Nuclear Fusion Stepping outside into the sun, you can feel its warmth and see the bright light. Our sun is just like countless other stars in our universe.

• The warmth and light are two types of energy released by the sun during nuclear fusion reactions deep inside the sun.
  • In a nuclear fusion reaction, two atoms collide and are fused together to make a new atom.
  • Nuclear fusions reactions also release a large amount of energy, some of which can be transformed into light and heat.

Review

• Q: What is the definition of nuclear fusion?
• A: A reaction that makes a new atom from other atoms.

Light Elements in Stars Hydrogen, as the smallest and simplest atom on the periodic table, has one proton, sometimes a neutron, and one electron. Helium is the next largest, which two protons, two neutrons, and two electrons.

Moving through the periodic table, the atoms get larger and become higher in mass because they are made up of more protons, neutrons, and electrons.

• Scientists call the first three elements on the periodic table, the smallest elements, the light elements.
  • These elements--hydrogen, helium, and lithium--have one, two, or three protons in their nuclei, respectively.

Review

• Q: What is the definition of light element?
• A: An atom with one, two, or three protons in its nucleus.

Heavy Elements in Stars As the early universe expanded after the big bang, clouds of light elements condensed into dense bodies. Due to high pressure and temperature, hydrogen started undergoing nuclear fusion, and stars were born.

In all but the smallest of these stars, additional nuclear fusion reactions took place, forming atoms with even more protons in their nucleus. In larger stars, the fusion reactions continued until iron was formed. These elements produced in stars by nuclear fusion are called heavy elements.

Review

• Q: What is the definition of a heavy element?
• A: An atom with four or more protons, up to the number of protons in iron.