The Stars

1. The Stars

2. The Sun is a Star

3. Surface of the Sun

4. Solar Prominence

5. Solar Corona

6. The Stars in the Sky Vary in Brightness • Distance • Size Vary in Color • Color = Temperature

7. Star Names • Proper star names mostly Arabic • Greek Letters, Numbers • Catalog Identifiers • Faint stars usually have no name

8. Constellations • 89 in number • Based on Near Eastern myths via the Greeks • Some 17th-18th century Invented Constellations • Now Have Formal Boundaries • Every star is in one and only one constellation • Constellations are entirely human inventions. They are not “Really” in the sky.

9. The Southern Cross

10. A Myth in the Autumn Sky

11. A Star Map

12. Western Constellations

13. Chinese Constellations

14. The Heavens Are Not Changeless • The Stars Move • Most of our constellations would have been unrecognizable to Neanderthal Man • The Solar System Moves • Very few of our nearby stars would have been visible to the first humans • Stars are Born, Live and Die • Many of our brightest stars did not exist in the days of the dinosaurs

15. Parallax and the Distances of Stars • Stars appear identical all over Earth • They do show slight parallax shift from opposite sides of Earth’s orbit

16. Parallax: pre-1997 • Parallax is tiny - was once used as argument against motion of the Earth • One second of arc = size of a quarter at 5 km (3 mi.) • Parallax angle of nearest star (4.3 l.y.) is 0.75” • Accuracy limited by Earth’s atmosphere • Fairly accurate to 30-40 l.y., rough to 100

17. Hipparcos • Named for ancient Greek astronomer who catalogued the stars • High Precision Parallax Collecting System • Launched by European Space Agency, 1989 • Data Collection 1989-1993 • Data Analysis 1993-1997

18. The Hipparcos Data • 118,218 stars measured: parallax and motion • 22,396 accurate to 10% - a 20-fold improvement • Stars out to 200-300 l.y. are known to within 10% • 30,000 more accurate to 20% • All pre-Hipparcos distance data is obsolete

19. Brightness of Stars • Variations in distance and intrinsic brightness • Scale based on one by Hipparcos 500 B.C. • Magnitude: Large Numbers = Fainter • One magnitude = 2.5 x • Five magnitudes = 100 x

20. Magnitudes • Planet around nearby star: 30 • Pluto: 13 • Faintest Naked-Eye Star: 6 • Big Dipper Stars: 2 • Sirius (Brightest Star) -1.6 • Venus -4 • Full Moon -12 • Sun -27

21. Absolute Magnitude • Altair and Deneb are about equally bright as seen from Earth • Altair is 16 l.y. away, Deneb 1600 • Hence Deneb must be about 10,000 times brighter

22. Absolute Magnitude • How bright a star would be at a distance of 32.6 l.y. (10 parsecs) • Sun: 4.5 (inconspicuous naked-eye star) • Altair: 2.2 • Deneb: -7.1 (bright as crescent moon) • Note: Deneb - Altair about 10 magnitudes = 100 x 100 = 10,000 times

23. Black-Body Radiation • Objects Emit Radiation Because They Are Hot • Why “Black”? Because None of the Radiation is Reflected from Some Other Source • The Sun Emits Black-Body Radiation, the Moon Does Not

25. Color = Temperature

26. Why Black-Body Radiation is so Important • Color is directly related to temperature • Temperature is the only determinant of color • Energy per unit area is the same if temperature is the same • If two stars have the same color and distance, difference in brightness is due to difference in size • Dwarf and giant stars are literally dwarfs or giants

27. Spectroscopy • Different atoms absorb or emit specific wavelengths of light • When light spread into a spectrum, the absorbed wavelengths show up as dark (missing) bands • These spectral lines are indicators of: • Chemical composition • Physical conditions

28. Atoms and Radiation

29. The Solar Spectrum

30. Spectral Lines are Affected By: • Electrical and Magnetic Fields • Number of Electrons Atoms Have Lost (Indicates Temperature and Pressure) • Motion (Doppler Effect) • Blue-shifted if Motion Toward Observer • Red-shifted if Motion Away From Observer

31. The Doppler Effect

32. The Doppler Effect

33. What the Doppler Effect Tells Us • Radial Motion • Rotation of Stars • Approaching side of star blue-shifted, receding side red-shifted • Unseen Companions (Stars or Planets) • Star oscillates around center of mass • Surface and Interior Motions • Changes in Size • Interior Oscillations

34. The Hertzsprung-Russell Diagram

35. How Stars Form • Collapsing gas and dust cloud • Protostar - mostly infrared

36. Main Sequence Stars • Brown Dwarf • Red Dwarf • Normal Star

37. All Objects Exist Because of a Balance Between Gravity and Some Other Force • People, Planets-Interatomic Forces • Normal Stars-Radiation • White Dwarfs-Electron Repulsion • Neutron Stars-Nuclear Forces • Black Holes-No Known Force

38. How Stars Die • Main Sequence Stars Brighten With Age • The More Massive a Star, the Faster it Uses Fuel • Giant Phase • White Dwarf • Supernova • Neutron Star - Pulsar • Black Hole

39. Historical Supernovae • 1006 - Chinese • 1054 - Chinese, European, Anasazi? • 1572 - Tycho’s Star • 1604 - Kepler’s Star • 1987 - Small Magellanic Cloud (170,000 l.y.)

40. Life (Briefly!) Near a Supernova • Sun’s Energy Output = 77 billion megatons/second • Let’s relate that to human scales. What would that be at one kilometer distance? • 77 x 1015 tons/(150 x 106km)2 = 3 tons • Picture a truckload of explosives a km away giving off a one-second burst of heat and light to rival the Sun

41. Now Assume the Sun Goes Supernova • Brightens by 100 billion times • Our 3 tons of explosive becomes 300,000 megatons • Equivalent to entire Earth’s nuclear arsenal going off one km away - every second • This energy output would last for days

42. Planetary Systems • Protoplanetary Disks • Accretion of Planets • Expulsion and Migration of Planets • About 120 extrasolar planets known • Our Solar System may be unusual?

43. Protoplanetary Disks in Orion