Download Stars: Other Suns

Stars: Other Suns
The Solar-Stellar
Connection
Physical Properties
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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