1. ASTR 1040 – September 19 Second Homework Due Thursday September 28
Next Observatory opportunity September 26
Planetarium on September 26
First Exam October 5
Website

2. 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

3. Parallax
I year cycle

4. The Parsec 1 parsec 360 degrees in circle 60 arcminutes per degree
1AU
1 arcsecond
360 degrees in circle
60 arcminutes per degree
60 arcseconds per arcminute
200,000AU = 1 parsec = 3x1016m
parsec parallax second

5. 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.

6. Answer
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.

7. Measure Parallax
distance to a star in parsecs = 1/(parallax in arcseconds)
e.g. measure .04” parallax, then distance is 25pc
Measuring Parallax was first successful way to measure
distances to stars after centuries of trying
Took high speed photography in 1890’s to do it.

8. Question
The parallax of an observed star is 0.1 arcseconds, how many light years is it away from Earth?
a. 1 light year
b. 3 light years
c. 10 light years
d. 30 light years
e. 75 light years

9. Question
The parallax of an observed star is 0.1 arcseconds, how many light years is it away from Earth?
a. 1 light year
b. 3 light years
c. 10 light years
d. 30 light years (10parsecs)
e. 75 light years

10. 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 A=4d2 d
Brightness is
Star Emits N photons
per second
photons/m2/s

11. 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 400’s

12. 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

13. Answer
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

14. 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 m=1
Polaris 2.51x fainter m=2
2.5x fainter than Polaris m=3
2.5x fainter than that m=4
etc

15. 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.

16. 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

17. Question
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

18. 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

19. 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 don’t understand it
Electromagnetic Theory (Maxwell’s Equations) 1860’s
Quantum Electrodynamics 1948 Feynman
Each photon has:
direction
wavelength
polarization

20. Light Waves l 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

21. Color RED 7000Å YELLOW 5500Å VIOLET 4000Å
Wavelength Determines Color of Light
Color is the eye’s response to different wavelengths
Color is a physiological effect
A photon can have any wavelength
RED 7000Å
YELLOW 5500Å
VIOLET 4000Å

22. Electromagnetic Spectrum
visible is tiny chunk of em spectrum

23. Parts of EM Spectrum Radio l > 1mm (107A)
Infrared 1mm> l > 10000A
Visible 10,000A > l > 3500A
Ultraviolet 3500A > l > 100A
X-ray 100A > l > 0.1A
Gamma-ray 0.1A > l