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Title: Universe 8e Lecture Chapter 17 Nature of Stars


1
Roger A. Freedman William J. Kaufmann III
Universe Eighth Edition
CHAPTER 17 The Nature of Stars
2
M 39 is an Open or Galactic Cluster
3
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

4
Determining a Stars L Rfrom a knowledge
of d T
5
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).
6
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.
7
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
8
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.

11
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.

12
Binary Star Krüger 60 (upper left hand corner)
About half of the stars visible in the night sky
are part of multiple-star systems.
13
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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15
Ch. 17, problem 66
16
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

17
Measured Masses of Main Sequence Stars in Binary
Systems
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19
Hertzsprung-Russell (H-R) Diagram
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21
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.

22
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24
Spectroscopy makes it possible to study binary
systems in which the two stars are close together.
k Ari
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27
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.

28
Light curves of eclipsing binaries provide
detailed information about the two stars Sizes,
effective temperatures, shapes, etc
29
NN Ser ESO
30
Light curves of eclipsing binaries provide
detailed information about the two stars.
31
Light curves of eclipsing binaries provide
detailed information about the two stars.
32
Light curves of eclipsing binaries provide
detailed information about the two stars.
33
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34
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.

35
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.

36
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.

37
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.

38
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.

39
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.

40
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).

41
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.

42
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.
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