Introduction to Binary Stars
Steve B. Howell
NOAO
Binary Stars ??
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Binary Stars ??
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Binary Stars Binary stars are
pairs of stars
orbiting each other
“connected’ by
their mutual
gravitational
interaction.
50% of stars are
binaries:
~100% O/B
~75% M
Obs. biases
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Binary Stars 
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Binary Stars may have orbital periods from
seconds to hours to thousands of years
They may contain stars of all kinds, sizes,
and masses
They may have circular or elliptical orbits
They are believed to have formed together same age (not capture)
Close binaries affect each other’s evolution
Long period binaries essentially act as
single stars
Binary Stars - Types
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Binary stars come in a few types
differentiated by their orbital period and/or
observational behavior. For example:
– Visual double (actually see both
components - LONG period). Do not
confuse with optical double
– Astrometric binaries - see the wobble on
the sky
Binary Stars - Types
An astrometric and visual
binary: Sirius A&B -- an A
star (A) and a white dwarf
(B)
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Has a ~50 year orbit. 8.6
light years from Earth
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Binary Stars - Types
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or
– Eclipsing binaries - one/two eclipses per
orbit
– Spectrum binaries - spectral properties
change
– Spectroscopic binaries - radial velocity
motions (short periods easier to find)
Binary Stars - Types
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Schematic and real eclipsing
binary light curves
Binary Stars - Types
Spectral
variations
over time due
to the binary
nature of this
star
Binary Stars - Types
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Or differentiated by other criteria– Close binaries w/ stellar distortions or
mass transfer
– Intrinsic brightness changes due to stellar
variability
– Binaries with components of very different
properties: color (WD+RD), mass (ExoPlanet), temperature
– Other
Binary Stars - Types
Binaries can be close (short
orbital period) and show light
variations due to …
Ellipsoidal shape and/or tidal
interaction
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Interactions such as mass
exchange / transfer
If the smaller (but more
massive) star in an IB is a
white dwarf, the binary is
called a cataclysmic variable
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Binary Stars - Types
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All combinations of the previous properties
are possible
 Many are unknown as they simply have not
been seen to vary (line of sight / observed)
 Binary Stars are the fundamental way in
which we understand stars (and star
systems): their formation, evolution & death.
 Remember --Three out of every two stars are
in a binary system
Binary Stars - What can we learn?
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Eclipsing
binaries are
the best as
the light
curve can
give stellar
masses, radii,
temperatures,
& age
estimates
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Binary Stars - What can we learn?
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Eclipsing (or not) Radial velocity curve
can give masses (or mass estimate)
RV amplitudes give K1 & K2
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Binary Stars - What can we learn?
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For a circular orbit
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Measure semi-major axis, a, from
projected orbit & the distance.
Relative positions about the center
give: M1/M2 = a2/a1 = K2/K1
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Kepler’s Law
Mass vs. radius and luminosity
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The Oddest Eclipsing Binary?
1987 Epsilon Aurigae “model”
from web site -->
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The Oddest Eclipsing Binary Life??
Elemarkhorsaurus from
the Epsilon Aurigae
Binary Star System
Caption: Ashraf, do you
want to meet my new pet?
Look, he is behind you and
friendly. He loves the
climate and his new home
in the Eps Aur binary
system. (From flickr)
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