1. The Solar-Stellar Connection
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 Kepler’s 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 => 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 star’s 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 => 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
Sun’s 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