Starlight and What it Tells Us
The Stars in the Sky
Vary in Brightness
• Distance
• Size
Vary in Color
• Color = Temperature
Star Names
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Proper star names mostly Arabic
Greek Letters, Numbers
Catalog Identifiers
Faint stars usually have no name
The Names of Sirius
• Alpha Canis Majoris (Bayer, 1603)
• 9 Canis Majoris (Flamsteed, 1725)
• BD -16 1591 (Bonner Durchmusterung 18591903)
• HR 2491 (Harvard Revised Catalog, 1908)
• HD 48915 (Henry Draper, 1918-1924)
• ADS 5423 (Aitken Double Star Catalog, 1932)
• HIP 32349 (HIPPARCOS, 1997)
The Heavens Are Not Changeless
• The Stars Move
– Most of our constellations would have been
unrecognizable to Neanderthal Man
• The Solar System Moves
– Very few of our nearby stars would have been
visible to the first humans
• Stars are Born, Live and Die
– Many of our brightest stars did not exist in the
days of the dinosaurs
Brightness of Stars
• Variations in distance and intrinsic
brightness
• Scale based on one by Hipparcos 500 B.C.
• Magnitude: Large Numbers = Fainter
– One magnitude = 2.5 x
– Five magnitudes = 100 x
Magnitudes
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Planet around nearby star:
Pluto:
Faintest Naked-Eye Star:
Big Dipper Stars:
Sirius (Brightest Star)
Venus
Full Moon
Sun
30
13
6
2
-1.6
-4
-12
-27
Absolute
Magnitude
• Altair and Deneb
are about equally
bright as seen from
Earth
• Altair is 16 l.y. away,
Deneb 1600
• Hence Deneb must
be about 10,000
times brighter
Absolute Magnitude
• How bright a star would be at a distance of
32.6 l.y. (10 parsecs)
• Sun: 4.5 (inconspicuous naked-eye star)
• Altair: 2.2
• Deneb: -7.1 (bright as crescent moon)
– Note: Deneb - Altair about 10 magnitudes = 100 x
100 = 10,000 times
Black-Body Radiation
• Objects Emit Radiation Because They Are Hot
• Why “Black”? Because None of the Radiation
is Reflected from Some Other Source
• The Sun Emits Black-Body Radiation, Mars
Does Not
• Close Example of pure Black-Body radiation:
Peephole in a pottery kiln
Black Body Radiation
What’s The Source of the Light?
Color = Temperature
Why Black-Body Radiation is so
Important
• Color is directly related to temperature
• Temperature is the only determinant of color
• Energy per unit area is the same if
temperature is the same
– If two stars have the same color and distance,
difference in brightness is due to difference in size
– Dwarf and giant stars are literally dwarfs or giants
Sirius and the Pup
Sirius and the Pup
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Sirius M = -1.5; Pup M = 8.5
10 magnitude difference
100 x 100 = 10,000 times brightness distance
Sirius and the Pup are same color, therefore
same temperature (Pup is hotter)
• Pup must have 1/10,000 the apparent area of
Sirius = 1/100 the diameter
Spectroscopy
• Different atoms absorb or emit specific
wavelengths of light
• When light spread into a spectrum, the
absorbed wavelengths show up as dark
(missing) bands
• These spectral lines are indicators of:
– Chemical composition
– Physical conditions
Atoms and Radiation
The Solar Spectrum
Spectra and Spectral Lines
• Continuous Spectrum: Incandescent solids or
liquids (steel mill) and dense hot gases (Sun’s
photosphere)
• Emission Spectrum: Thin hot gases (fireworks,
sodium or mercury vapor lights, Sun’s
chromosphere
• Absorption Spectrum: Light shining through
thin gases (Sun and star light)
How the Chromosphere Works
Spectral Lines are Affected By:
• Electrical and Magnetic Fields
• Number of Electrons Atoms Have Lost
(Indicates Temperature and Pressure)
• Motion (Doppler Effect)
• Blue-shifted if Motion Toward Observer
• Red-shifted if Motion Away From Observer
The Doppler Effect
What the Doppler Effect Tells Us
• Radial Motion
• Rotation of Stars
– Approaching side of star blue-shifted, receding
side red-shifted
• Unseen Companions (Stars or Planets)
– Star oscillates around center of mass
• Surface and Interior Motions
– Changes in Size
– Interior Oscillations
Spectral Classification of Stars
• W – very hot young stars expelling their outer
layers
• Main Sequence: O, B, A, F, G, K, M (hottest to
coolest)
– “Oh be a fine girl/guy, kiss me”
• Subdwarfs: L, T, Y (hottest to coolest)
• Chemically Peculiar Stars: C, N, R, S
• White Dwarfs: D
Spectral Signatures of Stars
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O: Ionized Helium
B: Neutral Helium
A: Strongest Hydrogen Lines
F: Ionized Calcium
G: Strongest Calcium Lines + Neutral Metals
K: Neutral Metals Dominate
M: Titanium Oxide
The Hertzsprung-Russell Diagram
The Main Sequence: O
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30,000-60,000 K (Blue-white)
Absolute Magnitude -5
1,000,000 times Sun’s Luminosity
16 times Sun’s Diameter
64 times Sun’s Mass
Lifetime: Less than a million years
Examples: Orion's Belt
The Main Sequence: B
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10,000-30,000 K (Blue-white)
Absolute Magnitude -3
20,000 times Sun’s Luminosity
7 times Sun’s Diameter
18 times Sun’s Mass
Lifetime: 10 million years
Examples: Spica
The Main Sequence: A
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Temperature: 7500-10,000 K (White)
Absolute Magnitude +0.5
40 times Sun’s Luminosity
2 times Sun’s Diameter
3 times Sun’s Mass
Lifetime: 600 million years
Examples: Vega, Sirius
The Main Sequence: F
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Temperature: 6000-7500 K (Yellow-White)
Absolute Magnitude +2.5
6 times Sun’s Luminosity
1.5 times Sun’s Diameter
1.7 times Sun’s Mass
Lifetime: 2.5 billion years
Examples: Procyon
The Main Sequence: G
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Temperature: 5000-6000 K (Yellow)
Absolute Magnitude +5
1 times Sun’s Luminosity
1 times Sun’s Diameter
1 times Sun’s Mass
Lifetime: 10 billion years
Examples: Sun, Alpha Centauri A
The Main Sequence: K
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Temperature: 3500-5000 K (Orange)
Absolute Magnitude +6
0.4 times Sun’s Luminosity
0.9 times Sun’s Diameter
0.8 times Sun’s Mass
Lifetime: 10 billion years
Examples: Alpha Centauri B
The Main Sequence: M
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Temperature: 2000-3500 K (Red)
Absolute Magnitude +10 to +15
0.04 times Sun’s Luminosity
0.5 times Sun’s Diameter
0.4 times Sun’s Mass
Lifetime: 5 trillion years
75% + of all stars
Examples: Barnard's Star, Proxima Centauri
Sub-Dwarfs
• L: 1300-2000 K, Borderline stars with alkali
metals and metal hydrides
• T: 700-1300 K, Substellar, methane in spectra
• Y <700 K, Substellar, ammonia in spectra
(predicted)