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by OC2

Astronomy & Astrophysics
Course Code
Clifford Orwin
Study Guide

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Lecture 1 ± Chapter 1
1 AU (astronomical unite): average distance between Earth and sun ± 150 mil km
Mercury: 0.4 AU
Jupiter: 5 AU
Neptune: 30 AU
Distance light travels in a year: 300 000 km/s
1 light year = 63 000 AU or 10 trillion km
Earth -> Sun = 8 light minutes
Nearest star = 4 light years
Galaxy: island of stars (billions) in space held together by gravity
nearest galaxy: Andromeda = 2.5 mil light years away
Universe: sum total of all matter and energy, that is, everything within and between
Star: glowing ball of gas, generates heat and light through nuclear fusion
Planet: orbits a star, shines by reflecting light ± rocky, icey, gaseous
Moons: orbits a planet
Comet: small, icy object that orbits a star ± has a tail
Solar system: star and all material orbiting around it
Nebula: interstellar cloud of gas or dust
Galaxy: large island of stars held together by gravity, orbit around a common centre
Equator: 1670 km/h rotating speed
Tropics: 1275 km/h
Spin axis tilted by 23.5 degrees (pointing to polaris)
-tilted in direction it orbits (counter clockwise)
-1 AU (150 mil km)
Sun moves relative to other stars ± speeds of 70 000 km/h
-stars take 230 mil years to orbit galaxy
1658: Bishop James Ussher ± Sunday, October 23, 4004 BC
-Bible = not literal
-radioactivity discovered in 1900
-1920: Earth thought to be 4 billion years old
Hubble: -universe expanding, galaxies moving away from us
-farther away something is, faster away it moves ± v=Hsub0d
-13.7 bil years old ± amount of hydrogen burned by oldest stars
-Hubble telescope ± observe galaxies 12.5 bil light years away = what they looked like 12.5 bil
years ago -> can see what universe looked like when it was 1 bil years old
observable universe: portion of the entire universe we can potentially observe ± 14 billion light years away
Big Bang produces H, He ± heavy elements in stars create stars/planets
-atoms made in stars, only simple atoms were in Big Bang, complex atoms created in stars
-stars die, expelled to form new stars/planets
nuclear fusion: process in which lightweight atomic nuclei smash together and stick to make heavier nuclei
Lecture 2 ± Chapter 4
RQHDQRWKHU´± Feynman
Solid: atoms/molecules are held tightly in place
Liquid: atoms/molecules remain together but move relatively easily
Gas: atoms/molecules move unconstrained
Plasma: free electrons move among positively charged ions ± atoms become increasingly ionized
-elegance, simplicity = beauty in science (explaining many things with one basic idea)
-beautiful things are not necessarily true
-total amount of energy was created at the moment of the creation of the Universe ± changes in Universe
are just energy changing into another form (First Law of Thermodynamics).
-energy can be converted into kinetic, potential, thermal, nuclear, geothermal, wind, sound, solar


