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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 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. 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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