Page:Illustrated Astronomy.pdf/13

 When light could finally escape, there were neither planets nor galaxies, and we had to wait other 200 million years before the first stars formed. During that time, the Universe was completely dark, just like a thick, darkened nocturne fog.

It is more than likely that the first formed stars were much hotter, larger, and brighter than our Sun. They needed fuel to shine, so they used both hydrogen and helium to do it. When they merged, they were creating new elements more complex, such as carbon, nitrogen, oxygen, and iron, at the same time when the Universe was releasing energy and light. These stars had very short lives, and when they died, they massively blew up, which is known as supernovae. Thus, they expelled to the cosmos the new materials they had formed, allowing the new generations of stars had access to tiny quantities of them.

Little by little, the dark Universe started to light up. After two hundred million years, the first galaxies showed up, and from then on, a new transformation began: the large amount of light that stars produce makes electrons split up inside hydrogen atoms, so the Universe is ionized again (as in the beginning), and that’s how it is up to date.

Going forward in time, the Universe continues to expand; new galaxies and stars are formed, some of them explode as supernovae and inject some other materials into the Universe; galaxies merge with each other, grow, and trigger the formation of new stars. That’s how the Milky Way, our Galaxy, arouse, which due to its shape is classified as a spiral type.

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