# GroupAssig5.html - Group Assignment #5: Luminosity,...

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Group Assignment #5: Luminosity, Temperatureand SizeIntroduction (Adapted from Pearson Prentice Hall Astro LectureTutorials)LowMedHighIn astronomy, luminosity is the amount of electromagnetic energy a body radiates perunit of time (so Joules per second, J/s, for example). A star's luminosity can bedetermined from two stellar characteristics: size and effective temperature. The formeris typically represented in terms of solar radii, R⊙, while the latter is represented inkelvins (K), but in most cases neither can be measured directly.Temperature (K)Luminosity (solar units)20,000 10,000 5,000SYXWTWe know that for objects that are approximately blackbodies:Hotter things are brighterBigger things are brighterPutting this together, we know that the luminosity of a star is⋆=⋆2⋆4?4????If you assume that all stars are the size of the Sun, then hotter ones are moreluminous just because they are hotter. We can see this visually (Figure A) whenplotting Luminosity versus Temperature, on something called the H–R diagram (namedafter Hertzsprung and Russell), and we would expect all stars to fall on the blue line.However, when we actually measure the sizes of stars and their temperatures we find
picture B.ABThe coolest main sequence stars are a lot smaller than the sun. The hottest mainsequence stars are a lot bigger than the sun!ObjectiveAside from building connections with your classmates, the purpose of this groupexercise is to familiarize you with the H–R Diagram, what it means, and how it is usedin Astronomy. By the end of this exercise, you will 1) have a better appreciation of theinterplay between observation and theory, and 2) interpret the data in an H–RDiagram, and 3) Identify the physical characteristics of stars that are used to create anH–R diagram, and describe how those characteristics vary among groups of stars.

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