Glossary term: Nucleus
Description: All the matter that we see around us consists of atoms, and each atom in turn consists of electrons surrounding a small, central nucleus. Atomic nuclei consist of protons, which are positively charged, and neutrons, which carry no electric charge. Although the protons repel each other due to their electric charge, there is an even stronger force, called the strong nuclear force or just the strong force, that holds the nucleus together. Nuclei with the same number of protons form atoms that belong to the same chemical element.
Nuclei are tiny, only about 1/100,000th of the size of an atom – so in a sense, most of the atom is empty space! The nucleus typically accounts for more than 99.9% of an atom's total mass. That mass at such tiny size makes nuclei very dense, with typical densities of a hundred million billion kilograms per cubic meter.
Nuclei are important in different areas of astrophysics. In the interior of stars, energy is set free as lighter nuclei (starting with hydrogen, whose nucleus is a single proton) fuse to form successively heavier nuclei – this is what makes stars shine. Nuclear fusion in stars can form heavy nuclei up to those of iron, with supernova explosions and the interior of certain cool stars able to form even heavier nuclei. Shortly after the Big Bang, a brief phase of "Big Bang nucleosynthesis" had hydrogen nuclei fusing to helium and traces of other elements. Neutron stars, as the remnants of the supernova explosions of massive stars, consist of mostly neutrons stacked to similar densities as those of nuclei. Certain types of atomic nuclei, stripped of their electrons, are emitted by stars as part of stellar winds, or travel the depths of space as cosmic rays.
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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher
The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".
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Related Diagrams
Nucleon
Caption: A nucleon is a constituent of an atomic nucleus. For normal matter, nucleons can be either protons or neutrons. The number of nucleons defines the mass number of a nucleus while the number of protons defines the atomic number. Atomic nuclei of a particular chemical element all have the same atomic number but can have different mass numbers. Nuclei with the same atomic number but different mass numbers are known as isotopes. Most elements have more than one isotope that occurs in nature although a few only have one isotope that is stable over long periods of time.
This diagram shows the nuclei of isotopes of the first six chemical elements (from left to right: hydrogen, helium, lithium, beryllium, boron and carbon). Beside each chemical element symbol are two numbers. The upper number is the mass number, the number of nucleons. The lower number for each is the atomic number, the number of protons.
Credit: Maria Cristina Fortuna/IAU OAE
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
