Starlight and What it Tells Us
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
•
•
•
•
•
•
•
•
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, the
Moon Does Not
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
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
The Solar Spectrum
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