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ASTR_2011Introduction to
Observational AstronomyDay-5
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Announcements•1st Quarter Observing Night• Next Thursday – Oct. 10
Start time will be 7:30pmStart set-up at ~6:30pm in Atrium
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MagnitudeOriginally 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 periods1st mag…visible in first2nd mag…visible in second3rd mag…visible in third4th mag…visible in fourth5th mag…visible in fifth6th mag…visible in sixth
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Modern definition of magnitude is based on light flux
1
212 log5.2
F
Fmm
Note that this compares two stars. If a “zero point” is defined then
CFm log5.2where C is the zero point offset
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Zero PointsN.R. Pogson, originator of the modern magnitude definition, proposed an average of the sixth magnitude stars in certain star catalogues .
Result: mSirius = -1.6
North Polar Sequence…system of “standard stars” with known magnitudes to compare against
mVega ≡ 0.0 but problems with variability and dust leads modern values to mVega = 0.03
Most common systems now are standard star systems
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Absolute magnitudeDefined 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
10
log5d
Mm
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Types of MagnitudeVisual Magnitude (mv)…measured over the visible spectrum
Monochromatic magnitude(ml)…measured over a narrow wavelength range
Bolometric magnitude (mbol)…measured over the entire E/M spectrum
Photographic magnitude (mpg)…magnitude measured with photographic plate
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Filter SystemsEarly 20th Century “filter” system was photographic (mpg) and visual (mv)
1950’s H. L. Johnson & W. W. Morgan added an ultraviolet (mU) and converted photographic to “blue” (mB) 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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FluxAmount of light energy per unit area per unit time in a specific wavelength band
dAdt
EF band
Recall that hEphoton
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Blackbody Spectra
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Two laws govern blackbody radiation
Wein’s Displacement LawWhere T is in Kelvin and l is in meters
T
0029.0max
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 l is in meters. h is Planck’s constant and k is Boltzmann’s constant
1
2)(
1
2)(
5
3
2
3
kThc
kTh
e
hcTB
ec
hTB
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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