ASTR 1200 Announcements

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ASTR 1200Announcements
Second problem set due Lecture Notes going up
on the websiteFirst Exam October 7
Website http//casa.colorado.edu/wcash/APS1200/AP
S1200.html
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Summary Sun as a Star

• Formed from cloud 4.6x109 years ago
• Collapsed to present size
• stabilized by nuclear reactions
• Emits 4x1026 W
• Runs on proton-proton chain and CNO cycle
• Now 20 brighter
• Turbulent upper envelope
• Magnetic Fields from Differential Rotation
• Sunspots, Corona, Solar Wind
• Activity Cycle 11 years

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Stars are grouped in Galaxies

• Sun and all the stars we see are part of Milky
  Way Galaxy
• We all orbit a common center
• Sun is 3x1020m from center of MW

You are here
Each star orbits center Disk Stability Again
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The Light Year
Light Travels at 300,000km/s (186,000miles/s
3x108m/s) Thats one foot per nanosecond One
Year is 3.15x107 seconds long In one year light
travels 3.15x107x3x108 1016m This is the
definition of a light year. Prox Cen is at 4ly.
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The Parsec
Astronomers use the parsec as a measure of
distance 1pc 3ly 1pc 3x1016m Origin of
parsec comes from method of measuring distance
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Each Star Orbits the Center
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How Long does that Take?
Takes about a hundred million years to
circumnavigate the galaxy
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Star Names

• Arabic Names
• Antares, Capella, Mira, etc.
• Constellations
• a Orionis, b Cygni, then 49 Ori, 50 Ori, etc.
• Catalogues HD80591, SAO 733421, etc
• RA and Dec just position in the sky

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Proper Motion
2003
All stars move Nearby stars move faster Appear
to move against fixed field Can Take Many
Years Use Old Photographic Plates
1900
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Parallax
I year cycle
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The Parsec
1 parsec
1AU
1 arcsecond
360 degrees in circle 60 arcminutes per degree 60
arcseconds per arcminute
200,000AU 1 parsec 3x1016m
parsec ---- parallax second
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Brightness
Around the sky stars vary in brightness and in
color.
Brightness is the result of two factors 1.
Intrinsic Luminosity 2. Distance
Each Sphere has area A4pd2
d
Brightness is
Star Emits N photons per second
photons/m2/s
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Brightness (2)
Brightness e.g. 10-12 Watts/m2 Simple and
easy to understand If your eye is 10-4m2, then
it collects 10-16W 4 stars at 10-12W/m2 together
have 4x10-12W/m2
But this would be too easy for astronomers. We
use a brightness system invented by Ptolemy in
the 400s
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The Magnitude System
Ptolemy Broke Stars into 5 magnitude groups m1
the brightest, m5 the faintest
In 1700s it was found this was a logarithmic
scale, as that is howthe naked eye responds.
Also, faintest were about 100x fainter
than brightest.
Break the factor of 100 into 5 equal factors
Start with Vega m1 Polaris 2.51x fainter
m2 2.5x fainter than Polaris m3 2.5x fainter
than that m4 etc
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Magnitudes (2)
Every 5 magnitudes is a factor of 100 m5 is 100
times fainter than m0 m10 is 100x100 10,000
times fainter than m0 m15 is (100)3 1million
times fainter than m0
Sun m-26.5 Full Moon m-13 Venus m-4 Sirius m
-1.5 Vega m1 Polaris m2 Faintest
Visible m6 Faintest Detected m28
Works only in the visible. Really inconvenient in
modern astronomy because we observe across the
spectrum from radio to gamma rays.
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Absolute Magnitude
The magnitude a star would have were it at 10pc
We see a star of magnitude m10 at 100 pc. What
would be its magnitude (M) if it were at 10 pc
instead of 100pc?
At 10 times closer the star would be 100x
brighter 5 magnitudes M 10-5 5
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Clicker

• A 5 magnitude difference means a factor of 100 in
  flux. By what factor do the fluxes differ between
  two stars of 20 magnitudes difference?
• 2.51
• 20
• 400
• 10,000
• 100,000,000

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Answer

• 5magnitudes difference is a factor of 100. By
  what factor do the fluxes differ between two
  stars of 20 magnitudes difference
• 2.51
• 20
• 400
• 10,000
• 100,000,000

