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orbit to the stars, and vice versa. If orbital
angular momentum is transferred to the star,
the orbit shrinks, while the stars spin faster.
If their separation shortens sufficiently, mass
transfers from the largest of the two stars onto
its companion.
Alternatively, mass transfer can be caused by the
natural process of stellar evolution. Stars expand
as they age and most become a red giant, many
hundreds of times the size of the Sun. If a star has
a close companion, as in many binary systems, this
expansion causes mass from one star to spill onto
the other. This transfer of gas makes the mass-
gaining star spin faster and contributes to a range
of diverse stellar phenomena. If the rate of mass
gain is fast enough, the companion star is unable to
accommodate the newly acquired mass and it also
expands and eventually fills its own Roche lobe. The
two stars enter a common envelope in which they
orbit inside a shared gaseous cocoon. Sometimes,
the stars merge; other times the gaseous envelope
can be ejected to form a nebula around what is left
of the two stars in the binary.
Oddballs and observing
Is it possible to see objects that are exchanging
mass? Put simply, yes and no. Modern astronomy,
with its ever more powerful telescopes that
can monitor large swaths of sky for anything
that changes in brightness, can detect binary
interactions directly. However, these interactions
are fleeting and so hence are rarely observed.
We do, however, often see the stars which are
products of binary interaction because these can
survive sometimes for billions of years.
One example is the peculiar set of stars called
blue stragglers. These stars were first identified in
globular clusters where all stars have the same age
and, correspondingly, a maximum possible mass.
More massive stars in the cluster should no longer
have a nuclear-burning core, and should instead
be dim white dwarfs.
Yet, many clusters have been discovered to
house stars which are too massive to still be
alive according to current stellar evolutionary
theories - fortunately, the binary evolution
channel naturally explains their continued
existence. At some point in their past, these
stars must have acquired some mass, possibly by
merging with another star, to give them a new
lease on life.
Other notable oddballs are those that are known
as chemically-peculiar stars. These stars are at
a stage in their life when they are not expected
to have certain chemical elements, such as
carbon and barium, at their surfaces. The only
explanation, again, is that they have ‘inherited’
the peculiar chemical gas from a companion star
that made these chemical elements in the past.
Today that companion is likely too faint to be
observed directly, but lurks undiscovered near our
chemically peculiar star.
Mass gain is not the only peculiar binary
phenomenon. The star that has its mass removed
is often stripped of its surface layers. In very
massive stars, this process makes unusual objects
called Wolf-Rayet stars. These are often rich in
helium, carbon and oxygen but because of a binary
interaction. The outer layers have been stripped
from the star and these gases are now detected
at the surface, when ordinarily they shouldn’t be.
When these type of stars run out of nuclear fuel,
they explode as the peculiar, hydrogen-free type
Ib and Ic supernovae, and may cause long gamma-
ray bursts.
It is not just massive stars that are prone to
alteration; binary interaction can also affect
stars with masses more like our Sun. In single-
star evolution, smaller-mass stars ignite helium
deep within their hydrogen-rich envelope. If this
envelope is removed by binary interactions just
before helium ignites, a new type of star called
a subdwarf-B star forms. What we see is the
exposed, nuclear-burning core of a red giant, and
we know this can only be made in binary stars.
Transients
Transient sources are bright objects in the sky
which appear, then disappear or dim, quickly
on astronomical timescales but fast enough to
be seen by astronomers. They may last from
seconds to years and they can often be seen at
all wavelengths, from radio to gamma ray. Many
transients are associated with binary stars and
mass transfer in binaries is the cause in many of
these cases. Perhaps the best-known are novae,
which is Latin for ‘new stars’.
In classical novae, material piles up on a white
dwarf until the increasing pressure triggers a
nuclear explosion on its surface. If the mass
of the white dwarf increases, it may reach the
Chandrasekhar mass and at this point the whole
star explodes as a type Ia supernova, enriching
the interstellar medium with iron in the process.
Notable
oddballs are
those that
are known as
chemically-
peculiar stars
If this envelope is removed by binary interactions
just before helium ignites, a new type of star
called a subdwarf-B forms
