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infrared, ultraviolet, x-ray, and gamma-ray wavelengths), and some are
highly variable, changing in brightness considerably over a short periods of
The current model of active galaxies suggests that such a galaxy may be
powered by a supermassive black hole at its center, from which pour forth
jets of hot gas in opposite directions at nearly the speed of light. The black
hole is surrounded by a disk of gas that is spiraling into it. As it does so, the
gas gives off huge amounts of radiation before disappearing inside the
black hole.
Scientists currently recognize many phenomena associated with active
galaxies, including quasars and blazars. Quasars are extremely distant
objects, some as far away as 12 billion light-years. A quasar is also
extremely luminous, perhaps a hundred or even a thousand times brighter
than a normal galaxy. Some scientists have suggested that quasars may be
highly active galactic nuclei, burning so brightly that they blot out all the
light from the galaxies’ stars. A blazar is believed to be an active galaxy that
has one of its jets pointed toward Earth, so that observers here on Earth are
looking directly into the jet of escaping energy.
Lower-luminosity active galaxies also exist. Astronomers suspect they
may be weaker versions of quasars or may be powered by star formation
rather than supermassive black holes.
Active galaxies are the subject of much debate among astronomers.
The study of these distant objects is one of astronomy’s greatest challenges,
and astronomers are continually making new discoveries about them, and
devising new theories to explain their characteristics.
Observe some regular, irregular, and
very peculiar galaxies.
Keycode: ES2809
ACTIVE GALAXY In this artist’s rendering, an active galaxy’s jets of gas are clearly seen
pouring from its nucleus.
Chapter 28 Stars and Galaxies