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

2. M37 Capella 五車二 Binary stars in Cygnus

3. M20 M42 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. 3,000 o C6,000 o C Higher temperature 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? violet §Violent collisions  high-energy light (short wavelength, high frequency, e.g. violet) red §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. Spectrum 光譜 : decomposing light into different colors Red: lower frequency, longer wavelength blue: higher frequency, shorter wavelength Sun’s spectrum

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

12. §= Energy of EM wave with wavelength per unit time per unit area emitted by a body at T. Boltzmanns constant Plancks constant §Intensity peak (Wien’s law) §Blue stars are hotter than red stars (higher surface T) Planck’s distribution for blackbody radiations 黑體輻射 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-Boltzmanns 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 ) Eg.: Sun: GVega ( 織女星 ): A OBAFGKM Oh! Be A Fine Girl (Guy)! Kiss Me! Group stars with similar spectra (temperature, elements) into same classes.

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

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

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

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

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

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

22. 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 radius ＝ n²a o a o = 5x10 -11 m Bohr’s radius http://id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html E 1 →E 2 E 1 →E 3 E 1 →E 4 E1E1 E2E2 E3E3 E4E4 Hydrogen atom: E n = -13.6eV/n² Transitions: emission or absorption of light at specific energies Absorption spectrum 吸收光譜

23. Different elements emit different spectral lines Emission spectrum 放射光譜 E 2 →E 1 E 3 →E 1 E 4 →E 1 E1E1 E2E2 E3E3 E4E4 http://www.colorado.edu/physics/2000/index.pl

24. 1.3* Stellar Spectrum

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

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

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

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

29. Spectral Class 恆星光譜型 TypeColor Surface Temperature O Blue > 25,000 K B Blue 11,000 - 25,000 A Blue 7,500 - 11,000 F Blue/White 6,000 - 7,500 G White/Yellow 5,000 - 6,000 K Orange/Red 3,500 - 5,000 M Red < 3,500

30. Spectrum of Betelgeuse 參宿四光譜 violet Hydrogen Alpha line (6563Å) No H lines?? red A typical Type M star (Red Giants) Metal lines TiO 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. Spectrum of Orion Nebula emission spectrum H lines What are these ? ~ 5890Å violet red Photos taken by Lee Wing Kit and Chan Wing Hang

33. §O 2+ (Earth’s atmosphere) 4959Å, 5007Å violet red Light pollution !! Street lamp ( sodium ) Photos taken by Lee Wing Kit and Chan Wing Hang

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

35. Atmospheric Absorption 大角 Aldebaran 心宿二 Antares 參宿四 Betelgeuse 天狼 Sirius 織女 Vega 金星 Venus 土星 Saturn 七姊妹星團 Pleiades 獵戶座大星雲 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. LowHighVery High

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

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

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

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

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

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

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 l 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: l Surface area l Surface temperature

49. §Convention: m Vega = 0 §Luminosity ~ 1/r 2 §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)

51. §Stefan-Boltzmanns law y = m x + b §Lines of constant R

52. 1.6 Hertzsprung-Russell diagram, H-R diagram ( 赫羅圖 )

53. NOT Cooler, NOT hotter 主序星 白矮星 超巨星

54. §Main sequence: A belt from upper left to lower right, 90% of all stars l Cool stars are faint and small; hot stars are bright and large §Giants at the upper right corner, they are cool but luminous l must have large surface area ~10-100 l supergiants have ~100-1000 §White dwarfs lie in the lower left, they are hot but faint l must be very small (~ size of Earth)

55. Figures courtesy HST/NASA Red Giant

56. 1.7 H-R Diagrams of Star Clusters

57.  Two kinds of clusters:  Open clusters ( 疏散星團 ) : contain 10-1000 stars; open, less densely populated, younger, mostly distributed close to the plane of our Galaxy Star clusters ( 星團 ) M45 PleiadesOpen Cluster M37 taken in CUHK 七姊妹

58. M13 taken in NA M3 taken in CC by Delphi Globular Clusters 球狀星團

59. Star clusters  Stars in the same star cluster are formed from the same cloud  they have similar ages and initial chemical compositions §But the stars differ in luminosity (mass) and surface temperature (color) l They are located at different positions on the H-R diagram

60. The Pleiades M45 七姊妹星團 From CUHK How do you explain the turnoff point? The Pleiades

61.  More massive members (more luminous) leave the main sequence and become giants, while lower mass members still lie on the main sequence  confirms the evolution picture that more massive stars have shorter lives on the main sequence massive stars

62.  Locating the turnoff point of a cluster’s H-R diagram  determines the cluster’s age by stellar evolution theory; the lower the turnoff point, the older it is

63. Summary §Light = EM waves, emitted when charges change speed; frequency, wavelength §Starlight: continuous spectrum, higher temperature →higher intensity, bluer spectral lines (absorption or emission) §Bohr model: spectral lines correspond to energy level separations §Doppler effect: red/blue shifts, broadening (T, rotation) §Spectral classes: OBAFGKM §Absolute/apparent magnitudes, luminosity §H-R Diagram: luminosity vs. surface temperature

64. 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

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