Transcript Day 4

Announcements
•Lab this week: Observing Double Stars. We
will adjourn to the Farm after lecture.
•Chapter 4 is spherical geometry and we are
skipping it.
•Homework: Chapter 5 # 1, 2, 5, 6, 8 & 9
•Dark Sky Observing Night next week on
Monday and Thursday. Activities start at 7:30
so set-up starts at 6:45pm…no lecture
Chapter 5: Quantifying Light
Light is an
Electromagnetic Wave
Basic Properties of Waves
Wavelength = l in meters
Frequency = n in cycles per second or Hertz (Hz)
Speed = c in meters per second
E  hn
c  nl
Each “color” is characterized
by its wavelength
Using c = ln we can see that the frequency of
visible light is in the 1014 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
1st mag…visible in first
2nd mag…visible in second
3rd mag…visible in third
4th mag…visible in fourth
5th mag…visible in fifth
6th mag…visible in sixth
Modern definition of magnitude
is based on light flux
 F2 
m2  m1  2.5 log  
F
 1
Note that this compares two stars. If a “zero point” is
defined then
m  2.5 log F  C
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: 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 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
d 
m  M  5 log  
 10 
Types of Magnitude
Visual 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 Systems
Early 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
Flux
Amount of light energy per unit area per unit time
in a specific wavelength band
Eband
F
dAdt
Recall that
E photon  hn
Blackbody Spectra
Also known as a
Planck curve
Two laws govern blackbody
radiation
Wein’s Displacement Law
Where T is in Kelvin and l
is in meters
lmax
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
0.0029

T
Bn (T ) 
2hn 3
hn

c  e kT  1


2hc 3
Bl (T ) 
hc
5
l  e lkT  1


2
Stellar Spectra
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