An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is usually surrounded by one or more electrons. Each electron is negatively charged.
The nucleus is positively charged, and contains one or more relatively heavy particles known as protons and neutrons.
During a nucleus fissions, an atom splits into several smaller fragments. These fragments, or fission products, are about equal to half the original mass.
Two or three neutrons are also emitted.
Many nuclei are radioactive. This means they are unstable, and will eventually decay by emitting a particle, transforming the nucleus into another nucleus, or into a lower energy state.
There are three main types of radiation
- Alpha radiation
- Beta radiation
- Gamma radiation
A star's energy comes from the combining of light elements into heavier elements in a process known as fusion, or "nuclear burning".
It is generally believed that most of the elements in the universe heavier than helium are created, or synthesized, in stars when lighter nuclei fuse to make heavier nuclei. The process is called nucleosynthesis.