Chapter 11 Surveying
the Stars
·
How bright the star appears to us depends on
both distance and luminosity.
·
Luminosity: Amount of power a star actually
radiates. (energy per second=Watts)
·
Apparent brightness: Amount of starlight that
reaches Earth. (energy per second per square meter)
·
Luminosity passing through each sphere is the
same
·
Parallaxis the apparent shift in position of a
nearby object against a background of more distant objects.
·
Parallax is measured by comparing snapshots
taken at different times and measuring the shift in angle to star
·
We determine the stars surface temperature from
its spectrum (color!).
·
Every object emits thermal radiationwith a
spectrum that depends on its temperature
·
Lines in a star’s spectrum correspond to a
spectral typethat reveals its temperature
·
About half of all stars are in binary systems.
·
The orbit of a binary star system depends on
strength of gravityand therefore on the mass of the two stars.
·
We can directly observe the orbital motions of
these stars
·
If not, we would have to use dopplereffect.
·
Luminosity: from brightness and distance We
measure mass using gravity Direct mass measurements are possible only for stars
in binary star systems
·
Temperature: from color and spectral type
·
Mass: from period (p) and average separation (a)
of binary-star orbit
·
Core pressure and temperature of a higher-mass
star need to be larger in order to balance gravity
·
Stars with low temperature and high luminosity
must have large radius
·
Massive blue stars die first, followed by white,
yellow, orange, and red stars
·
Pleiades now has no stars with life expectancy
less than around 100 million yearsMain-
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