1. 9 Stars … how I wonder what you are.

2. 9 Goals Stars are Suns. Are they: –Near? Far? –Brighter? Dimmer? –Hotter? Cooler? –Heavier? Lighter? –Larger? Smaller? What categories can we place them in?

3. 9 Angular Size Linear size = how big something really is –Meters, inches, light years, feet Angular size = how big something looks –Degrees, arcminutes, arcseconds, milliarcseconds Circle = 360 degrees 1 degree = 60 arcmin 1 arcmin = 60 arcsec 1 arcsec = 1000 mas

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5. 9 Distance One proof of a heliocentric Universe is stellar parallax. –Tycho Brahe saw no parallax. –Copernicus: stars too far away. Nearest star: Proxima Centauri Parallax angle = 0.76 arcsec –Tycho’s precision = 1 arcmin

6. 9 The Parsec What is the distance of an object with a parallax angle of 1 arcsec? Distance = 206,265 AU This distance is 1 parsec (pc) 1 pc = 206,265 AU = 3.3 ly 1 lightyear = distance light travels in one year.

7. 9 Concept Test If Star A has a parallax of 2 arcseconds, and Star B has a parallax of 0.25 arcseconds: a.Star A is closer to us than Star B. Both are farther from us than 1 pc. b.Star A is closer to us than Star B. Both are closer to us than 1 pc. c.Star A is closer to us than 1 pc. Star B is farther than 1 pc. d.Star B is closer to us than 1 pc. Star A is farther than 1 pc. e.Star B is closer to us than Star A. Both are farther away than 1 pc.

8. 9 Distances Closest star: Proxima Centauri parallax = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears

9. 9 Brightness How bright are they really? What is due to distance? What is due to luminosity? Luminosity: –Total energy radiated every second.

10. 9 Terms Brightness = How intense is the light I see from where I am. –Magnitude is numerical term for this. Luminosity = how much light is the thing really giving off.

11. 9 Magnitude Scale The SMALLER the number the BRIGHTER the star! –Every difference of 1 magnitude = 2.5x brighter or dimmer. –Difference of 2 magnitudes = 2.5x2.5 = 6.3x brighter or dimmer

12. 9 Magnitude vs. Brightness Mag. Difference Factors of 2.5Brightness Diff. 12.5 1 = 2.5 2.5 22.5 2 = 2.5 X 2.5 6.3 32.5 3 = 2.5 X 2.5 X 2.5 16 42.5 4 = 2.5 X 2.5 X 2.5 X 2.5 40 52.5 5 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 100 62.5 6 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 X 2.5 250

13. 9 Star light, star bright Sirius is magnitude -1.5 Polaris is magnitude 2.5 Is Sirius really more luminous than Polaris? No, Sirius is just closer. Example: light bulbs.

14. 9 Apparent and Absolute Apparent Magnitude = brightness (magnitude) of a star as seen from Earth.  m –Depends on star’s total energy radiated (Luminosity) and distance Absolute Magnitude = brightness (magnitude) of a star as seen from a distance of 10 pc.  M –Only depends on a star’s luminosity

15. 9 example Our Sun: –m = -26.8, –distance = 4.8 x 10 -6 pc So: M = 4.8 Polaris: –m = 2.5, –distance = 132 pc So: M = -3.1 Polaris is 1500 times more luminous than the Sun!

16. 9 Concept Test The most likely absolute magnitudes (M) for each is: a.A = 2.5, B = -2.5, C = 2.5 b.A = 2.5, B = 2.5, C = -2.5 c.A = -2.5, B = 2.5, C = 2.5 d.A = 2.5, B = 2.5, C = 2.5 e.None of the above. StarDistancemM A5 pc1.0 B10 pc2.5 C20 pc

17. 9 Stellar Temperatures How hot are stars? Thermal radiation and temperature. Different stars have different colors, different stars are temperatures. Different temp, different trace compositions Stellar Spectra Hot Cool

18. 9 Spectral Classification

19. 9 Stellar Masses How massive are stars? Kepler’s Laws – devised for the planets. Apply to any object that orbits another object. Kepler’s Third Law relates: – Period: “how long it takes to orbit something” –Semimajor axis: “how far you are away from that something” –Mass: “how much gravity is pulling you around in orbit” Where M is the Total Mass. Can calculate the mass of stars this way.

20. 9 Binary Stars Most stars in the sky are in multiple systems. Binaries, triplets, quadruplets, etc…. –Sirius –Alcor and Mizar –Tatooine The Sun is in the minority by being single.

21. 9 0.005 arcsec

22. 9 Stellar Masses How massive are stars? Most stars have masses calculated this way. Find: –The more massive the star, the more luminous it is. –The more massive the star, the hotter it is.

23. 9 Stellar Radii How big are stars? We see stars have different luminosities and different temperatures. Stars have different sizes. If you know: –Distance –Angular size Learn real size. 50 mas

24. 9 Betelgeuse Angular size = 50 mas Parallax = 7.6 mas = 0.0076 arcsec Apparent mag = 0.6 Distance = 1/parallax = 132 pc True size = distance * angular size = 1400 R sol Absolute Mag = m – 5log(d/10pc) = -5 –Our sun M ~5, Betelgeuse = 10,000x luminosity

25. 9 Angular versus Linear Supergiants, Giants and Dwarfs

26. 9 H-R Diagram Can order the stars by: –Temperature (or spectral type) –Luminosity (or absolute magnitude). And see where other qualities fall: –Mass –Radius

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28. 9 Luminosity Class The roman numerals. Stars at same temp can have different luminosities. Corresponds to different classes: MS, giant, supergiant. V III I

29. 9 Concept Test Which star is: 1.Hottest? 2.Coolest? 3.Faintest as seen from Earth? 4.Most luminous? Of Main Seq. Stars? 1.Most massive? 2.Most like the Sun? StarSpec Type mM AF0 V0.0 BG2 V10.04.4 CK5 III0.0-2.0 DF7 I-5.0 EK3 V5.06.5

30. 9 The Main Sequence Stars characterized by what holds them up. 90% held up by heat of Hydrogen fusion? 4H  He + Energy

31. 9 Main Sequence & Thermal Radiation The Main Sequence makes sense! Hotter stars are bluer – Wien’ Law Hotter stars are brighter – Stefan’s Law

32. 9 Homework #9 For Oct 8: Reread B16.1 – B16.5, Read B16.6 Redo Problem 21, correct mistakes, give reasons.