ASTR 1200
Announcements
Second problem set due next Tuesday in class.
Observatory Sessions all now at 8:30pm
Website
http://casa.colorado.edu/~wcash/APS1200/APS1200.html
Solar Structure
Magnetic Fields
• Stronger when closer
together
• Charged particles
follow field lines
• Magnetic field holds
energy
Solar Magnetic Fields
Sunspots Erupt in Groups
Solar Granules
• https://www.youtube.com/watch?v=Ekeys
TGSdi4
Solar Atmosphere
Solar Wind 5x105K
Corona 2x106K
Transition Region 105K
Chromosphere 104K
Photosphere 5500K
Solar Wind Passes Earth
Solar Wind Passes Earth
Coronal Mass Ejection
Differential Rotation
Rotates in 25 days at Equator
28 days Mid Latitude
30 days Poles
Rapidly Twists Up
Sunspot Cycle
During mid 1600’s sunspots became non-existent
Maunder Minimum
Summary: Sun as a Star
• Formed from cloud 4.6x109 years ago
• Collapsed to present size
– stabilized by nuclear reactions
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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
STARS
Stars are grouped in Galaxies
• Sun and all the stars we see are part
of Milky Way Galaxy
• Contains 1011 (100 billion) stars
You are here
20
• Sun is 3x10 m from center of MW
Each star orbits
center
Disk Stability Again
Each Star Orbits the Center
How Long does that Take?
r3
Orbital Period: P  2π
GM
r = 3x1020 m, M = 2x1042 kg, G = 6.7x10-11 m3 kg-1 s-2
P  6.28
3x10 
20 3
6.7 x1011 x2 x1042
15
8
P  3x10 s  10 yr
Takes about a hundred million years to
circumnavigate the galaxy
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
Distances to the Stars
• Closest Star, Proxima Centauri is 4x1016m
away. (Alpha Cen ~4.3x1016m)
• Need a more convenient unit
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
The Light Year
• Earth circumference: 0.13 light seconds
• Distance to Sun: 8 light minutes
• Distance to Pluto: 5.5 light hours
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.
Parallax
• How to measure the distance to stars?
• Look at how they appear to move relative
to each other
Parallax
I year cycle
The Parsec
Parsec (pc) ---- parallax second
1 parsec
1AU
1′′
360 degrees (360°) in circle
60 arcminutes (60′) per degree
60 arcseconds (60′′) per arcminute
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
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.
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.
Gaia
• Parallax for a billion
stars
• Distances out to 104 ly
• Launched Dec 2013
Proper Motion
2003
All stars move
Nearby stars appear to
move faster
1900
Appear to move against
fixed field
Can take many years
Use old photographic plates
Brightness
Around the sky stars vary in brightness and in color.
Brightness is the result of two factors
1. Intrinsic Luminosity
2. Distance
r
Each Sphere has
area A=4πr2
Brightness is
Star Emits N photons
per second
B
N
4πr 2
photons/m2/s
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
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 4x=1/(.5)2
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 400’s
The Magnitude System
Ptolemy Broke Stars into 5 magnitude groups
m=1 the brightest, m=5 the faintest
In 1700’s it was found this was a logarithmic scale, as that is how
the naked eye responds. Also, faintest were about 100x fainter than
brightest.
Break the factor of 100 into 5 equal factors:
Start with Vega
Polaris 2.51x fainter
2.5x fainter than Polaris
2.5x fainter than that
etc
m=1
m=2
m=3
m=4
1mag  5 100  2.51
Magnitudes (2)
Every 5 magnitudes is a factor of 100
m=5 is 100 times fainter than m=0
m=10 is 100x100 =10,000 times fainter than m=0
m=15 is (100)3 = 1million times fainter than m=0
Sun
m=-26.5
Full Moon
m=-13
Venus
m=-4
Sirius
m=-1.5
Vega
m=1
Polaris
m=2
Faintest Visible m=6
Faintest Detected m=28
Works only in the visible.
Really inconvenient in modern astronomy because we
observe across the spectrum from radio to gamma rays.
Absolute Magnitude
The magnitude a star would have were it at 10pc
We see a star of magnitude m=10 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
M  m  5 log10 d  5
M  10  5 log100 5  10  10  5  5