Stars

Suppose we have a large cloud of dust and gas .it is quite likely that because of random fluctuations or some other causes, some part of this cloud may come to be denser than its neighbourhood ,in that case, the surrounding regions would begin to gravitate towards the denser part. 
This is an accelerating process . denser the inner part, more is its attraction.
We therefore have a situation where the dust and the gas start falling inwards, leading to a compaction of the cloud . 
the following material is subject to collisions leading to heating up of the dense cloud.The gravitational energy is being converted into heat energy. the central part of this cloud will be the hottest .
When we say that a gas is hot, we also imply that its particles are moving with random velocity and continuously colliding with each other . in fact, temperature is a measure of the velocity of these particles.Then something new happens . at very high temperatures, atoms are ionized that we do not have neutral atoms anymore . the most abundant element is hydrogen, which after ionization is broken up into protons and electrons . through a well understood series of nuclear reactions and radioactivity, 4 protons are fused into one Helium nucleus resulting in a lot of energy.Energy is released because the mass of helium nucleus is significantly less than the mass of a Proton . 
this can happen only if the thermal energy of the protons is high enough to overcome the electrostatic repulsion between particles of like charges .as the temperature at the centre of the dense cloud Rises, the rate of the fusion reactions also increases . rising temperature leads to a high gas pressure which is enough to support the inward gravitational pressure . 
At that stage, the dense cloud is stabilized. it has become a star.Our sun is one such star. this Star would keep shining at about the same rate till the hydrogen supply in the core is exhausted.Notice the remarkable mechanism that gives long term stability to stellar furnaces. Any increase in the internal temperature that would increase the energy production would also increase the internal pressure . as a result the Star would expand, leading to a drop in the temperature and ,hence, the rate of energy production . the reverse would happen if the internal temperature decreased .stars are self regulating furnaces.
From the above discussion it should be clear that if the material from which stars are formed is about the same the only thing that controls the brightness of the star is its Mass.
Very heavy stars will be able to compress the centre to much higher temperature before the internal pressure Rises enough to counter the force of the gravitational compression.  a higher temperature also implies that the rate of reactions inside is much higher, resulting in the stars being much brighter. this is a sensitive function .
 it turns out that stars that are 10 times more massive than our Sun would produce energy and consume their hydrogen fuel at such a rate that they live a thousand times less than moderate stars like our Sun .
 even among stars the fat and the spendthrifts do not last long! While we talk about energy production in stars, let us not over look that the Stellar furnace is also creating heavier elements through fusion of light elements .
 we will not go at this time into the details of how and when that happens .we should only say that the stars are believed to be the generators of all chemical elements of mass equal to and greater than carbon.
In the end let me say that nothing is permanent does not even the 'saptarishi' . our life span is too short compared to that of the stars .even our recorded history of a few Thousand Years is minuscule in length .
we humans have not been here for a long enough period to see a significant change in the appearance of the heavens . yes ,the changing relative positions of Stellar conditions can be discerned.Life of star depends upon its mass. Strange as it might seem, bigger the mass shorter is its life! While a star with the mass like our Sun might Shine for about 10 billion years, very heavy stars might die in a few million years . To understand this we have to have a General understanding of the way stars are Bond, how they live and the way they Die.this process does not stop at making of helium from hydrogen but in later stages of the Stellar life, also leads to the synthesis of heavier elements . To be aware of big changes, we have to look at the largest sample of astronomical observations .there we do se reminents of supernova ,neutron stars and evidence of black holes.
 these are all stages of the way some stars and their lives sometimes to be born again in States completely unpredictable at the time of their birth.