如果將原子核放大到葡萄籽的大小

1、Atoms and StarlightChapter 7In the last chapter you read how telescopes gather light from the stars and how spectrographs spread the light out into spectra. Now you are ready to see what all the fuss is about. Spectra contain the secrets of the stars. Here you will find answers to four essential que

2、stions: What is an atom? How do atoms interact with light? What kinds of spectra do you see when you look at celestial objects? What can you learn from a stars spectrum? GuidepostThis chapter marks a change in the way you will look at nature. Up to this point, you have been thinking about what you c

3、an see with your eyes alone or aided by telescopes. In this chapter, you begin using modern astrophysics to search out secrets of the stars that lie beyond what you can see, and that leads to an important question about science: How can we understand the world around us if it depends on the atomic w

4、orld we cannot see?The analysis of spectra is a powerful tool, and in the chapters that follow you will use that tool to study the sun and stars.Guidepost (continued)I. AtomsA. A Model AtomB. Different Kinds of AtomsC. Electron ShellsII. The Interaction of Light and MatterA. The Excitation of AtomsB

5、. Radiation from a Heated ObjectC. Two Radiation LawsOutlineIII. Stellar SpectraA. The Formation of a SpectrumB. The Balmer ThermometerC. Spectral ClassificationD. The Composition of the StarsE. The Doppler EffectF. Calculating the Doppler VelocityG. The Shapes of Spectral LinesOutline (continued)Th

6、e Amazing Power of StarlightJust by analyzing the light received from a star, astronomers can retrieve information about a stars1. Total energy output2. Surface temperature3. Radius4. Chemical composition5. Velocity relative to Earth6. Rotation periodAtomic Structure An atom consists of an atomic nu

7、cleus (protons and neutrons) and a cloud of electrons surrounding it. Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud.Nuclear DensityIf you could fill a teaspoon just with material as dense as the matter in an atomic nucleus, it wou

8、ld weigh 2 billion tons!Different Kinds of Atoms The kind of atom depends on the number of protons in the nucleus.Helium 4Different numbers of neutrons different isotopes (同位素) Most abundant: Hydrogen (H), with one proton (+ 1 electron) Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.)Elect

9、ron Orbits Electron orbits in the electron cloud are restricted to very specific radii and energies. r1, E1r2, E2r3, E3 These characteristic electron energies are different for each individual element.Atomic Transitions An electron can be kicked into a higher orbit when it absorbs a photon with exac

10、tly the right energy. All other photons pass by the atom unabsorbed. Eph = E4 E1Eph = E3 E1(Remember that Eph = h*f)Wrong energy The photon is absorbed, and the electron is in an excited state.Color and TemperatureOrionBetelgeuseRigelStars appear in different colors, from blue (like Rigel) via yello

11、w (like our sun) to red (like Betelgeuse).These colors tell us about the stars temperature.Black Body Radiation (1)The light from a star is usually concentrated in a rather narrow range of wavelengths. The spectrum of a stars light is approximately a thermal spectrum called a black body spectrum.A p

12、erfect black body emitter would not reflect any radiation. Thus the name “black body”. Two Laws of Black Body Radiation1. The hotter an object is, the more energy it emits: F = s*T4 where s = Stefan-Boltzmann constant F = Energy Flux = Energy given off in the form of radiation, per unit time and per

13、 unit surface area J/s/m2 (J:焦耳)Energy FluxTwo Laws of Black Body Radiation2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases. Wiens displacement law: lmax 3,000,000 nm / TK(where TK is the temperature in Kelvin)Stellar SpectraThe spectra of star

14、s are more complicated than pure blackbody spectra.They contain characteristic lines, called absorption lines.With what we have learned about atomic structure, we can now understand how those lines are formed.Kirchhoffs Laws of Radiation (1)1.A solid, liquid, or dense gas excited to emit light will

15、radiate at all wavelengths and thus produce a continuous spectrum.Kirchhoffs Laws of Radiation (2)2. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum.Light excites electrons in atoms to higher energy statesTransition back to lower state

16、s emits light at specific frequenciesKirchhoffs Laws of Radiation (3)3.If light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.Light excites electrons in atoms to higher energy statesFrequencies corresponding to the transition energ

17、ies are absorbed from the continuous spectrum.The Spectra of StarsThe inner, dense layers of a star produce a continuous (blackbody) spectrum.Cooler surface layers absorb light at specific frequencies.= Spectra of stars are absorption spectra.Analyzing Absorption Spectra Each element produces a spec

18、ific set of absorption (and emission) lines.By far the most abundant elements in the Universe Comparing the relative strengths of these sets of lines, we can study the composition of gases.Lines of HydrogenBalmer lines of hydrogen can be seen at optical wavelengthsThe Balmer Linesn = 1n = 2n = 4n =

19、5n = 3HaHbHgThe only hydrogen lines in the visible wavelength rangeTransitions from 2nd to higher levels of hydrogen2nd to 3rd level = Ha (Balmer alpha line)2nd to 4th level = Hb (Balmer beta line)Observations of the H-Alpha LineEmission nebula, dominated by the red Ha lineAbsorption Spectrum Domina

20、ted by Balmer LinesModern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength The Balmer ThermometerBalmer line strength is sensitive to temperature:Almost all hydrogen atoms in the ground state (electrons in the n = 1 orbit) = few transitions from n = 2 = wea

21、k Balmer linesMost hydrogen atoms are ionized = weak Balmer linesMeasuring the Temperatures of StarsComparing line strengths, we can measure a stars surface temperature!Spectral Classification of Stars (1)TemperatureDifferent types of stars show different characteristic sets of absorption lines.Spec

22、tral Classification of Stars (2)Mnemonics (幫幫助記憶的方法助記憶的方法) to remember the spectral sequence:OhOhOnlyBeBoy,BadAAnAstronomersFineFForgetGirl/GuyGradeGenerallyKissKillsKnownMeMeMnemonicsStellar SpectraOBAFGKMSurface temperatureThe Composition of StarsFrom the relative strength of absorption lines (car

23、efully accounting for their temperature dependence), one can infer the composition of stars.The Doppler Effect (1)The light of a moving source is blue/red shifted byDl/l0 = vr/cl0 = actual wavelength emitted by the sourceDl = Wavelength change due to Doppler effectvr = radial velocityBlue Shift (to

24、higher frequencies)Red Shift (to lower frequencies)vrSound waves always travel at the speed of sound just like light always travels at the speed of light, independent of the speed of the source of sound or light.The Doppler Effect (2)The Doppler effect allows us to measure the component of the sources velocity along our line of sight.This component is called radial velocity, vr.The Doppler Effect (2)Examp