ASTR 1040 – September 19 Second Homework Due Thursday September 28 Next Observatory opportunity September 26 Planetarium on September 26 First Exam October 5 Website http://casa.colorado.edu/~wcash/APS1020/APS1020.html

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

Parallax I year cycle

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

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.

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

Electromagnetic Spectrum visible is tiny chunk of em spectrum

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