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Relativistic beaming (or Doppler beaming or Doppler boosting or headlight effect) is a factor in the apparent magnitude of an object when it is moving at relativistic speed relative to an observer. Any source moving toward or away from you appears brighter if it is approaching you and dimmer if it is moving away from you, and extreme speed makes this effect significant. The Doppler effect is one cause, shifting the EMR frequency and thus its photon energy. Another cause is that photons launched from something moving begin their journey with some movement in that direction: even if the EMR emitted is isotropic radiation in its source's own rest frame, someone receiving the photons in the direction the source is moving receives more photons, and the opposite happens to someone the source is moving away from. For example, if a source is approaching you at extremely close to the speed of light, even photons launched perpendicular to the direction toward you are moving nearly directly at you, according your own rest frame. In the source's rest frame, you are approaching so fast that you scoop up the photons before they've traveled much away from this line. (The source itself would hit you shortly after the photons if it hasn't been slowed along the way.) Time dilation is such that both you and the source experience these photons moving at the speed of light (c). If the approach is, for example, 0.5 c rather than 0.999999 c, the same thing happens to a lesser extent.
Particularly affected are relativistic jets, e.g., from quasars and active galactic nuclei, making them appear substantially brighter or dimmer, according to the direction of the jets relative to the observer.