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.

Light is an Electromagnetic Wave

Basic Properties of Waves Wavelength = in meters Frequency = in cycles per second or Hertz (Hz) Speed = c in meters per second

Each “color” is characterized by its wavelength Using c = we can see that the frequency of visible light is in the Hz range

Visible light is only a very small part of the Electromagnetic Spectrum

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

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

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

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

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

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

Johnson-Cousins Filter System

SDSS Filter System

Comparison of UBVRI and ugriz

Flux Amount of light energy per unit area per unit time in a specific wavelength band Recall that

Blackbody Spectra Also known as a Planck curve

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

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

Stellar Classification Scheme