20 magnitudes is four factors of 102,which is 108
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Nature of Light
Light is a flux of particles called photons Each
photon is both a particle and a wave (a packet of
waves) 250 years after Newton we still dont
understand it Electromagnetic Theory (Maxwells
Equations) 1860s Quantum Electrodynamics 1948
Feynman Each photon has direction wavelength
polarization
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Light Waves
lambda is lower case Greek L stands for length
Each photon is a sine wave moving at the speed of
light
Wavelength is usually measure in Angstroms 1Å
10-8cm 10-10m about the diameter of an
atom. And 10Å 1nm
Electric and Magnetic Fields Sloshing Back And
Forth
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Color
Wavelength Determines Color of Light Color is
the eyes response to different
wavelengths Color is a physiological effect A
photon can have any wavelength
RED 7000Å YELLOW 5500Å VIOLET 4000Å
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Electromagnetic Spectrum
visible is tiny chunk of em spectrum
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Parts of EM Spectrum
Radio l gt 1mm (107A) Infrared 1mmgt l gt
10000A Visible 10,000A gt l gt 3500A Ultraviolet 3
500A gt l gt 100A X-ray 100A gt l gt
0.1A Gamma-ray 0.1A gt l
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Clicker

• What range of wavelength can the average human
  eye see and what color is each side of the
  spectrum?A) 400nm-800nm, redder to bluerB)
  500nm-700nm, bluer to redderC) 400nm-700nm,
  bluer to redderD) 300nm-600nm, redder to
  bluerE) None of the above

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Answer

• What range of wavelength can the average human
  eye see and what color is each side of the
  spectrum?A) 400nm-800nm, redder to bluerB)
  500nm-700nm, bluer to redderC) 400nm-700nm,
  bluer to redderD) 300nm-600nm, redder to
  bluerE) None of the aboveAnswer C

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Speed of Light
Speed of Light c 3x108m/s
Thats a very odd statement
2 cars at 65mph
1 car at 130mph
Cover same distance in same amount of time The
Relative speeds are the same
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Relativity
.8c
.8c
Clearly Approaching each other at 1.6c
NO!!!
v always less than c if velocities ltlt c, then
vv1v2
per Einstein
(Concept of time and space changes)
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Frequency
Moves l during each cycle
Frequency is the number of cycles per second, n
Greek nu
Moves distance l for each of n cycles each second
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Frequency (2)
300MHz 1m wavelength
Yellow Light 600 trillion Herz
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Question

• An x-ray has a wavelength of 100Å
• (10nm, 1x10-8m). What is it's frequency, in
  cycles per second? (aka Hertz)
• A. 3x1016
• B. 1.5x1016
• C. 3x1013
• D. 1.5x1013

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Question

• An x-ray has a wavelength of 100Å(10nm,
  1x10-8m). What is it's frequency, in cycles per
  second? (aka Hertz)
• A. 3x1016
• B. 1.5x1016
• C. 3x1013
• D. 1.5x1013
• Answer A. (3E8m/s)/(1E-8m)3E16 Hz

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Energy of a Photon
Plancks Constant
h 6.63x10-34 J s
energy of yellow photon
Outside we have 1023 photons/m2/s hit us
Sunlight is 104 W/m2
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Question

• How many times more energy is there in an x-ray
  photon at 100A than the infrared light photons
  emitted by every living human? (Assuming 10,000nm
  wavelength of infrared light).
• A. Ten times as powerful.
• B. A hundred times more powerful.
• C. A thousand times more powerful.
• D. 1x1012 (a trillion) times more powerful.
• E. 1x1015 (a quadrillion) times more powerful.

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Question

• How many times more energy is there in an x-ray
  photon at 100A than the infrared light photons
  emitted by every living human? (Assuming 10,000nm
  wavelength of infrared light).
• A. Ten times as powerful.
• B. A hundred times more powerful.
• C. A thousand times more powerful.
• D. 1x1012 (a trillion) times more powerful.
• E. 1x1015 (a quadrillion) times more powerful.
• Answer C. 10,000nm/10nm 1000

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Spectroscopy
Spectrum is plot of number of photons as a
function of wavelength Tells us huge amounts
about nature of object emitting light.
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Thermal Radiation
Plancks Law
Temperature Determines Where Spectrum Peaks
Position of Peak Determines Color
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Blue is Hotter than Red
Optically Thick, But hot Sun almost white
hot Burner red hot Desk black hot Ice Cube
black hot
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Question

• A star with a temperature of 100,000K has what
  color to the naked eye?
• White
• Yellow
• Orange
• Red

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Wiens Law
Å
(T in Kelvin)
As T rises, l drops Bluer with temperature
300K 100,000A Earth 5500 5500 Sun 106 30 X-ray
source
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Question

• How many times smaller would the peak wavelength
  be for a star twice as hot as the Sun? (Remember
  the sun is 5500K)
• A. Twice as long
• B. Half as long
• C. Four times as long
• D. A fourth as long

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Question

• How many times smaller would the peak wavelength
  be for a star twice as hot as the Sun? (Remember
  the sun is 5500K)
• A. Twice as long
• B. Half as long
• C. Four times as long
• D. A fourth as long
• Answer B. Since peak wavelength is a function of
  the inverse of temperature, doubling the temp of
  a star would cause it's peak wavelength to cut in
  half.

