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: 30• Pluto: 13• Faintest Naked-Eye Star: 6• Big Dipper Stars: 2• Sirius (Brightest Star) -1.6• Venus -4• Full Moon -12• Sun -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