[AST201H1] - Midterm Exam Guide - Ultimate 239 pages long Study Guide!

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AST201H1
MIDTERM EXAM
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AST201 - The Sun: Part 1 (Chp 5.1-5.4) Feb 2/16
Note:
Light spectral can tell us distant objects of their
o chemical composition (emission & absorption lines)
otemperature (by color or thermal radiation)
omoving toward or away from us & rotation (Doppler effect – blue & redshift; also only if it is directly moving
away/toward us can we measure the speed. Can’t for diagonal or across our line of sight)
for rotation- faster the object is rotating, the broader the wavelength the spectral lines become
Emission and absorption spectra used to determine the composition of a cloud of gas
Dense objects like stars, planets, rock, people, light cannot easily pass thru them and light emitted inside them cannot easily
escape.
oThermal radiation (blackbody radiation) depends only on temperature so we cause use them to measure the
temperatures of distant objects
Object’s color
Cool star 3000K surface temp emits mostly red light (Betelgeuse, Antares)
Sun 5800K emits in green light but Sun looks yellow or white to our eyes bc it also emits other
colors throughout the visible spectrum
Hotter stars emit ultraviolet but appear blue-white in color bc our eyes can’t see ultraviolet light
The sun’s diameter is about 100 times Earth’s diameter
The sun is about 8 light minutes (150 million km) from Earth
Until 1925, people assumed the Sun was made of similar material to the Earth
In 1925, Cecilia Payne-Gaposchkin showed that in fact, the Sun was made mostly of hydrogen, a little helium, and tiny
amounts of other elements
How did she do it?
When you pass sunlight through a prism, it appears to separate into a perfectly smooth rainbow
Spectrum of Light
White is not a color
White light is the sum of all colors
A prism splits sunlight up into a spectrum
LIGHT IN EVERYDAY LIFE
How do we experience light?
Light carries radiative energy that it can exchange with matter
Power is the rate of energy transfer, measured in watts: 1 watt= 1 joule
The colors of light contain a great deal of info about the matter with which it has interacted
How do light and matter interact?
Matter can emit, absorb, transmit or reflect (scatter) light
PROPERTIES OF LIGHT
What is light?
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Light is an electromagnetic wave, but also comes in individual “pieces” called photons
Each photon has a precise wavelength, frequency, and energy
Shorter the wavelength, higher the frequency and energy
What is the electromagnetic spectrum?
From order of decreasing wavelength (increasing frequency & energy), the forms of light are: radio waves, microwaves,
infrared, visible light, ultraviolet, x-rays, gamma rays
Speed= wavelength x frequency
Frequency is cycles per second aka hertz
PROPERTIES OF MATTER
What is the structure of matter?
Matter is made of atoms, which are made of protons, neutrons, and electrons
Atoms of different chemical elements have different numbers of protons
Isotopes of a particular chemical element all have the same number of protons but different number of neutrons
Molecules are made from 2 or more atoms
What is the phases of matter?
The appearance of matter depends on its phase: solid, liquid, gas
Gas is always present along with solid or liquid phases
Solids sublimate into gas
Liquids evaporate into gas
At very high temperatures, molecular dissociation breaks up molecules and ionization strips electrons from atoms
An ionized gas is called a plasma
How is energy stored in atoms?
Electrons can exist at particular energy levels within an atom
Energy level transitions (in which an electron moves from one energy level to another) can occur only when the electron
gains or loses just the right amount of energy
LEARNING FROM LIGHT
What are the 3 basic types of spectra?
Continuous spectrum: looks like a rainbow of light
Emission line spectrum: we see light only with specific colors against a black background (spikes)
Absorption line spectrum: specific colors are missing from the rainbow (dips)
How does light tell us what things are made of?
Emission or absorption lines occur only at specific wavelengths that correspond to particular energy level transitions in
atoms or molecules
Every kind of atom, ion, and molecule produces a unique set of spectral lines, so we can determine composition by identifying
these lines
Note- Reflected Light Spectra : a red shirt absorbs blue light and reflects red light so its visible spectrum will look like the
spectrum of sunlight but with blue light missing
How does light tell us the temperature of planets and stars?
Objects such as planets and stars produce thermal radiation spectra, the most common type of continuous spectra
We can determine temperature from these spectra bc hotter objects emit more total radiation per unit area and emit photons
with a higher avg energy
How does light tell us the speed of a distant object?
The Doppler effect tells us how fast an object is moving toward or away from us
Spectral lines are shifted to:
oShorter wavelengths (a blueshift) in objects moving toward us
oLonger wavelengths (a redshift) in objects moving away from us
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