Strong (source)

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In astronomy in particular, for the description of celestial sources of radiation, strong is understood to refer to intensity when observed from the Earth or another particular body, relative to other sources relevant in the same observational context. In the optical (visual) range, "brightness" is synonymous, outside it "strong" is standard. Absolute strength, taking into account distance effects, and generally integrating over frequency and over all directions, is termed luminosity, and measured in units of energy per unit time. Thus, the Sun's luminosity is ~4×10³³ ergs/s, while its visual brightness as observed from the Earth is about -26th magnitude, or in energy flux integrated over frequencies, approximately 1380 watts/m².

In the contexts of signal detection and perception in science and technology, but also more generally, "strong" means intense, either relative to other signals of similar kind, or relative to the detection threshold of the measuring or observing system. Thus "Cygnus X-1 is one of the strongest hard X-ray sources in the sky", or "We consider only strong sources, having a signal to noise ratio above 20 in our detecting system".