  A galaxy contains vast areas of dust, clouds of elements light-years in diameter that are so massive, they fall together or collapse under the force of their own gravity.
When our galaxy was young, 10 billion years ago, these clouds were mostly made of hydrogen, with a little helium mixed in. Stars which formed in that early time lived, and died...and some of the biggest of these died explosively as supernovas.As a star burns, it fuses hydrogen atoms into helium, releasing energy in the process. Stars much larger than our sun are hot enough to fuse helium, and its byproducts, into more and more massive elements. The largest stars burn elements heavy enough to produce iron. When the first stars of this type died, 10 billion years ago, they exploded, spreading these heavier elements through space. Gradually, over billions of years, the interstellar clouds of gas and dust became seeded with these heavy elements. More and more stars continued to form, some burning out rapidly and exploding, others (the smaller ones) burning moderately for billions of years. In our neighbourhood, some 5 billion years ago, a vast cloud of interstellar matter contracted under gravity...and began to heat up. It moved together in swirls and eddies, spinning 
and clumping together as it contracted further and further. At the center, where it was densest, the heat was
great enough to cause hydrogen atoms to fuse together to form helium. This nuclear reaction began to give off vast amounts of energy, stopping any further contraction, and blowing away all the remaining hydrogen and dust clouds...and leaving behind the 'clumps' of heavier material, the planets. This was the birth of our sun and solar system, 5 billion years ago. (Diagram at right, and stage 1 below)
Our sun is average in size and temperature. For a star of its type, it can expect to continue burning hydrogen for about 10 billion years without changing much in appearance. (stages 1 through 2). Our sun at present is about halfway between stages 1 and 2, in the top right corner of the diagram. 
In about another 5 billion years, the sun's hydrogen will have been depleted  enough that it can no longer sustain itself under its own gravity, and it will start to contract again. (Stage 2 on the diagram) As it contracts, it heats up again, and may in fact start to burn helium. This may continue for a few hundred million years, but eventually the helium too gives out...and our sun is not massive enough to get hotter, and burn any of the heavier elements produced.At this point, the sun will collapse...but very quickly (in a few hours?) the outer, cooler materials that are falling inward will heat up explosively, and the sun will flare up in one last gasp of light and brilliance, called a nova. (Stage 3) The remaining core of the sun, compressed even further by the recently exploded outer layers, is now extremely dense...weighing tons per cubic centimeter, and can be called a white dwarf. It continues to shine, and will do so for hundreds of billions of years, slowly getting smaller and dimmer... 
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