When we look at the Universe we find that there are a seemingly innumerate number of stars, which vary in size, colour and luminosity. These variations are often due to the mass of the star, or what stage the star is in its lifetime. As a star ages its mass and luminosity will change, over periods of millions of years. However, when we look at stars in the night sky we find that a minority vary their luminosity over a period of days to a few weeks. These particular stars are catalogued as Variable Stars i. e. stars which vary in brightness.
This can then be measured and be given a numerical value, in which we can compare the data, over a period of a few nights. If a variable star is close enough, then it is actually possible to visually see the difference. Variable stars are classified into two main categories: extrinsic and intrinsic. It is classified as extrinsic if the variability is caused by external factors; often binary stars or large planets, which eclipse the light in the star system. The intrinsic classification is given to stars that physically change or where a stellar event occurs, such as a stellar flare.
 Although these classes can then be further broken down into more subsets, all variable stars are unified by their inherent mystery that surrounds them. As time progresses so does our understanding of variable stars, but this is only due to the massive advances in technology, within the past decade. Understanding these variables stars has become much more relevant in the previous years. Although often seen as an esoteric topic, studying variable stars has led to the discovery of various extrasolar planets; fuelling the search for extraterrestrial life and attracting the interest of both the public and astronomers alike.
Although not obvious, studying variable stars can often have other significant contributions to other fields. The purpose of this piece is to hopefully explore why the luminosity varies and also how understanding these variations can be used in other areas. Though certain variable stars differ in their behaviour, to ‘regular’ stars, they are still stars. This means that how they evolve in their lifetime is very similar to that of other stars (of similar mass).
Although the situations may vary, the laws of physics, throughout the Universe, remain constant; and so understanding them becomes a test of mankind’s grasp of the fundamental laws of physics. The majority of extrinsic variable stars are in fact binary stars and so will be explored first in this essay, where we will explore various ways a binary star’s luminosity can vary. The stars in these systems can vary widely and are an excellent example for variable star systems. These binary stars can also often impose events on its partner star, causing stellar events, which also vary the luminosity.
These stellar events, however, are not exclusive to binary stars and can also be observed with isolated stars. Certain changes in light, such as radio or x-rays can be attributed to neutron stars or Pulsars (where the light pulsates), which will lead on from binary star systems, where we will explore the various subtypes of neutron stars. Although these aren’t strictly classified as variable stars, the light from these systems do vary and so will also be investigated. From this we can then begin to develop the ideas behind intrinsic variable stars.
Many of these stars can be grouped together and therefore condensed and so will allow us to give a general overview of these stars. This will then allow us to then make conclusions for their uses and furthermore, their importance in astrophysics. Within our Universe there are a large number of stars which are a part of a binary, or multiple, star system. They range from stars which are so distant, it is only their mutual gravity which has kept them together, from stars which can be extremely close to each other, such that mass transfers can occur ; looked later at in this section.
However, the scale at which these binary stars are from Earth make it difficult to observe them separately. However, that is not to say that no binary systems can be viewed separately. They can be observed with powerful telescopes, but the correct orientation of the binary system is needed as well. These particular binary stars are known as ‘visual binaries;’ where the two stars are visible separately , as in figure 1. If two stars are extremely close, when observing the binary star system, it is often observed as a single light source and can be mistakenly taken as a single star.
If these stars could not be observed our estimates for the number of stars, in our Universe, would be inaccurate, by a large factor. However, these systems are not static. They are dynamic systems, under the influence of their mutual gravity. As such, similar to a star and planet, these stars will orbit each other. As these stars move along their orbit, relative to us, they can pass in front of each other. As they pass in front of each other they inevitably block out the light from their partner star, which causes a decrease in the total light from the system.
As an outside observer, we can observe this drop in luminosity. However, as we only view this as a single star, it seems as though the star has a decreased luminosity, when in actual fact it is due to the eclipsing, of its partner star.  As there are two stars, each star gets eclipsed by the other. As an observation, we see two periods of changes in luminosity. The rate at which this occurs is dependent on the orbital period. These are known as ‘eclipsing binaries,’ where “the viewer sees a double eclipse along a single plane. “