radians) ≈ D/d where D is diameter and d is distance.
★ Axial tilt: The angle between a planet's rotational axis and its orbital axis; Earth's tilt is 23.5°. The cause of Earth's seasons—NOT distance from the Sun.
★ Celestial equator: Earth's equator projected outward onto the celestial sphere. A great circle that divides the celestial sphere into northern and southern hemispheres.
★ Celestial pole: A point on the celestial sphere directly above Earth's geographic pole. Stars appear to circle the celestial poles due to Earth's rotation.
★ Celestial sphere: An imaginary sphere centered on the observer onto which all celestial objects are projected. A coordinate system for directions on the sky, not a representation of actual distances.
◇ Constellation: A named pattern of stars on the sky. Constellations are defined by directions (angles) on the celestial sphere; the stars in a constellation are not necessarily close together in space.
★ Ecliptic: The Sun's apparent yearly path across the celestial sphere, corresponding to the plane of Earth's orbit. Tilted by about 23.5° relative to the celestial equator; the zodiac constellations lie along it.
★ Equinox: A time when the Sun crosses the celestial equator, making day and night approximately equal in length worldwide. Occurs around March 20 and September 22; exact day length depends slightly on refraction and the Sun's finite size.
◇ Great circle: Any circle on a sphere whose plane passes through the sphere's center. The celestial equator, ecliptic, and horizon are all great circles.
◇ Horizon: The great circle that marks the boundary between directions above and below your local ground plane. Depends on your location on Earth.
★ Solstice: A time when the Sun reaches its farthest angular distance north or south of the celestial equator, producing the longest or shortest day. Occurs around June 21 and December 21; from Latin for 'Sun stands still'.
◇ Zenith: The point on the celestial sphere directly overhead for a given observer. Depends on your location on Earth.

Lecture 4: Moon Geometry

◇ Annular eclipse: A solar eclipse in which the Moon appears smaller than the Sun, leaving a ring ('annulus') of sunlight visible. Occurs when the Moon is near apogee in its elliptical orbit.
◇ Apogee: The point in the Moon's orbit when it is farthest from Earth (~406,700 km). At apogee, the Moon appears smallest; if a solar eclipse occurs, it cannot be total.
◇ Ascending node: The point where the Moon crosses the ecliptic plane moving from south to north. One of two points where the Moon's tilted orbit intersects the ecliptic.
◇ Blood moon: Colloquial term for the reddish appearance of the Moon during a total lunar eclipse. Caused by Earth's atmosphere filtering and refracting sunlight into the shadow zone.
◇ Conjunction: An alignment where two objects appear in nearly the same direction on the sky. New moon corresponds to conjunction of the Moon with the Sun as seen from Earth.
◇ Crescent (Moon): A Moon phase with less than half of the Earth-facing disk appearing illuminated. A waxing crescent occurs after new moon; a waning crescent occurs before new moon.
◇ Descending node: The point where the Moon crosses the ecliptic plane moving from north to south. One of two points where the Moon's tilted orbit intersects the ecliptic.
★ Eclipse: An astronomical event where one celestial body passes into the shadow of another. Solar eclipses occur when the Moon's shadow falls on Earth; lunar eclipses occur when the Moon passes through Earth's shadow.
◇ Eclipse season: A period of about 34–35 days during which eclipses are possible, occurring when the Sun is near one of the Moon's orbital nodes. Two eclipse seasons occur each year, about 173 days apart.
★ First quarter: The lunar phase when the Moon is 90° east of the Sun. Right half illuminated (as seen from the Northern Hemisphere); rises around noon.
★ Full moon: The lunar phase when the Moon is opposite the Sun, with essentially 100% of the Earth-facing side illuminated. Rises at sunset and sets at sunrise.
◇ Gibbous: A lunar phase between quarter and full, with more than half but less than all of the Moon's face illuminated. From Latin 'gibbosus' meaning 'humpbacked'.
★ Lunar eclipse: An eclipse that occurs when the Moon passes through Earth's shadow. Possible only at full moon when the Moon is near a node; visible from the entire night side of Earth.
★ New moon: The lunar phase when the Moon is between Earth and the Sun, with the illuminated side facing away from Earth. The Moon is lost in the Sun's glare during this phase.
★ Node: One of two points where the Moon's orbital plane intersects the ecliptic plane. Eclipses can only occur when the Moon is near a node during new moon (solar) or full moon (lunar) phase.
◇ Opposition: An alignment where two objects appear 180° apart on the sky. Full moon corresponds to opposition of the Moon relative to the Sun.
◇ Penumbra: The lighter, outer portion of a shadow where only part of the light source is blocked. The Moon passing through Earth's penumbra creates a subtle penumbral lunar eclipse.
◇ Perigee: The point in the Moon's orbit when it is closest to Earth (~356,500 km). At perigee, the Moon appears largest; total solar eclipses are possible.
★ Phase (lunar): The apparent shape of the Moon's illuminated portion as seen from Earth, determined by the Moon's position relative to the Sun. The eight traditional phases cycle over about 29.5 days.
◇ Saros cycle: A period of approximately 18 years, 11 days, and 8 hours after which similar eclipses repeat. Results from the alignment of the synodic, draconic, and anomalistic months.
★ Solar eclipse: An eclipse that occurs when the Moon's shadow falls on Earth. Possible only at new moon when the Moon is near a node; visible only from a limited area (the shadow path).
★ Synodic month: The time for the Moon to complete one cycle of phases (new moon to new moon), approximately 29.5 days. Longer than the sidereal month because Earth is also moving around the Sun.
★ Third quarter: The lunar phase when the Moon is 90° west of the Sun. Left half illuminated (as seen from the Northern Hemisphere); rises around midnight.
◇ Totality: The phase of a total eclipse when the Sun (solar eclipse) or Moon (lunar eclipse) is completely blocked or immersed in shadow. During total solar eclipses, the corona becomes visible.
★ Umbra: The darkest, central portion of a shadow where the light source is completely blocked. The Moon entering Earth's umbra creates a total or partial lunar eclipse.
◇ Waning: Decreasing in illumination; the Moon is waning between full moon and new moon. From Old English 'wanian' meaning 'to lessen'.
◇ Waxing: Increasing in illumination; the Moon is waxing between new moon and full moon. From Old English 'weaxan' meaning 'to grow'.

