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Ast 307  Nov. 2, 2010
ch. 17: Taking the
Measure of the Stars
Luminosity
is the total
amount of power that a
star radiates
Its units are energy per
second = Watts
The intensity of the star’s
light reaching us is called
the brightness
b
, or more
commonly,
flux
f
.
The
units are energy per unit
time per unit surface area.
Luminosity, Distance, and Flux
The apparent brightness of a star
depends on its luminosity & distance:
Luminosity
b =
flux =
4
!
(distance)
2
The InverseSquare Law of Light
Why? Because light spreads out in all
directions  in a spherical way  and
the surface area of a sphere is
4
!
d
2
.
(Divide a fixed luminosity evenly over
a larger surface area, and there will be
less energy per unit surface area.)
A star’s parallax is measured by
comparing images taken at
different times, and measuring
the shift of the nearby star
relative to more distant stars.
Measuring Stellar Distances
• The nearest stars have angular
shifts of less than 1 arcsecond.
• More distant stars have even
smaller parallaxes.
• The angular shift is inversely
proportional to the distance.
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View Full DocumentParallax and the Parsec
If
p
is in arcsec, then
d
is in parsecs, so that
a star with a p
arallax of 1 arcsec
is 1 parsec
distant
We choose to define a new unit of length:
1 parsec
"
206,265 A.U. = 3.26 light years
d
(
in pc
)
=
1
p
(
in
")
The parallax angle,
p,
is defined as half the total annual shift
The Scaled SmallAngle Formula
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 Fall '10
 Dinerstein
 Astronomy

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