How did our Milky Way Galaxy originate? What led to the formation of such a huge entity?

From the Big Bang came pristine elements, hydrogen and helium. Some kind of density variations amplified in time and caused the condensation of these two gasses which resulted into a great primordial cloud. It had the shape of a large sphere that was rotating and undergoing gravitational collapse. When it collapsed inward to a diameter of about 100,000 light-years, its increasing density and temperature caused fusion of hydrogen atoms. Nuclear process, in accordance of E=mc², was triggered – conversion of hydrogen into helium, which in turn released energy and light. Numerous stars came into being and the galaxy, hitherto dark, lighted up. Many of the original stars burned rapidly and died in convulsive explosions as supernovae after just a few million years. They simply blew up, one after another, spewing forth their star stuff. Their demise created new enriched interstellar clouds that contained first heavy elements, forged in their stellar interiors. From this debris came a second generation of stars. Our Sun was among them, with its surrounding matter that formed planets.

The entire Milky Way Galaxy rotates, making one complete revolution in a period of about 220 million years. But the stars and star clusters rotate at different speeds, depending on their distance from the galactic center. Our Sun, and the Earth with it, is rushing around the center at speed of about 290 kilometers per second.

Additional reading:
Milky Way Galaxy (Wikipedia)

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