AST201H1 Study Guide - Final Guide: Laser Guide Star, Spiral Galaxy, Nuclear Fusion

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Published on 7 Jan 2014
Astronomy 201 – Exam Notes
- Occam’s Razor tells us that all other factors being equal, simpler explanations are more likely to be true
- The universe is the totality of all space, time, energy and matter
- Light travels at a constant, finite speed (c = 300 million m/s)
- A light year is the distance that light travels in one year (1 ly = 9.46 trillion km)
- The universe is about 13.7 billion years old
- There is a limit on the amount of universe we can see because the universe is only 13.7 billion years
old and the light from other places would not have had enough time to travel to us
- What we are able to see is called the observable universe
- Looking very far away is like looking back in time because the light that we receive with the images has
taken lots of time to get to us and the object has changed/aged within that time
- 2.5 million light years away = seeing the object as it was 2.5 million years ago
- Stars: enormous spheres of very hot gas which produce energy by nuclear fusion – the sun
- Solar system: one or more planets orbiting a star
- Galaxy: a collection of billions of stars all orbiting a common centre
- The Milky Way galaxy is about 100,000 light years across
- Our nearest (large) neighbour galaxy is the Andromeda Galaxy
- The milky way and Andromeda are both spiral galaxies
- The milky way, Andromeda and a few dozen small galaxies make up the Local Group of galaxies, all of
which orbit one another
- All of the galaxy clusters in the universe are moving apart from one another
- Solar system < galaxy < galaxy (local) group < galaxy cluster < superclusters < wall or filament
- Ancient astronomy was often astrological in nature, was geocentric, earth centered
- For thousands of years, astronomers insisted on the Pythagorean ideal of circular motion
- Objects orbit around a common centre of mass
- The scientific method:
Keplers Laws:
Newton’s Laws:
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- Fgravity = GMm/r2
- an object is in free fall when the only external force acting on it is gravity
- at v = vorbital objects begin to orbit
- vescape – objects break free of the planet’s gravity
- escape speed doesn’t depend on the direction in which the object is thrown
- v > vescape = an unbound orbit
- vescape = 1.4 x vcircular
- light is a wave-particle – depending on how you observe it, it may act as either a particle or a wave
- wavelength is the distance from one peak to the next (or one trough to the next)
- water waves require a medium = water
- light is not a wave in something, it’s a wave OF something
- light is an electromagnetic wave
- electromagnetic radiation = light
- wavelength is the distance between adjacent peaks of the electric (and magnetic) field
- frequency is the number of times each second that the electric (and magnetic) field vibrates up and
down (or side to side) at any point
- all light travels with speed c = 5,300,000 km/s
- speed of light = wavelength x frequency
- wavelength translates into colour
- Light can have any wavelength
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- Different wavelengths show different physical conditions
- Emission: the process by which matter emits energy in the form of light – gives out energy (electrons
move down in energy shells/orbits
- Absorption: the process by which matter absorbs radiative energy – takes in energy so electrons move
up in energy shells/orbits, more active
- The energy that an atom absorbs or emits translates into energy lines and correspond to certain
elements and colours
- Spectra lines = chemical composition
- Low-density gas = each atom emits or absorbs light independently of the others and produce
EMISSION spectra – black line with coloured areas where it picks up light at certain wavelengths
- Dense objects (gas, liquid or solid) = emit some light at all wavelengths and is called a CONTINOUS
spectrum (all colours) – the particular continuous spectrum emitted by dense objects is called a
blackbody spectrum
- As a blackbody gets hotter it: Emits more light at shorter wavelengths and emits more light overall
- Low-density gas in front of a blackbody = ABSOPTION – some particles of light get scattered and so
the spectra is missing some wavelengths of light (full spectra with some black lines)
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