2 GuidepostSome chapters in textbooks do little more than present facts. The chapters in this book attempt to present astronomy as organized understanding. But this chapter is special. It presents us with a tool. The interaction of light with matter gives astronomers clues about the nature of the heavens, but the clues are meaningless unless astronomers understand how atoms leave their traces on starlight. Thus, we dedicate an entire chapter to understanding how atoms interact with light.This chapter marks a transition in the way we look at nature. Earlier chapters described what we see with our eyes and explained those observations using models and theories. With this chapter, we turn to modern astrophysics, the application of physics to the study of
3 Guidepost (continued) the sky. Now we can search out secrets of the stars that lie beyond the grasp of our eyes.If this chapter presents us with a tool, then we should use it immediately. The next chapter will apply our new tool to understanding the sun.
4 Outline I. Starlight A. Temperature and Heat B. The Origin of StarlightC. Two Radiation LawsD. The Color IndexII. AtomsA. A Model AtomB. Different Kinds of AtomsC. Electron ShellsIII. The Interaction of Light and MatterA. The Excitation of AtomsB. The Formation of a Spectrum
5 Outline (continued) IV. Stellar Spectra A. The Balmer Thermometer B. Spectral ClassificationC. The Composition of the StarsD. The Doppler EffectE. Calculating the Doppler VelocityF. The Shapes of Spectral Lines
6 The Amazing Power of Starlight Just by analyzing the light received from a star, astronomers can retrieve information about a star’sTotal energy outputSurface temperatureRadiusChemical compositionVelocity relative to EarthRotation period
7 Color and Temperature Orion Stars appear in different colors:• blue (like Rigel)• green / yellow (like our sun)• red (like Betelgeuse).These colors tell us about the star’s temperature.OrionBetelgeuseRigel
8 Black Body RadiationThe light from a star is mostly ultraviolent (sunburn!), visual (roygbiv!), and infrared (heat).The star’s light is nearly a black body spectrum.Two laws of blackbodies:1. The hotter an object is, the more luminous it is2. The hotter the object is the more the black body spectrum shifts towards shorter wavelengths.
9 The Color IndexThe color of a star is measured by comparing its brightness in the blue (B) band and the visual (V) band.A color index measures both B-band & V-band magnitude, and takes the difference (B minus V, or simply B – V).The hotter (more blue) a star, the smaller the color index:Blue stars: B – V = ,000 degrees Kelvin!Red stars: B – V = ,000 degrees KelvinB bandV band
10 Light and MatterSpectra of stars are more complicated than pure blackbody spectra because of characteristic absorption lines.To understand those lines, we need to understand atomic structure and the interactions between light and atoms.
11 Atomic Structurevideo clipvideo clipAn atom consists of an atomic nucleus The nucleus has positive protons and neutral 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.• The nucleus is so dense that a teaspoon of it would weigh about 2 billion tons!!
12 Different Kinds of Atoms The kind of atom depends on the number of protons in the nucleus:Different numbers of neutrons = different isotopesHydrogen (H), has one proton (and 1 electron).Helium (He), has 2 protons (and 2 neutrons & 2 electrons).Atoms can collide and bond into molecules, but only in “cool” stars.If an atom loses or gains electrons, it is called an ion.
13 Electron OrbitsElectron orbits in the electron cloud are restricted to very specific radii and energies.r3, E3r2, E2r1, E1These characteristic electron energies are different for each individual element.
14 Atomic Transitionsvideo clipAn electron can be kicked into a higher orbit in a collision or when it absorbs a photon with the right energy/wavelength.Ephoton = E3 – E1Ephoton = E4 – E1The photon is absorbed, and the electron is in an excited stateWhen the electron returns to the ground state it will emit a photon.The spectrum of a star forms as light passes outward through gases near its surface
15 Kirchhoff’s Laws of Radiation (1) A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.
16 Kirchhoff’s 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 states emits light at specific wavelengths
17 Kirchhoff’s 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 statesWavelengths of light corresponding to the transition energies are absorbed from the continuous spectrum.
18 The Spectra of Starsvideo clipInner, dense layers of a star produce a continuous (blackbody) spectrum.Cooler surface layers absorb light at specific frequencies.Therefore, spectra of stars are absorption spectra.
20 Analyzing Absorption Spectra Each element produces a specific set of absorption and emission lines.Comparing the relative strengths of these sets of lines, we can study the gases from stars.Where to start?With the most abundant elements in the universe!
21 Lines of HydrogenMost prominent lines in many astronomical objects are Balmer lines of hydrogen
22 Observations of the H-Alpha Line Emission nebula, dominated by the red hydrogen alpha (Hα) line.
23 Absorption Spectrum Dominated by Balmer Lines Modern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength
24 The Balmer Thermometer Balmer line strength tells us star temperatureIn medium temp. stars (10,000K), Balmer lines are strongIn cooler stars (<<10,000 K) almost all hydrogen atoms are in the ground state, so Balmer lines are weak.In hotter stars (>> 10,000K) most hydrogen atoms are ionized, so Balmer lines are weak
25 Measuring the Temperatures of Stars A star’s surface temperature is measured by comparing many linesA very hot star (40,000K) has weak Balmer lines and strong ionized helium lines.A very cool star (3,000K) has weak Balmer lines and strong titanium oxide lines.
26 Spectral Classification of Stars (1) Each spectral class divides into 10 subclasses (0 to 9)Different types of stars show different characteristic sets of absorption lines.Temperature
27 Spectral Classification of Stars (2) Mnemonics to remember the spectral sequence:OhOnlyBeBoy,BadAAnAstronomersFineFForgetGirl/GuyGradeGenerallyKissKillsKnownMeMnemonics
29 The Composition of Stars From the relative strength of absorption lines (carefully accounting for their temperature dependence), one can infer the composition of stars.
30 The Doppler EffectWaves from a source are shifted in observed frequency when the source/observer move toward each other.higher pitchlowerpitchLight of different frequency is seen as a different color.Increase in observed frequency is called a blue shift.Blue Shift (to higher frequencies)Red Shift (to lower frequencies)Decrease in observed frequency is called a red shift.
31 Doppler ShiftIf a star is moving toward Earth, the lines in its spectrum are shifted slightly toward shorter wavelength (higher frequency).This shifts the absorption lines toward the blue end of the spectrum, so it’s called a blue shift.If a star is moving away from Earth, the lines in its spectrum are shifted slightly toward the longer wavelength (lower frequency).This creates a red shift in the absorption spectrum.
32 New Terms temperature Kelvin temperature scale absolute zero thermal energyelectronblack body radiationwavelength of maximum intensity (λmax)color indexnucleusprotonneutronisotopeionizationionmoleculeCoulomb forcebinding energyquantum mechanicspermitted orbitenergy levelexcited atomground statecontinuous spectrumabsorption spectrum (dark-line spectrum)absorption lineemission spectrum (bright-line spectrum)emission lineKirchhoff’s lawstransitionLyman seriesBalmer seriesPaschen seriesspectral class or type