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Stefan-Boltzman Law
s 5.67x10-8 W/m2/K4
A is area in m2
T in Kelvins
Example The Sun
L 5.7x10-8 x 4 x 3.14 x (7x108m)2 x (5500K)4
4 x 1026 W
4x1026 Watts 100 billion billion MegaWatts!!
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Question

• If you were to double the temperature of the Sun
  without changing its radius, by what factor would
  its luminosity rise?
• 2
• 4
• 8
• 16
• 32

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Question

• If you were to double the temperature of the Sun
  without changing its radius, by what factor would
  its luminosity rise?
• 2
• 4
• 8
• 16 24
• 32

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Emission Lines
Electron Drops
Energy Levels of H
Photon Escapes
Can Only Happen Between Certain Pre-determined
orbitals
Spectrum of Hydrogen
Each Element Has Different Orbitals So Each
Element Has Different Lines
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Absorption Lines
Light moving through cold gas can have photons
removed. Creates dark wavelengths called
absorption lines
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Question

• A star is viewed through a far away hydrogen gas
  cloud, what kind of spectrum can we expect to
  see?A) Absorption only
• B) Emission only C) Continuum onlyD) Emission
  and ContinuumE) Absorption and Continuum

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Question

• A star is viewed through a far away hydrogen gas
  cloud, what kind of spectrum can we expect to
  see?A) Absorption only
• B) Emission only C) Continuum onlyD) Emission
  and ContinuumE) Absorption and Continuum

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Stars Come in Different Colors
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Stellar Temperature
Stars come in different sizes and
temperatures. Can the two be linked?
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Question

• You see three stars up in the sky. One is bigger
  than the others and red, one is yellow, and one
  is white. Which one peaks at a higher frequency?
• A)Red
• B)Yellow
• C)White
• D)I need to know how far away they are

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Question

• You see three stars up in the sky. One is bigger
  than the others and red, one is yellow, and one
  is white. Which one peaks at a higher frequency?
• A)Red
• B)Yellow
• C)White
• D)I need to know how far away they are

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Stellar Classification
Full range of surface temperatures from 2000 to
40,000K
Spectral Classification is Based on Surface
Temperature
O B A F G K M
Coolest
Hottest


Gal
Kiss Me
Oh Be A Fine
Guy
Each Letter has ten subdivisions from 0 to 9 0 is
hottest, 9 is coolest
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The Spectral Types
O Stars of Orion's Belt gt30,000 K Lines of ionized helium, weak hydrogen lines lt97 nm (ultraviolet)
B Rigel 30,000 K-10,000 K Lines of neutral helium, moderate hydrogen lines 97-290 nm (ultraviolet)
A Sirius 10,000 K-7,500 K Very strong hydrogen lines 290-390 nm (violet)
F Polaris 7,500 K-6,000 K Moderate hydrogen lines, moderate lines of ionized calcium 390-480 nm (blue)
G Sun, Alpha Centauri A 6,000 K-5,000 K Weak hydrogen lines, strong lines of ionized calcium 480-580 nm (yellow)
K Arcturus 5,000 K-3,500 K Lines of neutral and singly ionized metals, some molecules 580-830 nm (red)
M Betelgeuse, Proxima Centauri lt3,500 K Molecular lines strong gt830 nm (infrared)
All stars above 6,000 K look more or less white to the human eye because they emit plenty of radiation at all visible wavelengths. All stars above 6,000 K look more or less white to the human eye because they emit plenty of radiation at all visible wavelengths. All stars above 6,000 K look more or less white to the human eye because they emit plenty of radiation at all visible wavelengths. All stars above 6,000 K look more or less white to the human eye because they emit plenty of radiation at all visible wavelengths.
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Stellar Classification (2)
Sun G2 a Cen G2 K5 Sirius A1 Antares M1 Rigel
B8
O5 40,000K B5 15,500 A5 8500 F5 6580 G5 5520 K5 41
30 M5 2800
Letters are odd due to confusion in sorting out
temperature scale between 1900 and 1920