Lecture 5: From Ancient Skies to Kepler’s Laws

★ Aphelion: The point in a planet's orbit when it is farthest from the Sun. Earth reaches aphelion in early July.
★ Eccentricity (e): A measure of how elongated an ellipse is, ranging from 0 (circle) to just under 1 (very elongated). Earth's orbit has e ≈ 0.017 (nearly circular); Pluto's has e ≈ 0.25.
★ Ellipse: An oval shape defined by two foci; the sum of distances from any point to both foci is constant. A circle is a special ellipse where both foci coincide.
★ Empirical law: A pattern or relationship derived from observations, without a theoretical explanation for why it holds. Kepler's laws were empirical—they described what happens but not why.
★ Kepler's First Law: Planets orbit the Sun in ellipses, with the Sun at one focus. Replaced the ancient assumption of perfect circular orbits.
★ Kepler's Second Law: A line from the Sun to a planet sweeps out equal areas in equal times. Planets move faster when closer to the Sun, slower when farther.
★ Kepler's Third Law: The square of a planet's orbital period is proportional to the cube of its semi-major axis: $P^2 \propto a^3$. In solar system units, $P^2 = a^3$ (years, AU). Connects orbital size to orbital time.
★ Perihelion: The point in a planet's orbit when it is closest to the Sun. Earth reaches perihelion in early January.
◇ Retrograde motion: The apparent backward (westward) motion of a planet against the background stars. Caused by Earth overtaking outer planets or being overtaken by inner planets.
★ Semi-major axis (a): Half the longest diameter of an ellipse; the average distance from the orbiting body to the focus. For planetary orbits, it's the 'average' orbital radius used in Kepler's Third Law.

Lecture 6: Newton’s Revolution — From Patterns to Physics

★ Centripetal force: The net inward force required to keep an object moving in a circle: $F = mv^2/r$. For orbital motion, gravity provides the centripetal force.
◇ Gravitational constant (G): The fundamental constant in Newton's law of gravitation: $G = 6.67 \times 10^{-11}$ N·m²/kg². Determines the strength of gravity; measured by Cavendish in 1798.
★ Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force. Also called the law of inertia.
★ Newton's form of Kepler's Third Law: The relationship $P^2 = 4\pi^2 a^3 / (GM)$ that allows mass to be determined from orbital measurements. This is how we 'weigh' stars, planets, and galaxies.
★ Newton's Second Law: The acceleration of an object is proportional to the net force and inversely proportional to its mass: $\vec{F}_{net} = m\vec{a}$. Force causes acceleration, not velocity.
★ Newton's Third Law: For every action, there is an equal and opposite reaction. Forces come in pairs acting on different objects.
★ Orbital velocity: The speed required for a stable circular orbit: $v = \sqrt{GM/r}$. Depends on the central mass and orbital radius, not the orbiting object's mass.
★ Universal Gravitation: Every mass attracts every other mass with force $F = GMm/r^2$, where G is the gravitational constant. The same law governs falling apples and orbiting moons.

Lecture 7: The Cosmic Messenger — Light Carries Information

◇ Atmospheric window: A range of wavelengths that can pass through Earth's atmosphere without being absorbed. Visible and radio windows allow ground-based astronomy; other wavelengths require space telescopes.
★ Electromagnetic spectrum: The full range of electromagnetic radiation, from radio waves (long wavelength) to gamma rays (short wavelength). Visible light is a tiny slice; most of the spectrum is invisible to human eyes.
◇ Gamma rays: The highest-energy electromagnetic radiation, with wavelengths < 0.01 nm. Produced in extreme events: supernovae, pulsars, active galactic nuclei.
◇ Infrared (IR): Electromagnetic radiation between radio and visible, with wavelengths ~1 μm to 1 mm. Reveals warm dust and can penetrate through gas clouds that block visible light.
◇ Radio waves: Electromagnetic radiation with the longest wavelengths (> 1 mm), used to study cold gas and cosmic backgrounds. Radio telescopes can observe day or night, through clouds.
◇ Rayleigh scattering: The scattering of light by particles much smaller than the wavelength, with efficiency proportional to 1/λ⁴. Explains why the sky is blue (blue light scattered more than red) and sunsets are red.
◇ Ultraviolet (UV): Electromagnetic radiation between visible and X-rays, with wavelengths ~10–400 nm. Absorbed by Earth's ozone layer; requires space telescopes to observe.
◇ X-rays: High-energy electromagnetic radiation with wavelengths ~0.01–10 nm. Emitted by hot gas (millions of K) around black holes and in galaxy clusters.

Lecture 8: Reading the Glow — Temperature Written in Light

★ Blackbody spectrum: The characteristic continuous spectrum emitted by an idealized thermal radiator, determined only by temperature. The shape is universal; only the peak position and height change with temperature.
★ L-T-R Relation: The relationship $L = 4\pi R^2 \sigma T^4$ connecting luminosity, radius, and temperature. Knowing any two quantities allows calculation of the third.
★ Stefan-Boltzmann Law: The total power radiated by a blackbody is proportional to the fourth power of temperature: $L = 4\pi R^2 \sigma T^4$. Doubling temperature increases luminosity by 16×.
★ Wien's Law: The peak wavelength of a blackbody spectrum is inversely proportional to temperature: $\lambda_{peak} = b/T$. Hotter objects peak at shorter (bluer) wavelengths; b = 2.9 × 10⁶ nm·K.

