Astronomy 3: The Nature of the Universe - Midterm 1 Vocabulary

Comprehensive vocabulary flashcards covering the astronomical principles, observational concepts, physical laws of motion and gravity, light and matter interactions, telescopic properties, and solar system formation models presented in Astronomy 3 lectures 1 through 8.

Astronomical Unit (AU)

The average distance between the Earth and the Sun, equal to approximately 93 million miles or $1.49 \times 10^{11}\,\text{m}$.

Light-Year

The distance light can travel in one year at the speed of light ($c \approx 300,000\,\text{km/s}$), which equals nearly 10 trillion kilometers (about 6 trillion miles) or $9.46 \times 10^{15}\,\text{m}$.

Star

A very large, glowing ball of mostly hydrogen gas that generates heat and light by nuclear fusion deep inside.

Planet

A moderately large object that orbits a star and shines primarily by reflected light, with a rocky, icy, or gaseous composition.

Nebula

A huge interstellar cloud of gas and/or dust, composed mostly of hydrogen gas along with tiny solid grains like carbon or silicon.

Galaxy

An enormous "island of stars" far out in space, all held together by gravity and orbiting a common center.

Universe

The sum total of all matter and energy; that is, everything within and between all galaxies.

Constellation

A specific region of the sky defined by official borders established in 1928 by the International Astronomical Union (IAU), dividing the entire sky into 88 official areas.

Celestial Sphere

An imaginary two-dimensional sphere surrounding Earth upon which the stars, Sun, Moon, and planets appear to reside.

Ecliptic

The annual path of the Sun through the celestial sphere, which represents the projection of Earth's orbital plane against the sky.

Zenith

The point in the local sky directly overhead an observer, positioned at an altitude angle of $90^\circ$ above the horizon.

Horizon

The boundary between the Earth and the sky, consisting of all points located $90^\circ$ from the zenith.

Meridian

An imaginary half-circle in the local sky running from due north on the horizon, passing through the zenith, to due south on the horizon.

Circumpolar Stars

Stars situated close enough to an observer's celestial pole that their daily circles never cross below the horizon, remaining visible at all times of the night.

Precession

A roughly 26,000-year cyclical wobbling of Earth's rotation axis like a spinning top, which slowly alters the celestial coordinates of the equinoxes and celestial poles.

Nodes (Lunar Orbit)

The two points where the Moon's tilted orbital plane ($5.2^\circ$ relative to the ecliptic) intersects the ecliptic plane of Earth's orbit around the Sun.

Metonic Cycle

A repetitive astronomical cycle of approximately 19 years (equal to 235 lunar months), after which the Moon's phases recur on the exact same solar calendar dates.

Apparent Retrograde Motion

The apparent backward (westward) drift of a planet relative to the background stars that occurs when Earth laps an outer planet (or when an inner planet laps Earth).

Stellar Parallax

The slight apparent angular shift in a star's position against distant background stars caused by the changing viewing angle as Earth orbits the Sun.

Ptolemaic Model

A geocentric model developed by Ptolemy that explained planetary retrograde motion by having each planet move along a small circle (epicycle) whose center orbited Earth on a larger circle.

Ellipse

A closed geometric curve shaped like an elongated circle, characterized by two internal foci, a semi-major axis ($a$), and a semi-minor axis ($b$).

Eccentricity

A quantitative measurement of how much an ellipse deviates from a perfect circle, ranging from $e = 0$ for a perfect circle toward 1 for increasingly elongated shapes.

Kepler's First Law

The fundamental law stating that the orbit of each planet around the Sun is an ellipse with the Sun situated at one focus.

Kepler's Second Law

The principle stating that an imaginary line connecting a planet to the Sun sweeps out equal areas in equal intervals of time, causing the planet to travel fastest at perihelion and slowest at aphelion.

Kepler's Third Law

The law showing that more distant planets orbit at slower average speeds, governed by the mathematical relationship $p^2 = a^3$, where $p$ is orbital period in years and $a$ is semi-major axis in AU.

Occam's Razor

The scientific principle asserting that when evaluating competing hypotheses that explain observations equally well, the simplest explanation requiring the fewest assumptions is favored.

Scientific Method

An organized iterative methodology of acquiring knowledge through observations, question formulation, hypothesis development, testable predictions, and experiments.

Velocity

A vector quantity specifying both an object's speed and its direction of motion.

Acceleration

Any change in an object's velocity over time, involving changes in speed, changes in direction of motion, or both, measured in units of $\text{m/s}^2$.

Acceleration of Gravity ($g$)

The uniform rate of acceleration with which freely falling objects gain downward speed near Earth's surface in the absence of air friction, approximately equal to $9.8\,\text{m/s}^2$ (or $\approx 10\,\text{m/s}^2$).

Mass

The intrinsic quantity of matter contained within an object, which determines its resistance to changes in motion (inertia) and is measured in kilograms.

Weight

The downward gravitational force acting on an object's mass ($W = mg$), measured in Newtons or $\text{kg}\cdot\text{m/s}^2$.

