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Announcements
• Star Assignment 4,
due Monday March 22
Do Angel quiz,
• Quiz 2: Monday March 22
Light, Planets, Sun, Observations of stars
Chapters 6.4-5, 8-11, 15, 16.1-4
• Mid-Term 2: Wednesday March 24
Light, Planets, Sun, Observations of stars
Chapters 6.4-5, 8-11, 15, 16.1-4
Confusions from last time
• Waves in the Sun
• Neutrinos
Objectives
• Describe what is meant by Luminosity and
Brightness
• Describe how to measure a star’s
luminosity, brightness, distance, mass and
surface temperature
• Use stellar spectra to determine the surface
temperature and composition of stars
Luminosity & Brightness
• Luminosity = Rate of Energy Loss
= Energy radiated per second
• Apparent Brightness (Flux)
= Energy per second per unit area
Farther away -> Fainter appears
Apparent Brightness (Flux)
= Luminosity/4 p (Distance)2

Area of sphere
Luminosity, Brightness & Distance
Luminosity, Brightness & Distance
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Luminosity
1. Measure Brightness: Use detector
(Eye, Film, Light meter, CCD)
2. Measure Distance: Use Parallax
Measuring Brightness
Measures TOTAL
Energy impinging
on telescope per
second
Light
from
star
Telescope
focuses
Light onto
Detector
which
records
it onto
Why build big Telescopes?
To collect MORE light
Computer
disk
Must divide by
AREA of
Telescope to get
BRIGHTNESS
= energy per
second per unit
area
Measuring
Distance
by
Parallax
Number of Stars vs Luminosity
100
HIGH
1
0.01
Luminosity
0.0001
LOW
Surface Temperature
1. Color
2. Spectrum
Color & Temperature
Hotter -> Bluer
max  2,900,000nm/T(K)
Math Insight 6.2
Hotter -> Bluer
Temperature & Spectrum:
Hydrogen Thermometer
Visible (Balmer) lines of H formed when
electron in 2nd level absorbs a photon and
jumps up to a higher level, or when an
electron in a higher level emits a photon and
jumps down to the 2nd level.
Temperature & Spectrum:
Hydrogen Thermometer
Hot Stars: most electrons ionized
(knocked free from atom)
3
2
1
Visible (Balmer) lines of H due to
Electron transitions to 2nd energy level
Medium Temperature Stars (~104K) ->
hot enough for collisions to knock
electron up to 2nd level, not hot enough to
knock electron free of atom.
Cool Stars: most electrons in
Lowest Energy (ground) state
Temperature & Spectrum:
Hydrogen Thermometer
Only gas at Intermediate Temperature
(~104 K) has large fraction of H atoms with
excited electrons in the 2nd energy level
Colder gas:
electrons in lowest enery
(ground) state
Hotter gas:
energetic collisions know
electrons out of atom (ionized)
Hot
helium
hydrogen
Vega
Spectral
Classes
Sun
calcium+
calcium
Cool
Spectral Classification
• Movie: Cecelia Payne ( h m)
• Activity: Try it yourself
Spectral Classes
• Originally classified by strength of hydrogen lines:
A, B, C, D
• Now converted to a Temperature Sequence
O, B, A, F, G, K, M, L
hotest stars
strongest
hydrogen lines
coolest stars
Can you think of a good mnemonic for remembering this
order?
Stellar Masses
Use Newton’s Version of Kepler’s 3rd Law
3
3
D (AU) V P
M1  M 2  2

P (yrs) 4 pG
•

•
•
Must have TWO binary stars orbiting
around each other.
Measure Velocity (by Doppler Shift)
Measure Period (by light curve)
Doppler Shift
Stellar
Masses
What should we do with our
information?
• Luminosities
• Surface Temperatures
• Masses
Look for Relationships