Lecture 3 – The Sky

1. Lecture 3 – The Sky Constellations, Star Names, and Magnitudes

2. Announcements • Homework Assignment 1 is due now. • Homework 2 – Due Monday, February 5 • Unit 5: Problem 2, Test Yourself 3 • Unit 6: Review Questions 1, 3 • Unit 9: Review Question 2, Test Yourself 1 • Unit 11: Review Question 2, Test Yourself 3

3. The Constellations • Origins • Star patterns named by ancient peoples after gods, goddesses, animals, monsters, and mythic heroes. • The constellations used by Western Culture today originated in Mesopotamia around 3,000 B.C.

4. The Constellations • Only certain stars were part of the pattern and belonged to the constellation. Faint stars were not part of any constellations. • Frequently constellations were named for their brightest star. • Of the constellations defined by the Babylonians, Egyptians, and Greeks, 48 are still used today.

5. The Constellations • The ancient civilizations we got the 48 classical constellations from couldn’t see the entire southern sky. • During the Age of Sail, European explorers added 40 modern constellations in the “uncharted” areas of the sky.

6. The Constellations • The 48 classical and the 40 modern constellations make up the 88 official constellations used by astronomers today. • The term constellation also now has a new meaning: • No longer refers to the pattern of stars itself. • Now refers to a well defined region of the sky that contains the traditional star pattern. • Everything inside that region of the sky is now part of the constellation, like a “celestial state”.

7. The Constellations

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9. What Is An Asterism? • In addition to the 88 official constellations there are several unofficial but popular star patterns. • Called asterisms. • Examples: • The Big Dipper • The Little Dipper • The Northern Cross • The Great Square

10. Daily Grade 3 – Question 1 • Which statement below most accurately describes modern constellations? • They are 88 well defined regions in the sky. • They are 88 connect-the-dot mythological sky figures. • They are 13 connect-the-dot mythological sky figures along the ecliptic. • They are 13 well defined sky regions along the ecliptic.

11. 3D Constellations • The stars of a constellation only appear to be close to one another • Usually, this is only a projection effect. • The stars of a constellation may be located at very different distances from us.

12. The Celestial Sphere • Many ancient cultures, including the Greeks and Romans, assumed that the Earth was stationary in the center of the Universe. • Given this assumption, they concluded that the Sun, Moon, planets, and stars were attached to gigantic crystalline spheres that surrounded the Earth. • Today we call this imaginary globe the Celestial Sphere.

13. Celestial Sphere Concepts THE CELESTIAL SPHERE REPRESENTS THE APPARENT VIEW OF THE UNIVERSE AS SEEN FROM THE EARTH. • A huge globe of stars surrounding the Earth. • An ancient concept of the Cosmos. IT IS A USEFUL CONCEPT TODAY BECAUSE: • it is understandable in terms of spherical geometry. • it can be used to extend map concepts for the Earth to the sky. • it is useful in navigation. RELATIONSHIP TO THE VISIBLE SKY • The visible sky is 1/2 the celestial sphere. • Half of the celestial sphere is above the horizon, while the other half is below the horizon.

14. 0 The Celestial Sphere Zenith = Point on the celestial sphere directly overhead Nadir = Point on the c.s. directly underneath (not visible!) Celestial equator = projection of Earth’s equator onto the c. s. North celestial pole = projection of Earth’s north pole onto the c. s.

15. Angular Measure In The Sky • 360o = A COMPLETE CIRCLE • 1o = 60’ (minutes) • 1’ = 60” (seconds)

16. 0 Latitude Affects Visible Sky • What we see in the sky depends on our latitude. • At the north pole, the north celestial pole is at the zenith. • At the equator, the north celestial pole is at the horizon. • The altitude of the north celestial pole = latitude.

17. 0 The Sky From Abilene – 32.4o N • North Celestial Pole is 32.4o above the northern horizon. • Celestial Equator is 57.6o above the southern horizon. • Circumpolar stars lie within 32.4o of the North Celestial Pole. • Rising & Setting stars are visible from 32.4o away from the South Celestial Pole. • Stars within 32.4o of the South Celestial Pole are never visible from Abilene.

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19. Looking North • The stars appear to circle around the North Celestial Pole (near Polaris) once in 24 hours. • Stars within the angle of the observer's latitude away from the North Celestial Pole never rise or set. • They are always above the horizon (circumpolar stars).