Lecture 9: Decoding Starlight — Spectral Lines and Chemical Fingerprints

★ Balmer series: The set of hydrogen emission/absorption lines corresponding to transitions to/from the n=2 energy level. Includes Hα (656 nm, red), Hβ (486 nm, blue-green), Hγ (434 nm, violet).
★ Energy level: A discrete allowed energy state for an electron in an atom; electrons can only occupy specific levels. Transitions between levels produce photons with specific energies (and wavelengths).
◇ Excited state: Any energy level above the ground state; atoms in excited states tend to emit photons and drop to lower levels. An atom can be excited by absorbing a photon or through collisions.
◇ Ground state: The lowest energy level of an atom; the n=1 level for hydrogen. Atoms 'prefer' the ground state; excited atoms quickly return to it.
★ Kirchhoff's First Law: A hot, dense gas or solid produces a continuous spectrum (all wavelengths). Examples: the interior of a star, a glowing filament.
★ Kirchhoff's Second Law: A hot, low-density gas produces an emission-line spectrum (bright lines at specific wavelengths). Examples: neon signs, nebulae.
★ Kirchhoff's Third Law: A cool gas in front of a hot source produces an absorption-line spectrum (dark lines at specific wavelengths). This is what we see in stellar spectra—absorption lines from the cooler outer atmosphere.
★ OBAFGKM: The spectral classification sequence for stars, ordered from hottest (O, ~40,000 K) to coolest (M, ~3,000 K). Mnemonic: 'Oh Be A Fine Girl/Guy, Kiss Me.' Based on line strengths, which depend on temperature.
★ Spectral line: A bright or dark feature at a specific wavelength in a spectrum, corresponding to an atomic transition. Each element has a unique pattern of lines—its 'fingerprint'.

Lecture 10: Motion Revealed — Doppler Effect and the Astronomer’s Toolkit

★ Angular resolution: A telescope's ability to distinguish closely-spaced objects; limited by $\theta \approx 1.22\lambda/D$. Larger aperture and shorter wavelength give finer resolution.
★ Blueshift: A shift to shorter wavelengths, indicating the source is approaching the observer. $\lambda_{obs} < \lambda_0$; negative $\Delta\lambda$.
★ Doppler effect: The change in observed wavelength due to relative motion between source and observer. Approaching sources are blueshifted; receding sources are redshifted.
◇ Interferometry: Combining signals from multiple telescopes to achieve the resolution of a single telescope the size of their separation. Used extensively in radio astronomy; enabled imaging of a black hole's shadow.
★ Light-gathering power: A telescope's ability to collect light, proportional to the area of its primary mirror: $\propto D^2$. Doubling diameter quadruples the light collected.
★ Radial velocity: The component of an object's velocity along the line of sight (toward or away from the observer). This is what Doppler measurements detect; transverse motion produces no Doppler shift.
◇ Rayleigh criterion: The formula $\theta = 1.22\lambda/D$ for diffraction-limited angular resolution. Sets the fundamental limit; atmospheric seeing often dominates for ground telescopes.
★ Redshift: A shift to longer wavelengths, indicating the source is receding from the observer. $\lambda_{obs} > \lambda_0$; positive $\Delta\lambda$.
★ Rest wavelength (λ₀): The wavelength of a spectral line as measured in the laboratory, with no relative motion. The reference against which Doppler shifts are measured.
◇ Transverse velocity: The component of an object's velocity perpendicular to the line of sight (across the sky). Produces proper motion but no Doppler shift.

Lecture 11: Our Cosmic Backyard — Solar System Architecture & Formation

★ Frost line: The distance from the Sun (~3 AU) beyond which water ice could condense in the early solar nebula. Explains why rocky planets are close and gas/ice giants are far.
★ Gas giant: A large planet composed primarily of hydrogen and helium; Jupiter and Saturn. No solid surface; formed beyond the frost line where more material was available.
★ Ice giant: A large planet composed primarily of heavier volatiles (water, ammonia, methane); Uranus and Neptune. Distinguished from gas giants by their composition and smaller size.
◇ Kuiper Belt: A region of icy bodies beyond Neptune, from ~30 to ~50 AU. Home to Pluto, Eris, and many other dwarf planets and comets.
★ Nebular hypothesis: The theory that the solar system formed from a rotating disk of gas and dust around the young Sun. Explains common orbital direction, planetary spacing, and composition gradient.
◇ Oort Cloud: A hypothetical spherical shell of icy objects at ~10,000–100,000 AU, the source of long-period comets. Never directly observed; inferred from comet orbits.
◇ Protoplanetary disk: A rotating disk of gas and dust around a young star from which planets form. Observed around many young stars; confirms the nebular hypothesis.
★ Terrestrial planet: A rocky planet with a solid surface; Mercury, Venus, Earth, and Mars in our solar system. Small, dense, close to the Sun; formed inside the frost line.

Lecture 12: Climates and Exoplanets

★ Bond albedo: The fraction of total incoming stellar energy reflected back to space by a planet, averaged over wavelength and direction. Energy balance uses absorbed fraction $(1-A)$, where $A$ is Bond albedo.
★ Bulk density: Average density of a planet, computed from mass and radius. $\rho = M/(\tfrac{4}{3}\pi R^3)$; used to infer broad composition classes (rocky vs gas-rich).
★ Chemical disequilibrium: A state where reactive atmospheric gases coexist away from thermodynamic equilibrium and require continuous replenishment. Coexisting O$_2$ and CH$_4$ is a canonical biosignature candidate.
◇ Effective emission level: The characteristic altitude in an atmosphere from which most thermal radiation escapes to space. If greenhouse opacity increases, this level shifts higher and colder, so the surface must warm to restore balance.
★ Equilibrium temperature: The temperature a planet would have if absorbed sunlight balanced emitted thermal radiation, ignoring atmospheric greenhouse warming. For a planet at distance d with albedo A: $T_{\mathrm{eq}} \propto [(1-A)/d^2]^{1/4}$.
★ Exoplanet: A planet orbiting a star other than the Sun. Over 5,000 confirmed as of 2024; many are unlike anything in our solar system.
◇ Exoplanet atmosphere: The gaseous envelope around an exoplanet, inferred remotely through spectra and transit/eclipse observations. Atmospheric composition constrains climate, chemistry, and possible biosignatures.
★ Greenhouse effect: The warming of a planet's surface when atmospheric gases absorb and re-emit infrared radiation. CO₂, H₂O, CH₄ are key greenhouse gases; Venus is an extreme example.
★ Greenhouse gas: A gas that absorbs and re-emits infrared radiation, tending to warm a planet's surface. Key examples include H$_2$O, CO$_2$, CH$_4$, and N$_2$O.
★ Habitable zone: The range of orbital distances from a star where liquid water could exist on a planet's surface. Also called the 'Goldilocks zone'; depends on stellar luminosity.
◇ Hot Jupiter: A gas giant exoplanet orbiting very close to its star (< 0.1 AU), with orbital periods of days. Easiest to detect but puzzling—gas giants should form far from their stars.
◇ Insolation flux: The stellar energy flux received by a planet at its orbit. Often reported in Earth-flux units to compare irradiation levels across exoplanets.
★ Minimum mass ($M_p\sin i$): The lower bound on planet mass measured by radial velocity when inclination is unknown. If a planet also transits, then $\sin i \approx 1$ and radial velocity gives near-true mass.
★ Orbital inclination (i): The tilt of an orbit relative to the observer's line of sight. Transits require nearly edge-on geometry ($i \approx 90^{\circ}$).
★ Planetary atmosphere: A layer of gases surrounding a planet that can absorb, emit, and transport energy. Atmospheric thickness and composition control greenhouse warming strength.
★ Planetary climate: The long-term thermal and atmospheric state of a planet, set by energy input, energy loss, and atmospheric/oceanic processes. In this lecture: sunlight in, infrared out, plus greenhouse feedbacks.
★ Planetary habitability: The potential of a planet to support life, commonly evaluated first by the possibility of stable liquid water. Habitable zone membership is a starting screen, not a guarantee of habitability.
★ Radial velocity method: Detecting exoplanets by measuring the Doppler shift caused by the star's wobble around the system's center of mass. Gives minimum planet mass; most effective for massive planets close to their stars.
◇ Runaway greenhouse: A feedback loop where rising temperatures evaporate more water, which traps more heat, leading to extreme warming. May explain Venus's current state; a cautionary tale for Earth's climate.
★ Transit depth: The fractional drop in stellar brightness during a transit. To first order, depth $\approx (R_p/R_*)^2$, so depth constrains planet radius.
★ Transit method: Detecting exoplanets by measuring the tiny dimming when a planet passes in front of its star. Gives planet size; requires edge-on orbital alignment.
◇ Transit probability: The chance that an exoplanet system is aligned so a transit is visible from Earth. Approximately scales as $R_*/a$ for circular orbits, so close-in planets transit more often.
★ Transmission spectroscopy: Measuring wavelength-dependent starlight filtering through a transiting planet's atmosphere to identify atmospheric gases. Compare in-transit and out-of-transit spectra to isolate atmospheric absorption features.

