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Interacting binary stars

  • Dirk Goës
  • Apr 2
  • 7 min read

Stars that pull each other apart, merge, and explode

A thermonuclear explosion on the surface of a white dwarf in a close binary system.
A still from an animation simulating a thermonuclear explosion occurring on the surface of a white dwarf star (right) while still pulling in gas from its companion red giant star (left) in binary star system T Corona Borealis (Credit: NASA's Goddard Space Flight Center Conceptual Image Lab).    

By Dirk Goës


A binary star system is one where two stars are gravitationally bound together and orbit each other in either circular or elliptical orbits. At least 50% of stars visible in the night sky and at least 50% of all stars are part of a multiple star system consisting of two or more stars orbiting each other.  Binary stars, also known as double stars, are a favourite viewing target for many amateur astronomers.  If you are interested in observing double stars through a telescope a good place to start is here. 

Figure 1: The basic layout of a binary star system. The two stars are in orbit around a common point called the centre of mass.  The size of the stars and the size and shape of the orbits will vary with each system (Credit: Swinburne University of Technology). 
Figure 1: The basic layout of a binary star system. The two stars are in orbit around a common point called the centre of mass.  The size of the stars and the size and shape of the orbits will vary with each system (Credit: Swinburne University of Technology). 

Some binary systems are simply two stars orbiting each other quietly. In many cases, however, the two stars interact.  The most common example of this is by transferring mass from one star to the other.  This transfer of mass can, for example, result in periodic thermonuclear explosions or in a supernova explosion which destroys one or both stars.  These types of systems are called interacting binaries and occur when the stars are orbiting each other closely and when one or both stars are in an advanced state of evolution. 


The transfer of mass can occur when one of the stars in the binary system expands to overfill its gravitational domain.  The gravitational domain of a star is called its Roche-Lobe.  Any gas outside the Roche-Lobe of the star may flow to its companion under the right conditions. 

Figure 2: A conceptual diagram showing how matter in the form of gas can flow from one star to another in a binary star system (Credit: Einstein Online and G. Nelemans). 
Figure 2: A conceptual diagram showing how matter in the form of gas can flow from one star to another in a binary star system (Credit: Einstein Online and G. Nelemans). 

This article describes four examples of binary systems.  One is the brightest star system in the sky, a non-interacting binary.  The other three are interacting binary systems including a recurrent nova, an X-ray binary and a supernova. 


Sirius

Figure 3: A Hubble Space Telescope image of the Sirius binary star system.  Sirius A is the bright star that dominates the image.  Sirius B can be seen in the bottom left third of the image.  The cross like pattern are diffraction spikes, a side effect of the telescopes imaging system (Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)). 
Figure 3: A Hubble Space Telescope image of the Sirius binary star system.  Sirius A is the bright star that dominates the image.  Sirius B can be seen in the bottom left third of the image.  The cross like pattern are diffraction spikes, a side effect of the telescopes imaging system (Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)). 

Sirius is the brightest star in the night sky and together with its companion Sirius B is an example of a non-interacting binary system.  The two stars orbit each other over a period of about 50 years and on average are separated by 20 times the distance between the Earth and the Sun.  The two stars quietly orbit each other without exchanging any material. 


Sirius is a Sun-like star that is about twice as big as the Sun.  Sirius B is a white dwarf star with a similar radius to the Earth but the mass of the Sun.  A white dwarf is the left-over remanent of a star that has exhausted its supply of nuclear fuel and shed its outer layers.  It is the final stage in the evolution of a Sun-like star. 

Figure 4: A Schematic diagram of binary star system Sirius.  The main-sequence star (MS) is 2 times the mass of the Sun and 1.7 times its radius.  The white dwarf (WD) is the mass of the Sun but only a small percentage of its radius (8%) showing that it is a very dense object.  On average the two stars are 20 astronomical units (AU) apart but the distance ranges from 8 AU to 32 AU due to their elliptical orbit.  The two components orbit each other every 50 years.    Not to scale (Diagram: Dirk Goës and data from Wikipedia). 
Figure 4: A Schematic diagram of binary star system Sirius.  The main-sequence star (MS) is 2 times the mass of the Sun and 1.7 times its radius.  The white dwarf (WD) is the mass of the Sun but only a small percentage of its radius (8%) showing that it is a very dense object.  On average the two stars are 20 astronomical units (AU) apart but the distance ranges from 8 AU to 32 AU due to their elliptical orbit.  The two components orbit each other every 50 years.    Not to scale (Diagram: Dirk Goës and data from Wikipedia). 

