| Cataclysmic Variable |
The following description includes additional technical comments in brackets, the non-physicists amongst you
can skip the comments in brackets.
The majority of stars are not solitary like the Sun but occur in close groups in which the stars orbit about one
another, bound together by gravity. Most of these stars occur in pairs as binary stars. Binary stars may exhibit
all sorts of phenomena that are unknown in single star systems. One of the most dramatic is stellar accretion, in
which the recipient stars either pulls material from its neighbouring star and adds it to its own mass, or the
donor star throws out some of its material which is then passively scooped up by the companion.
The picture above shows a type of accreting binary system, known as a cataclysmic variable. In a cataclysmic
variable, the two stars probably start out quite far apart, orbiting one another once every few months or years,
but one of the stars eventually exhausts its own nuclear fuel. Having run out of nuclear fuel, the core of the star
condenses into a very hot and very dense remnant, only about the size of the Earth, called a white dwarf. As
the star stops burning fuel and 'dies' leaving the white dwarf remnant, much of its material is thrown out into
space, forming a planetary nebula (so-called because in a telescope one sees a ring of gas around the white
dwarf, rather like a ring of planetary material). The material that is thrown out into space forms a common
envelope around both stars and imposes frictional drag on the system, causing it to lose orbital energy and so
the stars move closer together, forming a close binary. The surrounding nebula is eventually ejected from the
system (though some of it may fall onto the companion star, increasing its mass).
In a close binary system, the stars really start to feel the tug of gravity from the companion star (remember that
stars are very massive and so have strong gravitational fields). This is especially true when one of the stars is a
white dwarf, since white dwarfs are so dense with a lot of mass concentrated in a small volume, that they
generate a very strong gravitational field. Also if the donor star is old, perhaps a red giant, it has swollen to ten
times its previous size and the outer layers of material are only loosely bound to it. (The donor star is filling its
Roche lobe - the maximum extent it can have before material at its surface is more strongly attracted to the
white dwarf than it is to the donor star). This arrangement favours mass transfer - the white dwarf pulls a stream
of material away from its companion star onto itself. The white dwarf is essentially a parasitic star, feeding off its
companion.
In the case of a cataclysmic variable, the material does not fall directly onto the white dwarf (it has too much
angular momentum, that is it is rotating too fast) but instead orbits the white dwarf and spreads out into a thin
disc of material that slowly falls inwards (as it slowly loses angular momentum) onto the white dwarf. Such a disc
is called an accretion disc as the white dwarf accretes material from it.
The accretion disc is actually a huge engine for converting gravitational potential energy into radiation, and for
transporting angular momentum (the bulk of the material spirals onto the white dwarf, losing angular
momentum, but some of the material spirals outwards as it gains angular momentum - effectively transporting
angular momentum away from the material that becomes accreted).
Essentially what is happening is that as material spirals into the white dwarf, it is losing gravitational energy, just
as a falling apple does, and this energy is converted into heat, so the disc becomes very hot and so glows
brightly (it is a disc of plasma).
A typical accretion disc of this type is about the same diameter as the Sun and the rotating material orbits the
central white dwarf at speeds of about 1000 km per second, but moves inwards at only about 300 metres per
second. The total mass of the disc is only about one ten thousand millionth of a solar mass (10 x E-10 SM).
Notice that where the stream of material from the donor star impacts on the disc, there is a hot spot - a local
region which is hotter and brighter than the rest of the disc.
Why cataclysmic variable? Though the disc shown in the picture above is in a so-called steady state, however
accretion discs are also prone to transient instabilities during which there is a dramatic outburst of radiation as
the rate of accretion suddenly speeds up for a while, possibly increasing the brightness of the disc 100-fold in
one day. Thus the brightness of the star is seen to vary.
How does a Cataclysmic Variable develop?
If, in a binary system, the two stars are close together in their orbits, then as the older companion expands to a
red giant stage (or even a supergiant stage), it may push its tenuous outer envelope far from its own centre of
gravity and closer to the centre of gravity of its younger companion. The red giant is said to overflow its Roche
lobe. (The Roche lobe is the boundary which is the furthest distance material can be from one star before
being more strongly attracted to the other star in a binary system). When this happens, the material, which is
now loosely bound to the red giant, will be attracted toward the younger companion star, which pulls the
material from the red giant with its gravitational field. Material starts to stream from the bloated red giant onto
the smaller companion - a process known as mass transfer.
