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lecture03

# lecture03 - Astronomy Picture of the Day The Small...

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Astronomy Picture of the Day The Small Magellanic Cloud

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Homework 1 is due in the P20A drop box on the 4th floor of Reines Hall by 8:30 on friday morning Writing out the answers by hand is fine (pencil is ok) Make sure to show your work for the mathematical questions! Eric will go over the solutions in discussion section on friday Reminder: homework
Black-body radiation: review An ideal black-body absorbs all light incident on it If we compare 2 black-body emitters having the same size but different temperatures, the hotter black-body will: emit more radiation overall (i.e., it will be more luminous) emit more photons at any given wavelength have a spectrum peaking at shorter wavelengths have a bluer color The wavelength at which a star’s spectrum peaks is a measure of its surface temperature

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T = 7000 K T = 5800 K T = 4500 K The Spectrum of Black-Body Radiation
λ max T = 2.9 × 10 7 Å K A relationship between the temperature of a black-body emitter and the wavelength at which its spectrum peaks: Wien’s Law Example question: If a star’s spectrum peaks at a wavelength of 2900 Å, what is its surface temperature?

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Luminosity and Brightness Luminosity means the amount of energy radiated by an object per second (the same thing as Power) Luminosity can be measured in Watts Brightness refers to how bright an object appears as seen by an observer This depends on both the object’s luminosity and on its distance from the observer
L = σ T 4 × surface area

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lecture03 - Astronomy Picture of the Day The Small...

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