The Stars and the Sun I. Colors of stars

1. The Stars and the Sun I.Colors of stars Chu Ming-chung 朱明中 Department of Physics The Chinese University of Hong Kong mcchu@phy.cuhk.edu.hk http://www.phy.cuhk.edu.hk/gee/mctalks/mcpdp.html

2. Capella 五車二 M37 Binary stars in Cygnus

3. M42 M20 M8 M57 All taken in CUHK What information are carried in star light?

4. Colors of Stars • 1.1 Starlight • 1.2* Light-matter interaction • 1.3* Stellar spectrum • 1.4 Doppler effect • 1.5 Stellar luminosity • 1.6 H-R Diagram • 1.7 H-R Diagrams for star clusters

5. + + 1.1 Starlight • What is light? • When the velocities of moving charged particles are changed, electromagnetic radiation (EM radiation) 電磁輻射 (light is a kind of EM radiation) is emitted in the form of waves ( EM waves電磁波). • Thermal motion of particles in a star → light http://www.colorado.edu/physics/2000/applets/fieldwaves.html

6. Different bands of EM waves

7. Higher temperature 6,000oC 3,000oC Why do hot materials give out light? What happens if temperature rises further?

8. Charged particles in a hot gas (e.g. inside a star) move around rapidly and undergo many collisions • their velocities are changed in the collisions • light (EM waves in general) is emitted • Why are the colors of light different for different temperature? • Violent collisions high-energy light (short wavelength, high frequency, e.g. violet) • Gentle collisions low-energy light (long wavelength, low frequency, e.g. red)

9. In a star, the temperature is very high, both violent and gentle collisions occur • it gives out electromagnetic radiation of all wavelengths • starlight can be decomposed into a continuous spectrum 連續光譜 (like a rainbow)

10. Sun’s spectrum Spectrum 光譜: decomposing light into different colors Red: lower frequency, longer wavelength blue: higher frequency, shorter wavelength

11. Blue stars are hotter than red stars As temperature rises: 1. light intensity increases, 2. the color of light shifts towards high frequency (blue) side

12. Planck’s distribution for blackbody radiations 黑體輻射 • = Energy of EM wave with wavelength  per unit time per unit area emitted by a body at T. Boltzmanns constant Plancks constant • Intensity peak (Wien’s law) • Blue stars are hotter than red stars (higher surfaceT) Just for reference!

13. Cooler stars are dimmer and redder • Total radiation power for all ’s • = = energy per unit time per unit area emitted by a body at temperature T • Stefan-Boltzmanns const. • Luminosity of a blackbody sphere • Color:

14. ‘colors’ emitted by different ‘stars’. Eg. Sun’s radiation peaks at ~ 0.5 microns You emit light too! What ‘color’ is the light you emitted? Ans.: Body temperature ~ 300 K ~ 1/20 Sun’s surface temperature. Therefore, human’s radiation peaks at 20x 0.5=10 microns.

15. Spectral classification (光譜分類) Group stars with similar spectra (temperature, elements) into same classes. Eg.: Sun: G Vega (織女星): A Oh! Be AFine Girl (Guy)! Kiss Me!

16. M42 M20 M8 M57 http://apod.nasa.gov/apod/ap010729.html All taken in CUHK What information are carried in star light?

17. Examples of using non-visible light Use infrared telescopes to detect planets directly – 2 found already so far! http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/ssc2005-09b.shtml Illustration courtesy NASA/Spitzer Infrared Space Telescope

18. Planetary Eclipses in Infrared http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/ssc2005-09b.shtml Illustration courtesy NASA/Spitzer Infrared Space Telescope

19. Examples of using non-visible light Found even asteroid belt around HD 69830 using Infrared telescope http://www.spitzer.caltech.edu/Media/releases/ssc2005-10/ssc2005-10b.shtml Illustration courtesy NASA/Spitzer Space Telescope

20. Examples of using non-visible light Gamma ray telescope Positron Clouds near the galactic center How do we know there are positrons？

21. 1.2* Light-matter interaction http://www.colorado.edu/physics/2000/index.pl

22. radius＝n²ao ao= 5x10-11 mBohr’s radius Hydrogen atom: En= -13.6eV/n² Transitions: emission or absorption of light at specific energies E4 E3 E2 E1 →E3 E1 →E2 E1 →E4 Absorption spectrum 吸收光譜 E1 Bohr’s model of atoms • Electrons have wave properties(de Broglie) • de Broglie wavelength • Bohr: circular orbits only standing wave orbits are stable • Only discrete energies allowed http://id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html

23. E4 E3 E2 E3 →E1 E2 →E1 E4 →E1 E1 Emission spectrum 放射光譜 Different elements emit different spectral lines http://www.colorado.edu/physics/2000/index.pl