As material streams from the red giant onto its smaller companion, conservation of angular momentum causes
the stars to spiral in toward one another and the distance between them decreases. This makes matters worse,
because as the distance between the two stars decreases, the red giant's Roche lobe decreases in radius and
the red giant overflows its Roche lobe more and more, dumping more and more material onto its companion
star. Within a few years, the red giant will have dumped almost its entire envelope onto its companion, but the
companion star cannot possibly add this much material to its own atmosphere this quickly and the excess
material forms a greatly distended envelope that encloses both stars.
This gaseous envelope imposes drag on the two stars as they move in their orbits around one another. This
braking effect causes the stars to move even closer together. Like a propeller the two stars spin faster and
faster (like an ice-skater pulling in their arms) and spin off the excess material which expands out as a particular
type of planetary nebula.
When all this excess material has been spun off, the old red giant is now reduced to its stellar core remnant,
which will be white dwarf. Nuclear burning will cease as the star has lost its fuel. However, the white dwarf may
gain a new lease of life. Since it is compact and very dense, it has a very strong gravitational field, whilst its
companion is now somewhat overweight and distended with the extra material it has acquired. Furthermore, the
two stars are now very close together. Now the young companion overflows its Roche lobe and material starts
to stream back toward the white dwarf. This is generally a more leisurely and controlled affair than when the red
giant rapidly dumped its envelope, and the jet of material starts to orbit the white dwarf and eventually it forms
an accretion disc. As material slowly spirals onto the white dwarf, the material heats up more and more as it
accumulates on the surface of the white dwarf. Every 10 000 to 100 000 years or so, enough material
accumulates to generate sufficient heat and pressure for nuclear fusion and the material suddenly ignites as it
burns, blasting much of it into space. This causes the star system to brighten by as much as 100 000 times
over a period as short as a few days. Such a star is called a nova, and is undergoing a nova outburst.
These nova outbursts grant the white dwarf a new lease of life, as it feeds like a parasite upon its companion
star. Eventually, however, the white dwarf may put on more weight than it can handle, as it adds the material to
itself - if the mass of the white dwarf exceeds the Chandrasekhar limit (about 1.4 solar masses) then it is in big
trouble! It can no longer support itself and it suddenly collapses in a tremendous supernova explosion (a type I
supernova). Although the white dwarf may possibly collapse to a neutron star or black hole, the most likely
outcome appears to be its total destruction as it detonates in a tremendous fireball!
can skip the comments in brackets.
The majority of stars are not solitary like the Sun but occur in close groups in which the stars orbit about one
another, bound together by gravity. Most of these stars occur in pairs as binary stars. Binary stars may exhibit
all sorts of phenomena that are unknown in single star systems. One of the most dramatic is stellar accretion, in
which the recipient stars either pulls material from its neighbouring star and adds it to its own mass, or the
donor star throws out some of its material which is then passively scooped up by the companion.
The picture above shows a type of accreting binary system, known as a cataclysmic variable. In a cataclysmic
variable, the two stars probably start out quite far apart, orbiting one another once every few months or years,
but one of the stars eventually exhausts its own nuclear fuel. Having run out of nuclear fuel, the core of the star
condenses into a very hot and very dense remnant, only about the size of the Earth, called a white dwarf. As
the star stops burning fuel and 'dies' leaving the white dwarf remnant, much of its material is thrown out into
space, forming a planetary nebula (so-called because in a telescope one sees a ring of gas around the white
dwarf, rather like a ring of planetary material). The material that is thrown out into space forms a common
envelope around both stars and imposes frictional drag on the system, causing it to lose orbital energy and so
the stars move closer together, forming a close binary. The surrounding nebula is eventually ejected from the
system (though some of it may fall onto the companion star, increasing its mass).
In a close binary system, the stars really start to feel the tug of gravity from the companion star (remember that
stars are very massive and so have strong gravitational fields). This is especially true when one of the stars is a
white dwarf, since white dwarfs are so dense with a lot of mass concentrated in a small volume, that they
generate a very strong gravitational field. Also if the donor star is old, perhaps a red giant, it has swollen to ten
times its previous size and the outer layers of material are only loosely bound to it. (The donor star is filling its
Roche lobe - the maximum extent it can have before material at its surface is more strongly attracted to the
white dwarf than it is to the donor star). This arrangement favours mass transfer - the white dwarf pulls a stream
of material away from its companion star onto itself. The white dwarf is essentially a parasitic star, feeding off its
companion.