Newton's First Law of Motion

The law of inertia, establishing that an object at rest or moving at constant velocity will remain in that state unless acted on by a net external force.

Newton's Second Law of Motion

The dynamic law stating that force equals mass times acceleration ($F = ma$), or that force equals the time rate of change of momentum.

Newton's Third Law of Motion

The physical law establishing that for every exerted force, there is always an equal and opposite reaction force acting on the other interacting object.

Conservation of Angular Momentum

The principle stating that an object's angular momentum ($m \times v \times r$) remains unchanged unless an external torque acts on it, requiring a decrease in radius ($r$) to cause an increase in rotational velocity ($v$).

Thermal Energy

A subtype of kinetic energy representing the total collective kinetic energy of all moving particles within a substance, depending on both temperature and particle density.

Gravitational Potential Energy

The energy stored in an object due to its position in a gravitational field ($\text{P.E.} = mgh$ on Earth), which converts into kinetic energy as the object falls or as a gas cloud collapses.

Universal Law of Gravitation

Isaac Newton's formulation stating that every mass attracts every other mass with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers ($F_g = G \frac{M_1 M_2}{d^2}$).

Escape Velocity

The minimum speed required for an object to permanently overcome the gravitational pull of a celestial body ($v_{\text{esc}} = \sqrt{\frac{2GM}{R}}$), equal to approximately $11.1\,\text{km/s}$ from Earth's surface.

Newton's Version of Kepler's Third Law

The generalized orbital equation $p^2 = \frac{4\pi^2}{G(M_1 + M_2)} a^3$, which enables the determination of celestial masses by measuring orbital period and average distance.

Tidal Friction

The friction caused by Earth rotating ahead of its gravitational tidal bulges, which gradually slows Earth's rotation and transfers angular momentum outward to the Moon.

Photon

A localized packet or particle of electromagnetic radiation carrying a discrete quantity of energy given by $E = hf = \frac{hc}{\lambda}$.

Electromagnetic Spectrum

The full continuum of light arranged by wavelength or frequency: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves.

Thermal Radiation

Continuous electromagnetic radiation emitted by opaque, dense objects whose spectrum and peak emission wavelength depend solely on the object's temperature.

Stefan-Boltzmann Law

A radiation law establishing that hotter objects radiate significantly more total power per unit surface area, scaling with the fourth power of temperature ($F = \sigma T^4$).

Wien's Law

A physical law stating that the peak wavelength of thermal emission from an object is inversely proportional to its absolute temperature ($\lambda_{\text{peak}} = \frac{2.9 \times 10^6\,\text{nm}\cdot\text{K}}{T}$).

Doppler Effect

The shift in observed wavelength of light caused by motion toward or away from an observer, producing a blueshift for approaching objects and a redshift for receding objects.

Light-Collecting Area

The measure of a telescope's ability to capture photons, which scales directly with the square of the objective mirror or lens diameter ($\text{Area} \propto D^2$).

Angular Resolution

The smallest angular separation at which two separate objects can be distinguished by an optical system, scaling inversely with telescope diameter ($1/D$).

Reflecting Telescope

A telescope design that utilizes curved mirrors instead of primary lenses to gather and focus light onto a detector or eyepiece.

Adaptive Optics

An observational technology that monitors atmospheric turbulence and rapidly modifies mirror curvature hundreds of times per second to neutralize atmospheric blurring.

Interferometry

An astronomical technique that combines light from multiple separated telescopes to achieve the angular resolution of a single telescope spanning the entire baseline.

Terrestrial Planets

The four inner planets of the solar system—Mercury, Venus, Earth, and Mars—which are small, rocky, dense, possess solid surfaces, and have few or no moons.

Jovian Planets

The four outer planets—Jupiter, Saturn, Uranus, and Neptune—which are large, low-density gas and ice worlds rich in hydrogen, helium, and hydrogen compounds, with numerous moons and ring systems.

Galilean Moons

The four large moons of Jupiter—Io, Europa, Ganymede, and Callisto—originally observed and recorded by Galileo Galilei in 1610.

Nebular Theory

The standard scientific model stating that the solar system formed 4.6 billion years ago from the gravitational collapse and flattening of a rotating interstellar cloud of gas and dust.

Frost Line

The critical distance from the Sun in the solar nebula inside of which temperatures were too high for hydrogen compounds to freeze, restricting condensation to metals and rocks.

Planetesimals

Solid chunks of condensed rock, metal, or ice in the solar nebula that acted as building blocks and collided to form planets.

Accretion

The process by which microscopic particles and small planetesimals stick together and gravitationally accumulate to form larger planetary bodies.

Giant Impact Hypothesis

The prevailing hypothesis stating that Earth's Moon formed from the orbital debris created when a Mars-sized planetesimal collided with the newly forming Earth.

Radiometric Dating

A technique used to determine the age of rocks and meteorites by analyzing the relative proportions of radioactive parent isotopes (e.g., Potassium-40) and stable daughter isotopes (e.g., Argon-40).