2. Prepare your scantron: Fill in your name and fill the bubbles under your name. LAST NAME FIRST, First name second Put your 4-digit code instead of “ IDENTIFICATION NUMBER ”. --- (The last 4 digits of your OleMiss ID.) Question # 1: answer A Question # 2: answer A Question # 3: answer B Setup: Please take a moment to mute your cell phone! Use a pencil, not a pen!

3. A star ’ s absolute magnitude is how bright it would look from 10 pc (32 light years) away. How bright it looks (m) How bright it really is (M) Apparent and absolute brightness Recall: A parsec (pc) is a unit of distance, 3.26 light years. Why this unit? Because the orbit of Earth looks 1 arc sec radius from the distance of 1 pc. The Sun: M = 5 mg (an average star) Solar neighborhood (~100 pc)

4. Star A and star B have the same absolute magnitude M = 5 mg Star B is 10 pc away: apparent magnitude m = 5 mg Star A is 25 pc away: apparent magnitude m = 7 mg The rule: 2.5 times as far away, looks 2 mg dimmer A B Looks like: A B 10 pc 25 pc Distance makes stars dimmer Reality:

5. Distance modulus  Centauri m=1.3 mg, distance=1.3 pc => absolute M=5.7 mg : a faint star looking bright; distance modulus = 4.4 mg (close to us)  Crucis m=1.6 mg, distance=27 pc => absolute M=-0.6 mg distance modulus = - 2.2 mg (not that close)  Centauri m=2.6 mg, distance=120 pc => absolute M=-2.8 mg : bright star looking faint; distance modulus = - 5.4 mg (far from us - still in the solar neighborhood)  Centauri m=3.5 mg, distance=5200 pc => absolute M=-17.1 mg : whole star cluster looking faint; distance modulus = - 13.6 mg (very far from us, halfway to the center of the Galaxy) The distance modulus Relation: M (abs. magn.), m (appt. magn.), d (distance in pc) Meaning of relation: the farther a star, the fainter it looks M - m = 5 - 5  lg d The name of “ M - m ” is: the “ distance modulus ” How to use this?

6. Questions coming …

7. sec 30 Question 4 29 How bright would the Sun look in the sky from a distance of 10 parsec? A -12 mg (as bright as the full Moon). B 1 mg (as bright as the brightest stars in the sky). C 5 mg (barely visible to the naked eye). D 15 mg (very faint). E Invisible: we cannot see that far. 282726252423222120 19 181716151413121110 9 8 7 6 5 4 3 2 1 0 Next question coming …

8. sec 30 Question 5 29 Which of the following data of Sirius cannot be directly measured from Earth, only calculated? A Its apparent brightness, which is m =  1.5 mg. B Its absolute brightness, which is M = + 1.5 mg. C Its speed of motion in the sky. D The color of its light. 282726252423222120 19 181716151413121110 9 8 7 6 5 4 3 2 1 0 Next question coming …

9. sec 30 Question 6 29 Polaris, the North Star, is m = 2 mg, not particularly bright. Its absolute magnitude is M = - 4 mg. If you imagine Polaris placed where the Sun is now, would it look brighter, or fainter than the Sun? A Ten thousand times brighter. B Somewhat brighter. C About the same. D Somewhat fainter. E Ten thousand times fainter. 282726252423222120 19 181716151413121110 9 8 7 6 5 4 3 2 1 0

10. How do we know the distance to the stars? (Closeby stars, for a start.) Closeby stars seem to move on tiny circles, once a year Reflection of the motion of Earth around the Sun Most (far-away) stars move very little Measure parallax: the closer the star, the larger the circle Parallax = 1 as ↔ distance 1 parsec (pc) 1 pc = 3.26 light years Formula: distance[pc] = 1/parallax[as] Practical limit - precision: Good telescope can measure where the middle of the blurred image of a star is with a precision of 1/1000 as Can use parallax method up to distances of 2-300 pc works only for the closest stars – in the Solar Neighborhood. Parallax

11. Questions coming …

12. sec 30 Question 7 29 What is parallax? A Far away stars appear dimmer in the sky. B Stars make one circle around the whole sky in one year. C Stars move in tiny circles in the sky, once a year. D The telescope must follow the star ’ s apparent daily motion. 282726252423222120 19 181716151413121110 9 8 7 6 5 4 3 2 1 0 Next question coming …

13. sec 45 Question 8 403530252019181716151413121110 9 8 7 6 5 4 3 2 1 0 Next question coming … How large an effect is parallax? A Very large: stars move all the way around the sky in a year. B Large. You can see it by the naked eye without any difficulty. C Tiny. We need a very precise telescope to detect the parallax of stars. D Extremely tiny. Not even the largest telescopes can see the parallax of stars because stars are so far away.

14. sec 45 Question 9 403530252019181716151413121110 9 8 7 6 5 4 3 2 1 0 Polaris, the North Star, has its parallax measured as 0.01 arc seconds. How far is it? A One parsec. (That would be ~ 3 light years.) B A tenth of a parsec. (That would be ~ 1/3 of a light year.) C A hundred parsecs. (That would be ~ 300 light years.) D Ten parsecs. (That would be ~ 30 light years.) E A thousand parsecs. (That would be ~ 3000 light years.)

15. Enjoy the pictures … And now enjoy the pictures … These pictures have already been posted in the first presentation called ‘Intro”.