-energy measured in joules
-three basic types of energy: kinetic (motion), potential (stored), radiative (light)
-thermal energy: total kinetic energy of many particles (depends on
temperature and density)
Law of Conservation of Energy: energy cannot be created or destroyed, only transform itself from one form
into another ± total energy in universe remains constant
Gravitational Potential Energy: how far a mass can fall as a result of gravity (as h decreases, v increases)
-temperature is a measure of the average kinetic energy of the particles ± LIDOOSDUWLFOHVZHLJKWKHVDPHLWV
-the more atoms there are (even if two things are the same temperature), the more heat there is
-atoms are divisible (e.g., plasma)
Lecture 3 ± Chapter 4
speed=distance/time (how far an object will travel in certain time)
velocity=speed with an associated direction
acceleration: how velocity changes over time
-if speed is constant but direction of velocity changes, acceleration changes
momentum=mass x velocity
-momentum can only be changed when a force is applied to an object (where force=mass x
mass=total amount of matter
weight=mass times acceleration of gravity (force on the scale)
Law of inertia: everything in the universe wants to move in a straight line at a constant velocity
-velocity of zero means that if something is stopped ,it wants to stay stopped
-when you see a changing velocity, something is being acted on by a force
Angular momentum: present when object rotates or moves on curved path
-force that changes angular momentum = torque
-acceleraction by gravity: for each second an object falls (ignore air resistance) it increases in speed by
If gravity is so weak, why does it seem to control the Universe?
Because gravity only pulls, and never pushes. So as you add more stuff, you get more gravity. It never
cancels out.
2. They sweep out equal areas in equal times
3. Period squared is distance cubed
-when you are nearer to the sun, you move more quickly
-the more stuff there is, the more gravity there is (from the stuff)
-further away you are from stuff, the less gravity you feel
-the smaller something is, the less gravity it has, and the farther away from the sun you are, the less gravity
there is and things rotate more slowly
-weightlessness is due to falling and not hitting anything ± still subjected to gravity, still have mass
Newton: brought together the heavens and earth, saying physical laws that applied on earth also apply in
space = astrophysics
1. Objects move at a constant velocity or stay at rest until an outside force is acted upon the object
2. Force = mass x acceleration
3. For any force, there is always an equal and opposite force
Law of Conservation of Momentum: total momentum of a system does not change unless an eternal force is
applied to one of the objects in the system
Law of Conservation of Angular Momentum: in the absence of external torque, the total angular
momentum of a system must remain constant
-angular momentum = mass x velocity x radius from sun
Rotational Angular Momentum: as r decreases, the v must increase to compensate
-earth loses some r.a.m. because tides create friction which slow orbit


hyperbolic route
-objects orbit their Common Centre of Mass (the point at which both two objects would balance of
-sun: its mass is so large that this point still lies within the star itself
significantly larger object ± this is the primary method we use to calculate the mass of other planets
Orbital Energy = Gravitation Pot. Energy + Kinetic Energy
-orbits cannot change spontaneously through Gravitational Encounters ± where one object passes closely to
another and thus can feel each others gravitational effects
-gravitational encounters can be used to speed up satellites as they travel through space, as it gains
orbital energy from the planet which it passes
-atmospheric drag only affects low orbit satellites around earth (few hundred Km in altitude)
-transforms its Orbital Energy into Thermal Energy which is evidenced by satellites burning up as
they plummet towards the earth during reentry
does not affect Vescape)
1RWH(DUWV9HNPV40,000 Km/hr)
Universal Law of Gravitation:
-force is directionally proportional to the mass of both objects
-strength of force decreases by square of the distance
-if one would double the distance between two objects, force decreases by a factor of four (2^2=4)
Lecture 4 ± Chapter 2.1-2.2
Constellations: a region of the sky with well-defined borders, not just patterns of stars (we use the stars to
find the constellations)
-88 constellations set by International Astronomical Union in 1928
The Celestial Sphere: ancient Greeks believed that all stars were the same distance from earth and lied on a
µFHOHVWLDOVSKHUH± not true, we lack depth perceptions b/c stars are so far away so they appear to be close
north celestial pole: point directly over (DUWV1RUWK3ROH
celestial sphere
ecliptic: path the Sun follows as it appears to circle around celestial sphere once each year ±
Milky Way: circles around celestial sphere so you can only see part of it at a time
-formed by interstellar gas and dust of the Milky Way Galaxy (we are halfway from the center so
we can see it)
-dark lanes appear in it when strong clouds block the light of the stars behind it
-kept much of the galaxy hidden until technology conqured it
The Local Sky: the half of the celestial sphere you can see at a time (other half hidden by ground)
-meridian: imaginary half circle stretching from horizon due south through zenith to the horizon
due north
-can pinpoint position of object in local sky by knowing direction along horizon (azimuth ±
degrees clockwise from due north) and altitude above horizon
Angular Sizes and Distances: we cannot judge true sizes of objects we see, only angular sizes
angular size of an object: the angle it appears to span in your field of view ± does not tell by itself
REMHFWVtrue size because it also depends on distance
angular distance between a pair of objects: angle that appears to separate them
1 degree = 60 arcminutes
1 arcminute=60 arcseconds
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