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AST201H1 Study Guide - Midterm Guide: Orbital Period

Astronomy & Astrophysics
Course Code
Stefan Mochnacki
Study Guide

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* The definition of "Universe": totality of all space, time, energy, matter; has no outside
* The age and size of the Universe: 13.7 billion years old
- we look back in time when observing things far away, we see how things were when the light left it
* The observable Universe: the contents and the limit to which we can see in the universe, cant see past
the observable universe
* Light travel time and the light year
- light year: distance light travels in a year
- light year: almost ten trillion km
- galaxy hundred billion light years away: light from there takes 100 billion light years to reach us
* The contents of the Universe: planets, asteroids, comets, moons, stars, galaxies, solar system
* Our location in the Universe: in solar system, milky way galaxy
* Gravity and motion
- speed: rate at which object moves
- Velocity: speed and direction
- acceleration: changes in velocity (either speed or direction), same for all objects, mass of object does
not matter
Driving in a straight line is constant velocity, and no acceleration
Objects dropped on the moon fall at the same time, acceleration of gravity same for all objects on moon
* Forces and Newton's Laws
First law: objects move at constant velocity if no net force acts on it to change direction/speed (spacecraft,
Second law: force = mass x acceleration
Third law: for every force there is an equal and opposite reaction force (rocket shooting up, earth/person)
* Mass vs. weight
Mass: amount/quantity of matter in an object
Weight: the force acting on object and mass
* Free-fall: astronauts weightless in space because they are in constant freefall, we experience free fall all
the time as well (jumping off chair), planets, moons in free fall
Gravity does not allow for any acceleration = free fall, objects in orbit are in constant state of free fall
* Momentum: only net force can change momentum , causes acceleration
Mass x velocity
Angular momentum: rotational momentum of objects that spin or rotate (planet, particle moving in
circle). Particles rot. Momentum = mass x velocity x radius
Conservation of momentum: momentum between two objects cannot change without net force acting on
them, exchange momentum, not lose
Reason why objects move at constant velocity without force (Car, elevator)
Angular momentum also cannot change without external force (torque), stays constant
Why earth continue to orbit sun in same pattern/motion, will only change with torque (like tides)
Angular momentum is why objects rotate faster with a decrease in radius
Force is the rate of change in momentum
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* Conservation of energy and types of energy: can be exchanged, change types/forms but not destroyed
or created
Kinetic energy: motion
Radiative energy: light
Stored or potential energy
Thermal energy: collective kinetic energy of many particles (in air, or a rock)
Contracting gas cloud converts gravitational potential energy to thermal energy
The accreting disk formed from conservation of angular momentum
temperature is average kinetic energy
an object cant just get back the type of energy it lost, it is always converted into something else unless it
goes back to the position where it gained that energy
* Mass-energy equivalence
Mass is potential energy, mass releases energy
In a nuclear bomb mass of the hydrogen is converted into energy, can calculate using e=mc2
* Kepler's Laws, especially the third law: apply to all orbiting objects
First law: planets have elliptical orbits with the sun at one focus
Second law: planets sweep out equal areas in equal amounts of time
Third law: distant planets orbit the sun at lower average speeds, the farther you are the longer you take to
orbit and the slower you orbit
(Period)2 =(Average distance from sun)3
Square of planets orbital period = cube of avg. distance from sun
A single planets orbital period and distance is all we need to know to determine suns mass
If planets orbits are deviated then a force is acting on them
Shrinking suns radius increases velocity to keep angular momentum constant
* Newton's law of gravity
every mass attracts every other mass
attraction directly proportional to product of masses, inversely proportional to distance between
relationship between orbital period + avg orbital distance tells us total mass of the system
suns size does not affect earths orbit. If the mass is same then force of gravity will be the same
spacecrafts can gain orbital energy by taking it from a planet
* The nature of light
* Colours, wavelengths, and frequencies
* The electromagnetic spectrum: entire range of wavelengths of light
* The interaction of light and matter, especially on an atomic scale: light interacts with charged
particles through magnetic fields, photons: particles of light
* Properties of atoms
Molecules have two or more atoms
Isotope same number of protons but different number neutrons
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