Light and Atoms. ASTR 1120 General Astronomy: Stars & Galaxies. ASTR 1120 General Astronomy: Stars & Galaxies !ATH REVIEW: #AST CLASS: "OMEWORK #1

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1 ASTR 1120 General Astronomy: Stars & Galaxies!ATH REVIEW: Tonight, 5-6pm, in RAMY N1B23 "OMEWORK #1 -Due THU, Sept. 10, by 5pm, on Mastering Astronomy CLASS RECORDED STARTED - INFO WILL BE POSTED on CULEARN ASTR 1120 General Astronomy: Stars & Galaxies #AST CLASS: - Light: general properties - Matter: general properties $ODAY: - Matter: general properties (cont.) - Interaction between light and matter Light and Atoms Light interacts with atoms in specific ways Allows us to measure properties of the gas such as composition & temperature The key: the spectrum of an object (intensity as a function of wavelength) Our goals for learning: How can we use spectral lines to determine the composition of a distant object? How can we determine the temperature of distant objects? Can we use spectra to tell us how fast something is moving?

2 Energy Levels in Atoms Electrons in atoms do NOT orbit around the nucleus like little planets - their position better described by probability waves However, they do move in different energy states some electrons in a given atom have more energy than others These energy states are quantized there are only certain energies that the electrons are allowed to have. This is quantum physics. Example of electron energy states in a hydrogen atom Lower level is lower energy. Units: 1 electronvolt (ev) = 1.6 x Joules = TINY Each electron in each element has its own particular pattern of energy levels: elemental fingerprints! How do electrons move between levels? Electrons can move between levels if they are given or give out the exact amount of energy corresponding to the difference in the energy levels. For hydrogen, if an electron at level 1 (Ground state) is given more than 13.6 ev of energy, the electron will fly free (ionize) Example: Energy jumps A, B and C allowed; D is not possible for this atom. E ionizes the atom with an energy gain of >3.4 ev Where does that energy come from (energy increase) or go to (energy decrease)?? The energy change between levels is equal to the energy of the photon. Larger energy jumps will be SHORTER wavelength photons! PHOTONS!

3 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: The Crab nebula: remains of an exploded star (supernova) shows bright emission lines from various elements Hydrogen Alpha (H!) ^ N=3 to n=2 energy jump at nm The universe is mostly red!!

4 Continuous Absorption Hot solids/liquids/dense gases emit a continuous rainbow of light Blackbody Radiation If light with a continuous spectrum shines through a cloud of COOL gas with electrons in low-energy states, the gas can absorb photons OF THE RIGHT ENERGIES to move electrons to excited states Resulting spectrum shows DARK LINES of absorption. Corresponds to wavelengths where the atom has absorbed a photon and excited an electron to a higher energy state Why don t we see those atoms re-emit the same photon when they de-excite? Atoms WILL emit these photons again and electrons fall back to ground state, BUT photons will be scattered in all directions and so most will be lost from our sight What causes spectral lines? A. Black body radiation. B. Electron energy transitions in the atom. C. The Doppler shift of moving objects. D. High frequency electromagnetic waves. E. Protons and neutrons spinning in an atom.

5 What causes spectral lines? What do we see at position 1? A. Black body radiation. B. Electron energy transitions in the atom. C. The Doppler shift of moving objects. D. High frequency electromagnetic waves. E. Protons and neutrons spinning in an atom. A. Absorption B. Continuous C. Emission What do we see at position 2? What do we see at position 3? A. Absorption 1 2 A. Absorption 1 2 B. Continuous C. Emission 3 B. Continuous C. Emission 3

6 1) Hot solid, liquid, or dense gas (continuum spectrum) 2) Continuous spectrum viewed through a cooler gas (absorption line spectrum) 3) Thin, hot gas (emission line spectrum) Kirchoff s Laws Solar (as seen from Earth) Where could the dark lines in the Solar spectrum be coming from? A. Absorption in the Sun s atmosphere B. Emission from the Sun s atmosphere C. Absorption in the interior of the Sun D. Emission from the interior of the Sun E. Absorption by the glass mirrors in the telescope used to collect the light Where could the dark lines in the Solar spectrum be coming from? A. Absorption in the Sun s atmosphere B. Emission from the Sun s atmosphere C. Absorption in the interior of the Sun D. Emission from the interior of the Sun E. Absorption by the glass mirrors in the telescope used to collect the light

7 "16 4 million Year-old Cluster + Ionized Nebula + Surviving cloud Rules for Emission by Blackbody Objects 1. Hotter objects emit more total radiation per unit surface area. " Stephan-Boltzmann Law " E is proportional to T 4 2. Hotter objects emit bluer photons (with a higher average energy.) " Wien Law " # max = 2.9 x 10 6 / T(Kelvin) [nm] "uman seen wi% an in&a-red camer'

8 Which is the hottest star? One that appears: A. Red B. Yellow C. Blue D. White E. They are all the same temperature. They just look different colors Which is the hottest star? One that appears: A. Red B. Yellow C. Blue D. White E. They are all the same temperature. They just look different colors Quick guide to thermal spectra (be familiar with these) 3 K (coldest natural things): # max = 1mm = Microwaves 300 K (people, planets, warm dust): # max = 10-5 m = 10,000 nm, IR ,000K (stars): # max = 10-6 m to 10-7 m = 1000 to 100 nm, IR visible UV What is this object? Let s use its spectral information to determine what it is. 300,000-30,000,000K: weird and intense places (UV through X-rays/gamma rays)

9 What is this object? What is this object? Reflected Sunlight: Continuous spectrum of visible light is like the Sun s except that some of the blue light has been absorbed - object must look red Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K What is this object? What is this object? Carbon Dioxide: Absorption lines are the fingerprint of CO 2 in the atmosphere Ultraviolet Emission s: Indicate a hot upper atmosphere

10 What is this object? Measuring velocities without a stopwatch: the Doppler Shift Familiar shift in pitch of SOUND: higher when approaching, lower when receding Mars! Similar shift in frequency of light: higher frequency (blueshift) when approaching, lower frequency (redshift) when receding Most easily used with absorption or emission lines where you know the zero-velocity (rest) wavelengths. Then, measure redshift or blueshift to get the velocity away or towards you. Reading/Assignment Ch. 5 sec. 5.3, 5.4, 5.5 Homework #1 on Mastering Astronomy due on Thursday, 09/10, by 5pm, online