The binary system

However, these charged particles will begin to interact, with the magnetic fields, and funnel along the magnetic field lines, to the magnetic poles. As these particles accelerate to the poles they can be ejected into space, creating columns of stellar material, at the poles. [15] If the rotational axis is not aligned to that of its magnetic poles, as it rotates, the ejected material will seem to revolve around the white dwarf and so the luminosity of the system would begin to gain and drop, as parts of the ejected columns come into and out of view.

The ideas of material being ejected at the magnetic poles are not exclusive to Cataclysmic variables, where the white dwarfs have significant magnetic fields. This occurs throughout the Universe, with the most notable example being black holes. Black holes are remnants of large massive stars. Their gravity is so immense that even light cannot escape. As material begins to form a disk around the black hole large magnetic fields can begin to build up. This results in two massive jets of material being ejected, at the poles of the black hole. These massive jets accelerate the material, causing it to heat up greatly and emit light.

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They heat up so much that the light released can be as energetic as light from the x-ray region, of the electromagnetic spectrum. Events such as these can be easily found, when viewing the Universe in x-ray light, as they are often brighter than most other x-ray sources in the Universe. [16] This idea is also very similar to neutron stars can vary in luminosity. Cataclysmic variables can also contain neutron stars, rather than white dwarves, where the same processes will occur. Neutron stars also have various subtypes, which we are only just beginning to understand.

Neutron stars are an intermediate object, which lie between white dwarfs and black holes. At the end of its lifetime, if the star is of sufficient mass, rather than forming a white dwarf, as in lesser mass stars, the core of the star can no longer withstand its own weight. Consequently, the core begins to collapse even further, forcing the subatomic particles to merge into neutrons. This process releases so much energy that it generates a supernova. The result is a collapsed core of a star, comprised of only neutrons. As these have collapsed down, their actual size is very small.

Neutron stars are often found between the ranges of five to twenty kilometres. This decrease in radius has radical effects on the neutron star. As the core collapses, to conserve angular momentum, the neutron star must begin to rotate at a higher pace. This is similar to an ice skater; as they spin and bring their arms towards them their rotation increases in speed. However, these neutron stars have originated from stars many times larger than our sun, to a size of a small city. Consequently, some neutron stars can be rotating at a rate of 1000 times a second.

[17] These neutron stars can also be extremely hot, as they will have retained the heat, from the stellar core, which would have reached into the many millions of degrees. This heat is then radiated into space, as x-ray light. When these are first born they are incredibly hot and can easily be observed, when viewing in x-ray light. However, neutron stars are, in essence, a sphere of neutrons. Within them, they have no processes which can generate energy, as in a star. Their temperature cannot be sustained, with the constant radiating of light.

Over long periods of time they begin to cool down and subsequently radiate less energetic light. The luminosity of these neutron stars will decrease as they cool, until eventually they are the same temperature as deep space; a few degrees above absolute zero. However, this cooling down takes billions of years; a period such that the Universe is not yet old enough so that cool neutron stars exist and the existence of cool neutron stars would only be able to be inferred from their gravitational effects. Over their active lifetime, neutron stars can turn into Pulsars.

The reason why this occurs has many theories, although it is though that it is an evolutionary process, where only specific neutron stars can become Pulsars. [18] Pulsars are a subtype of neutron stars. Similar to Cataclysmic variables, particles can get caught within the Pulsar’s magnetic field and stream out of the poles, creating powerful beams of light. Similar to white dwarfs, if the rotational axis is not aligned with the magnetic poles the Pulsar acts like lighthouse where we can only view the Pulsar if the beam of light is pointed directly at us, as in figure 1.

As it rotates it seems as though it pulsates, hence the name. As stated previously, these can be rotating at rates up to 1000 time per second. The variations of luminosity in these systems are due to their orientation, relative to the observer. If the magnetic and rotational axes are aligned we can never view this pulsating effect and so can never deduce they are Pulsars; only when the orientation is correct can we observe the lighthouse effect.

These variations of the ‘off and on’ light, from Pulsars, are too quick for human eyes to detect, but they can be detected with sensitive instruments. The variable luminosity of Pulsars are much more systematic and structured and so can be used a reference points. As neutron stars take billions of years to cool, the rotation periods of Pulsars are exceptionally consistent, over long periods of time, and so therefore the pulses of light provide a visual means of consistent timing. This allows Pulsars to become incredibly accurate astronomical ‘clocks,’ which we can then utilise.