Lecture 13: Are We Alone?

◇ Abiogenesis: The natural process by which life arises from non-living matter. How common is this? We have one example (Earth) and don't know the probability.
★ Biosignature: A detectable sign of life, such as atmospheric gases (O₂, CH₄) that are unlikely without biological processes. A key target for future telescope observations of exoplanet atmospheres.
★ Drake Equation: A framework for estimating the number of communicating civilizations in the galaxy: N = R* × fp × ne × fl × fi × fc × L. Not a calculation but a way to organize our ignorance about astrobiology.
★ Fermi Paradox: The apparent contradiction between the high probability of extraterrestrial civilizations and the lack of evidence for them. 'Where is everybody?' — Enrico Fermi, 1950.
◇ Great Filter: A hypothetical barrier in the evolution of life that makes advanced civilizations extremely rare. Could be in our past (we passed it) or our future (we haven't reached it yet).
◇ SETI: Search for Extraterrestrial Intelligence; the scientific effort to detect signals from alien civilizations. Primarily searches for artificial radio or optical signals.

Alphabetical Index

For quick lookup, here’s an alphabetical listing of all Module 1 terms:

◇ Abiogenesis: The natural process by which life arises from non-living matter. How common is this? We have one example (Earth) and don't know the probability.
★ Absorption: Light removed from a beam by intervening material, often at specific wavelengths where atoms absorb photons. Absorption lines appear dark against a brighter continuum.
★ Angular resolution: A telescope's ability to distinguish closely-spaced objects; limited by $\theta \approx 1.22\lambda/D$. Larger aperture and shorter wavelength give finer resolution.
★ Angular size: The angle an object subtends as seen by an observer, measured in degrees, arcminutes, or arcseconds. Angular size depends on physical size and distance; for small angles, θ (radians) ≈ D/d where D is diameter and d is distance.
◇ Annular eclipse: A solar eclipse in which the Moon appears smaller than the Sun, leaving a ring ('annulus') of sunlight visible. Occurs when the Moon is near apogee in its elliptical orbit.
★ Aphelion: The point in a planet's orbit when it is farthest from the Sun. Earth reaches aphelion in early July.
◇ Apogee: The point in the Moon's orbit when it is farthest from Earth (~406,700 km). At apogee, the Moon appears smallest; if a solar eclipse occurs, it cannot be total.
◇ Ascending node: The point where the Moon crosses the ecliptic plane moving from south to north. One of two points where the Moon's tilted orbit intersects the ecliptic.
★ Astronomical Unit (AU): The average distance from Earth to the Sun, approximately $1.5 \times 10^{11}$ m. A convenient unit for solar system distances. Neptune is ~30 AU from the Sun.
◇ Atmospheric window: A range of wavelengths that can pass through Earth's atmosphere without being absorbed. Visible and radio windows allow ground-based astronomy; other wavelengths require space telescopes.
★ Axial tilt: The angle between a planet's rotational axis and its orbital axis; Earth's tilt is 23.5°. The cause of Earth's seasons—NOT distance from the Sun.
★ Balmer series: The set of hydrogen emission/absorption lines corresponding to transitions to/from the n=2 energy level. Includes Hα (656 nm, red), Hβ (486 nm, blue-green), Hγ (434 nm, violet).
★ Biosignature: A detectable sign of life, such as atmospheric gases (O₂, CH₄) that are unlikely without biological processes. A key target for future telescope observations of exoplanet atmospheres.
★ Blackbody: An idealized object that absorbs all incident radiation and emits a characteristic spectrum determined only by its temperature. Stars approximate blackbodies. The blackbody spectrum peaks at $\lambda_{\text{peak}} = b/T$ (Wien's law).
★ Blackbody spectrum: The characteristic continuous spectrum emitted by an idealized thermal radiator, determined only by temperature. The shape is universal; only the peak position and height change with temperature.
◇ Blood moon: Colloquial term for the reddish appearance of the Moon during a total lunar eclipse. Caused by Earth's atmosphere filtering and refracting sunlight into the shadow zone.
★ Blueshift: A shift to shorter wavelengths, indicating the source is approaching the observer. $\lambda_{obs} < \lambda_0$; negative $\Delta\lambda$.
★ Bond albedo: The fraction of total incoming stellar energy reflected back to space by a planet, averaged over wavelength and direction. Energy balance uses absorbed fraction $(1-A)$, where $A$ is Bond albedo.
★ Bulk density: Average density of a planet, computed from mass and radius. $\rho = M/(\tfrac{4}{3}\pi R^3)$; used to infer broad composition classes (rocky vs gas-rich).
★ Celestial equator: Earth's equator projected outward onto the celestial sphere. A great circle that divides the celestial sphere into northern and southern hemispheres.
★ Celestial pole: A point on the celestial sphere directly above Earth's geographic pole. Stars appear to circle the celestial poles due to Earth's rotation.
★ Celestial sphere: An imaginary sphere centered on the observer onto which all celestial objects are projected. A coordinate system for directions on the sky, not a representation of actual distances.
★ Centripetal force: The net inward force required to keep an object moving in a circle: $F = mv^2/r$. For orbital motion, gravity provides the centripetal force.
★ Chemical disequilibrium: A state where reactive atmospheric gases coexist away from thermodynamic equilibrium and require continuous replenishment. Coexisting O$_2$ and CH$_4$ is a canonical biosignature candidate.
◇ Conjunction: An alignment where two objects appear in nearly the same direction on the sky. New moon corresponds to conjunction of the Moon with the Sun as seen from Earth.
◇ Constellation: A named pattern of stars on the sky. Constellations are defined by directions (angles) on the celestial sphere; the stars in a constellation are not necessarily close together in space.
◇ Cosmic Web: The large-scale structure of the universe: galaxies arranged in filaments, walls, and clusters, separated by vast voids. The cosmic web is still evolving—gravity continues to pull matter from voids into filaments and clusters.
