Stars: Other Suns

1
Stars Other Suns

• The Solar-Stellar Connection

2
Physical Properties

• Luminosity
• Mass
• Size (diameter/radius)
• Surface temperature
• Chemical composition

3
Distances

• Heliocentric stellar parallax
• Inverse relation Smaller parallax, greater the
  distance
• Hipparcos satellite measured over 100,000 stars
  precisely (1 mas), over 1 million with less
  precision

4
Luminosities

• Measure flux at earth
• Imagine a sphere with radius equal to distance to
  star catches all flux from star
• Apply inverse square law for light
• Watch out for interstellar dust! (dims the
  starlight)

5
Masses

• Measure directly only with binary systems of
  stars (lots!)
• Revolve around center of mass
• Apply Keplers 3rd law to get sum of masses from
  orbital period, separation (need distance!)

6
Sizes

• Angular diameters mostly too small (mas!) to
  measure directly
• New optical techniques work on some stars (more
  to come!)
• Angular diameter distance gt physical diameter

7
Chemical Compositions

• Examine spectra (most show absorption lines)
• Match darks lines to those for known elements
• Gives composition of photosphere only (mostly H,
  He)

8
Temperatures

• From color (hottest, bluish white coolest,
  reddish)
• Or from wavelength at peak in the continuous
  spectra
• Assume radiate like blackbodies(many do!)

9
Mass-luminosity relation

• A stars mass and luminosity are related a
  little more mass means a lot more luminosity!
• Luminosity directly proportional to Mass4 M x
  M x M x M!
• Ex 2 solar mass star is L 2 x 2 x 2x 2 16
  solar luminosities!

10
Stellar lifetimes

• Fuel reserve depends on mass fuel use depends on
  luminosity
• Lifetime depends on reserve/use, or M/M4 or 1/M3
• More mass gt shorter lifetime (by a lot!)

11
Stellar ages

• Lifetime is total span of active life from fusion
  reactions
• Age is time elapsed since fusion began
• Suns lifetime about 10 billion years age about
  5 billion years (middle age!)

12
Hertzsprung-Russell Diagram

• Plot of stellar luminosities (low to high) versus
  surface temperature (hot to cool)
• A sorting tool stars fall into different regions
• Main sequence, giants, supergiants, white dwarfs