Chapter 16: Analyzing Starlight

1. Chapter 16:Analyzing Starlight Astronomy 2010

2. Astronomy 2010

3. Starlight • stars: bright lights in the sky • like our sun? different? in what ways? why? • Nearest: Proxima Centauri 4.2 LY • 100,000 years for fastest spacecraft • red dwarf, 7% of Sun's diameter • 1/18,000 as bright as Sun • nearest star of a triple system • observable from southern hemisphere • Alpha Centauri 4.3 LY • 40% brighter than Sun (G2 V spectral class) • yellow orange (main sequence) • magnitude -0.3 apparent, +4.4 absolute Astronomy 2010

4. All Stars are Different • colors: blue-white to red • brightness: bright to very faint • Orion: Constellation with many different star types • Betelgeuse: orange-red supergiant • Rigel: blue-white supergiant Astronomy 2010

5. Betelgeuse A red supergiant star Astronomy 2010

6. Key Properties of Stars • Sun's key properties • mass = 333,400 x Earth mass • surface temperature: color yellow  5860 K • composition: spectrum  mostly hydrogen • size = 110 x Earth diameter • luminosity = 3.8 x 1026 watts (spectral type G2V) • Magnitude – 26.7 apparent, +4.8 absolute • Deduce: core “burns” hydrogen, converting it to helium by thermonuclear fusion. • Stars: how do we infer mass, temperature, chemical composition, size from observations? Astronomy 2010

7. Key Properties of Stars • Stars: how do we infer mass, temperature, chemical composition, size from observations? • once we know distance, we can say a lot • Chapter 18: how we measure distance • assume for now we know the distance • Distance Units: • 1 AU handy unit for distances in Solar system • light year: distance light travels in one year, 9.46 x 1017 m • New unit: parsec (pc) • 1 pc = 3.26 LY is roughly the average distance between stars • 1 kiloparsec = kpc = 1,000 parsecs is roughly the size of galaxy • natural unit for measuring distances – see Ch. 18 Astronomy 2010

8. Star Brightness • Star brightness specified with the magnitude system. • Devised by the Greek astronomer Hipparchus around 150 B.C.E. • brightest stars into the first magnitude class, • next brightest stars into second magnitude class, • and so on, until he had all of the visible stars grouped into six magnitude classes. • dimmest stars were of sixth magnitude. • brighter objects have smaller magnitudes than fainter objects! • magnitude system was based on how bright a star appeared to the unaided eye. Astronomy 2010

9. Extra magnitude? • Some objects go beyond Hipparchus' original bounds of magnitude 1 to 6. • Very bright objects can have magnitudes of 0 or even negative numbers. • Very faint objects have magnitudes greater than +6. • Remember: brighter objects have smaller magnitudes than fainter objects! Astronomy 2010

10. Apparent Magnitude • Apparent brightness of a star observed from the Earth is called the apparent magnitude. • The apparent magnitude is a measure of the star's flux received by us. • Examples of apparent magnitudes: • Sun = -26.7, • Moon = -12.6, • Venus = -4.4, • Sirius = -1.4, • Vega = 0.00, • faintest naked eye star = +6.5, • brightest quasar = +12.8, • faintest object = +27 to +28. Astronomy 2010

11. Absolute Magnitude • Measure of star luminosity. • Luminosity is the total amount of energy radiated by the star every second • If you measure a star's apparent magnitude and know its absolute magnitude, you can find the star's distance  • If you know a star's apparent magnitude and distance, you can find the star's luminosity • A quantity that depends on the star itself, not on how far away it is • Provides information about the structure of the star – this is the real luminosity • More important quantity than the apparent brightness • need the distance to determine the absolute magnitude Astronomy 2010

12. Star Luminosity vs Temperature • stars are luminousbecause • they are hot • they are large • or both! • Luminosity of an object = the amount of energy every square meter produces multiplied by its surface area. • Luminosity = s x T4, • Luminosity of a star increases very quickly with even slight increases in the temperature. Astronomy 2010

13. Star Luminosity vs Size • Luminosity ~ surface area. • 1,000 watt bulb has same luminosity as a row of ten 100 watt bulbs • Luminosity of a bigger star larger than a smaller star at the same temperature. • From the apparent brightness, temperature, and distance of a star, one can determine its size. Astronomy 2010

14. Absolute vs Apparent • Star brighter if closer • brightness fades with distance • inverse square law • if stars were all the same brightness than apparent luminosity would measure distance Astronomy 2010