◇ Crescent (Moon): A Moon phase with less than half of the Earth-facing disk appearing illuminated. A waxing crescent occurs after new moon; a waning crescent occurs before new moon.
◇ Dark Energy: The unknown cause of the accelerating expansion of the universe, comprising ~68% of the cosmic energy budget. Often modeled as a cosmological constant (Λ), but its physical origin remains one of the deepest mysteries in physics.
◇ Dark Matter: Invisible matter that does not emit, absorb, or reflect light but exerts gravitational influence. Dark matter outweighs ordinary matter ~5:1 and is essential for explaining galaxy rotation curves and cosmic structure formation.
◇ Descending node: The point where the Moon crosses the ecliptic plane moving from north to south. One of two points where the Moon's tilted orbit intersects the ecliptic.
★ Doppler effect: The change in observed wavelength due to relative motion between source and observer. Approaching sources are blueshifted; receding sources are redshifted.
★ Drake Equation: A framework for estimating the number of communicating civilizations in the galaxy: N = R* × fp × ne × fl × fi × fc × L. Not a calculation but a way to organize our ignorance about astrobiology.
★ Eccentricity (e): A measure of how elongated an ellipse is, ranging from 0 (circle) to just under 1 (very elongated). Earth's orbit has e ≈ 0.017 (nearly circular); Pluto's has e ≈ 0.25.
★ Eclipse: An astronomical event where one celestial body passes into the shadow of another. Solar eclipses occur when the Moon's shadow falls on Earth; lunar eclipses occur when the Moon passes through Earth's shadow.
◇ Eclipse season: A period of about 34–35 days during which eclipses are possible, occurring when the Sun is near one of the Moon's orbital nodes. Two eclipse seasons occur each year, about 173 days apart.
★ Ecliptic: The Sun's apparent yearly path across the celestial sphere, corresponding to the plane of Earth's orbit. Tilted by about 23.5° relative to the celestial equator; the zodiac constellations lie along it.
◇ Effective emission level: The characteristic altitude in an atmosphere from which most thermal radiation escapes to space. If greenhouse opacity increases, this level shifts higher and colder, so the surface must warm to restore balance.
★ Electromagnetic spectrum: The full range of electromagnetic radiation, from radio waves (long wavelength) to gamma rays (short wavelength). Visible light is a tiny slice; most of the spectrum is invisible to human eyes.
★ Ellipse: An oval shape defined by two foci; the sum of distances from any point to both foci is constant. A circle is a special ellipse where both foci coincide.
★ Emission: Light produced and sent out by a source, often at specific wavelengths determined by atomic transitions. Emission lines appear bright against a darker background.
★ Empirical law: A pattern or relationship derived from observations, without a theoretical explanation for why it holds. Kepler's laws were empirical—they described what happens but not why.
★ Energy level: A discrete allowed energy state for an electron in an atom; electrons can only occupy specific levels. Transitions between levels produce photons with specific energies (and wavelengths).
★ Equilibrium temperature: The temperature a planet would have if absorbed sunlight balanced emitted thermal radiation, ignoring atmospheric greenhouse warming. For a planet at distance d with albedo A: $T_{\mathrm{eq}} \propto [(1-A)/d^2]^{1/4}$.
★ Equinox: A time when the Sun crosses the celestial equator, making day and night approximately equal in length worldwide. Occurs around March 20 and September 22; exact day length depends slightly on refraction and the Sun's finite size.
◇ Excited state: Any energy level above the ground state; atoms in excited states tend to emit photons and drop to lower levels. An atom can be excited by absorbing a photon or through collisions.
★ Exoplanet: A planet orbiting a star other than the Sun. Over 5,000 confirmed as of 2024; many are unlike anything in our solar system.
◇ Exoplanet atmosphere: The gaseous envelope around an exoplanet, inferred remotely through spectra and transit/eclipse observations. Atmospheric composition constrains climate, chemistry, and possible biosignatures.
◇ Extinction: The dimming of light by dust through a combination of absorption and scattering. Extinction is wavelength-dependent: blue light is extinguished more than red (interstellar reddening).
★ Fermi Paradox: The apparent contradiction between the high probability of extraterrestrial civilizations and the lack of evidence for them. 'Where is everybody?' — Enrico Fermi, 1950.
★ First quarter: The lunar phase when the Moon is 90° east of the Sun. Right half illuminated (as seen from the Northern Hemisphere); rises around noon.
★ Flux (F): The amount of light energy arriving at a detector per unit time per unit area. Measured in erg s⁻¹ cm⁻². Flux depends on both luminosity and distance: $F = L/(4\pi d^2)$.
★ Frequency (ν): The number of wave cycles passing a point per second, measured in Hertz (Hz). Higher frequency means higher energy photons: $E = h\nu$.
★ Frost line: The distance from the Sun (~3 AU) beyond which water ice could condense in the early solar nebula. Explains why rocky planets are close and gas/ice giants are far.
★ Full moon: The lunar phase when the Moon is opposite the Sun, with essentially 100% of the Earth-facing side illuminated. Rises at sunset and sets at sunrise.
◇ Gamma rays: The highest-energy electromagnetic radiation, with wavelengths < 0.01 nm. Produced in extreme events: supernovae, pulsars, active galactic nuclei.
★ Gas giant: A large planet composed primarily of hydrogen and helium; Jupiter and Saturn. No solid surface; formed beyond the frost line where more material was available.
◇ Gibbous: A lunar phase between quarter and full, with more than half but less than all of the Moon's face illuminated. From Latin 'gibbosus' meaning 'humpbacked'.
◇ Gravitational constant (G): The fundamental constant in Newton's law of gravitation: $G = 6.67 \times 10^{-11}$ N·m²/kg². Determines the strength of gravity; measured by Cavendish in 1798.
