Lecture 5

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School

University of Virginia *

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1210

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Astronomy

Date

Oct 30, 2023

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docx

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Uploaded by ProfessorLightningPuppy25

Announcements ● HW 2 due Wednesday @ 11:59 PM ● Telescope lab sign ups available Learning from Light Types of Spectra in Astronomy ● Continuous spectra (emitted by sufficiently dense objects) ● Continuous spectrum with dark lines (absorption spectrum) ● Bright line spectra Bohr Model of the Atom ● Toy model of an atom ● Electrons orbit the nucleus at discrete energy levels ● Orbits are given by numbers, n, based on their energy level ● To change orbits, an electron must change energy ● Higher energy states correspond to higher n levels ● It’s like rungs on a ladder Absorption ● To go from a lower level to a higher level, the electron must increase in energy ● It can do this by absorbing a photon that has exactly the amount of energy needed to go from the lower level to the upper level Emission ● To go from a higher level to a lower level, the electron must give up energy (emit a photon) ● The emitted photon has exactly the difference between the two levels Ionization ● It is possible to give the electron though energy to completely escape from the aton. The atom is then said to be ionized. ○ The amount of energy required to ionize an atom is called the ionization energy

○ The ionization energy can be provided by photons or by collisions ○ The resulting atom is left with a net positive charge Energy Level Transitions ● The only allowed changed in energy are those corresponding to a transition between energy levels ○ The energy stored in an electron is measured in “eV” or electron volts Chemical Fingerprints ● Each type of atom has a unique spectral fingerprint. The correspond to its allowed electron levels ● Observing the fingerprints in a spectrum tells us which kinds of atoms are present What is temperature? ● Temperature is a direct measurement of the internal motions of atoms and molecules in an object ○ The higher an object’s temperature, the faster its particles are moving ○ The temperature of an object species the average thermal energy (or heat) of the particles it contains Measuring Temperature ● Scientists often use the Kelvin scale: ○ A change in temperature of 1 K is the same as a change of 1 o C ○ 0k (absolute zero) is the point at which all molecular motion ceases ○ 0k = 273 o C = -459 o F ○ Absolute zero is the lowest possible temperature ○ For comparison, the temperature of the atmosphere of the earth is about 300K Thermal Radiation ● Nearly all large or dense objects emit thermal radiation, including stars, planets, and people ● An object’s thermal radiation spectrum depends on only one property: its temperature Properties of Thermal Radiation ● Hotter objects emit more light at all frequencies per unit area ● Hotter objects emit photons with a higher average energy Wien’s Law ● The higher the temperature, the shorter the wavelength of peak radiation: ○ Lambda max = 2.9E 6 /T ● Lambda max is the wavelength of the peak, in nm, and T is the temperature in Kelvin Speed of a Distant Object

● The Doppler Shift can tell you about the relative motion of distant objects because photons behave as waves

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