15. Famous stars Magnitudes and Distances of some stars(from the precise measurements of the Hipparcos mission) Star ApparentMagnitude Distance(pc) AbsoluteMagnitude Luminosity(relative to Sun) Sun -26.74 4.84813×10-6 4.83 1 Sirius -1.44 2.6371 1.45 22.5 Arcturus -0.05 11.25 -0.31 114 Vega 0.03 7.7561 0.58 50.1 Spica 0.98 80.39 -3.55 2250 Barnard's Star 9.54 1.8215 13.24 1/2310 Proxima Centauri 11.01 1.2948 15.45 1/17700 • Most famous apparently bright stars are also intrinsically bright (luminous). • Can be seen from great distances away. • Most nearby stars are intrinsically faint. • Not necessarily representative of all stars… Astronomy 2010

16. Color and Temperature • Stars are dense hot balls of gas • Their spectrum is close to that of a perfect thermal radiator • Which produces a smooth continuous spectrum • So called blackbody spectrum. • Color of stars depends on their temperature: • hotter stars are bluer • cooler stars are redder.  Astronomy 2010

17. Color and Temperature • One can observe the stars through different filters to get an approximate temperature. • Filter allows only a narrow range of wavelengths (colors) through. • By sampling the star's spectrum at two different wavelength ranges (“bands”), one can determine if the spectrum is that a hot, warm, cool, or cold star. • Hot stars have surface temperatures around 60,000 K while cold stars have surface temperatures around 3,000 K. Astronomy 2010

18. Star’s Color Temperature Astronomy 2010

19. B-V Color Index • Measure of the temperature based on apparent color. • Based on two different filters. • A blue (B) filter that only lets a narrow range of colors or wavelengths through centered on the blue colors. • A “visible”' (V) filter that only lets the wavelengths close to the green-yellow band through. • A hot star has a B-V color index close to 0 or negative, while a cool star has a B-V color index close to 2.0. Other stars are somewhere in between. • Defined as the difference in magnitude between the B and V bands. Astronomy 2010

20. Spectra of Stars • primary reason stellar spectra look different is stars have different temperatures • hydrogen most abundant element – most stars show hydrogen absorption lines • hottest may not • so hot that hydrogen is completely ionized • coolest stars • hydrogen atoms are all in lowest state • no hydrogen transitions seen Astronomy 2010

21. Spectral Classes O F M B G A K Astronomy 2010

22. Astronomy 2010

23. Doppler Effect (a) (b) v Observer B Observer A Source Observer B Observer A Source • Case (a) • Object (source) moving towards observer A at velocity “v” • Observer “A” sees compressed wave, I.e. shorter wavelength, higher frequency. • Observer “B” see stretched wave, I.e. longer wavelength, lower frequency. • Case (b) • Stationary source • Observer “A” and “B” see same wavelength. Astronomy 2010

24. Doppler Effect with Stars • Motion of the light source (star) causes the spectral lines to shift positions. • An object's motion causes a wavelength shift • Dl= lnew - lrest • Depends on speed and direction of moving object. • Shift given by: • Dl = lrest × Vradial / c, • c is the speed of light, • lrest is the wavelength measured if object is at rest. • Vradial is object speed along the line of sight. Astronomy 2010

25. Red and Blue Shift • If object is moving towardyou, the waves are compressed, • So their wavelength is shorter. • Lines are shifted to shorter (bluer) wavelengths. • This is called a blueshift. • If the object is moving away from you, the waves are stretched out, • So their wavelength is longer. • The lines are shifted to longer (redder) wavelengths. • This is called a redshift. Astronomy 2010

26. Spectral Shifts • Doppler effect doesn’t affect overall color of an object unless it is moving at a significant fraction of the speed of light (VERY fast!). • For an object moving toward us, the red colors will be shifted to the orange and the near-infrared will be shifted to the red, etc. All of the colors shift. • The overall color of the object depends on the combined intensities of all of the wavelengths (colors). Astronomy 2010

27. Spectral Shifts • Sun spectrum at 3 speeds (0, 0.01c, 0.1c). • Hydrogen-alpha line (at 656.3nm) is shown. • Objects in our galaxy move at speeds much less than 0.01c. • Doppler-shifted continuous spectrum for the Sun moving at 0.01c almost indistinguishable from the Sun at rest. Astronomy 2010

28. Spectral Shifts (cont’d) • Doppler shift of spectral lines measurable even for slow speed. • Astronomers can detect spectral line doppler shifts for speeds as small as 1 km/sec or lower (less than 3.33410-6c). Astronomy 2010

29. Proper Motion • Doppler effect provides speed along the line of sight. • Most stars move at an angle to our line of sight. • We measure this by watching stars move over time. Astronomy 2010

30. Doppler Shift Astronomy 2010

31. Summary • A wealth of information is contained in the spectra of stars. • Astronomers can learn about: • luminosity • surface temperature • composition • radial motion from the doppler shift • rotation from broadening of spectral lines Astronomy 2010