Finding the absolute Magnitude

1. Finding the absolute Magnitude • To figure out absolute magnitude, we need to know the distance to the star • Then do the following Gedankenexperiment: • In your mind, put the star from its actual position to a position 10 pc away • If a star is actually closer than 10pc, its absolute magnitude will be a bigger number, i.e. it is intrinsically dimmer than it appears • If a star is farther than 10pc, its absolute magnitude will be a smaller number, i.e. it is intrinsically brighter than it appears

2. Measuring the Sizes of Stars • Direct measurement is possible for a few dozen relatively close, large stars • Angular size of the disk and known distance can be used to deduce diameter

3. Indirect Measurement of Sizes • Distance and brightness can be used to find the luminosity: L  d2 B(1) • The laws of black body radiation also tell us that amount of energy given off depends on star size and temperature: L R2  T4 (2) • We can compare two values of absolute luminosity L to get the size

4. Sizes of Stars • Dwarfs • Comparable in size, or smaller than, the Sun • Giants • Up to 100 times the size of the Sun • Supergiants • Up to 1000 times the size of the Sun • Note: Temperature changes!

5. Classification of the Stars: Temperature Class Temperature Color Examples O 30,000 K blue B 20,000 K bluish Rigel A 10,000 K white Vega, Sirius F 8,000 K white Canopus G 6,000 K yellowSun,  Centauri K 4,000 K orange Arcturus M 3,000 K red Betelgeuse Mnemotechnique: Oh, Be AFine Girl/Guy, Kiss Me

6. The Key Tool to understanding Stars: the Hertzsprung-Russell diagram • Hertzsprung-Russell diagram is luminosity vs. spectral type (or temperature) • To obtain a HR diagram: • get the luminosity. This is your y-coordinate. • Then take the spectral type as your x-coordinate, e.g. K5 for Aldebaran. First letter is the spectral type: K (one of OBAFGKM), the arab number (5) is like a second digit to the spectral type, so K0 is very close to G, K9 is very close to M.

7. Constructing a HR-Diagram • Example: Aldebaran, spectral typeK5III, luminosity = 160 times that of the Sun L 1000 Aldebaran 160 100 10 1 Sun (G2V) O B A F GK M Type … 01234567890123456789 012345…

8. The Hertzprung-Russell Diagram • A plot of absolute luminosity (vertical scale) against spectral type or temperature (horizontal scale) • Most stars (90%) lie in a band known as the Main Sequence

9. Hertzsprung-Russell diagrams … of the closest stars …of the brightest stars

10. Star Formation(Compare: Solar System Formation)

11. Where Stars come from: the Interstellar Medium • Gas • Single atoms and molecules • Mostly hydrogen (90%), 9% helium; deficient in heavier elements • Dust • Microscopic clumps of atoms/molecules • Size ~ 107 m, similar to the wavelength of visible light • Composition is not well known • Temperature depends on the proximity of stars, typically ~100 K • Density is very low! • Gas: about 1 atom/cm3 D; Dust: even less dense

12. How do we know it’s there? • Cold gas or dust doesn’t glow • they are dark • We might “see” them blocking light of other objects (Dark Nebulae) • Gas & Dust clouds are very dilute • they might not be blocking other object’s light totally • Usually they will reduce (redden) the light of other objects

13. Reminder: Kirchhoff’s Laws Cool gas absorbs light at specific frequencies  Dark Lines: “fingerprints of the elements”

14. Looking Through Dust Clouds

15. Seeing Through Gas and Dust • EM radiation is appreciably scattered or absorbed only by particles with size comparable to its wavelength (or larger) • Gas • Emission and absorption lines • Doesn’t block EM radiation • Dust • Grain size is comparable to the wavelength of visible light • Dims visible light and high frequency EM radiation • Transparent to longer wavelength radio and infrared radiation, though

16. Scattering in Earth’s Atmosphere

17. Pleiades M20 Dust Clouds • What happens to the blue light scattered by the dust clouds? • It’s still there, and sometimes can be seen

18. Nebulae • Any irregularly shaped cloud of gas and dust • May be bright or dark, depending on temperature • Types: • Emission (bright) Nebulae • Dark Nebulae • Reflection Nebulae • Historic Remark: Only some of the 109 “nebulae” catalogued by Charles Messier in 18th Century are actual nebulae; most are star clusters and galaxies

19. Dark Nebulae • Classic Example: Horsehead Nebula in Orion Can’t see what’s behind a dark nebula, that’s why we see it!

20. Dark Nebulae • Dark Nebulae do emit light of their own, though • Temperatures ~ 10 to 100 K; black body radiation peaks in the radio to infrared frequencies fpeakin infrared frequencies

21. Dark Nebulae • Now you see it Now you don’t • (infrared frequencies)(visible frequencies) Rho Ophiuchi (infrared) Rho Ophiuchi (visible light)

22. Emission Nebulae • Regions of hot glowing gas • Temperatures ~ 8000K • Made to glow by ultraviolet radiation emitted by new O- or B-type (hot) stars located inside • Emission lines from the nebula are easily distinguished from the continuous spectrum and absorption lines of stars within • Color predominantly red, the color of a particular hydrogen emission line (the “H line”)

23. Emission Nebulae Example: Orion Nebula (M 42) • hot glowing gas Temperatures ~ 8000K • Made to glow by ultraviolet radiation emitted by young O- or B-type (hot) stars located inside • Color predominantly red, the color of a particular hydrogen emission line (“H”)