Evolution of Binary Stars &
Star Clusters
Binary Stars:
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If sufficiently close, stars in binary systems make affect each other’s
evolution. The two stars generally must be separated by no more than a few
Astronomical Units. Stars in binary systems with larger separations will evolve
independently of each other.
Close binaries can affect each other by exchanging mass during specific
evolutionary phases in each star’s life.
Each star in a binary is bounded by a region in which that star's gravity is
dominant. These regions are called Roche lobes, after the French scientist
who first identified them.
When a star evolves, it may expand and “overfill" its Roche lobe.
The more massive star will evolve first. As it expands into a red giant, and, if
the two stars are sufficiently close, the outer layers of the red giant will pass
beyond the star’s Roche lobe.
Mass from the outer layers of the expanding star can be captured by the
gravity of the second star. This matter will fall onto the second star, increasing
that star’s mass.
A wide variety of strange phenomena may occur as a consequence of mass
exchange between stars in binary systems:
 Novae – When the “receiving” star is a white dwarf, fresh hydrogen can
build up, leading to a fusion event on the surface. The star’s brightness
increases by a million fold, as the surface layer is blasted off the white
dwarf into space.
 Supernovae – If the receiving star is a white dwarf and collects too
much new gas, the entire white dwarf detonates, leading to a vast
explosion. White dwarfs cannot have a mass larger than 1.4 M
 Altered spectral type – A normal star may become a more massive
(bluer, hotter) star by collecting gas from its companion, shortening its
evolution as well.
 Coalescence – The binary combines into one star.
If stars in a binary system exchange mass, each star will evolve differently
than if it was isolated.
Star Clusters:
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All stars within a cluster form at the same time from the same interstellar gas
cloud. Therefore, all stars in a cluster have the same age and chemical
composition.
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When a cluster forms, the stars all lie along the main sequence.
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Individual stars subsequently evolve according to their initial mass. More
massive stars “leave” the main sequence first, when they consume all of the
hydrogen in their cores.
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Within the cluster HR diagram, stars evolve away from the main sequence,
through the subgiant branch, towards the red giant phase. The point on the
main sequence where stars are “leaving” is termed the main-sequence
turnoff.
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All stars above the turnoff have evolved away from the main sequence.
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By comparing the observed main-sequence turnoff with theoretical models,
astronomers can determine the age of the cluster, and by inference, the age
of all stars which are members of the cluster.