Introduction to Binary StarsIntroduction to Binary Stars

Steve B. Howell

NOAO

Binary Stars ??Binary Stars ??

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Binary Stars ??Binary Stars ??

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Binary Stars - Binary Stars -

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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

Binary Stars - Binary Stars -

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 - TypesBinary Stars - Types

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 - TypesBinary Stars - Types

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An astrometric and visual binary: Sirius A&B -- an A star (A) and a white dwarf (B)

Has a ~50 year orbit. 8.6 light years from Earth

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Chandra Image

Binary Stars - TypesBinary Stars - Types

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 - TypesBinary Stars - Types

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Schematic and real eclipsing binary light curves

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Binary Stars - TypesBinary Stars - Types

Spectral variations over time due to the binary nature of this star

Binary Stars - TypesBinary Stars - Types

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 (Exo-Planet), temperature

– Other

Binary Stars - Types Binary Stars - Types

Binaries can be close (short orbital period) and show light variations due to …

Ellipsoidal shape and/or tidal interaction

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 - TypesBinary Stars - Types

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?Binary Stars - What can we learn?

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?Binary Stars - What can we learn? Eclipsing (or not) Radial velocity curve

can give masses (or mass estimate)

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RV amplitudes give K1 & K2

Binary Stars - What can we learn?Binary Stars - What can we learn? 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

Kepler’s Law

Mass vs. radius and luminosityMass vs. radius and luminosity

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The Oddest Eclipsing Binary?The Oddest Eclipsing Binary?

1987 Epsilon Aurigae “model” from web site -->

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The Oddest Eclipsing Binary Life??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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