Download 07 September: The Solar System in a Stellar Context

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Distances to stars; parsecs and light years
Another way of expressing
distances in the solar system
Distance to Sun in terms of light travel time
d=vt (like driving to Des Moines)
The fastest anything can travel is speed of light = c = 2.9979E+08
Distance to Sun = 1 au = 1.496E+11 meters (see Appendix 1), so
light travel time from Sun is
t=d/c =1.496E+11/2.9979E+08 =
t=499.02 sec
A little over 8 minutes
The Sun could have exploded 7 minutes ago, and we
would not have gotten the news yet!
Let’s repeat the calculation with the
Voyager 1 spacecraft, 103.75 au from
Voyager is a long ways out there
• Light takes over
14 hours to reach
Voyager from
• Round-trip time is
over a day!
Summary: an alternative description of
the size of the solar system
• Inner solar
system is
light minutes
in extent
• Outer solar
system is
light hours to
a light day
The Stars
“e quindi uscimmo a riveder le stelle”
Last words of Dante’s Inferno
The Stars are other
“L’amor che move il Sole
E le altre stelle”… Dante, end
Of Paradiso
Or…the Sun is the
closest star
Basic Questions about Stars
How far away are they?
How hot are they?
How massive are they
What are they made of?
Do they have planets too?
16 Cygni
Most Basic Method of Stellar
Distance Determination
Trigonometric parallax: an
ordinary surveying technique
Figure 16.1 of textbook
Next time:
• How far away are
the stars (compared
to solar system
• What units do we
use to describe their