Universe 8e Lecture Chapter 17 Nature of Stars

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Roger A. Freedman William J. Kaufmann III
Universe Eighth Edition
CHAPTER 17 The Nature of Stars
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M 39 is an Open or Galactic Cluster
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Review of Previously Covered Concepts

• The distance (d) to a star can be determined from
  a measurement of the stars parallax (p).

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Stellar Parallax As Earth moves from one side of
the Sun to the other, a nearby star will seem to
change its position relative to the distant
background stars. d 1 / p d distance to
nearby star in parsecs p parallax angle of that
star in arcseconds
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Some Nearby Stars
Proxima Centauri p 0.772 arcsec, d 1/p
1.3 pc
Barnards Star p 0.545 arcsec, d 1/p
1.83 pc Sirius A/B p 0.379 arcsec,
d 1/p 2.64 pc
1 pc 206,265 AU 3.26 LY
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Review of Previously Covered Concepts

• The distance (d) to a star can be determined from
  a measurement of the stars parallax (p).
• The intrinsic brightness or luminosity (L) of a
  star can be determined from a measurement of the
  stars apparent brightness (b) and a knowledge of
  the stars distance.

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If a stars distance is known, its luminosity can
be determined from its brightness.

• As you get farther and farther away from a star,
  it appears to get dimmer.
• Luminosity, L, doesnt change
• Apparent brightness, b, does change following the
  inverse square law for distance.
• b L / (4pd2)

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If a stars distance is known, its luminosity can
be determined from its brightness.

• A stars luminosity can be determined from its
  apparent brightness if its distance is known
• L 4p d 2 b
• L