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26 Mar 2011
AST 201 1
Lecture 2 – The Universe
Universe: the totality of all space, time, energy, and matter, no outside; roughly 13.7
billion years old, therefore, we can only see things that are less than this far away, as we
are looking back in time;Universe” usually refers to the observable universe
Light Year: distance light travels in one year = 9.46 trillion km
Speed of light (c): 300 million m/s
Planets: spherical bodies which orbit stars directly; can be rocky, gassy, liquid or icy;
now planets must have cleared their orbit
Stars: enormous spheres of very hot gas which produce energy through nuclear fusion;
produces more energy than it takes in
Solar system: one or more planets orbiting a star; ours is roughly 1ly in length
Galaxy: a collection of billions of stars all orbiting a common centre; we are in the Milky
Way Galaxy, our nearest neighbour is the Andromeda Galaxy, both of them are spiral
Local Group: a few dozen or so small galaxies make up ours, they orbit each other
Galaxy Clusters: whole bunch of galaxy groups, galaxies are all organized into groups
and the groups are all moving away from one another
Super Clusters: enormous groups of galaxies all orbiting one another; 1000s or millions
of galaxies, there is nothing in the universe much bigger; ours is called Virgo Super
cluster because many of its galaxies appear to us in the direction of the constellation
Science: the process of trying to find natural explanations for natural phenomena by
repeatedly following a cycle of hypothesis, prediction, and observation or experiment; for
something to be scientific it must be falsifiable (could be proven false if it is so)
Lecture 2 – Readings (1, 3.4, 3.5)
Rotation: spin
Orbit: revolution around the sun
Astronomical Unit (AU): Earths average orbital distance; 150 million km
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AST 201 2
Ecliptic Plane: the flat plane which is defined by Earths orbital path
Axis Tilt: 23 ½ degree tilt of Earths axis from a line perpendicular to the ecliptic plane;
it point almost directly at the North Star
Occam’s Razor: idea that scientists should prefer the simpler of two models that agree
equally well with observations
Lecture 3 – The Physics of Astronomy
Speed: rate at which object moves; speed = distance/time ; units of m/s (i.e. 10 m/s)
Velocity: speed and direction (i.e. 10 m/s east)
Acceleration: any chance in velocity, units of speed/time (m/s2)
Gravity (g): the acceleration of gravity is the SAME for all falling objects regardless of
Momentum: = mass x velocity
Net Force: changes momentum, which generally means an acceleration
Angular Momentum: rotational momentum of a spinning or orbiting object; = mass x
velocity x radius
Mass: the amount of matter in an object
Weight: the force that acts upon an object; therefore while you are weightless in free fall,
your mass does not change
Newtons first law of motion: an object moves at a constant velocity unless a net force
acts to change its speed or direction
Newtons second law of motion: force = mass x acceleration
Newtons third law of motion: for every force there is always an equal and opposite
reaction force
Conservation of Momentum: the total momentum of interacting objects cannot change
unless an external force is acting on them; interacting objects exchanged momentum
through equal and opposite forces
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AST 201 3
Conservation of Angular Momentum: the angular momentum of an object cannot
change unless there is an external twisting force (torque) acting on it; therefore, angular
momentum stays constant (however, increase in radius=decrease of velocity and vice
Energy: makes matter move; is conserved but it can transfer from one object to another,
change form; three basic types: kinetic (motion), radiative (light), potential/stored; it
cannot be created or destroyed, it can change forms, the total energy content of the
Universe was determined in the Big Bang and remains the same today
Thermal Energy: the collective kinetic energy of many particles (i.e. in a rock, air, etc);it
is related to temperature but it is not the same; it is the total kinetic energy of all the
particles in a substance; it therefore depends on density and temperature (i.e. a pot of hot
water has more thermal energy than a hotter stove)
Temperature: is the average kinetic energy of many particles in a substance
Gravitational potential energy: on Earth it depends on: objects mass (m), strength of
gravity (g), and the distance the object could potentially fall; in space an object or gas
cloud has more gravitational potential energy when it is spread out, than when it contracts
(a contracting cloud converts gravitational potential energy to thermal energy,
conservation of angular momentum leads to a disk being formed)
Mass-Energy: mass is a form of potential energy; concentrated energy can spontaneously
turn into particles; a small about of mass can release a great deal of energy; right after the
Big Bang radiation was the dominantmass in the Universe; E=mc2 (Energy
released/absorbed=mass lost/gained x speed of light, squared)
Lecture 4 – Gravity, Light, Electromagnetic Waves and Spectrum
Keplers First Law: Orbits are ellipses; with the sun at one focus and nothing at
the other
Keplers Second Law: The are swept out in 30 day periods are all equal; as a
result of angular momentum; his second law basically states that speed x radius= constant
Keplers Third Law: period squared (in years) = average distance from the Sun
cubed (in AU) or P2= A3; or more generally: period squared = a constant x total mass x
(average distance between two bodies) cubed; this means that more distant planets orbit
the Sun at slower average speeds
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