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 Concepts Covered in Previous Class

• Determining a stars L and R from a knowledge
  of its d and T
• Determining a stars Luminosity Class from an
  analysis of its spectrum

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Determining a Stars L Rfrom a knowledge
of d T
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LUMINOSITY CLASS Based on the width of spectral
lines, it is possible to tell whether the star is
a supergiant, a giant, a main sequence star or a
white dwarf. These define the luminosity classes
shown on the left occupying distinct regions on
the HR diagram. The complete spectral type of the
Sun is G2 V. The G2 part tells us Teff, the V
part tells us to which sequence or luminosity
class the star belongs. Example M5 III is a red
giant with Teff 3500K, M0 (or L100 Lsun).
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HR Diagram This template will be used in the
upcoming test. Please become familiar with it. We
will do a few examples in class of how to read
off the temperature, luminosity and size of a
star given a full spectral type.
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HR Diagram I expect you to know which of the
gray sequences is which luminosity class. From
top to bottom Ia, luminous supergiants Ib,
supergiants III, giants V, main
sequence Examples
G2V The Sun
M5III
B4Ib
M5Ia
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Today we will learn

• 17-8 How we can estimate a stars distance from
  its spectrum (spectroscopic parallax)
• 17-9 How we can use binary stars to measure the
  masses of stars
• 17-10 How we can learn about binary stars in very
  close orbits
• 17-11 What eclipsing binaries are and what they
  tell us about the sizes of stars

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Binary star systems provide crucial information
about stellar masses.

• Double star a pair of stars located at nearly
  the same position in the night sky.
• Optical double stars stars that lie along the
  same line of sight, but are not close to one
  another.
• Binary stars, or binaries stars that are
  gravitationally bound and orbit one another.
• Visual binary binaries that can be resolved
• Spectroscopic binary binaries that can only be
  detected by seeing two sets of lines in their
  spectra
• Eclipsing binary binaries that cross one in
  front of the other.

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Binary star systems provide crucial information
about stellar masses.

• Double star a pair of stars located at nearly
  the same position in the night sky.
• Optical double stars stars that lie along the
  same line of sight, but are not close to one
  another.
• Binary stars, or binaries stars that are
  gravitationally bound and orbit one another.
• Visual binary binaries that can be resolved
• Spectroscopic binary binaries that can only be
  detected by seeing two sets of lines in their
  spectra
• Eclipsing binary binaries that cross one in
  front of the other.

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Binary Star Krüger 60 (upper left hand corner)
About half of the stars visible in the night sky
are part of multiple-star systems.
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Mizar A, z1 UMa or Zeta-one Ursae Majoris,
a0.01, P 20.5 d Courtesy Navy Prototype
Optical Interferometer
http//leo.astronomy.cz/mizar/article.htm
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Ch. 17, problem 66
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Ch. 17, problem 66

• Whats measured observationally?
• Orbital period (P 87.7 yr)
• Parallax (p 0.2 arcsec)
• Apparent length of semimajor axis as seen on the
  sky (a 4.5 arcsec)
• What can we determine from this information?
• Distance (d) in parsecs
• Actual length of semimajor axis (a) in AU
• Total mass of this binary system (M1 M2) in
  solar masses

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Measured Masses of Main Sequence Stars in Binary
Systems
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Hertzsprung-Russell (H-R) Diagram
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Binary star systems provide crucial information
about stellar masses.

• Double star a pair of stars located at nearly
  the same position in the night sky.
• Optical double stars stars that lie along the
  same line of sight, but are not close to one
  another.
• Binary stars, or binaries stars that are
  gravitationally bound and orbit one another.
• Visual binary binaries that can be resolved
• Spectroscopic binary binaries that can only be
  detected by seeing two sets of lines in their
  spectra
• Eclipsing binary binaries that cross one in
  front of the other.

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Spectroscopy makes it possible to study binary
systems in which the two stars are close together.
k Ari
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Binary star systems provide crucial information
about stellar masses.

• Double star a pair of stars located at nearly
  the same position in the night sky.
• Optical double stars stars that lie along the
  same line of sight, but are not close to one
  another.
• Binary stars, or binaries stars that are
  gravitationally bound and orbit one another.
• Visual binary binaries that can be resolved
• Spectroscopic binary binaries that can only be
  detected by seeing two sets of lines in their
  spectra
• Eclipsing binary binaries that cross one in
  front of the other.

