Transcript Stars PowerPoint Slides

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
•
2
Fusion reactions! (E = mc )
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)