why the universes...

Because of universal gravitation (萬有引力) and nuclear reaction(核反應).

We know that most of the stars in the universe are fixed stars like the sun. Only the planets in solar system are the stars that we can see and not fixed stars. Therefore we only simply take about the formation of the fixed stars in the universe.

Gravity can gather many many different element in the space near by together and also keep the stars in their own orbits. As the gravity increases, the mass of the elements increase, temperature and preause increases. To some extent, nuclear reaction happens and generate light and heat. These are the stars (or fixed stars) we saw in the sky at night.

First, you have to know what is stars.

What is a Star?
A star is a ball of gas held together by its own gravity. The force of gravity is continually trying to cause the star to collapse. This is counteracted by the pressure of hot gas and/or radiation in the star's interior. This is called hydrostatic support. During most of the lifetime of a star, the interior heat and radiation is provided by nuclear reactions near the center; this is phase of the star's life is called the main sequence. Before and after the main sequence, the heat sources differ slightly: before the main sequence the star is contracting, and is not yet hot nor dense enough in the interior for the nuclear reactions to begin. During this phase, hydrostatic support is provided by the heat generated during contraction; after the main sequence, most of the nuclear fuel in the center has been used up. The star now requires a series of less efficient nuclear reactions for internal heat, before finally collapsing when these no longer generate sufficient heat to support the star against its own gravity.
Conclusion:
A star is a massive, luminous ball of plasma that is held together by its own gravity.

Then, you'll see that stars are actually planets! And, you should know, that there are lots of planets in the universe!

For further information, go to the following links:
http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html
http://en.wikipedia.org/wiki/Stars

因為宇宙大爆炸後

產生左好多物質

例如氣體、粒子、原子

呢d野經過幾百萬年時間

不停地聚集係一齊

不停地旋轉

不停地加熱

最後就形成左恆星 ( Star )

所有今時今日就有咁多恆星