ASTR 1120 ASTR 1120 General Astronomy: General Astronomy: Stars & Galaxies 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 CLASS RECORDED STARTED - INFO WILL BE POSTED on CULEARN
ASTR 1120 ASTR 1120 General Astronomy: General Astronomy: Stars & Galaxies Stars & Galaxies � AST CLASS: - Light: general properties - Matter: general properties � ODAY: - Matter: general properties (cont.) - Interaction between light and matter
Light and Atoms 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 spectrum of an object (intensity as a function of wavelength)
Our goals for learning: • How can we use spectral lines to determine the composition composition of a distant object? • How can we determine the temperature temperature of distant objects? • Can we use spectra to tell us how fast fast something is moving?
Energy Levels in Atoms 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 Example of electron energy states in a hydrogen atom hydrogen atom states in a • Lower level is lower energy. • Units: 1 electron- volt (eV) = • 1.6 x 10 -19 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 does that energy come from (energy Where increase) or go to (energy decrease)?? increase) or go to (energy decrease)?? PHOTONS! PHOTONS! • The energy change between levels is equal to the energy of the photon. • Larger energy jumps will be SHORTER wavelength photons!
Emission Spectra Emission Spectra • 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! FINGERPRINT of the elements in the gas! • Will eventually lose thermal energy through emitting photons, and cool!
• Each atom has a different set of energy levels � different spectrum • spectra using a diffraction grating
Spectrum shows bright emission lines from various The Crab nebula: elements remains of an exploded star (supernova)
Most common visible light Most common visible light emission line: emission line: • Hydrogen Alpha (H � ) ^ • N=3 to n=2 energy jump at 656.3 nm • The universe is mostly red!!
Continuous Spectrum Continuous Spectrum • Hot solids/liquids/dense gases emit a continuous rainbow of light – Blackbody Radiation Blackbody Radiation –
Absorption Spectrum Absorption Spectrum • 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 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
Clicker Question Clicker Question What causes spectral lines? spectral lines? What causes 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.
Clicker Question Clicker Question What causes spectral lines? spectral lines? What causes 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.
Clicker Question What do we see at position 1? A. Absorption Line 1 2 Spectrum B. Continuous Spectrum 3 C.Emission Line Spectrum
Clicker Question What do we see at position 2? A. Absorption Line 1 2 Spectrum B. Continuous Spectrum 3 C.Emission Line Spectrum
Clicker Question What do we see at position 3? A. Absorption Line 1 2 Spectrum B. Continuous Spectrum 3 C.Emission Line Spectrum
Kirchoff’ ’s s Laws Laws Kirchoff 1) Hot solid, liquid, or dense gas ( continuum spectrum) 2) Continuous spectrum viewed through a cooler gas 1 2 ( absorption line spectrum) 3) Thin, hot gas 3 ( emission line spectrum)
Solar Spectrum (as seen from Solar Spectrum (as seen from Earth) Earth)
Clicker Question Clicker Question Where could the dark lines in could the dark lines in Where the Solar spectrum be coming the Solar spectrum be coming from? 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
Clicker Question Clicker Question Where could the dark lines in could the dark lines in Where the Solar spectrum be coming the Solar spectrum be coming from? 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
� 16 4 million Year-old Cluster + Ionized Nebula + Surviving cloud
Rules for Emission by Blackbody Rules for Emission by Blackbody Objects 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 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 �
Clicker Question Which is the hottest star? Which is the hottest star? One that appears: One that appears: A. Red B. Yellow C. Blue D. White E. They are all the same temperature. They just look different colors
Clicker Question Which is the hottest star? Which is the hottest star? One that appears: 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 Quick guide to thermal spectra (be familiar with these) (be familiar with these) • 3 K (coldest natural things): � max = 1mm = Microwaves Microwaves • 300 K (people, planets, warm dust): � max = 10 -5 m = 10,000 nm, IR IR • 3000-30,000K (stars): � max = 10 -6 m to 10 -7 m = 1000 to 100 nm, 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? • Let’s use its spectral information to determine what it is.
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
What is this object? What is this object? 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
What is this object? What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere
What is this object? What is this object? Mars! Mars!
Measuring velocities without a Measuring velocities without a stopwatch: the Doppler Shift stopwatch: the Doppler Shift • Familiar shift in pitch of SOUND: higher when approaching, lower when receding • 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 Reading/Assignment • Ch. 5 sec. 5.3, 5.4, 5.5 • Homework #1 on Mastering Astronomy due on Thursday, 09/10, by 5pm, online
Recommend
More recommend
