Which objects form the foundation of the International Celestial Reference Frame (ICRF)?

Very distant, extragalactic radio sources, such as quasars

The celestial reference frame against which all optical astrometry is measured is defined by the International Celestial Reference Frame (ICRF). This framework is established by utilizing extremely distant, extragalactic radio sources, primarily quasars, because they are assumed to have negligible proper motion over human timescales. This assumption provides the necessary stability for defining the fixed coordinate system. Any uncertainty in the assumed stillness or precise location of these quasar anchors, or systematic errors in the Very Long Baseline Interferometry (VLBI) data used to define them, will directly propagate as systematic errors into all derived optical star catalogs.