AST201 Definitions SEM22013
Star A large glowing ball of gas that generates heat and light though nuclear fusion in its
core. Our Sun is a star.
Planet A moderately large object that orbits a star and shines primarily by reflecting light
from its star. According to a definition approved in 2006, an object can be considered a
planet only if it: 1. Orbits a star, 2. Is large enough for its own gravity to make it round, and 3.
Has cleared most other objects from its orbital path. An object that meets the first two criteria
but has not cleared its orbital path, like Pluto, is designated a dwarf planet.
Dwarf Planet Orbits a star, 2. Is large enough for its own gravity to make it round but had
not cleared its orbital path.
Moon (or Satellite) An object that orbits a planet. The term satellite can refer to any object
orbiting another object
Asteroid A relatively small and rocky object that orbits a star
Comet A relatively small and ice-rich object that orbits a star
Small Solar System Body An asteroid, comet, or other object that orbits a star but is too
small to qualify as a planet or dwarf planet
COLLECTIONS OF ASTRONOMICAL OBJECTS:
Solar System The sun and all the material that orbits it, including planets, dwarf planets, and
small solar system bodies. Although the term solar system technically refers only to our own
star system (solar mean “of the sun”), it is often applied to other star systems.
Star System A star (sometimes more than one star) and any planets and other material that
Galaxy A great island of stars in space, containing from a few hundred million to a trillion or
more stars, all held together by gravity and orbiting a common center
Cluster (or Group) of Galaxies A collection of galaxies bound together by gravity. Small
collections (up to a few dozen galaxies) are generally called groups, while larger collections
are called clusters AST201 Definitions SEM22013
Superclusters A gigantic region of space where many individual galaxies and many groups
and clusters of galaxies are packed more closely together than elsewhere in the universe.
Universe (or Cosmos) The sum total of all matter and energy – that is, all galaxies and
everything between them
Observable Universe The portion of the entire universe that can be seen from earth, at
least in principle. The observable universe is probably only a tiny portion of the entire
ASTRONOMICAL DISTANCE UNITS:
Astronomical Unit (AU) The average distance between earth and the sun, which is about
150 million km. More technically, 1 AU is the length of the semimajor axis of earth’s orbit.
Light Year The distance that light can travel in 1 year, which is about 9.46 trillion km.
TERMS RELATING TO MOTION:
Rotation The spinning of an object around its axis. For example, earth roates once each
day around its axis, which is an imaginary line connecting the north pole to the south pole
through the center of earth.
Orbit (Revolution) The orbital motion of one object around another. For example, earth
orbits around the sun once each year.
Expansion (Of The Universe) The increase in the average distance between galaxies as time
progresses AST201 Definitions SEM22013
Chapter One Definitions: (Refer to ASTRONOMICAL OBJECTS AND UNITS)
Milky Way Used both as the name of our galaxy and to refer to the band of light we see in
the sky when we look into the plane of the Milky Way galaxy
Axis Tilt (of a planet in our solar system)The amount by which a planet’s axis is tilted with
respect to a line perpendicular to the ecliptic plane
Ecliptic Plane The plane of earth’s orbit around the sun
Chapter Three Definitions:
Model (scientific) A representation of some aspect of nature that can be used to explain
and predict real phenomena without evoking myth, magic, or the supernatural.
Geocentric model Any of the ancient Greek models that were used to predict planetary
positions under the assumptions that earth lay in the center of the universe
Ptolemaic model The geocentric model of the universe developed by Ptolemy in about 150
Chapter Four Definitions:
Speed The rate at which an object moves. Its units are distance divided by time, such as
m/s or km/h.
Velocity The combination of speed and direction of motion. It can be stated as a speed in a
particular direction, such as 100km/hr due north.
Acceleration The rate at which an object’s velocity changes. Its standard units are m/s^2.
Momentum The product of an object’s mass and velocity
Force Anything that can cause a change in momentum
Weightless A weight of zero, as occurs during free fall
Freefall The condition in which an object is falling without resistance. Objects are
weightless when in free fall. AST201 Definitions SEM22013
Conservation of momentum (law of) The principle that, in absence of net force, the total
momentum of a system remains constant
Conservation of angular momentum (law of) The principle that, in the absence of net
torque (twisting force), the total angular momentum of a system remains constant
Kinetic energy Energy of motion, given by the formula 1/2mv^2
Radiative energy Energy carried by light. The energy of a photon is Planck’s constant times
its frequency, or h x f.
Potential energy Energy stored for later conversion into kine