1. Astronomy 4230 天 文 学 概 论 A Brief Course of Astronomy

2. Lecture 3 Stellar Spectrum

3. Stars have different colours ! Colours depend on surface temperature Hot stars appear to be blue Cooler stars appear to be red ‘Measure’ colours by filters Measuring Star Colours

4. White Light Spectrum Prisms disperse light into its component colors Prism

6. Light consists of particles Newton (c.1670) Light as waves Christiaan Huygens (1678) Seemingly a ‘either – or’ situation Particles cannot behave like waves Waves cannot behave like particles Two opposing views

7. 1801 Thomas Young Double-slit experiment Include graphic Demonstrates wave nature of light, rejects particle picture The wave picture gets a boost

8. In what medium do light waves travel ? Concept of ether (check spelling !!) Concept of fields An unexpected solution : Complete theory of electricity and magnetism by James Clerk Maxwell (c.1860) allows for electro-magnetic waves to travel in vacuum with speed of light A slight problem

9. Waves are described by two numbers: Wavelength (): Distance between wave crests. Frequency (f): Number of wave crests passing per second. The wave speed, c, is the product of these: c =  f Measuring Waves

10. Wavelength () Speed (c) Frequency (f) (# waves/second)

11. Ocean waves:  = 100 m, f = 0.1/second; wave speed: c = 10 m/second (36 km/hr) Speed depends on water depth, salinity, etc. Sound waves (A 440):  = 0.73 m, f = 440/second; wave speed: c = 320 m/second (1150 km/hr) For sound, “frequency” = “pitch”. Examples of Waves

12. Can treat light as an Electromagnetic Wave Fluctuation in the intensity of electric and magnetic fields. Travels through a vacuum at the speed of light. Doesn’t need a medium to “wave” in. Speed of light is a constant for all light waves: c = 300,000 km/sec Independent of wavelength or frequency. Light as electromagnetic waves

13. Electromagnetic Wave

14. Wavelengths: 400 – 700 nanometers (nm) 1 nm = 10-9 meters Frequencies: 7.51014–4.31014 waves/second Visible Spectrum: Red Orange Yellow Green Blue Indigo Violet 700 nm ------------- 550 nm ------------ 400 nm R O Y G. B I V Visible Light Waves

15. 电磁辐射是以变化的电磁场传递能量、具有特定波长和强度的波（波动性）。电磁辐射是以变化的电磁场传递能量、具有特定波长和强度的波（波动性）。 波长范围：＜0.01Å – 30 m 1 Ångstrom = 10-10 m (波长λ)×(频率ν) =光速c = 3×1010 cms-1

16. 根据波长由长到短，电磁辐射可以分为射电、红外、光学、紫外、X射线和g射线等波段，可见光又可分解为七色光。根据波长由长到短，电磁辐射可以分为射电、红外、光学、紫外、X射线和g射线等波段，可见光又可分解为七色光。

17. The Electromagnetic Spectrum

18. A spectrum is the distribution of photon energies coming from a light source: How many photons of each energy are emitted by the light source? Spectra are observed by passing light through a spectrograph: Breaks the light into its component wavelengths and spreads them apart (dispersion). Uses either prisms or diffraction gratings. 光谱(电磁波谱)

19. 太阳光谱 M17中恒星形成区的热气体辐射谱

20. Kirchoff’s Laws 1) A hot solid or hot, dense gas produces a continuous spectrum. 2) A hot, low-density gas produces an emission-line spectrum. 3) A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum.

21. Continuous Spectrum Emission-line Spectrum Cloud of Hydrogen Gas Absorption-line Spectrum Continuum Source

22. 黑体(blackbody) 能吸收所有的外来辐射（无反射）并全部再辐射的理想天体。 Absorbs at all wavelengths. As it absorbs light, it heats up. Characterized by its Temperature. 黑体辐射(blackbody radiation) 具有特定温度的黑体的热辐射。 大部分正常恒星的辐射可以近似地用黑体辐射来表示。 Emits at all wavelengths (continuous spectrum) Energy emitted depends on the Temperature. Peak wavelength also depends on Temperature. 黑体辐射(blackbody radiation)