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Light curves of eclipsing binaries provide
detailed information about the two stars Sizes,
effective temperatures, shapes, etc
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NN Ser ESO
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Light curves of eclipsing binaries provide
detailed information about the two stars.
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Light curves of eclipsing binaries provide
detailed information about the two stars.
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Light curves of eclipsing binaries provide
detailed information about the two stars.
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Key Ideas

• Measuring Distances to Nearby Stars Distances to
  the nearer stars can be determined by parallax,
  the apparent shift of a star against the
  background stars observed as the Earth moves
  along its orbit.
• Parallax measurements made from orbit, above the
  blurring effects of the atmosphere, are much more
  accurate than those made with Earth-based
  telescopes.
• Stellar parallaxes can only be measured for stars
  within a few hundred parsecs.
• The Inverse-Square Law A stars luminosity
  (total light output), apparent brightness, and
  distance from the Earth are related by the
  inverse-square law. If any two of these
  quantities are known, the third can be
  calculated.

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Key Ideas

• The Population of Stars Stars of relatively low
  luminosity are more common than more luminous
  stars. Our own Sun is a rather average star of
  intermediate luminosity.
• The Magnitude Scale The apparent magnitude scale
  is an alternative way to measure a stars
  apparent brightness.
• The absolute magnitude of a star is the apparent
  magnitude it would have if viewed from a distance
  of 10 parsecs. A version of the inverse-square
  law relates a stars absolute magnitude, apparent
  magnitude, and distance.

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Key Ideas

• Photometry and Color Ratios Photometry measures
  the apparent brightness of a star. The color
  ratios of a star are the ratios of brightness
  values obtained through different standard
  filters, such as the U, B, and V filters. These
  ratios are a measure of the stars surface
  temperature.
• Spectral Types Stars are classified into
  spectral types (subdivisions of the spectral
  classes O, B, A, F, G, K, and M), based on the
  major patterns of spectral lines in their
  spectra. The spectral class and type of a star is
  directly related to its surface temperature O
  stars are the hottest and M stars are the
  coolest.
• Most brown dwarfs are in even cooler spectral
  classes called L and T. Unlike true stars, brown
  dwarfs are too small to sustain thermonuclear
  fusion.

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Key Ideas

• Hertzsprung-Russell Diagram The
  Hertzsprung-Russell (H-R) diagram is a graph
  plotting the absolute magnitudes of stars against
  their spectral typesor, equivalently, their
  luminosities against surface temperatures.
• The positions on the H-R diagram of most stars
  are along the main sequence, a band that extends
  from high luminosity and high surface temperature
  to low luminosity and low surface temperature.

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Key Ideas

• On the H-R diagram, giant and supergiant stars
  lie above the main sequence, while white dwarfs
  are below the main sequence.
• By carefully examining a stars spectral lines,
  astronomers can determine whether that star is a
  main-sequence star, giant, supergiant, or white
  dwarf. Using the H-R diagram and the inverse
  square law, the stars luminosity and distance
  can be found without measuring its stellar
  parallax.

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Key Ideas

• Binary Stars Binary stars, in which two stars
  are held in orbit around each other by their
  mutual gravitational attraction, are surprisingly
  common. Those that can be resolved into two
  distinct star images by an Earth-based telescope
  are called visual binaries.
• Each of the two stars in a binary system moves in
  an elliptical orbit about the center of mass of
  the system.
• Binary stars are important because they allow
  astronomers to determine the masses of the two
  stars in a binary system. The masses can be
  computed from measurements of the orbital period
  and orbital dimensions of the system.

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Key Ideas

• Mass-Luminosity Relation for Main-Sequence Stars
  Main-sequence stars are stars like the Sun but
  with different masses.
• The mass-luminosity relation expresses a direct
  correlation between mass and luminosity for
  main-sequence stars. The greater the mass of a
  main-sequence star, the greater its luminosity
  (and also the greater its radius and surface
  temperature).

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Key Ideas

• Spectroscopic Observations of Binary Stars Some
  binaries can be detected and analyzed, even
  though the system may be so distant or the two
  stars so close together that the two star images
  cannot be resolved.
• A spectrum binary appears to be a single star but
  has a spectrum with the absorption lines for two
  distinctly different spectral types.

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Key Ideas

• A spectroscopic binary has spectral lines that
  shift back and forth in wavelength. This is
  caused by the Doppler effect, as the orbits of
  the stars carry them first toward then away from
  the Earth.
• An eclipsing binary is a system whose orbits are
  viewed nearly edge-on from the Earth, so that one
  star periodically eclipses the other. Detailed
  information about the stars in an eclipsing
  binary can be obtained from a study of the
  binarys radial velocity curve and its light
  curve.