◇ Great circle: Any circle on a sphere whose plane passes through the sphere's center. The celestial equator, ecliptic, and horizon are all great circles.
◇ Great Filter: A hypothetical barrier in the evolution of life that makes advanced civilizations extremely rare. Could be in our past (we passed it) or our future (we haven't reached it yet).
★ Greenhouse effect: The warming of a planet's surface when atmospheric gases absorb and re-emit infrared radiation. CO₂, H₂O, CH₄ are key greenhouse gases; Venus is an extreme example.
★ Greenhouse gas: A gas that absorbs and re-emits infrared radiation, tending to warm a planet's surface. Key examples include H$_2$O, CO$_2$, CH$_4$, and N$_2$O.
◇ Ground state: The lowest energy level of an atom; the n=1 level for hydrogen. Atoms 'prefer' the ground state; excited atoms quickly return to it.
★ Habitable zone: The range of orbital distances from a star where liquid water could exist on a planet's surface. Also called the 'Goldilocks zone'; depends on stellar luminosity.
◇ Horizon: The great circle that marks the boundary between directions above and below your local ground plane. Depends on your location on Earth.
◇ Hot Jupiter: A gas giant exoplanet orbiting very close to its star (< 0.1 AU), with orbital periods of days. Easiest to detect but puzzling—gas giants should form far from their stars.
◇ Hubble Constant (H₀): The current expansion rate of the universe, approximately 70 km/s/Mpc. A galaxy 1 Mpc away recedes at ~70 km/s; one 10 Mpc away recedes at ~700 km/s. The inverse of H₀ gives a rough estimate of the universe's age.
★ Ice giant: A large planet composed primarily of heavier volatiles (water, ammonia, methane); Uranus and Neptune. Distinguished from gas giants by their composition and smaller size.
★ Inference: Drawing conclusions about quantities we cannot directly access (like a star's temperature) from quantities we can measure (like its color). Inference requires a model—a physical relationship connecting observable to unobservable.
◇ Infrared (IR): Electromagnetic radiation between radio and visible, with wavelengths ~1 μm to 1 mm. Reveals warm dust and can penetrate through gas clouds that block visible light.
◇ Insolation flux: The stellar energy flux received by a planet at its orbit. Often reported in Earth-flux units to compare irradiation levels across exoplanets.
◇ Interferometry: Combining signals from multiple telescopes to achieve the resolution of a single telescope the size of their separation. Used extensively in radio astronomy; enabled imaging of a black hole's shadow.
★ Inverse-square law: The principle that flux decreases with the square of distance: double the distance, quarter the brightness. $F = L/(4\pi d^2)$. This geometric spreading is why distance determination is central to astronomy.
◇ Ionized: An atom that has lost one or more electrons, leaving it with a net positive charge. Ionization requires energy (heat or UV radiation). Ionized gas emits different spectral lines than neutral gas.
★ Kepler's First Law: Planets orbit the Sun in ellipses, with the Sun at one focus. Replaced the ancient assumption of perfect circular orbits.
★ Kepler's Second Law: A line from the Sun to a planet sweeps out equal areas in equal times. Planets move faster when closer to the Sun, slower when farther.
★ Kepler's Third Law: The square of a planet's orbital period is proportional to the cube of its semi-major axis: $P^2 \propto a^3$. In solar system units, $P^2 = a^3$ (years, AU). Connects orbital size to orbital time.
★ Kirchhoff's First Law: A hot, dense gas or solid produces a continuous spectrum (all wavelengths). Examples: the interior of a star, a glowing filament.
★ Kirchhoff's Second Law: A hot, low-density gas produces an emission-line spectrum (bright lines at specific wavelengths). Examples: neon signs, nebulae.
★ Kirchhoff's Third Law: A cool gas in front of a hot source produces an absorption-line spectrum (dark lines at specific wavelengths). This is what we see in stellar spectra—absorption lines from the cooler outer atmosphere.
◇ Kuiper Belt: A region of icy bodies beyond Neptune, from ~30 to ~50 AU. Home to Pluto, Eris, and many other dwarf planets and comets.
★ L-T-R Relation: The relationship $L = 4\pi R^2 \sigma T^4$ connecting luminosity, radius, and temperature. Knowing any two quantities allows calculation of the third.
★ Light-gathering power: A telescope's ability to collect light, proportional to the area of its primary mirror: $\propto D^2$. Doubling diameter quadruples the light collected.
★ Light-year: The distance light travels in one year—approximately $10^{18}$ cm or about 63,000 AU. A unit of distance, not time. Proxima Centauri is 4.2 light-years away.
◇ Lookback time: The time light takes to travel from a distant object to us—we see the object as it was that long ago. Looking at the Andromeda Galaxy (2.5 million light-years away), we see it as it was 2.5 million years ago.
★ Luminosity (L): The total light energy emitted by a source per unit time—its intrinsic brightness. Measured in erg/s or solar luminosities ($L_\odot$). Luminosity is what the object emits; flux is what we receive.
★ Lunar eclipse: An eclipse that occurs when the Moon passes through Earth's shadow. Possible only at full moon when the Moon is near a node; visible from the entire night side of Earth.
★ Minimum mass ($M_p\sin i$): The lower bound on planet mass measured by radial velocity when inclination is unknown. If a planet also transits, then $\sin i \approx 1$ and radial velocity gives near-true mass.
★ Model: A mathematical relationship encoding physical assumptions that connects what we measure to what we want to know. Models can be tested by comparing their predictions to new observations.
★ Nebular hypothesis: The theory that the solar system formed from a rotating disk of gas and dust around the young Sun. Explains common orbital direction, planetary spacing, and composition gradient.
◇ Neutral: An atom with equal numbers of protons and electrons, having no net electric charge. Neutral hydrogen emits at 21 cm (radio); ionized hydrogen emits at 656 nm (optical).