[20] By measuring the pulses of light we can begin to define a second by the number of light pulses, of a particular Pulsar, which we can use to calibrate other clocks and time keeping instruments, to a universal standard. Without these accurate clocks, our measurement of time would slowly become unsynchronised, causing great inaccuracies with orbiting satellites and in space exploration, for example, where synchronisations of times are vital. As mentioned previously, neutron stars have very strong magnetic fields and are considered to have the strongest fields, in our Universe.

A subtype of neutron stars, called Magnetars, have extremely strong magnetic fields and their fields are thought to be at the upper theoretical limit, which a neutron star’s magnetic field can be. For comparison, the magnetic field strength of the Earth is approximately 50 micro-Tesla, while a Magnetar can reach strengths of 1011 Tesla; that is ten followed by eleven zeroes. Their magnetic fields are so strong that at a distance of 1000 kilometres, due to the magnetic properties of water, it would begin to destroy human tissue and at the distance of the Moon, credit cards would stop functioning, due to their magnetic strip.

[21] Magnetars are still relatively new discoveries; astronomically speaking. As such their understanding is still relatively premature, although our understanding has increased since their discovery. Although there are no ‘nearby’ Magnetars, their structure and lifetimes can be inferred, due to various events that they produce. As their magnetic fields are so intense, they can become tangled and chaotic. Eventually, there comes a point that the field lines become so tangled that they break and rearrange themselves, into a simpler configuration.

However, this breaking and rearrangement of field lines releases a torrent of energy, that can dwarf the output of the world nuclear arsenal and even our Sun. [22] On the 27th December 2004, a catalogued astronomical object, previously unknown as a Magnetar, underwent a magnetic field line configuration. This particular object, catalogued SGR 1806-20, was 50000 light years away; approximately half the diameter of our Milky Way. However, despite its distance, the amount of energy released was so intense that they managed to cause a sizeable effect here, on Earth.

Various satellites and detectors were blinded and damaged, due to the amount of gamma rays. Our atmosphere was also affected and bloated up, as it was hit. At a distance on ten light years, the Ozone layer would have been destroyed. Fortunately, no known Magnetars exist within our cosmological neighbourhood. [23] Unlike Pulsars, the luminosity of a Magnetar is not consistent and systematic, but rather a single event, which increases its luminosity exponentially, brighter than any Pulsar or star, with a duration of less than a second.

These events are very rare and furthermore, Magnetars do not remain Magnetars for a long time, relatively speaking, as their magnetic field begins to weaken over time. Consequently, astronomers do not know if an object is a Magnetar, until it reconfigures its magnetic field, and so there are most likely many unknown Magnetars, as regular observations cannot differentiate between them and usual neutron stars. However, there are some variable stars which are instantly recognisable, given enough observations.

Unlike certain variables stars, like Binary stars (looked at earlier on), some stars vary intrinsically and are hence classified as ‘Intrinsic Variable Stars. ‘ The luminosity in these systems varies due to some intrinsic factor that is inherent in their nature, as opposed to an external factor. A type of intrinsic variable star is known as an RR Lyrae Variable. These stars also vary their luminosity periodically, but by different means to that of binary systems. RR Lyraes are more strictly known as ‘Pulsating Variables.

‘ These variables change their luminosity which causes a pulsating effect, similar to a light beacon. This pulsation is also directly related to the magnitude of the changes in size, of the star. A normal, hydrogen fusing, star generates trillions of photons per second. The energy created at the core of these stars is so great that it forms a radiation pressure. The energy pushes on the outer layers, of the star. Left at its own accord, the star would eventually be destroyed, due to this light pressure. However, stars contain a colossal amount of mass, creating a large gravitational field.

The stars own gravity pulls on the layers of the star, pulling it inwards, towards the centre. If this was left on its own accord the star would also eventually be destroyed. However, most stars find an equilibrium between these two factors, of gravity and radiation pressure, resulting in a relatively stable star. However, in pulsating variables, there is a loss of equilibrium. As gravity begins to pull on the layers, the star begins to contract under its own gravity. As a result, pressures and temperatures in the core begin to rise.

This increases the rate of fusion and therefore increases the amount of energy formed. This, in turn, increases the radiation pressure until it overcomes gravity and begins to bloat back out. As it bloats out, the pressure decreases and gravity begins to take over, as the pulsating variable oscillates between these two stages. [24] This process only occurs for particular types of stars. A Hertzsprung-Russell diagram is a diagram which plots a star’s luminostiy against its temperature. On this diagram there is a strip, named the “Instability Strip. “

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