Stars

1
Stars

• Other Suns

2
Physical Properties

• Luminosity
• Mass
• Diameter (radius)
• Must know distance to find out these properties!

3
Physical Properties

• Surface temperature
• Chemical composition
• Analyze spectra to infer these properties
  distance not required.

4
Distances

• Direct Heliocentric stellar parallaxes (AU as
  baseline)
• Smaller parallax, greater distance
• Inverse relation Distance is inversely
  proportional to parallax angle
• Precise parallaxes of many stars by Hipparcos
  satellite

5
Luminosities

• Measure flux at earth
• Imagine sphere with radius equal to distance
  area collects stars luminosity
• Inverse-square law Flux inversely proportional
  to distance squared

6
Masses

• Find Binary systems (lots!)
• Apply Newtons version of Keplers 3rd
• Need Distance, orbital period separation,
  center of mass
• Get Mass of each star

7
Diameters (Radii)

• Current techniques can measure angular diameters
  directly for some stars
• Angular diameter inversely proportional to
  distance
• Need distance to find physical diameter

8
Diameters (Radii)

• Infer From luminosity, surface temperature
• Assume Radiates like blackbody temperature
  gives flux at surface
• Luminosity From surface flux and area gt infer
  radius (area 4 p R2 for sphere of radius R)

9
Composition

• Analyze spectra (most contain absorption lines)
• Match dark lines to those for known elements
• Gives composition of photosphere only

10
Surface Temperatures

• From color Bluish-white (hottest) to reddish
  (coolest)
• From peak in continuous spectrum or matching
  continuous spectrum to that of a blackbody
• Assume radiate somewhat like blackbodies (Planck
  curve)

11
Energy

• Fusion reactions! (E mc2)PP Chain, CNO cycle
• In high-temperature cores (above ignition
  temperatures)
• Energy flows to surface (radiation, convection ),
  radiated into space

12
Spectral Classes

• Temperature sequence from hottest (O) to coolest
  (M)
• Based on intensities of certain dark lines of
  specific elements (especially Balmer series of
  hydrogen)
• Related to colors of stars (continuous spectra)

13
Hertzsprung-Russell Diagram

• Graph of stellar luminosities (need distances!)
  versus surface temperatures (colors or spectral
  types)
• See patterns among stars gt different physical
  features
• Main sequence, giants, supergiants, white dwarfs

14
Luminosity Classes

• Pattern on H-R digram
• Same spectral types (surface temperatures) but
  different luminosities!
• Infer different surface areas and so different
  radii Supergiants, giants, main sequence

15
Mass-Luminosity Relation

• Graph luminosities versus masses (from binary
  systems)
• Pattern Larger masses have much greater
  luminosities
• Luminosity directly proportional to mass to the
  4th power (L M4)

16
Lifetimes

• Fuel reserve Directly proportional to mass
• Use Directly proportional to luminosity
• Lifetime Reserve/Use or M/M4 or 1/M3 gt more
  mass, shorter lifetime!

17
Ages

• Lifetime Total span of active life from fusion
  reactions
• Age Time elapsed since fusion reactions began
• Suns lifetime 10 Gy suns age, 5 Gy when age
  lifetime, star dies (no more fusion)