20. Circumpolar Constellations • Never Rise Or Set • Are Always Above The Horizon • Circumpolar Region Depends On Latitude

21. Daily Grade 3 – Question 2 • At what location on Earth is an observer that has the South Celestial Pole directly overhead? • At Earth's equator (0 degrees latitude). • At Earth's North Pole (90 degrees North latitude). • At Earth's South Pole (90 degrees South latitude). • At 45 degrees North latitude.

22. Naming The Stars • The brightest stars have had proper names for thousands of years. • Typically from Arabic • Islamic astronomers produced many detailed star charts during the middle ages. • Also many stars have names that are from the Greek • Greek astronomers also produced many star charts.

23. Naming the Stars • Proper names are often a literal description of the star’s location in the constellation: • Betelgeuse – “Shoulder of the giant” • Rigel – “Foot” • Deneb – “Tail” • Procyon – “Before the dog” • Algol – “Eye of the ghoul” • Star proper names also sometimes describe the star. • Sirius – “Scorching” • Antares – “Rival of Mars” • Kochab – “Star”

24. Scam Alert! • NO organization has been given official legal power to name the stars. • Astronomers consider the names designated by the International Astronomical Union official, but the IAU has no true legal authority to name stars (or demote planets, for that matter). • Companies that sell star names as gifts are a SCAM!

25. Bayer Letter Names • Johann Bayer • Bavararian lawyer • Published a sky atlas: Uranometria. • Assigned lower case greek letters to brighter stars in each constellation. • Usually assigned in rough order of brightness. • Alpha = brightest star. • Beta = second brightest • Gamma = third brightest • And so on… • Astronomers continue to use these “Bayer letters”

26. Bayer Letter Names • A star’s Bayer Letter Name is: • It’s greek letter first… • …followed by the possessive form of the constellation name. • Example: The star Rigel Kentarus is the brightest star in the constellation Centarus: • Alpha Centauri

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28. Daily Grade 3 – Question 3 • What is the most likely Greek letter name of the second brightest star in the constellation Lyra? • Alpha Lyrae. • Beta Lyrae. • Gamma Lyrae. • Delta Lyrae.

29. The Magnitude Scale • Invented by the Greek Astronomer Hipparchus in the 2nd century B.C. • Called the brightest stars in the night sky “stars of the first magnitude.” • Second brightest were “stars of the second magnitude.” • All the way down to “stars of the sixth magnitude.”

30. The Magnitude Scale • Very useful system, but not perfect • Technically, it refers to Apparent Visual Magnitude • Brightness in visible wavelengths as seen from Earth • Does NOT tell how bright the star actually is (absolute magnitude) because it does not take into account how far away the star is from us. • Still useful because it is related to the brightness that you “see” (even through a telescope).

31. The Magnitude Scale • Used for centuries, and then refined by modern astronomers. • 1st magnitude stars are 100 times brighter than 6th magnitude stars. • So 5 “steps” in the magnitude scale corresponds to a 100× difference in brightness. • What brightness difference corresponds to 1 step in magnitude? • Need to know what number you have to multiply by itself five times to get 100. • That number is 2.5118864315095…. (usually rounded to 2.512 when used in calculations). • So 1 step in magnitude corresponds to a 2.512× difference in brightness.

32. The Magnitude Scale

33. 0 The Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): mv = -1.42Full moon: mv = -12.5Sun: mv = -26.5

34. The Magnitude Scale This thing in parentheses here is an exponent. So say mB – mA is a number we’ll call X. You would read this equation as “2.512 raised to the X power.” • Here’s the actual equation: IA/IB = (2.512)(mB-mA) This number right here is “how bright star A is compared to star B” If it is >1, then star A is brighter than star B. If it is <1, then star A is fainter than star B.

35. Daily Grade 3 – Question 4 • The apparent visual magnitude of star A is 2 and the apparent visual magnitude of star B is 1. Based on this information which statement below must be true? • Star A emits more light than star B. • Star B emits more light than star A. • Star A is closer than star B. • Light output and distance cannot be determined from a star's apparent visual magnitude alone.

36. Announcements • Read Units 9 and 11 • Test 1 is already approaching • Wednesday, February 7