T Coronae Borealis

Figure 5: Two stills from an animation simulating the recurrent nova T Coronae Borealis.  On the left (A) the white dwarf star is pulling material, mainly hydrogen, from its red giant star companion.  Once a critical threshold is reached the accumulated material on the white dwarf explodes as shown on the right (B) (Credit: NASA's Goddard Space Flight Center Conceptual Image Lab).
Figure 5: Two stills from an animation simulating the recurrent nova T Coronae Borealis.  On the left (A) the white dwarf star is pulling material, mainly hydrogen, from its red giant star companion.  Once a critical threshold is reached the accumulated material on the white dwarf explodes as shown on the right (B) (Credit: NASA's Goddard Space Flight Center Conceptual Image Lab).

T Coronae Borealis (T CrB), also known as the Blaze Star, is an interacting binary system where the mass transfer from the red giant star to the white dwarf star results in a thermonuclear explosion on the surface of the white dwarf approximately every 80 years.  The most recent explosion was observed in 1946. 


Based on an observed pre-eruption dip in brightness the next explosion was predicted to occur in 2024 but did not take place.  It is now predicted to possibly occur in 2025 or as late as 2027.  In its quiescent state T CrB is not visible to the naked eye however when the explosion occurs it will be approximately as bright as Alphecca the brightest star in the constellation of Corona Borealis as shown in figure 6. 

Figure 6: The constellation of Corona Borealis as viewed from the southern hemisphere with the location of T Coronae Borealis marked with a dashed circle.  From Sydney Australia Corona Borealis can been seen in the evening sky from June to August (Credit: Stellarium). 
Figure 6: The constellation of Corona Borealis as viewed from the southern hemisphere with the location of T Coronae Borealis marked with a dashed circle.  From Sydney Australia Corona Borealis can been seen in the evening sky from June to August (Credit: Stellarium). 

T CrB is a type of interacting binary system also known as a cataclysmic variable or a recurrent nova.  They are characterised by the rapid accretion of hydrogen rich gas onto the surface of a white dwarf pulled in from the binary companion which is either a main sequence star or a red giant.  Once a critical amount of material has been accumulated on the surface of a white dwarf a thermonuclear explosion occurs.  This results in a tremendous increase in brightness that will slowly fade away over several months.  The accumulation of material will continue, and the cycle will repeat. 

Figure 7: A Schematic diagram of binary star system T Coronae Borealis.  The red giant star (RG) is 1.1 times the mass of the Sun but 64 times its radius.  The white dwarf (WD) is 1.37 times the mass of the Sun.  The two stars are 0.54 astronomical units (AU) apart.  The two components orbit each other every 228 days.  Not to scale (Diagram: Dirk Goës.  Data from Linford et al. (2019) and Wikipedia).
Figure 7: A Schematic diagram of binary star system T Coronae Borealis.  The red giant star (RG) is 1.1 times the mass of the Sun but 64 times its radius.  The white dwarf (WD) is 1.37 times the mass of the Sun.  The two stars are 0.54 astronomical units (AU) apart.  The two components orbit each other every 228 days.  Not to scale (Diagram: Dirk Goës.  Data from Linford et al. (2019) and Wikipedia).

Cygnus X-1

Figure 8: An artists impression of Cygnus X-1, an X-ray binary star system consisting of a blackhole pulling material of its companion, a blue supergiant star (Credit: ESA/Hubble). 
Figure 8: An artists impression of Cygnus X-1, an X-ray binary star system consisting of a blackhole pulling material of its companion, a blue supergiant star (Credit: ESA/Hubble). 

Cygnus X-1 is an example of a type of interacting binary system called an X-ray binary.  It consists of a black hole sucking material off its companion star, a blue supergiant.  The material drawn from the blue supergiant is heated to millions of degrees as it forms an accretion disk around the blackhole and emits X-rays in the process. 


Cygnus X-1 was the first blackhole ever detected and was co-discovered by Australian astronomer Louise Webster in 1972.  While the blackhole itself is not visible its presence was determined and calculated from the X-ray emissions. 


