ASTR 1120 ASTR 1120 General Astronomy: General Astronomy: Stars & Galaxies Stars & Galaxies � ATH REVIEW: � AST CLASS: Tonight, 5-6pm, in RAMY N1B23 - Light: general properties - Matter: general properties � OMEWORK #1 � ODAY: -Due THU, Sept. 10, by 5pm, on Mastering Astronomy - Matter: general properties (cont.) - Interaction between light and matter CLASS RECORDED STARTED - INFO WILL BE POSTED on CULEARN Light and Atoms Our goals for learning: • Light interacts with atoms in specific • How can we use spectral lines to determine ways the composition of a distant object? • How can we determine the temperature of • Allows us to measure properties of the distant objects? gas such as composition & temperature • Can we use spectra to tell us how fast something is moving? • The key: the spectrum of an object (intensity as a function of wavelength)
Example of electron energy Energy Levels in Atoms states in a hydrogen atom • Lower level is lower energy. Electrons in atoms do NOT “orbit” around the nucleus like little planets - their position better described by • Units: 1 electron- probability waves volt (eV) = • 1.6 x 10 -19 Joules = However, they do move in different “ energy states ” – TINY some electrons in a given atom have more energy than others Each electron in each element has its own particular These energy states are “quantized”– there are only pattern of energy certain energies that the electrons are allowed to levels: elemental have. This is quantum physics. fingerprints! How do electrons move between levels? Where does that energy come from (energy Electrons can move increase) or go to (energy decrease)?? between levels if they are given or give out the exact amount of energy PHOTONS! corresponding to the difference in the energy levels. • The energy change between levels is For hydrogen, if an equal to the energy electron at level 1 of the photon. (Ground state) is given more than 13.6 eV of energy, the electron will • Larger energy jumps fly free (ionize) will be SHORTER Example: Energy jumps A, B and C wavelength photons! allowed; D is not possible for this atom. E ionizes the atom with an energy gain of >3.4 eV
Emission Spectra • Each atom has a different set of energy levels � different spectrum • spectra using a diffraction grating • Emission for thin, hot gas where electrons are “excited” (in high energy states). Gas glows in specific colors. – This is our FINGERPRINT of the elements in the gas! • Will eventually lose thermal energy through emitting photons, and cool! Most common visible light emission line: • Hydrogen Alpha (H � ) ^ Spectrum shows bright • N=3 to n=2 energy jump emission lines from various The Crab nebula: at 656.3 nm elements remains of an exploded star (supernova) • The universe is mostly red!!
Continuous Spectrum Absorption Spectrum • If light with a continuous spectrum shines through a cloud of • Hot solids/liquids/dense gases emit a COOL gas with electrons in low-energy states, the gas can continuous rainbow of light absorb photons OF THE RIGHT ENERGIES to move electrons to excited states – Blackbody Radiation Clicker Question What causes spectral lines? • Resulting spectrum shows DARK LINES of absorption. – Corresponds to wavelengths where the atom has A. Black body radiation. absorbed a photon and excited an electron to a higher B. Electron energy transitions in the atom. energy state C. The Doppler shift of moving objects. • Why don’t we see those atoms re-emit the same D. High frequency electromagnetic waves. photon when they de-excite? E. Protons and neutrons spinning in an – Atoms WILL emit these photons again and electrons atom. fall back to ground state, BUT photons will be scattered in all directions and so most will be lost from our sight
Clicker Question Clicker Question What do we see at position 1? What causes spectral lines? A. Black body radiation. B. Electron energy transitions in the atom. A. Absorption C. The Doppler shift of moving objects. Line 1 2 Spectrum D. High frequency electromagnetic waves. B. Continuous E. Protons and neutrons spinning in an Spectrum atom. 3 C.Emission Line Spectrum Clicker Question Clicker Question What do we see at position 2? What do we see at position 3? A. Absorption A. Absorption Line Line 1 1 2 2 Spectrum Spectrum B. Continuous B. Continuous Spectrum Spectrum 3 3 C.Emission C.Emission Line Line Spectrum Spectrum
Kirchoff’s Laws Solar Spectrum (as seen from 1) Hot solid, liquid, or dense gas Earth) ( continuum spectrum) 2) Continuous spectrum viewed through a cooler gas 1 2 ( absorption line spectrum) 3) Thin, hot gas 3 ( emission line spectrum) Clicker Question Clicker Question Where could the dark lines in Where could the dark lines in the Solar spectrum be coming the Solar spectrum be coming from? from? A. Absorption in the Sun’s atmosphere A. Absorption in the Sun’s atmosphere B. Emission from the Sun’s atmosphere B. Emission from the Sun’s atmosphere C. Absorption in the interior of the Sun C. Absorption in the interior of the Sun D. Emission from the interior of the Sun D. Emission from the interior of the Sun E. Absorption by the glass mirrors in the E. Absorption by the glass mirrors in the telescope used to collect the light telescope used to collect the light
Rules for Emission by Blackbody � 16 Objects 4 million 1. Hotter objects emit more total radiation per Year-old unit surface area. Cluster � Stephan-Boltzmann Law + � E is proportional to T 4 Ionized Nebula 2. Hotter objects emit bluer photons (with a + higher average energy.) Surviving cloud � Wien Law � � max = 2.9 x 10 6 / T(Kelvin) [nm] � uman seen wi � an in � a-red camer �
Clicker Question Clicker Question Which is the hottest star? Which is the hottest star? One that appears: One that appears: A. Red A. Red B. Yellow B. Yellow C. Blue C. Blue D. White D. White E. They are all the same temperature. E. They are all the same temperature. They just look different colors They just look different colors Quick guide to thermal spectra (be familiar with these) What is this object? • 3 K (coldest natural things): � max = 1mm = Microwaves • 300 K (people, planets, warm dust): � max = 10 -5 m = 10,000 nm, IR • 3000-30,000K (stars): � max = 10 -6 m to 10 -7 m • Let’s use its spectral information to determine = 1000 to 100 nm, what it is. IR – visible – UV • 300,000- 30,000,000K: weird and intense places (UV through X-rays/gamma rays)
What is this object? What is this object? Reflected Sunlight: Thermal Radiation: Continuous spectrum of Infrared spectrum peaks visible light is like the at a wavelength Sun’s except that some of corresponding to a the blue light has been temperature of 225 K absorbed - object must look red What is this object? What is this object? Carbon Dioxide: Ultraviolet Emission Lines: Absorption lines are the Indicate a hot upper fingerprint of CO 2 in the atmosphere atmosphere
What is this object? Measuring velocities without a stopwatch: the Doppler Shift • Familiar shift in pitch of SOUND: higher when approaching, lower when receding • Similar shift in frequency of light: Mars! higher frequency (blueshift) when approaching, lower frequency (redshift) when receding • Most easily used with absorption Reading/Assignment or emission lines where you know the zero-velocity • Ch. 5 sec. 5.3, 5.4, 5.5 (rest) wavelengths. Then, measure redshift or • Homework #1 on Mastering Astronomy blueshift to get due on Thursday, 09/10, by 5pm, online the velocity away or towards you.
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