ASTR_2011Introduction to

Observational AstronomyDay-5

Announcements•1st Quarter Observing Night• Next Thursday – Oct. 10

Start time will be 7:30pmStart set-up at ~6:30pm in Atrium

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

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

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

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

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

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

Johnson-Cousins Filter System

SDSS Filter System

Comparison of UBVRI

and UGRIZ

FluxAmount of light energy per unit area per unit time in a specific wavelength band

dAdt

EF band

Recall that hEphoton

Blackbody Spectra

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

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