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Published byAgnes Terry Modified over 9 years ago
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Announcements No lab this week’s due to Tuesday’s observing night Homework: Chapter 5 # 1, 2, 5, 6, 8 & 9 Dark Sky Observing Night next week on Wednesday. Set-up starts at 6:45pm Algol observing this Saturday/Sunday. Starts around 8:00pm Saturday night and goes to around 1:00am Sunday morning.
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Light is an Electromagnetic Wave
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Basic Properties of Waves Wavelength = in meters Frequency = in cycles per second or Hertz (Hz) Speed = c in meters per second
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Each “color” is characterized by its wavelength Using c = we can see that the frequency of visible light is in the 10 14 Hz range
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Visible light is only a very small part of the Electromagnetic Spectrum
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Magnitude Originally devised by Hipparchus around 140 BC. Based on when stars become visible after sunset. Sunset to astronomical twilight (complete dark) is divided into six equal time periods 1 st mag…visible in first 2 nd mag…visible in second 3 rd mag…visible in third 4 th mag…visible in fourth 5 th mag…visible in fifth 6 th mag…visible in sixth
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Modern definition of magnitude is based on light flux Note that this compares two stars. If a “zero point” is defined then where C is the zero point offset
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Zero Points N.R. Pogson, originator of the modern magnitude definition, proposed an average of the sixth magnitude stars in certain star catalogues. Result: m Sirius = -1.6 North Polar Sequence…system of “standard stars” with known magnitudes to compare against m Vega ≡ 0.0 but problems with variability and dust leads modern values to m Vega = 0.03 Most common systems now are standard star systems
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Absolute magnitude Defined as the magnitude of the object if it was located at a distance of 10 parsecs. This gives a distance (d) relationship between apparent magnitude (m) and absolute magnitude (M). Distance is measured in parsecs
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Types of Magnitude Visual Magnitude (m v )…measured over the visible spectrum Monochromatic magnitude(m )…measured over a narrow wavelength range Bolometric magnitude (m bol )…measured over the entire E/M spectrum Photographic magnitude (m pg )…magnitude measured with photographic plate
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Filter Systems Early 20 th Century “filter” system was photographic (m pg ) and visual (m v ) 1950’s H. L. Johnson & W. W. Morgan added an ultraviolet (m U ) and converted photographic to “blue” (m B ) for the UBV system Alan Cousins work with GaAs photomultipliers which are sensitive in red and IR adds R and I for Johnson-Cousins UBVRI system J. A. Smith et al use mostly non-overlapping filter system for SDSS…ugriz
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Johnson-Cousins Filter System
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SDSS Filter System
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Comparison of UBVRI and ugriz
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Flux Amount of light energy per unit area per unit time in a specific wavelength band Recall that
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Blackbody Spectra Also known as a Planck curve
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Two laws govern blackbody radiation Wein’s Displacement Law Where T is in Kelvin and is in meters Planck’s Law with B in Watts per square meter per Hz per steradians or Watts per square meter per meter per steradians, T in Kelvin, c in meters per second and is in meters. h is Planck’s constant and k is Boltzmann’s constant
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Stellar Classification Annie Jump Cannon developed a stellar classification system based on temperature and the women of Harvard Observatory classified hundreds of thousands of stars. The project spanned several decades and was funded by a grant from the widow of Henry Draper. The resulting catalogue is the Henry Draper Catalogue
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Stellar Classification Scheme
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