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Fall 2004
Dr. Mike Fanelli
Hints for Assigned Problems
Chapter 17
PROBLEM 17- 4: A star is determined to have a surface temperature twice that
of the Sun, and a luminosity 64 times greater. What is this star’s radius,
expressed in solar units ?
HINT: Problem 4 is an application of the radius – luminosity – temperature
relation for stars, described in the “More Precisely” box 17-2, in your text. Since
you are given two of these values, the third is found using that relation.
PROBLEM 10: Given a star with an apparent magnitude of 10.0, and an
absolute magnitude of 2.5, you are asked to find the distance to the star.
HINT: A star appears fainter if it is located further away, just like any luminous
object. The magnitude of a star represents its brightness, either its perceived
brightness, known as its apparent magnitude, or its actual, true, brightness,
known as its absolute magnitude. More luminous stars have smaller
magnitudes, an unusual attribute of the magnitude system. The star described in
this problem appears relatively faint, at 10th magnitude. This star must be at a
distance larger than 10 parsecs, since absolute magnitudes are defined as the
magnitude a star would have if located at a distance of10 parsecs from Earth.
The quoted absolute magnitude is 2.5, a much brighter value, than the observed
magnitude, which = 10.0. Read More Precisely 17-1 carefully, and use the
formula relating magnitude and distance to estimate the distance.
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