★ New moon: The lunar phase when the Moon is between Earth and the Sun, with the illuminated side facing away from Earth. The Moon is lost in the Sun's glare during this phase.
★ Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force. Also called the law of inertia.
★ Newton's form of Kepler's Third Law: The relationship $P^2 = 4\pi^2 a^3 / (GM)$ that allows mass to be determined from orbital measurements. This is how we 'weigh' stars, planets, and galaxies.
★ Newton's Second Law: The acceleration of an object is proportional to the net force and inversely proportional to its mass: $\vec{F}_{net} = m\vec{a}$. Force causes acceleration, not velocity.
★ Newton's Third Law: For every action, there is an equal and opposite reaction. Forces come in pairs acting on different objects.
★ Node: One of two points where the Moon's orbital plane intersects the ecliptic plane. Eclipses can only occur when the Moon is near a node during new moon (solar) or full moon (lunar) phase.
★ OBAFGKM: The spectral classification sequence for stars, ordered from hottest (O, ~40,000 K) to coolest (M, ~3,000 K). Mnemonic: 'Oh Be A Fine Girl/Guy, Kiss Me.' Based on line strengths, which depend on temperature.
★ Observable: A quantity that can be directly measured—brightness, position, wavelength, timing. Astronomers have remarkably few types of observables; nearly everything else is inferred.
◇ Oort Cloud: A hypothetical spherical shell of icy objects at ~10,000–100,000 AU, the source of long-period comets. Never directly observed; inferred from comet orbits.
◇ Opposition: An alignment where two objects appear 180° apart on the sky. Full moon corresponds to opposition of the Moon relative to the Sun.
★ Orbital inclination (i): The tilt of an orbit relative to the observer's line of sight. Transits require nearly edge-on geometry ($i \approx 90^{\circ}$).
★ Orbital velocity: The speed required for a stable circular orbit: $v = \sqrt{GM/r}$. Depends on the central mass and orbital radius, not the orbiting object's mass.
★ Parsec (pc): The distance at which 1 AU subtends an angle of 1 arcsecond; approximately 3.26 light-years or $3.1 \times 10^{16}$ m. Derived from 'parallax arcsecond.' The nearest star is about 1.3 pc away.
◇ Penumbra: The lighter, outer portion of a shadow where only part of the light source is blocked. The Moon passing through Earth's penumbra creates a subtle penumbral lunar eclipse.
◇ Perigee: The point in the Moon's orbit when it is closest to Earth (~356,500 km). At perigee, the Moon appears largest; total solar eclipses are possible.
★ Perihelion: The point in a planet's orbit when it is closest to the Sun. Earth reaches perihelion in early January.
★ Phase (lunar): The apparent shape of the Moon's illuminated portion as seen from Earth, determined by the Moon's position relative to the Sun. The eight traditional phases cycle over about 29.5 days.
★ Photon: A discrete packet of electromagnetic energy; the quantum of light. Each photon carries energy $E = h\nu$, where $h$ is Planck's constant and $\nu$ is frequency.
◇ Planck's constant (h): A fundamental constant that relates photon energy to frequency: $h = 6.63 \times 10^{-27}$ erg·s. The appearance of $h$ in an equation signals that quantum mechanics is involved.
★ Planetary atmosphere: A layer of gases surrounding a planet that can absorb, emit, and transport energy. Atmospheric thickness and composition control greenhouse warming strength.
★ Planetary climate: The long-term thermal and atmospheric state of a planet, set by energy input, energy loss, and atmospheric/oceanic processes. In this lecture: sunlight in, infrared out, plus greenhouse feedbacks.
★ Planetary habitability: The potential of a planet to support life, commonly evaluated first by the possibility of stable liquid water. Habitable zone membership is a starting screen, not a guarantee of habitability.
◇ Protoplanetary disk: A rotating disk of gas and dust around a young star from which planets form. Observed around many young stars; confirms the nebular hypothesis.
★ Radial velocity: The component of an object's velocity along the line of sight (toward or away from the observer). This is what Doppler measurements detect; transverse motion produces no Doppler shift.
★ Radial velocity method: Detecting exoplanets by measuring the Doppler shift caused by the star's wobble around the system's center of mass. Gives minimum planet mass; most effective for massive planets close to their stars.
◇ Radio waves: Electromagnetic radiation with the longest wavelengths (> 1 mm), used to study cold gas and cosmic backgrounds. Radio telescopes can observe day or night, through clouds.
★ Ratio method: A problem-solving technique that compares two situations using the same physical law, canceling constants and simplifying calculations. Avoids big-number pain: instead of computing absolute values, compare ratios.
◇ Rayleigh criterion: The formula $\theta = 1.22\lambda/D$ for diffraction-limited angular resolution. Sets the fundamental limit; atmospheric seeing often dominates for ground telescopes.
◇ Rayleigh scattering: The scattering of light by particles much smaller than the wavelength, with efficiency proportional to 1/λ⁴. Explains why the sky is blue (blue light scattered more than red) and sunsets are red.
★ Redshift: A shift to longer wavelengths, indicating the source is receding from the observer. $\lambda_{obs} > \lambda_0$; positive $\Delta\lambda$.
◇ Redshift (Cosmological): The stretching of light wavelengths due to the expansion of space. More distant galaxies show larger redshifts because light has traveled through more expanding space to reach us.
★ Rest wavelength (λ₀): The wavelength of a spectral line as measured in the laboratory, with no relative motion. The reference against which Doppler shifts are measured.
◇ Retrograde motion: The apparent backward (westward) motion of a planet against the background stars. Caused by Earth overtaking outer planets or being overtaken by inner planets.
◇ Rotation Curve: A plot of orbital speed versus distance from a galaxy's center. Flat rotation curves—where speed stays constant at large radii—are the primary evidence for dark matter halos around galaxies.
