1/18/2015
Stars
Brightness - Magnitudes
Magnitude - brightness value
__________Magnitude (m) - how bright it appears to us on Earth
Magnitude 1 = brightest
2 - not so bright
3 - fainter
4 - even fainter
etc.
Larger value of Magnitude => ____________!!!!!
Some examples The Sun
m = -26.8
Sirius
m = -1.4
Betelgeuse
m = 0.4
Vega
m = 0.0
Polaris
m = 2.0
Antares
m = 1.1
Arcturus
m = -0.4
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1/18/2015
Motion of Earth – “shifts” star positions
_____________ shift.
Relationship between ________ shift-__________?
Measure shift angle
Shift is small - ____________ (1 _________ = 1/3600º)
Parallax Formula -
d=
p = parallax shift (measured in arcseconds, ")
d = Distance (units?) - _______ (___) - for parallax second.
Measure p, put into formula, get d in parsecs.
Apparent Magnitude  actual brightness  distance
Must standardize magnitude scale
Distance = ___ pc, measure magnitude
___________ Magnitude (M) - the magnitude a star
would have if it were ____ pc away from us.
M = true measure of a star’s brightness!
M works the same as m - larger value  fainter star
But M is meaningful!
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1/18/2015
The Sun
m = -26.8
M = 4.83
Sirius
m = -1.4
M = 1.4
Betelgeuse
m = 0.4
M = -5.6
Vega
m = 0.0
M = 0.5
Polaris
m = 2.0
M = -4.6
Antares
m = 1.1
M = -5.3
Arcturus
m = -0.4
M = -0.3
_____________= energy output of a star (in Watts)
Black Body laws E = s T4 = Watts/meter2 (hotter  brighter)
Size of the star is important! (wider  brighter)
Combined together gives
L = 4 p R 2 s T4
Yuck. Simplify - if L, R, T measured in terms of the Sun,
then we have
L = R 2 T4
L - R - T formula
Stellar Spectra
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1/18/2015
Harvard College
Observatory “calculators”
Annie Jump Cannon
Classify spectrum by prominent features
Hydrogen - A, B, C, D, ……
Oops, is that correct?
Nope…..
Stars are
70% Hydrogen
28% Helium
2% other elements
So why are their spectra different?
_________________
Spectra variations mainly due to ________________
Rearrange the spectra classifications according to
_____________ (_____ to _______) and you get…...
The __________________System
A TEMPERATURE Scale - ordered from high to low
OBAFGKM
OBAFGKMRNS
Add in a few more recently discovered types
OBAFGKMLT
Further subdivisions - B0, B1, B2, …B8, B9, A0, A1….
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1/18/2015
Stellar Spectra
Ordered by temperature
Bringing it all together
Temperature & Absolute Magnitude (Luminosity) related?
Henry Norris Russell and Ejnar Hertzsprung
Created an Hertzsprung - Russell Diagram - or
H-R Diagram
_____________________
90% of all stars
Range of T, L values
Sun is there as well
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________________
Relatively low T (red)
Relatively large L
Pretty big R (giant)
__________________
Temperature high to low
Luminosity - all high
Fairly rare
__________________
Low L
High T (white)
Small R (dwarf)
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OBAFGKMLT – temperatures
Luminosity Classification System Ia, Ib - Supergiants
II, III, IV - Giants
V - Main Sequence
White Dwarf
Examples:
Sun G2 V
Betelgeuse M2 Ib
Sirius A0V
Arcturus
K2 III
Mass
Mass - ____________________ Law (a special version for stars)
Two stars orbiting each other - Binary Stars
Distances between stars, period of orbits (a, P in the formula)
Are they really binary stars? No fakes allowed!!
__________Binary System - fake, they just look like they are close
__________Binary System - true binary
Physical Binary System
___________ Binary - see the star(s) move in orbit
One movie
another movie
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1/18/2015
_____________ Binary- spectra reveals binary nature
___________Binary - light variation reveals binary nature
Mass - Luminosity
Trend seen in Main Sequence Stars
High Mass - High Luminosity
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