ASTR 1200 Announcements

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ASTR 1200Announcements
Second problem set due next Tuesday in
class. Observatory Sessions all now at 830pm
Website http//casa.colorado.edu/wcash/APS1200/AP
S1200.html
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Solar Structure
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Magnetic Fields

• Stronger when closer together
• Charged particles follow field lines
• Magnetic field holds energy

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Solar Magnetic Fields
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Sunspots Erupt in Groups
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Solar Granules

• https//www.youtube.com/watch?vEkeysTGSdi4

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Solar Atmosphere
Solar Wind 5x105K
Corona 2x106K
Transition Region 105K
Chromosphere 104K
Photosphere 5500K
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Solar Wind Passes Earth
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Solar Wind Passes Earth
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Coronal Mass Ejection
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Differential Rotation
Rotates in 25 days at Equator 28 days Mid
Latitude 30 days Poles
Rapidly Twists Up
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Sunspot Cycle
During mid 1600s sunspots became
non-existent Maunder Minimum
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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
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Stars are grouped in Galaxies

• Sun and all the stars we see are part of Milky
  Way Galaxy
• Contains 1011 (100 billion) stars
• Sun is 3x1020m from center of MW

You are here
Each star orbits center Disk Stability Again
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Each Star Orbits the Center
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How Long does that Take?
Orbital Period
r 3x1020 m, M 2x1042 kg, G 6.7x10-11
m3 kg-1 s-2
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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Distances to the Stars

• Closest Star, Proxima Centauri is 4x1016m away.
  (Alpha Cen 4.3x1016m)
• Need a more convenient unit

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The Light Year

• Speed of light is constant
• c 3x108 m/s (300 million meters/second)
• Measure distance by how long it takes light to
  travel

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The Light Year

• Earth circumference 0.13 light seconds
• Distance to Sun 8 light minutes
• Distance to Pluto 5.5 light hours

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The Light Year

• One year is 3.15x107 seconds long
• In one year light travels
  (3.15x107 s) x (3x108 m/s) 1016m
• This is the definition of a light year.
• Proxima Centauri is at 4ly.

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Parallax

• How to measure the distance to stars?
• Look at how they appear to move relative to each
  other

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Parallax
I year cycle
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The Parsec
Parsec (pc) ---- parallax second
1 parsec
1''
1AU
360 degrees (360) in circle 60 arcminutes (60')
per degree 60 arcseconds (60'') per arcminute
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Measure Parallax

• Distance in parsecs 1/(parallax in arcseconds)
• If parallax is .04'' d 1/0.04 25 pc
• 1 pc 3x1016 m 3.3 ly
• Measuring Parallax was first successful way to
  measure distances to stars after centuries of
  trying

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Question

• Based on the definition of a parsec , if star A
  has a parallax of 0.5 arcseconds and star B has a
  parallax of 0.75 arcseconds which one is farther
  from the Earth?
• A. Star B is farther away because it has a
  higher parallax
• B. Star A is farther away because it has a lower
  parallax
• C. All stars are the same distance away from the
  Earth
• D. It is impossible to tell from this
  information.

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Question

• Based on the definition of a parsec , if star A
  has a parallax of 0.5 arcseconds and star B has a
  parallax of 0.75 arcseconds which one is farther
  from the Earth?
• A. Star B is farther away because it has a
  higher parallax
• B. Star A is farther away because it has a lower
  parallax
• C. All stars are the same distance away from the
  Earth
• D. It is impossible to tell from this
  information.

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Gaia

• Parallax for a billion stars
• Distances out to 104 ly
• Launched Dec 2013

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Proper Motion
2003
All stars move Nearby stars appear to move
faster Appear to move against fixed field Can
take many years Use old photographic plates
1900
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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 A4pr2
r
Brightness is
Star Emits N photons per second
photons/m2/s
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Question

• If the distance between Earth and the Sun were
  cut in half, how much brighter would the sun
  appear in our sky?
• a. 2x brighter
• b. 4x brighter
• c. 8x brighter
• d. 16x brighter

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Question

• If the distance between Earth and the Sun were
  cut in half, how much brighter would the sun
  appear in our sky?
• a. 2x brighter
• b. 4x brighter
• c. 8x brighter
• d. 16x brighter
• Brightness is a function of the inverse square of
  distance, so if distance was cut by half it would
  get brighter by 4x1/(.5)2

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Brightness
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