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AST201H1 Study Guide - Midterm Guide: Emission Spectrum, Escape Velocity, Redshift

Astronomy & Astrophysics
Course Code
Michael Reid
Study Guide

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AST201 Mid-Term Test #1
Universe the totality of all space, time, energy and matter
o About 13.7 billion years old
o Unobservable universe is not limited by lack of technology; rather, the laws of physics prevents it this means there is a
limit imposed by nature to how far we can see
How big is the universe?
o Big problem because (a) it can't be measured; and (b) you must measure the distance with your eyes without touching
Solar system one or more planets orbiting a star (e.g. planets and their moons; asteroids; comets)
Galaxy a collection of billions of stars all orbiting a common center
o Milky Way Galaxy and Andromeda Galaxy are spiral galaxies
Star a large glowing ball of very hot gas that generates heat and light (energy) through nuclear fusion in its core (e.g. Sun)
o Born when gravity compresses the material into a cloud to the point where the center becomes dense enough and hot
enough to generate energy by nuclear fusion, the process in which lightweight atomic nuclei smash together and stick (or
fuse) to make heavier nuclei
o Star "lives" as long as it can shine with energy from fusion; "dies" when it exhausts its usable fuel
Moon (or satellite) an object that orbits a planet
Local Group a group of galaxies among 40 galaxies
Galaxy clusters groups of galaxies with more than a few dozen members
Light travels at a constant, finite speed c = 300 million m/s
Light year the distance light travels in one year
o 1 light year = 9.46 trillion km
If there was a galaxy 100 billion light years away from you, and given that the universe is only 13.7 billion years old, the light
would not have reached you yet
When we see Andromeda (i.e. 2.5 million light years away), we see it as it WAS 2.5 million years ago
A cupboard that is 0.0000000000002 light years away we see it as it was 0.0000000000002 years ago
Astronomical unit (AU) the average distance from Earth to the Sun
o 1 AU = 150 million km
Ancient astronomy was often astrological in nature
o Astrology making observations in the sky and correlating them to make predictions
Ancient astronomy was geocentric, or Earth-centred
For thousands of years, astronomers insisted on the Pythagorean ideal of circular motion
Planet = wanderers (in Greek) because they move in space
Nicholas Copernicus (1473-1543)
o Sun-centred model of solar system
Galileo Galilei (1564-1642)
o First to use telescope to look at sky saw craters on the moon which shattered the belief that everything in space was
o First evidence that something orbited something other than the Earth
o Published a book in Italian so that the commoners could understand it too (before it was only Latin)
Used characters that resembled himself and the pope
o Contributed the idea that if you have a scientific theory, you should check it against reality (scientific method)
Johannes Kepler (1571-1630)
o Took Galileo's data and codified them into a couple laws of planetary motion
Isaac Newton (1642-1727)
o Invented three laws of motion; contributed earliest observations of optics

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Kepler's Laws of Planetary Motion
1ST LAW the orbit of each planet around the Sun is an ellipse with the Sun at one focus
o Closest at the point called perihelion; furthest at point called aphelion
2ND LAW as a planet moves around its orbit, it sweeps out equal areas in equal times
o Planet travels faster when nearer to the Sun; slower when further away from the Sun
3RD LAW more distant planets orbit the Sun at slower average speeds; p2 = a3
o p = period, time taken to complete one orbit; a = orbital semimajor axis, a half of the longest axis of an ellipse
o More distant planets move more slowly
Hallmarks of Science
Modern science seeks explanations for observed phenomena that rely solely on natural causes
Science progresses through the creation and testing of models of nature that explain the observations as simply as
As scientific model must make testable predictions about natural phenomena that will force us to revise or abandon the
model if the predictions do not agree with observations
Occam's Razor principle holding that scientists should prefer the simpler of two models that agree equally well with
Falsifiable something that can be tested to determine whether true of false
Scientific Method
A TEST (experiment or additional observation)
o If test DOES NOT support hypothesis revise hypothesis or make new one
o If test DOES support hypothesis make additional predictions and test them
Theory a model of some aspect of nature that has been rigorously tested and has
passed all tests to date
o Often misused phrase
o In science, nothing is superior to a theory
CHAPTER 4: MAKING SENSE OF THE UNIVERSE Understanding Motion, Energy, and Gravity
Acceleration when velocity is changing in any way, either in speed or direction or both
Acceleration of gravity the acceleration of a falling object; approximately = 9.8 m/s2
Momentum product of an object's mass and velocity; momentum = mass x velocity
Force anything that can cause a change in momentum
Net force overall force to which an object responds; equal to the rate of change in the object's momentum, or
equivalently to the object's mass x acceleration
Angular momentum momentum attributable to rotation or revolution; equals m x v x r
Free-fall condition in which an object is falling without resistance; objects are weightless when in free-fall
o Only external force acting on it is gravity

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The Four Fundamental Forces
Strong nuclear force holds atomic nuclei together
Electromagnetism makes atoms cling to each other
Weak nuclear force breaks atoms apart
Gravity makes masses attract one another
o Because gravity's range is infinite, everything in the universe is aware of your mass
o Unique because it only ever adds up there is no "anti-gravity"; the more electric charges you add, the greater the
mass becomes
o Gravity only pulls, never pushes
Newton's Three Laws of Motion
1ST LAW an object at rest will remain at rest, and a moving object will remain at constant velocity unless acted upon by
a net force to change its speed or direction
2ND LAW force = mass x acceleration (Fnet = ma)
3RD LAW for every action force, there is always an equal and opposite reaction force
Conservation Laws
Conservation of momentum principle that, in the absence of net force, the total momentum of a system remains
Conservation of angular momentum principle that, in the absence of net torque (twisting force), the total angular
momentum of a system remains constant
Conservation of energy principle that energy (including mass-energy) can be neither created nor destroyed, but can
only change from one form to another
Types of Energy
Kinetic energy of motion
Radiative energy carried by light
Potential energy stored for later conversion into kinetic energy
Thermal collective kinetic energy, as measured by temperature, of the many individual particles moving within a
Temperature a measure of the average kinetic energy of particles in a substance
Gravitational potential energy that an object has by virtue of its position in a gravitational field; an object has more
gravitational potential energy when it has a greater distance that it can potentially fall
Mass-energy potential energy of mass, which has an amount E = mc2
Universal Law of Gravitation
ONE: every mass attracts every other mass through the force called gravity
TWO: the strength of the gravitational force attracting any two objects is directly proportional to the product of their masses
THREE: the strength of gravity between two objects decreases with the square of the distance between their centers
o Ex. Double the distance between two objects weakens the force of gravity by a factor of 4
, where G = 6.67 x 10-11m3/(kg x s2) gravitational constant
Bound orbits orbits in which an object goes around another object over and
over again
Unbound orbits paths that bring an object close to another object just once
Center of mass the point at which two objects would balance if they were
somehow connected
Escape velocity the speed necessary for an object to completely escape the
gravity of a large body such as a moon, planet, or star
o Doesn't depend on the direction in which the object is thrown
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