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# The U - ASTR 1060 - Class Notes - Week 4

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The U - ASTR 1060 - Class Notes - Week 4

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ASTR 1060 – Week 4 Notes “Astronomy” Chapter 5: Radiation and Spectra 5.1 The Behavior of Light •  Intro o  Light holds important info we can use to learn more about things in space
o  Light/radiation created through atomic level processes
o  Radiation – general term for waves that radiate outward from a source
•  Maxwell’s Theory of Electromagnetism o  Discovered many connections between electricity and magnetism
o  Atomic parts
§   Proton – positive charge, inside nucleus §   Neutron – neutral/no charge, inside nucleus §   Electron – negative charge, outside of nucleus o  Like charges repel each other, opposite charges are attracted to each other
o  Magnetism – deals with moving charged particles, alignment of motion causes
certain material to be magnetic o  Field – the action of forces that one object exerts on another (ex: magnetic field (moving), gravitational field, electric fields (stationary), etc.) o  Electromagnetic radiation – family of electromagnetic disturbances •  The Wave-Like Characteristics of Light o  Wave – a disturbance moving outward from the point of origin
o  Electromagnetic waves are able to travel through vacuums, fields generate each
other; all move at same speed (speed of light) through empty space o  Speed of light – 3.0 x 10 8 m/s o  Wave properties §   Wavelength – horizontal length of one cycle (from peak to peak); different wavelengths generate different colors §   Frequency – number of wave cycles per second §   Wavelength and frequency are inversely proportional •  Light as a Photon o  Photon – packet of electromagnetic energy
o  Light can behave as both a wave and a particle (photon); “wave-particle duality”
o  Waves and particles are complete opposites (wave spread out, particle one place
at a time) •  Propagation of Light o  Inverse square law – the apparent brightness of a source gets weaker as distance from the source increases 5.2 The Electromagnetic Spectrum •  Types of Electromagnetic Radiation o  Electromagnetic spectrum – range of electromagnetic radiation in the universe
o  Gamma rays – carry a lot of energy, dangerous to living tissues, generated in the
deep center of stars, absorbed by earth’s atmosphere o  X-rays – more energetic that visible light, penetrate soft tissue but not bone fr eque
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ASTR 1060 – Week 4 Notes o  Ultraviolent (UV) rays – also referred to as “black light”, cause sunburn
o  Visible light – wavelengths between 400-700nm, what we can perceive with the
human eye, violet has shortest wavelength, red has longest o  Infrared (IR) – heat radiation, absorbed by water and CO 2 molecules o  Microwave – used in sort-wave communication and appliances
o  Radio waves – anything longer than microwaves, large category, divided into
subsections (ex: radar, AM, FM, TV) •  Radiation and Temperatures o  Temperature often determines which types of electromagnetic radiation are emitted from different objects o  Hotter temperature = faster movement of molecules within a substance
o  As atoms move, their electron collisions give off electromagnetic radiation
•  Radiation Laws o  Blackbody – object that doesn’t scatter or reflect any radiation and instead absorbs all electromagnetic energy that falls on it o  Power – energy coming off per second (measured in watts)
o  Wien’s Law – max wavelength = (3 x 10
6 )/T o  Energy flux – power emitted per square meter
o  Stefan-Boltzmann law – energy flux = (5.67 x 10
-8 ) (temp) 4 5.3 Spectroscopy in Astronomy •  Properties of Light o  Reflection – bouncing off of a surface
o  Refraction – passing from one transparent material into another
o  Dispersion – phenomenon where different wavelengths of light are dispersed
differently when refracted; used in prisms o  Spectrometer – tool used to disperse light and form the spectrum •  The Value of Stellar Spectra o  Improved spectrometer shows spectra with black bands where color is missing
o  Each element absorbs different colors, and depending on what bands are
missing, we are able to identify elements from spectroscopy o  Hot gases only emit certain lines of color at specific wavelengths
o  Spectral signatures – different emissions that allow substances to be identified
•  Types of Spectra o  Continuous – array of all wavelengths or colors of the rainbow
o  Absorption – series of dark lines/missing colors from the continuous spectrum
o  Emission – series of bright lines where only certain wavelengths are present
5.4 The Structure of the Atom •  Probing the Atom o  Electrons discovered by JJ Thompson through the cathode ray tube experiment
o  Nucleus discovered by Earnest Rutherford through gold foil experiment
•  The Atomic Nucleus o  Simplest atom is Hydrogen (single proton) fr eque
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ASTR 1060 – Week 4 Notes o  Type of element determined by number of protons, charge of element determined by number of electrons (ion), mass determined by protons + neutrons (isotope) o  Isotopes – same element with a different number of neutrons, resulting in a different mass •  The Bohr Atom o  Electrons orbit the nucleus in specific energy levels
o  E = hƒ (h- Planck’s constant)
5.5 Formation of Spectral Lines •  The Hydrogen Spectrum o  Hydrogen atoms absorb light at only certain wavelengths and produce dark lines at those wavelengths o  Each type of atom has its own unique pattern of electron orbits, no two set are exactly alike •  Energy Levels of Excitation o  Ground state – lowest possible energy level of an atom
o  Excitation – when an atom absorbs energy and moves to a higher energy level
o  Excited state – higher energy level than an atom’s ground state
o  Jumps emit photons with wavelengths that correspond to the energy differences
o  Lyman series – transitions to or from the ground state
o  Balmer series – transitions to or from the first excited state
o  Paschen series – transitions to or from the second excited state
o  Brackett series – transitions to or from the third excited state
•  Ionization o  Ionization – removing an electron from an atom
o  Ionization energy – the minimum amount of energy required to remove one
electron from an atom in its ground state 5.6 The Doppler Effect •  Motion Affects Waves o  Doppler effect – if a light source is approaching or receding from the observer, the light waves will be crowed more closely together or spread out o  Radical velocity – a motion towards or away from an observer (not sideways)
o  Example with sirens: they change pitch when moving away from you
•  Color Shifts o  Blueshift – when wavelength decreases
o  Redshift – when wavelength increases
o  The greater the motion toward or away from us, the greater the Doppler shift
“Astronomy” Chapter 17: Analyzing Starlight 17.1 The Brightness of Stars •  Luminosity o  Luminosity – total amount of energy at all wavelengths that it emits per second
o  Expressed in terms of the sun’s luminosity (L
sun )

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##### Description: "Astronomy" OpenStax Text Chapter 5: Radiation and Spectra Chapter 17: Analyzing Starlight
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