Transcript PH1600: Introductory Astronomy

PH1600: Introductory Astronomy
Lecture 2
What is this?
PH1600: Introductory Astronomy
Lecture 2: Universe Scale, and Light
School: Michigan Technological University
Professor: Robert Nemiroff
Online Course WebCT pages:
http://courses.mtu.edu/
This class can be taken online ONLY, class
attendance is not required!
You are responsible for…
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Read the wikipedia entries
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APODs posted during the semester
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Anything from those wikipedia pages
(excluding higher level math) can appear on
quizzes or tests, even if I never mention them
during my lecture(s)
This quiz period covers Chapter 2
APOD review every week during lecture
Completing the Quizzes
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Chapter 1 quiz is now due
Chapter 2 quiz is due soon
See WebCT at http://courses.mtu.edu/ for
details
Wikipedia pages covered:
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Earth’s atmosphere
Light
Black body
Wein’s Law
Stefan Boltzman Law
4
Cosmic Scale: Powers of Ten
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Web sites
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Secret Worlds: The Universe Within
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Atlas of the Universe
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http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/
http://www.atlasoftheuniverse.com/
Scales covered (small to large):
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Quarks, protons, atomic nuclei, electrons and
atoms, molecules, DNA, cell nuclei, cells, leaves,
trees, towns, states, Earth, Moon orbit, Earth orbit,
Solar System, Sun, solar neighborhood, spiral
arms, Milky Way Galaxy, Local Group of Galaxies,
Virgo Cluster, neighboring superclusters, visible
universe
Earth’s Atmosphere
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Thins out higher up
Shuttle orbits at ~ 400 km
Aurora occur ~ 100 km
A Blue Crescent Moon from Space
Credit: Expedition 13 Crew,
International Space Station, NASA
APOD: 2007 March 20
http://imagine.gsfc.nasa.gov/docs/introduction/emsurface.html
Transparency of the Earth’s Atmosphere
The Nature of Light
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Electromagnetic radiation
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Colors of visible light: ROY G BIV
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Oscillating electric and magnetic fields
Red, Orange, Yellow, Green, Blue, Indigo,
Violet
E=hν
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E is energy of light (kg m/sec2)
h is Planck’s constant
 h=6.63 x 10-34 kg m/sec
ν is frequency (1/sec)
http://en.wikipedia.org/wiki/Image:Prisma.gif
The Nature of Light
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Also, E=hc/λ
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c is the speed of light
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λ is the wavelength of light
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c=3 x 108 m/sec
The distance between peaks
Quick Problem: What is the energy
of a red photon?
E=(6.63 x 10-34 kg m/sec)x(3.00 x 108 m/sec)/
(650 x 10-9 m)= 3.06 x 10-19 kg m / sec2
Black-body radiation
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Emitted by almost everything
Called “black” because reflected
light not included
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All incoming light assumed absorbed,
like something that is perfectly black
Temperature determines which
colors glow brightest
http://commons.wikimedia.org/wiki/Image:Blackbody-lg.png
Black Body Spectra
Black-body radiation: Wien’s Law
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Would you rather pick up a nail glowing red hot or
blue hot?
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In words: “blue hot is hotter than red hot”
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In math: λmax T = constant = 0.29 cm K
 T is temperature measured in Kelvin (K)
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Kelvin is a unit of temperature (see book)
λmax is the brightest wavelength that the black body emits
The Sun is “green” hot
Magma Bubbles from Mt. Etna
Credit & Copyright: Marco Fulle (Stromboli online)
APOD: 2005 October 2
Black-Body radiation:
Stefan-Boltzman Law
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In words: “hot stuff glows more”
In math: E = σ T4
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E is the energy emitted per surface area per
second
T is temperature in Kelvins
T4 = T x T x T x T
σ is the Stefan-Boltzmann constant
Volcano and Aurora in Iceland
APOD: 2004 June 13
Black-Body radiation:
Stefan-Boltzman Law
 Example:
Two stars, same size, one is red hot,
the other is blue hot.
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Blue hot star
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star emits more energy?
much more?
Assume Tblue = 2 x Tred
Eblue/Ered = (σ Tblue4)/(σ Tred4) = 24 = 16 times more!
Black-body Luminosity Law
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L=PSemt=4πR2σT4
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L is the total energy emitted per sec
π is pi = 3.14156…
R is the radius of the body
σ is the Stefan-Boltzmann constant
T is temperature in Kelvins
In words: “Bigger and hotter stuffs glows
more.”
Open Cluster NGC 290: A Stellar Jewel Box
Credit: ESA & NASA; Acknowledgement: E. Olszewski (U. Arizona) HST
APOD: 2006 May 1
Black-body Luminosity Law
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L=4πR2σT4 :
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Example: Two stars have the same color (and hence
T). One star is twice the radius R of the other.
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Which star has higher luminosity L?
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The bigger star
How much more?
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Lbig/Lsmall=4πRbig2σTbig4 /L=4πRsmall2σTsmall4
= (Rbig/Rsmall)2 = (2/1)2 = 4.
Albireo: A Bright and Beautiful Double
Credit & Copyright: Richard Yandrick (Cosmicimage.com)
APOD: 2005 August 30