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A merger between two white dwarfs may also
trigger the same type of supernovae. Because of
their homogeneity, these explosions can be used
as ‘standard candles’ such that we can calculate
their distance accurately and use them to
deduce that the expansion of the Universe
is accelerating.
Transients are also associated with the mergers
of stars, so not just when mass is transferred but
when the two objects combine to leave behind
one massive star. As this happens, the energy
that was in the orbit of the two stars goes into
the newly formed single object, causing it to
expand and brighten dramatically. The resultant
transient event is known as a red nova and an
example of this was observed in 2008 when the
stars in the binary system V1309 Scorpii spiralled
in and crashed into each other. The star is now a
single entity, surrounded by a dust cloud that was
ejected during the merger.
Similarly, when two neutron-stars merge, not
only does the process produce the heavy elements
found in our Galaxy, such as gold, platinum and
uranium, but the amalgamation produces radiation
that we observe in the optical and infrared as a
bright kilonova transient and a short gamma-ray
burst at higher energy.
Gravitational radiation has recently been
detected from merging stellar-mass black holes,
validating Einstein’s theory of General Relativity,
although the origin of such systems remains a
matter of some debate.
A binary future
Binary stars are as vital to astrophysics today
as they were in the 17th century to Kepler and
Newton. They can be used to measure the
expansion rate of the Universe and can help us
to understand the formation of the chemical
elements when the Universe was young.
Astronomers have even found planets around
binary stars, providing yet more intrigue as to how
planets form and survive. To help us solve these
challenging questions we have an exponentially
increasing volume of astronomical data from
surveys like ESA’s Gaia.
The challenge now is to understand stars of all
kinds - single, binary and multiple - to establish
how they were formed, how they evolved and
what brings about their end. Detailed modelling
of binary stars is a major challenge which will
require the best of astronomy, astrophysics,
applied mathematics and computer science on
many levels.
Coupled with state of the art observational
surveys, our models of binary stars will explain
how physics works in the most extreme
environments we can imagine. Binary stars are key
to pushing the boundaries of our knowledge and
will remain so for a long time to come.
For a more detailed review of binary stars, the authors have written the
2016 Dawes Review paper ‘The impact of companions on stellar evolution’,
available at
and
About the authors
Dr Robert Izzard
is an STFC Rutherford fellow at the Institute of
Astronomy, University of Cambridge, where he works on understanding
binary stellar evolution. He was previously a professor at die Rheinische
Friedrich-Wilhelms-Universität Bonn, a Marie-Curie fellow at l’Université
libre de Bruxelles and an NWO fellow at de Universiteit Utrecht. In
September 2017 he will join the astrophysics group at the University of
Surrey.
Dr Orsola De Marco
holds a professorship at Macquarie University, in
Sydney, Australia, where she works on observations and simulations of
interacting binary stars. She completed her PhD at University College
London and held positions at the Swiss Polytechnic, Zurich, and the
American Museum of Natural History, New York. She was an Australian
Research Council Future Fellow.
Chandra X-ray Image of
Mira (left pic) showing
Mira A, a highly evolved
red giant star, and Mira B,
a white dwarf. Mira A is
losing gas rapidly from its
upper atmosphere via a
stellar wind. Mira B exerts
a gravitational tug that
creates a gaseous bridge
between the two stars.
Gas from the wind and
bridge accumulates in an
accretion disk around Mira
B and collisions between
rapidly moving particles in
the disk produce X-rays.
Pic (right): artist’s
impression of the binary
system.
Models of
binary stars
will explain
how physics
works in the
most extreme
environments
we can
imagine
