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  d 2 B (1) The laws of black body radiation also tell us that amount of energy given off depends on star size and temperature: L  R 2  T 4 (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 TemperatureColorExamples O30,000 Kblue B20,000 KbluishRigel A10,000 KwhiteVega, Sirius F8,000 KwhiteCanopus G6,000 KyellowSun,  Centauri K4,000 KorangeArcturus M3,000 KredBetelgeuse Mnemotechnique : Oh, Be A Fine 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 type K5III, luminosity = 160 times that of the Sun O B A F G K M Type … 0123456789 0123456789 012345… 1 10 100 1000 L Aldebaran Sun (G2V) 160

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/cm 3 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. Dust Clouds What happens to the blue light scattered by the dust clouds? It’s still there, and sometimes can be seen M20 Pleiades

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 f peak in infrared frequencies

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

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  ”)