23. Planck定律 温度为T的黑体在单位面积、单位时间、单位频率内、向单位立体角发射的能量为 不同温度黑体的辐射谱

24. Wien定律 黑体辐射最强处的波长lpeak与温度之间的关系为 lpeakT＝0.29 (cm K) 高温黑体主要辐射短波，低温黑体主要辐射长波。

25. Relates peak wavelength and Temperature: Wien’s Law • In Words: • “Hotter objects are BLUER” • “Cooler objects are REDDER”

26. Example: Radiation from various objects with different temperature Gas cloud Young star Sun Cluster

27. 同一天体的不同波段的辐射来自不同（温度）的区域和物理过程。

28. Example 1: The Sun 光学 紫外 射电 X射线

29. Example 2: The Spiral Galaxy M81 光学中红外远红外 X射线紫外射电

30. Energy emitted per second per area by a blackbody with Temperature (T): Stefan-Boltzmann Law • is Boltzmann's constant (a number). • In Words: • “Hotter objects are Brighter at All Wavelengths”

32. Heat a piece of iron from 300K to 600K Temperature increases by 2× Brightness increases by 24 = 16× Peak wavelength shifts towards the blue by 2× from ~10m in the mid-Infrared to ~5m in the near-Infrared. Hotter objects get brighter at all wavelengths andgetbluer in color. Examples I

33. Person: Body Temperature = 310 K Peak wavelength = 9400 nm (infrared) Typical adult emits about 100 Watts of infrared light. Sun: surface temperature = 5770 K Peak wavelength = 503 nm (visible light) Emits about 3.81026 Watts of mostly visible light, infrared and ultraviolet. Examples II

34. Infrared Camera ImageWavelength ~2200nm (2.2 microns)

35. A hot, low-density gas emits a non-continuous emission-line spectrum. Emits only at particular wavelengths, giving the appearance of bright, discrete “emission lines”. Darkness in between the emission lines. Emission-Line Spectra

36. 19th chemists century noticed that each element, heated into an incandescent gas in a flame, emitted unique emission lines. Mapped out the emission-line spectra of known atoms and molecules. Used this as a tool to identify the composition of unknown compounds. They did not, however, understand how it worked.

37. Hydrogen Helium Oxygen Neon Iron

38. Light from a continuous spectrum through a vessel containing a cooler gas shows: A continuous spectrum from the lamp crossed by of dark “absorption lines” at particular wavelengths. The wavelengths of the absorption lines exactly correspond to the wavelengths of emission lines seen when the gas is hot! Light is being absorbed by atoms in the gas. Absorption-Line Spectrum

39. Lamp emits light at all energies Lamp Light Absorbed by Hydrogen Atoms in the Cloud Cloud of Hydrogen Gas Continuum Source

41. The Solar Spectrum

42. Why does each element have a characteristic line spectrum? Answer: It reflects the detailed structure of the atom. Depends on the number and arrangement of electrons in orbit around the nucleus. Discovering why unlocked the secrets of the atom. Why does it work?

43. 1H proton electron neutron Simple Atoms (Schematic) 4He

44. Electrons cannot orbit just anywhere around a nucleus: Can only orbit in discrete orbitals. Each orbital corresponds to a particular energy of the orbiting electron. If an electron does not have exactly the right energy, it cannot be in an orbital (all or nothing). Details are dictated by quantum mechanics. Looking inside the Atom

45. An atom of Hydrogen (1H) consists of: A single proton in the nucleus. A single electron orbiting the nucleus. First orbital: Ground State (n=1) Lowest energy orbital the electron can reside in. Higher orbitals: Excited States (n=2,3,...) Higher orbits around the nucleus. Come at specific, exact energies. Hydrogen: The Simplest Atom

46. Continuum n= n=1 (Ground State) n=3 (2nd excited state) n=2 (1st excited state) n=4 n=5 Energy Level Diagram of 1H

47. n=1 n=3 n=6

48. Emission Lines: When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less. Emission & Absorption Lines

49. 62 52 42 n=32 n=6 n=1 (Ground State) n=3 (2nd excited state) n=2 (1st excited state) n=4 n=5 Larger Jump = More Energy = BluerWavelength

50. 氢原子光谱