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Normal binary stars: BOB Chap. 7

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Sirius A. Sirius B. Chandra X-ray Observatory image (NASA/CfA) Sirius: at once the brightest, most famous, and most fruitful binary star system. ... – PowerPoint PPT presentation

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Title: Normal binary stars: BOB Chap. 7


1
Normal binary stars BOB Chap. 7
  • Review
  • Measurement of the physical properties of stars.
  • Stellar mass the special role of binaries.
  • The data stellar luminosity, effective
    temperature, and radius vs. mass.

Sirius B
Sirius A
Chandra X-ray Observatory image (NASA/CfA)
Sirius at once the brightest, most famous, and
most fruitful binary star system. One component
isnt normal, though.
2
Stellar luminosity, mass and radius
  • What can we measure, to test theories of stellar
    structure?
  • Luminosity (total power output) measure total
    flux at Earth at all wavelengths, and distance
    to star, via trigonometric parallax, expanding
    cluster parallax.
  • Temperature, from spectrum (more subtle than it
    sounds).
  • Radius of isolated stars stellar interferometry
    (e.g. Hanbury-Browns intensity interferometer,
    or CHARA).
  • Mass measure speeds, sizes and orientations of
    orbits in multiple star systems, most helpfully
    in binary star systems.
  • Observations of certain binary star systems can
    also help in the determination of radius and
    temperature.
  • So binary stars play a special role in stellar
    astrophysics.

3
Luminosity-temperature relation for binary stars
(mostly eclipsing) with well-determined orbits
Compiled by Oleg Malkov (1993), based mostly on
work over many decades by Dan Popper.
4
Binaries from which one gets useful mass
measurements
  • Most normal stars turn out to be members of
    binary systems.
  • Resolved visual binaries see stars separately,
    measure orbital axes and speeds directly. There
    arent too many of these.
  • Astrometric binaries only brighter member seen,
    with periodic wobble in the track of its proper
    motion.
  • Spectroscopic binaries unresolved (relatively
    close) binaries told apart by periodically
    oscillating Doppler shifts in spectral lines.
    Periods days to years.
  • Spectrum binaries orbital periods longer than
    period of known observations.
  • Eclipsing binaries orbits seen nearly edge on,
    so that the stars actually eclipse one another.
    (Most useful.)

5
Stellar masses determined for binary systems
  • If orbital major axes (relative to center of
    mass) or radial velocity amplitudes are known, so
    is the ratio of masses
  • If the period, P, and the sum of major axis
    lengths, are known, Keplers
    third law can give masses separately

6
Stellar masses determined for binary systems
  • If only radial velocities are known, the sum of
    masses (from Keplers third law) is
  • If orientation angle of orbit, i, is known, this
    allows separate determination of the masses
    thats why eclipsing binaries are so important
    (sin i must be close to unity in such cases).

7
Other uses for totally-eclipsing binary systems
  • Duration of eclipses and shape of light curve can
    be used to determine radii of starsRelativ
    e depth of primary (deepest) and secondary
    brightness minima of eclipses can be used to
    determine the ratio of effective temperatures of
    the stars

Flux
Time
8
Radius-mass relation for binary stars with
well-determined orbits
Compiled by Malkov (1993), based mostly on work
over many decades by Popper.
9
Luminosity-mass relation for binary stars with
well-determined orbits
Data from Popper (1980 ARAA 18, 115)
10
Temperature-mass relation for binary stars with
well-determined orbits
Compiled by Malkov (1993), based mostly on work
over many decades by Popper.
11
Why do we think these results apply to stars in
general? Well
?
  • Stars within 25 parsecs of the Sun (Gliese and
    Jahreiss 1991)
  • Nearest and Brightest stars (Allen 1973)
  • Pleiades X-ray sources (Stauffer et al. 1994)
  • Binary components with measured temperature and
    luminosity (Malkov 1993)

12
Stars within 30 pc (100 light years)
13
Astrometric Binary
14
Spectroscopic Binary

15
Geometric Projection of Binary Orbit
16
Radial Velocity vs. Time for Double-lined SB in
Circular Orbit
17
Radial Velocity vs. Time for Double-lined SB in
Elliptical orbit (e0.4)
18
Mass-Luminosity Relation
19
Eclipsing Binaries
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