Stars

Other Suns

Physical Properties

•Luminosity•Mass•Diameter (radius)Must know distance to find

out these properties!

Physical Properties

•Surface temperature•Chemical composition

Analyze spectra to infer these properties;

distance not required.

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

Luminosities•Measure flux at earth•Imagine sphere with radius equal to distance; area collects star’s luminosity

•Inverse-square law: Flux inversely proportional to distance squared

Masses•Find: Binary systems (lots!)•Apply: Newton’s version of Kepler’s 3rd

•Need: Distance, orbital period & separation, center of mass

•Get: Mass of each star

Diameters (Radii)•Current techniques can measure angular diameters directly for some stars

•Angular diameter inversely proportional to distance

•Need distance to find physical diameter

Diameters (Radii)•Infer: From luminosity, surface temperature

•Assume: Radiates like blackbody; temperature gives flux at surface

•Luminosity: From surface flux and area => infer radius (area = 4 π R2 for sphere of radius R)

Composition•Analyze spectra (most contain absorption lines)

•Match dark lines to those for known elements

•Gives composition of photosphere only

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)

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

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)

Hertzsprung-Russell Diagram

•Graph of stellar luminosities (need distances!) versus surface temperatures (colors or spectral types)

•See patterns among stars => different physical features

•Main sequence, giants, supergiants, white dwarfs

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

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)

Lifetimes

•Fuel reserve: Directly proportional to mass

•Use: Directly proportional to luminosity

•Lifetime = Reserve/Use or M/M4 or 1/M3 => more mass, shorter lifetime!

Ages

•Lifetime: Total span of active life from fusion reactions

•Age: Time elapsed since fusion reactions began

•Sun’s lifetime: 10 Gy; sun’s age, 5 Gy; when age = lifetime, star dies (no more fusion)