Welcome to AY 204

1. Welcome to AY 204 Solar System Astronomy Prof: Dr. Jimmy Irwin Meeting Room: Gallalee 338 Class time: TR 9:30-10:45 AM Course website: http://pages.astronomy.ua.edu/jairwin/AY204/AY204.html

2. Notes & Reminders • Wednesday, August 29 is the last day to register or add a course. • Wednesday, August 29 is also the last day to drop a course without a grade of “W”.

3. Notes & Reminders • Class notes now posted online after each class (http://pages.astronomy.ua.edu/jairwin/AY204/AY204.html). • Lectures also available on Tegrity. • Both can be reached via Blackboard.

4. Notes & Reminders Homework #1 is due Thursday, September 6. Tip: No need to carry seven digits in your calculations! Only carry 2-3 digits in most instances (occasionally more, depending on the accuracy of your input quantities)

5. A Universe in motion • We are not “sitting still.” • We are moving with the Earth. • and not just in one direction The Earth rotates around its axis once every day. 0.5 km/s

6. Rotation vs. Revolution Rotation - spinning of an object about its own axis (e.g., a spinning top) - Earth spins on its axis once per day Revolution- movement of an object around another object (e.g., a ball tied to a string) - orbit of the Earth around the Sun every 365 days

7. The Earth orbits around the Sun once every year 30 km/s… The Earth’s axis is tilted by 23.5o

8. Our Sun moves relative to the other stars … Our Sun and local stars orbit around the center of the Milky Way Galaxy every 230 million years. 220 km/s…

9. Galaxies have orbital motions in clusters HST Few hundred km s-1

10. All of this together with the expansion of the Universe

11. Are we ever sitting still? No! We cannot even define what “still” means!

12. Let’s Get Some Practice Multiplying/dividing large numbers need not be difficult or always require a calculator.

13. Back of the Envelope Calculations Sometimes, astronomers want to get quick, rough answers to problems. The same can be applied to every day life. “Back of the Envelope” Calculations It’s a good idea to try this before you start a homework problem to get a rough idea of what answer to expect before you plug numbers into your calculator.

14. Back of the Envelope Our national debt is about 21 Trillion USD. Assume we pay 3% interest per year. How much is that?

15. Back of the Envelope Bryant-Denny Stadium opened in 1929. How many people have attended home football games in the Stadium? and How much money has the University collected in ticket sales during that time (in real 2018 dollars)?

16. Back of the Envelope How many pitches have been thrown in the history of Major League Baseball? and What total distance have these pitches covered (in miles)?

17. Back of the Envelope Use this technique to get a rough estimate of your final answer on homework problems. Ask yourself if your final answer makes sense. Great way to catch errors in your homework – have an idea ahead of time of what your answer should be

18. Units In this course, you can use either of the following two systems: MKScgs* Length meters centimeters Mass kilograms grams Time seconds seconds *preferred by astronomers Whichever system you choose, you MUST BE CONSISTENT with this units throughout the entire problem (more on this later)

19. Light • Astronomy: almost everything we know comes from using light as a messenger. • Speed of light: c = 3 x 1010 cm/s • 1 light-year = 9.46 x 1017 cm • Examples • Sun to Earth 8 light-minutes (10-5 lyr) • = 1 astronomical unit (1.5 x 1013 cm) • Earth to Saturn? ~1 ½ light-hours • Nearest star? 4.4 lyr • Andromeda Galaxy 2.5 million lyr • distance = velocity x travel time

20. Luminosity: Amount of power an object radiates (energy per second) Intrinsic property of an object Apparent brightness (or flux): Amount of light that reaches Earth (energy per second per square centimeter) Depends on observers location

21. Luminosity Versus Apparent Brightness Two observers looking at a 100 Watt light bulb X Observer 2 10 meters X Observer 1 2 meters Both observers agree the luminosity of the bulb is 100 Watts. The observers measure a different apparent brightnessfor the bulb.

22. Luminosity passing through each sphere is the same Area of sphere = 4 (radius)2 Divide luminosity by area to get brightness Think of equal amounts of paint needed to paint walls of different sizes --- the larger wall gets a thinner layer of paint.

23. The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4 (distance)2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4 (distance)2 x (Brightness) This makes finding the distances to astronomical objects crucial!

24. How would the apparent brightness of Neptune change if it were 3 times closer? • It would be 1/3 as bright • It would be 1/6 as bright • It would be 1/9 less bright • It would be 9 times brighter

25. How would the apparent brightness of Neptune change if it were 3 times closer? • It would be 1/3 as bright • It would be 1/6 as bright • It would be 1/9 less bright • It would be 9 times brighter

26. The (nutty) Magnitude System • Ancient Greeks (Hipparchus) said “I’ll order the stars by how bright they appear to my eye”: 1, 2, 3, 4, 5, 6 (bigger numbers are fainter) • The eyes (and ears) are approximately logarithmic detectors • In modern times we convert magnitudes to linear brightness scale • m = -2.5 log10(brightness) + constant • Constant set to preserve (roughly) the historical magnitudes of stars

27. The (nutty) Magnitude System • Vega: 0.0 • Sirius: -1.5 • Polaris (North star): 2.0 • Faintest visible with naked eye: 6 • Faintest visible with largest telescope: ~27 • Jupiter (peak): -2.7 • Venus (peak): -4.4 • Full Moon: -12.7 • Sun: -26.7 Note: these are magnitudes in the V band

28. Working with Magnitudes m = -2.5 log(brightness) + constant brightness = flux = F Question: how much brighter is Sirius than Polaris? msirius - mpolaris = -2.5 log(FSirius) + 2.5 log(FPolaris) = -2.5 log(FSirius/FPolaris) -1.5 - 2.0 = -2.5 log(FSirius/FPolaris) FSirius/Fpolaris = 10(-3.5/-2.5) = 25.1 (Sirius is 25.1x brighter) Rule of thumb: 5 mag difference is a factor of 100 difference in brightness, 10 mag --> (100*100)= 10,000

29. Chapter 2Discovering the Universe for Yourself

30. What does the universe look like from Earth? • With the naked eye, we can see more than 2000 stars as well as the Milky Way, the Large and Small Magellanic Clouds, and the Andromeda Galaxy (barely).

31. The Milky Way A band of light making a circle around the celestial sphere. What is it? Our view into the plane of our galaxy.

32. Constellations • A constellation is a region of the sky. Eighty-eight constellations fill the entire sky. Many southern constellations have lame names (i.e., Telescopium, Sextans) Official boundaries agreed upon by the International Astronomical Union

33. The Celestial Sphere North celestial poleis directly above Earth’s North Pole. South celestial poleis directly above Earth’s South Pole. Celestial equatoris a projection of Earth’s equator onto sky.

34. The Celestial Sphere The ecliptic is the Sun’s apparent path through the celestial sphere over the course of a yearthrough 12 signs of the zodiac.

35. We measure the sky using angles. 360° in a circle 1° = 60 arcminutes = 3600 arcseconds

36. Angular Size Physical size = distance * angular size (or r = d * θ) Here, θ MUST be in units of radians, where there are 2π radians in a 360 degree circle. 1 radian = 57.3° 1” = 4.85 x 10-6 radians

37. Angular Size Example Physical size = distance * angular size (or r = d * θ) An object at a distance of 1.5 x 1013 cm subtends an angle of 30 arcminutes in the sky. What is this object’s physical size? r = distance x angular size = 1.5 x 1013 cm * [(30 arcminutes)*(60 arcseconds/arcminute) *(4.85 x 10-6 radians/arcsecond)] r = 1.3 x 1011 cm