1	Astro 1050 Wed. Sep. 18, 2002 Today: Homework #1, Tools of Astronomy (Recall Scientific Method Discussion)
2	Homework #1 Question 1: (1 point) If light takes about eight minutes to travel from the Sun to the Earth, and about forty minutes to travel from the Sun to Jupiter, how many astronomical units (AU) is Jupiter from the Sun? 1. 1 AU 2. 2 AU 3. 4 AU *4. 5 AU 5. 10 AU
3	Homework #1 Question 2: (1 point) What is the ratio of the mass of Earth to the mass of the moon? (Hint: look in appendix A of the text for these values.) *1. About 100. 2. About 10. 3. About 1000. 4. About 6. 5. About 600.
4	Homework #1 Question 3: (1 point) What do you get when you multiply together 3 x 10³², 2 x 10⁵⁶, 0.5 x 10¹⁸, and 3 x 10¹⁰⁰ ? *1. 9 x 10²⁰⁶ 2. This problem does not have a sensible answer! 3. 8.5 x 10²⁰⁶ 4. 9 x 10²⁰⁷ 5. 9 x 10²⁰⁵
5	Homework #1 Question 4: (1 point) If we have two stars in fictional constellations, and the two stars are named Alpha Buffalo and Beta Cowboy, which is brighter and by what factor? The apparent visual magnitude of Alpha Buffalo is 3.2 and the the apparent visual magnitude of Beta Cowboy is 2.7. 1. Alpha Buffalo is brighter by a factor of 1.6. *2. Beta Cowboy is brighter by a factor of 1.6. 3. Beta Cowboy is brighter by a factor of 1.3. 4. Alpha Buffalo is brighter by a factor of 1.3. 5. Beta Cowboy is brighter by a factor of 2. 6. Alpha Buffalo is brighter by a factor of 2.
6	Homework #1 Question 5: (1 point) Which of the following sets of stars have ALL been the "north star" (or pretty nearly) at one time or another? 1. Polaris, Vega, Rigel *2. Polaris, Vega, Thuban 3. Polaris, Rigel, Sirius 4. Polaris, Vega, Alpheratz 5. Polaris, Vega, Deneb
7	Homework #1 Question 6: (1 point) One of the nearest and brightest stars in the (southern) sky is Alpha Centauri. Assume that its radius is the same as that of the sun. The distance to Alpha Centauri can be found in Appendix A of the text. Assuming a perfect telescope and no atmospheric turbulence, what is the angular diameter (not radius!) of the star as seen from Earth? 1. 7 x 10^-4 arcseconds. 2. 7 x 10^-2 arcseconds. 3. 3.5 x 10^-2 arcseconds. 4. 3.5 x 10^-3 arcseconds. *5. 7 x 10^-3 arcseconds.
8	Chapter 5: Astronomical Tools Properties of light are fundamental Almost everything we know about the universe outside our solar system comes from interpreting the light from distant objects.
9	Radiation: Two very different kinds Something that “radiates”, or spreads out in “rays” High speed particles (eg. high speed neutrons ejected from a disintegrating atomic nucleus) Electromagnetic radiation: Towards shorter “wavelength” and higher energy: Visible light, Ultraviolet light, X-Rays, Gamma-Rays Towards longer “wavelength” and lower energy: Visible light, Infrared radiation, microwaves, radio waves
10	Properties of Light Light has both wave and particle properties Travels like a wave Interacts with matter like a particle: “photon” Full explanation involves quantum mechanics For most cases we can just choose the right “model” from the above two choices Photons, unlike particles in other kinds of “radiation,” are particles of “pure energy”
11	Light is an electromagnetic wave Changing electric fields generates magnetic fields Changing magnetic fields generates electric fields Can set up a cycle where one field causes the other: The E and B fields oscillate in strength, and the disturbance moves forward. To describe the wave you need to specify Direction it is moving Strength of the fields (its intensity) Frequency or Wavelength of the oscillation (u and l are inversely related) Orientation of the electric E field: up or sideways (polarization) You do not need to specify its speed In a vacuum all lightwaves move at the same speed c = 3´10⁸ m/s
12	The Electromagnetic Spectrum Radio waves Microwaves Infrared Visible Ultra-violet X-Rays Gamma rays
13	Relationship between Energy and Wavelength of Light Short wavelength Þ High energy photons Long wavelength Þ Low energy photons Intensity µ total energy (per area per second) µ (# of photons per area per second) ´ (energy per photon) Example with falling rain: Amount of rain µ (# of raindrops) ´ (volume per drop)
14	Why is energy per photon so important? Modified example: Hailstorm (with your car outside in it) Threshold for damage to car set by size of individual hailstones Below threshold hailstones cause no dents Above threshold they cause bigger dents Number of dents = number of hailstones bigger than threshold Very unlikely two small hailstones can hit exactly together to cause dent Real life example: Ultra-Violet light hitting your skin Threshold for chemical damage set by energy (wavelength) of photons Below threshold (long wavelengths) energy too weak to cause chemical changes Above threshold (short wavelength) energy photons can break apart DNA molecules Number of molecules damaged = number of photons above threshold Very unlikely two photons can hit exactly together to cause damage
15	Numerical Relationship between wavelength and photon energy Inverse relationship: Smaller l means more energetic c = speed of light = 3.00 ´ 10⁸ m/s h = Planck’s constant = 6.63 ´ 10^-34 joule/s Note: Joule is a unit of energy 1 Joule/second = 1 Watt Energy of a single photon of 0.5 mm visible light? Seems very small, but this is roughly the energy it takes to chemically modify a single molecule. Photons from a 100 W lightbulb (assuming all 100W goes into light?)
16	Optical Telescopes and Cameras Properly curved lenses and mirrors can form “Images” All the light leaving one point on object gets “reassembled” at one point on the image.
17	Refracting vs. Reflecting Telescopes
18	Why do astronomers need large telescopes? Large telescopes can collect more light Can detect fainter objects Have more light for specialized analysis. Large telescopes can form more detailed images Waves spread out as they go through an opening. The larger the opening, the less they spreads out. The longer the wavelength the more they spread out Angle of spread q µ l/D where D is Diameter of telescope Radio telescopes have to be much bigger than visible ones
19	Kinds of measurements made with telescopes Measure brightness of objects (photometry) Record images using electronic “CCD” detectors Split it into different wavelengths with “spectrometers”
20	Dark Side of the Moon What is wrong with this picture? Front: Not all primary colors (eg, pink, magenta), also refraction angles inconsistent Back: Spectrum is Convergent – I think done for art’s sake
21	Observing over the entire electromagnetic spectrum Different phenomena produce different wavelength waves Ordinary stars: Visible light Cool planets or dust clouds: Infrared light Moving charged particles, cool molecules: Radio waves Very hot objects: X-Rays and Gamma Rays Quasars: ALL wavelengths Only visible, some IR, and radio make it through atmosphere Need to observe from space for other wavelengths Going into space also lets you obtain more detailed images On Earth telescope size isn’t only limit on image resolution Temperature fluctuations in atmosphere cause “seeing” (blurring)
22	For Next Time: