The heavier elements in the universe were formed by

The heavier elements in the universe were formed by fusion reactions. Fusion reaction is a type of nuclear reaction where two or more nuclei combine or collide to form an element with a higher atomic number. This happens when the collision is in a very high speed. This reaction is the source of power of the Sun and other stars in the universe. The most common example of this reaction that happens in space is the fusion of hydrogen isotopes - Deuterium and Tritium - forming a much heavy element which is helium. For this reaction to occur, the nuclei of the atoms should be very close with each other so that the nuclear forces would bind them together.

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