◇ Runaway greenhouse: A feedback loop where rising temperatures evaporate more water, which traps more heat, leading to extreme warming. May explain Venus's current state; a cautionary tale for Earth's climate.
◇ Saros cycle: A period of approximately 18 years, 11 days, and 8 hours after which similar eclipses repeat. Results from the alignment of the synodic, draconic, and anomalistic months.
★ Scientific notation: A way of writing numbers as a coefficient times a power of ten, e.g., $3.0 \times 10^8$ m/s. Essential for handling the extreme scales in astronomy without drowning in zeros.
★ Semi-major axis (a): Half the longest diameter of an ellipse; the average distance from the orbiting body to the focus. For planetary orbits, it's the 'average' orbital radius used in Kepler's Third Law.
◇ SETI: Search for Extraterrestrial Intelligence; the scientific effort to detect signals from alien civilizations. Primarily searches for artificial radio or optical signals.
◇ SI prefix: Prefixes that denote powers of ten: kilo (10³), mega (10⁶), giga (10⁹), micro (10⁻⁶), nano (10⁻⁹), etc. Allows compact notation: 1 km = 1000 m; 1 nm = 10⁻⁹ m.
★ Solar eclipse: An eclipse that occurs when the Moon's shadow falls on Earth. Possible only at new moon when the Moon is near a node; visible only from a limited area (the shadow path).
★ Solstice: A time when the Sun reaches its farthest angular distance north or south of the celestial equator, producing the longest or shortest day. Occurs around June 21 and December 21; from Latin for 'Sun stands still'.
★ Spectral line: A bright or dark feature at a specific wavelength in a spectrum, corresponding to an atomic transition. Each element has a unique pattern of lines—its 'fingerprint'.
★ Spectroscopy: The technique of spreading light into its component wavelengths to analyze composition, temperature, and motion. Spectroscopy transformed astronomy from cataloging what's there to understanding what it's made of.
★ Spectrum: Brightness measured as a function of wavelength—the distribution of light across different colors/energies. A spectrum contains far more information than a single brightness measurement.
★ Speed of light (c): The speed at which light travels in vacuum: $c = 3 \times 10^{10}$ cm/s. A universal constant—the speed limit of the universe. Nothing with mass can reach this speed.
◇ Standard candle: An object whose intrinsic luminosity can be determined independently, allowing distance to be calculated from observed brightness. Examples: Cepheid variables (period → luminosity), Type Ia supernovae (consistent peak brightness).
★ Stefan-Boltzmann Law: The total power radiated by a blackbody is proportional to the fourth power of temperature: $L = 4\pi R^2 \sigma T^4$. Doubling temperature increases luminosity by 16×.
★ Synodic month: The time for the Moon to complete one cycle of phases (new moon to new moon), approximately 29.5 days. Longer than the sidereal month because Earth is also moving around the Sun.
★ Terrestrial planet: A rocky planet with a solid surface; Mercury, Venus, Earth, and Mars in our solar system. Small, dense, close to the Sun; formed inside the frost line.
◇ Thermal radiation: Light emitted by an object due to its temperature—all objects above absolute zero emit thermal radiation. Hotter objects emit more light at shorter wavelengths (Wien's law).
★ Third quarter: The lunar phase when the Moon is 90° west of the Sun. Left half illuminated (as seen from the Northern Hemisphere); rises around midnight.
◇ Totality: The phase of a total eclipse when the Sun (solar eclipse) or Moon (lunar eclipse) is completely blocked or immersed in shadow. During total solar eclipses, the corona becomes visible.
★ Transit depth: The fractional drop in stellar brightness during a transit. To first order, depth $\approx (R_p/R_*)^2$, so depth constrains planet radius.
★ Transit method: Detecting exoplanets by measuring the tiny dimming when a planet passes in front of its star. Gives planet size; requires edge-on orbital alignment.
◇ Transit probability: The chance that an exoplanet system is aligned so a transit is visible from Earth. Approximately scales as $R_*/a$ for circular orbits, so close-in planets transit more often.
★ Transmission spectroscopy: Measuring wavelength-dependent starlight filtering through a transiting planet's atmosphere to identify atmospheric gases. Compare in-transit and out-of-transit spectra to isolate atmospheric absorption features.
◇ Transverse velocity: The component of an object's velocity perpendicular to the line of sight (across the sky). Produces proper motion but no Doppler shift.
◇ Ultraviolet (UV): Electromagnetic radiation between visible and X-rays, with wavelengths ~10–400 nm. Absorbed by Earth's ozone layer; requires space telescopes to observe.
★ Umbra: The darkest, central portion of a shadow where the light source is completely blocked. The Moon entering Earth's umbra creates a total or partial lunar eclipse.
★ Universal Gravitation: Every mass attracts every other mass with force $F = GMm/r^2$, where G is the gravitational constant. The same law governs falling apples and orbiting moons.
◇ Waning: Decreasing in illumination; the Moon is waning between full moon and new moon. From Old English 'wanian' meaning 'to lessen'.
★ Wavelength (λ): The spatial period of a light wave—the distance between successive crests. Measured in nanometers (nm) for visible light, centimeters for radio. Related to frequency by $c = \lambda\nu$.
◇ Waxing: Increasing in illumination; the Moon is waxing between new moon and full moon. From Old English 'weaxan' meaning 'to grow'.
★ Wien's Law: The peak wavelength of a blackbody spectrum is inversely proportional to temperature: $\lambda_{peak} = b/T$. Hotter objects peak at shorter (bluer) wavelengths; b = 2.9 × 10⁶ nm·K.
◇ X-rays: High-energy electromagnetic radiation with wavelengths ~0.01–10 nm. Emitted by hot gas (millions of K) around black holes and in galaxy clusters.
◇ Zenith: The point on the celestial sphere directly overhead for a given observer. Depends on your location on Earth.
◇ ΛCDM: The standard cosmological model: Λ (cosmological constant/dark energy) + CDM (Cold Dark Matter). Fits observations remarkably well with only six free parameters, but leaves dark matter and dark energy unexplained.