Baryonic Matter

By definition, baryonic matter should only include matter composed of baryons. In other words, it should include protons, neutrons and all the objects composed of them (i.e. atomic nuclei), but exclude things such as electrons and neutrinos which are actually leptons.

In astronomy, however, the term ‘baryonic matter’ is used more loosely, since on astronomical scales, protons and neutrons are always accompanied by electrons (in appropriate numbers for astronomical objects to possess all but zero net charge). Astronomers therefore use the term ‘baryonic’ to refer to all objects made of normal atomic matter, essentially ignoring the presence of electrons which, after all, represent only ~0.0005 of the mass. Neutrinos, on the other hand, are (correctly) considered non-baryonic by astronomers.

Another slight oddity in the usage of the term baryonic matter in astronomy is that black holes are included as baryonic matter. While the matter from which black holes form is mainly baryonic matter, once swallowed by the black hole, this distinction is lost. For example, a theoretical black hole constructed purely out of photons (which are bosons and clearly not baryons) is indistinguishable from one made from normal baryonic matter. This is often referred to as the ‘black holes have no hair’ theorem which simply states that black holes do not have properties such as baryonic or non-baryonic.
Objects in the Universe composed of baryonic matter include:

Study Astronomy Online at Swinburne University
All material is © Swinburne University of Technology except where indicated.