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A star which falls in this strip will become a pulsating variable star. Matthew Templeton, from the American Association of Variable Star Observers, stated that “In all other stars, the energy that is created is dissipated and the pulsations die away. ”  This means that for stars that don’t fall in this strip the energy, from the increase in temperatures and pressure, gets distributed out and the pulsations slowly diminish in amplitude and the star stops pulsating and an equilibrium is found.
“All stars have this hydrogen ionization layer, but it is only in stars that lie along the instability strip where the layer is at the right depth that the pulsations are self sustaining. ”  As Matthew Templeton said, for stars in this strip, the depths of the different layers in these stars allow the pulsations to become self-sustaining and hence forming a pulsating variable. However, not every single star becomes a pulsating variable. Antara R. Basu-Zych, from the High Energy Astrophysics Science Archive Research Centre, said that “Only sometimes, the star overshoots and becomes a little too large and pressure balance is disturbed.
”  Not every single star, in this strip, will become a pulsating variable star; however they all have the ability to be. For these stars, when it has contracted, there is a great increase in the light produced and hence a greater increase in the luminosity. In its bloated stage, the radiation pressure is much less and therefore less light is also produced, creating a decrease in luminosity.
Their pulsating periods range from a few hours to approximately a day. Subsequently, this variable luminosity is visible and apparent, over a period of a few days.
 Another type of pulsating variable is known as a Cepheid Variable. The process is the same a RR Lyrae, but these stars are much larger and so their periods are much longer and can last many weeks. This means that the variations in luminosity are much greater as well.  However, not all intrinsic variables are pulsating variable stars. Some variable stars vary because they are young and beginning to form. These are known as T Tauri stars. T Tauri stars are young, intermediate mass, stars. Stars will form from gas and dust clouds, where the necessary elements are present, for stellar formation.
Many protostars can begin forming from the same dust cloud, creating stellar ‘nurseries,’ where many young stars can form. Consequently, the dust cloud, which surrounds the stars, can become heated by the new stars. As the luminosity of these stars varies, the amount of heating the dust cloud receives also varies and so the existence of T Tauri stars can be inferred from the effects they cause. However, as these stars are not particularly large, their luminosity is often quite low and so they are often quite faint.
As a result, to view these, large telescopes are needed to observe and study them, in full detail.  When we look at our own Sun, it fluctuates between eleven year periods, known as the solar maximum and minimum. As the maximum, the Sun is at its most active, creating many coronal mass ejections (solar flares) and sunspots. At this maximum, the magnetic field is at its strongest and sunspots are places where the magnetic field lines protrude from the surface. The protrusions cause the surface to decrease in temperature and hence become darker, as shown in figure 5.
T Tauri stars are young and energetic. As such, their magnetic fields are very complex. This causes many field lines to extend beyond the surface, causing many sunspots. As a result the total luminosity of the system decreases, due to the lower temperature of the sunspots. As the T Tauri star rotates various sunspots come in and out of view, varying the luminosity over time. Furthermore, the dust and gas can obstruct the view, of the star, causing even more changes in luminosity. The light curves of these stars are often quite asymmetric, due the rotation of the star and movement of gas and dust.
 While T Tauri stars vary due to their infancy, other stars vary due to their massive size. Similar to pulsating variables, with some massive stars there is an imbalance between the radiation pressure and gravity. However, in these cases the radiation pressure takes over and gravity never overcomes this pressure. Massive stars that exhibit this effect are called Wolf-Rayet stars. Wolf-Rayet stars can be in the region of twenty or more solar masses in size.  As a result their cores are very dense and so a lot of energy is created.
The surface temperature of these can be 50000 degrees Centigrade; for comparison, the surface temperature of our Sun is 6000 degrees Centigrade.  The rate of energy production is so great that the pressure, pushing on the outer layers, overcomes gravity. Consequently, large amounts of stellar material are blown away in violent and energetic winds, causing the Wolf-Rayets to lose mass at a regular rate. This process can continue indefinitely, until eventually the star tears itself apart, ending its life prematurely, in a supernova.
Unlike other large stars, the product of this is not a neutron star or a white dwarf. The process is so energetic that nothing remains, leaving only a nebula. As stellar material is blown away, around its stellar neighbourhood, there can be large and irregular changes in luminosity, as fusion occurs within the material being ejected, before a final and dramatic increase in luminosity, in the form of a supernova.  This essay is by no means a complete list of all the variable stars. However, it demonstrates the various different processes by which a star system can change its luminosity, when viewed here on Earth.
When we consider the variety of stars in our Universe, it is no surprise that there are also many different types of variable stars. The classification of variable stars is somewhat vague, as the word ‘variable’ can denote many things and it is inevitable that all stars, in some point in their lifetime, will vary. For example, neutron stars, looked at in this essay, are not strictly classed as variable stars, despite under examination we find that their luminosity does vary. This is, in part, the reasoning behind the choice of the title.
Purely looking at stars which were classified as variable stars would have restricted greatly the range of celestial bodies explored. As we’ve seen throughout this essay, star systems can vary due to effects of other stars, such as Cataclysmic variables, or vary due to factor inherent in their own nature, as with Wolf-Rayet stars. In all cases, this has caused an alteration in luminosity. It has become evident that the term ‘variable luminosity’ is quite vague and can have a variety of definitions, determined by how it is looked at. As such, the sensible conclusion was to cover all the different ways what it could mean.
Despite there being a large leap forward, with our understanding in variable stars, it is still incomplete. Matthew Templeton, from the American Association of Variable Star Observers, stated that “We also still have to build stellar models relatively simply usually making very simple approximations of stellar structure, since 3-dimensional models of stars still require an enormous amount of computational power. ”  The largest limiter to our understanding is indeed money and finance; something that is unlikely to change, in the near future.
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