The blue supergiant star and the blackhole orbit each other every 5.6 days and the distance between them is less than the distance between the Sun and Mercury.  The blackhole is 21 times the mass of the Sun.  The blue super giant is 41 times the mass of the Sun and 22 times the size of the Sun. 

Figure 9: A schematic diagram of binary system Cygnus X-1.  The Blackhole (BH) is 21 times the mass of the Sun.  The Blue Supergiant (BSG) is 41 times the mass of the Sun and its radius 22 times the radius of the Sun.  The two components are 0.2 astronomical units (AU) apart.  For comparison Mercury is 0.39 AU from the Sun.  The two components orbit each other every 5.6 days.  Not to scale (Diagram: Dirk Goës and data from Miller-Jones et al. (2021)). 
Figure 9: A schematic diagram of binary system Cygnus X-1.  The Blackhole (BH) is 21 times the mass of the Sun.  The Blue Supergiant (BSG) is 41 times the mass of the Sun and its radius 22 times the radius of the Sun.  The two components are 0.2 astronomical units (AU) apart.  For comparison Mercury is 0.39 AU from the Sun.  The two components orbit each other every 5.6 days.  Not to scale (Diagram: Dirk Goës and data from Miller-Jones et al. (2021)). 

Supernova 2014J

Figure 10: A Hubble Space Telescope image of Supernova 2014J (SN 2014J) and its location in Galaxy M82, which is in the constellation Ursa Major (Credit: NASA and ESA). 
Figure 10: A Hubble Space Telescope image of Supernova 2014J (SN 2014J) and its location in Galaxy M82, which is in the constellation Ursa Major (Credit: NASA and ESA). 

In 2014 astronomers observed a supernova explosion in Galaxy M82 and named it SN 2014J.  At approximately 11.5 million light years away, it was the closest supernova of its type recorded since 1972.  Specifically, it was a Type Ia Supernova which involves the explosion of a white dwarf in an interacting binary system.  Observations of Type Ia Supernovae were famously used to determine that the universe is expanding at an accelerating rate. 


A Type Ia Supernova is different to a core collapse supernova which is caused by a massive star exploding and caving in to become a neutron star or black hole.  The most recent nearby core collapse supernova was observed in 1987 and named SN 1987A.  It occurred in the Large Mallengenic Cloud at approximately 168,000 light years away.   


A Type Ia Supernova is thought to occur from a couple of different binary star configurations.  One is like T Coronae Borealis, described above, where a white dwarf is pulling material from a red giant.  However, in this case the material being accumulated on the surface of the white dwarf causes its mass to exceed the Chandrasekar Limit of 1.4 times the mass of the Sun.  This results in a thermonuclear explosion and the complete destruction of the white dwarf. 


The second possible configuration is when the binary system consists of two white dwarfs orbiting each other.  In this scenario the two white dwarfs may spiral into each other causing them to merge.  The merger results in a Type Ia Supernova explosion. 


A study by Margutti et al. (2014) found that a lack of X-Ray emissions from this explosion indicate that the second scenario, a white dwarf merger, is a more likely cause of SN 2014J.  This is because in the first scenario the shock wave from the explosion would be expected to heat up the surrounding material (pulled from the companion star) to a temperature that produces X-Rays. 

Figure 11: A schematic diagram of the possible binary system (the progenitor) that resulted in the Type Ia Supernova named SN 2014J.  The system consists of two White Dwarfs (WD) in a close binary orbit.  Not to scale (Diagram: Dirk Goës and data from Margutti et al. (2014)). 
Figure 11: A schematic diagram of the possible binary system (the progenitor) that resulted in the Type Ia Supernova named SN 2014J.  The system consists of two White Dwarfs (WD) in a close binary orbit.  Not to scale (Diagram: Dirk Goës and data from Margutti et al. (2014)). 

Many more binaries

This article has described a small selection of binary and interacting binary star systems.  However, there are many more and many variations on those described above.  For example, you may like to investigate black hole binaries or black hole-neutron star binaries and how the merger of these objects generate gravitational waves. 


If you are interested in observing binary systems, you may like to train your telescope onto T Corona Borealis and see if you can one of the first to spot the nova outburst.  Or you could attempt to observe U Geminorum, an interacting binary that has an outburst approximately every 105 days.  Or you could attempt to spot Sirius B in the glare of Sirius. 














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