104 04MagnConst

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Published on November 13, 2007

Author: Abbott

Source: authorstream.com

Setup:  Prepare your scantron: Fill in your name and fill the bubbles under your name. Put your 3-digit code in place of “identification number” LAST NAME FIRST, First name second Question # 1: answer A Question # 2: answer B Question # 3: answer C Setup Please note that the scantrons must be turned in by each student in person, not by a classmate! Please take a moment to mute or switch off your cell phones! Use a pencil, not a pen! Reading assignment: Have you read it? Chapter 1: pp. 1-25 Brightness of stars:  Brightness of stars Understand brightness of stars: the magnitude scale A first magnitude star: m = 1mg A second magnitude star: m = 2mg A third magnitude star: m = 3mg A fifth magnitude star: m = 5mg,, just visible to the naked eye The Andromeda Galaxy m = 3mg in total An m = 10mg, star, need an amateur telescope to see it An m = 17mg, star, need a professional telescope to see it Magnitudes mean brightness: • The larger the number, the fainter the object • One magnitude difference means a lot dimmer (2.5 times) Stars in the Big Dipper:  Stars in the Big Dipper 1.9 mg 2.2 mg 4.0 mg 3.3 mg 5.6 mg Questions coming …:  Questions coming … Question 4:  sec 30 Question 4 29 The magnitude of a star tells us … A How far the star us from us. B How bright the star appears in the sky. C How bright the star is in reality. D How large the star appears in the sky. E How large the star is in reality. 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Next question coming … Question 5:  sec 30 Question 5 29 Which one is brighter, a 1mg star or a 5mg star, and how much? A The magnitude of a star does not refer to brightness at all, it refers to size. B The 1mg star is much brighter. C The 1mg star is a little brighter. D The 1mg star is much fainter. E The 1mg star is a little fainter. 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Next question coming … Question 6:  sec 30 Question 6 29 Do you need a telescope to see Pluto whose brightness is a 15mg? A No, it can be seen by the naked eye. B Yes, but even binoculars will suffice. C Yes, a small amateur telescope is needed. D A fairly large amateur telescope or a small professional telescope is needed. E Pluto is only observable in the largest professional telescopes in the world. 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Exercises on magnitudes 1:  How bright do you think Orionis is?   Exercises on magnitudes 1 How bright do you think Orionis is? This is how Orion looks to a naked-eye observer in complete darkness.  Orionis : 2.0 mg   Orionis : 4.3 mg Orionis is one of the brightest stars in the sky. How bright do you think it is?  Orionis: 0.5 mg Exercises on magnitudes 2:  Exercises on magnitudes 2 Capella is 0 mg. How bright do you think the following are? Pleiades 2 mg Aldebaran 1 mg Jupiter - 2 mg Venus - 4 mg International Space Station 1 mg Apparent and absolute brightness:  Star A and star B have the same apparent magnitude (m=5mg). Star A is 10 pc away: absolute magnitude M=5mg Star B is 25 pc away: absolute magnitude M=3mg The rule: 2.5 times as far away, looks 2mg dimmer 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. The Sun: M = 5mg (an average star) Stars around the South Pole:  Stars around the South Pole How far are all these stars? (See next page for answers.) Distance modulus:  Distance modulus Centauri m=1.3mg, distance=1.3 pc => absolute M=5.7mg : a faint star looking bright; distance modulus = 4.4 mg (close to us) Crucis m=1.6mg, distance=27 pc => absolute M=-0.6mg distance modulus = - 2.2 mg (not that close) Centauri m=2.6mg, distance=120 pc => absolute M=-2.8mg: bright star looking faint; distance modulus = - 5.4 mg (far from us - still in the solar neighborhood) Centauri m=3.5mg, distance=5200 pc => absolute M=-17.1mg: 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  log d The name of “M - m” is: the “distance modulus” How to use this? Questions coming …:  Questions coming … Question 7:  sec 30 Question 7 29 How bright would the Sun look in the sky from a distance of 10 parsec? A -12mg (as bright as the full Moon). B 1mg (as bright as the brightest stars in the sky). C 5mg (barely visible to the naked eye). D 15mg (very faint). E Invisible: we cannot see that far. 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Next question coming … Question 8:  sec 30 Question 8 29 Which of the following data of Sirius cannot be directly measured from Earth, only calculated? A Its apparent brightness, which is m =  1.5mg. B Its absolute brightness, which is M = + 1.5mg. C Its distance from us, which is 9 light years. D The color of its light. E Its speed of motion in the sky. 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Next question coming … Question 9:  sec 30 Question 9 29 Polaris, the North Star, is m = 2mg, not particularly bright. Its absolute magnitude is M = - 4mg. 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. 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

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