The Solar-Stellar Connection Stars: Other Suns The Solar-Stellar Connection

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

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

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)

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!)

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

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

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

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!

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!)

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!)

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