24. 1.3* Stellar Spectrum

25. Dark lines Atoms in the atmosphere absorb light of particular frequencies Light source emitting a continuous spectrum Absorption spectrum 吸收光譜

26. C11 + spectrograph + CCD Stellar light absorbed selectively byatoms in stellaratmosphere Stellar atmosphere colder than interior Grating and CCD Stellar Spectrum http://apwww.smu.ca/~ishort/Astro/ Photos taken by Lee Wing Kit and Chan Wing Hang

27. H lines violet red Hydrogen Alpha line (6563Å) All are H lines !! 1Å=10-10m Spectrum of Vega 織女星 Photos taken by Lee Wing Kit and Chan Wing Hang in CUHK

28. 紅 Compared with Vega’s Violet RED Spectrum of Sirius天狼星光譜 Both are Type A Stars Photos taken by Lee Wing Kit and Chan Wing Hang

29. Spectral Class 恆星光譜型

30. red Metal lines TiO Hydrogen Alpha line (6563Å) No H lines?? A typical Type M star (Red Giants) Spectrum of Betelgeuse 參宿四光譜 violet Photos taken by Lee Wing Kit and Chan Wing Hang

31. Orion獵戶座 參宿四 Betelgeuse 獵戶座大星雲 Orion Nebula Photos taken by Lee Wing Kit and Chan Wing Hang

32. emission spectrum What are these? ~5890Å violet red H lines Spectrum of Orion Nebula Photos taken by Lee Wing Kit and Chan Wing Hang

33. 4959Å, 5007Å Light pollution!! violet red Street lamp (sodium) • O2+ (Earth’s atmosphere) Photos taken by Lee Wing Kit and Chan Wing Hang

34. violet 金星Venus red H different 土星Saturn Why are they so similar? Spectra of Planets Photos taken by Lee Wing Kit and Chan Wing Hang

35. Atmospheric Absorption Telluric Lines Photos taken by Lee Wing Kit and Chan Wing Hang

36. Absorption lines => elements • Intensity peak position => surface temperature • Strengths of absorption lines => also surface temperature • Hydrogen as an example: Very high temperatures => electrons leave the atoms (ionized) ; low temperatures, electrons stay at the ground state. Low High Very High

37. Measure the absorption line intensities of Balmer lines (巴耳末線) [electrons transit from the 2nd level to higher levels] • We can know the number of atoms in which the electrons are at the 2nd level • Hence get an estimate of the surface temperature Intensities depend on the number of electrons at the 2nd level 2nd level

39. Spectrum of Sun Taken from NOAO/AURA/NSF webpage http://www.noao.edu/image_gallery/html/im0649.html

40. Examples of Spectral Method Transit method: observe the planetary transit →small periodic dimming of star light, new absorption lines →elements in the planetary atmosphere Eg. HD209458: Na detected in planetary atmosphere Photo and animation courtesy NASA/STScI

41. stationary source moving source moving source v=0 v=0.4 v=1 1.4 Doppler effect (多普勒效應) http://www.tmeg.com/esp/p_doppler/doppler.htm

42. Spectrum of object at rest Spectrum taken for approaching object Spectrum taken for receding object v Blue shifted藍移 Red shifted紅移 Light emitted by the source will have wavelength decreased (blue shifted) in front of its motion and increased (red shifted) behind it.

43. Doppler effect demonstration http://sci.esa.int/content/doc/16/28950_.htm

44. Width of a spectral line may be affected by • Natural broadening- quantum effect, very small • Doppler broadening - Doppler shifts due to random thermal motions of atoms. • For • Total width

45. E.g., H line of the sun • 1000 times > natural broadening • Rotational broadening - light coming from a rotating star is Doppler shifted

46. Eg. see different shifts on different sides of Saturn’s ring: rotation speed

47. 1.5 Stellar luminosity (恆星光度)

48. Magnitudes and Luminosity • Apparent magnitude m (視星等): measures the luminosity (B) of starlight received on earth. • 5 magnitudes = 100 times • Absolute magnitude M (絕對星等): measures the luminosity a star would have if it was placed at a distance of 10 pc (~33 light years) away. • Luminosity (光度) B: Total amount of energy that the star radiates in one second. It is determined by a combination of two factors: • Surface area • Surface temperature

49. Convention: mVega= 0 • Luminosity ~ 1/r2 • Distance modulus • Comparing apparent and absolute magnitudes gives distance r

50. a hot star with a large surface area must be luminous • a cool star with a small surface area must be dim • a cool star could be luminous if it is very large (not much radiation is emitted per unit area, but the total radiation rate is large because its has a large surface area for light emission)