In the case of a cataclysmic variable, the material does not fall directly onto the white dwarf (it has too much
angular momentum, that is it is rotating too fast) but instead orbits the white dwarf and spreads out into a thin
disc of material that slowly falls inwards (as it slowly loses angular momentum) onto the white dwarf. Such a disc
is called an accretion disc as the white dwarf accretes material from it.
The accretion disc is actually a huge engine for converting gravitational potential energy into radiation, and for
transporting angular momentum (the bulk of the material spirals onto the white dwarf, losing angular
momentum, but some of the material spirals outwards as it gains angular momentum - effectively transporting
angular momentum away from the material that becomes accreted).
Essentially what is happening is that as material spirals into the white dwarf, it is losing gravitational energy, just
as a falling apple does, and this energy is converted into heat, so the disc becomes very hot and so glows
brightly (it is a disc of plasma).
A typical accretion disc of this type is about the same diameter as the Sun and the rotating material orbits the
central white dwarf at speeds of about 1000 km per second, but moves inwards at only about 300 metres per
second. The total mass of the disc is only about one ten thousand millionth of a solar mass (10 x E-10 SM).
Notice that where the stream of material from the donor star impacts on the disc, there is a hot spot - a local
region which is hotter and brighter than the rest of the disc.
Why cataclysmic variable? Though the disc shown in the picture above is in a so-called steady state, however
accretion discs are also prone to transient instabilities during which there is a dramatic outburst of radiation as
the rate of accretion suddenly speeds up for a while, possibly increasing the brightness of the disc 100-fold in
one day. Thus the brightness of the star is seen to vary.
How does a Cataclysmic Variable develop?
If, in a binary system, the two stars are close together in their orbits, then as the older companion expands to a
red giant stage (or even a supergiant stage), it may push its tenuous outer envelope far from its own centre of
gravity and closer to the centre of gravity of its younger companion. The red giant is said to overflow its Roche
lobe. (The Roche lobe is the boundary which is the furthest distance material can be from one star before
being more strongly attracted to the other star in a binary system). When this happens, the material, which is
now loosely bound to the red giant, will be attracted toward the younger companion star, which pulls the
material from the red giant with its gravitational field. Material starts to stream from the bloated red giant onto
the smaller companion - a process known as mass transfer.
As material streams from the red giant onto its smaller companion, conservation of angular momentum causes
the stars to spiral in toward one another and the distance between them decreases. This makes matters worse,
because as the distance between the two stars decreases, the red giant's Roche lobe decreases in radius and
the red giant overflows its Roche lobe more and more, dumping more and more material onto its companion
star. Within a few years, the red giant will have dumped almost its entire envelope onto its companion, but the
companion star cannot possibly add this much material to its own atmosphere this quickly and the excess
material forms a greatly distended envelope that encloses both stars.
This gaseous envelope imposes drag on the two stars as they move in their orbits around one another. This
braking effect causes the stars to move even closer together. Like a propeller the two stars spin faster and
faster (like an ice-skater pulling in their arms) and spin off the excess material which expands out as a particular
type of planetary nebula.
When all this excess material has been spun off, the old red giant is now reduced to its stellar core remnant,
which will be white dwarf. Nuclear burning will cease as the star has lost its fuel. However, the white dwarf may
gain a new lease of life. Since it is compact and very dense, it has a very strong gravitational field, whilst its
companion is now somewhat overweight and distended with the extra material it has acquired. Furthermore, the
two stars are now very close together. Now the young companion overflows its Roche lobe and material starts
to stream back toward the white dwarf. This is generally a more leisurely and controlled affair than when the red
giant rapidly dumped its envelope, and the jet of material starts to orbit the white dwarf and eventually it forms
an accretion disc. As material slowly spirals onto the white dwarf, the material heats up more and more as it
accumulates on the surface of the white dwarf. Every 10 000 to 100 000 years or so, enough material
accumulates to generate sufficient heat and pressure for nuclear fusion and the material suddenly ignites as it
burns, blasting much of it into space. This causes the star system to brighten by as much as 100 000 times
over a period as short as a few days. Such a star is called a nova, and is undergoing a nova outburst.
These nova outbursts grant the white dwarf a new lease of life, as it feeds like a parasite upon its companion
star. Eventually, however, the white dwarf may put on more weight than it can handle, as it adds the material to
itself - if the mass of the white dwarf exceeds the Chandrasekhar limit (about 1.4 solar masses) then it is in big
trouble! It can no longer support itself and it suddenly collapses in a tremendous supernova explosion (a type I
supernova). Although the white dwarf may possibly collapse to a neutron star or black hole, the most likely
outcome appears to be its total destruction as it detonates in a tremendous fireball!