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Quasi-Stellar Objects
In the 1950s and 1960s, astronomers found several dim points of light to corresponded with radio sources coming from outside the Milky Way galaxy. In 1963, one of these radio sources, designated 3C273, was the first object to be identified as a "Quasi-Stellar" or "star-like" radio source. Astronomer Maarten Schmidt calculated 3C273 was an astounding 2 billion light-years away. No ordinary star could ever be bright enough to see from Earth at such a distance, so it had to be a galaxy. Yet at every resolution, the radio and light waves seemed to come from a single point rather than a hazy patch, as a galaxy should. Schmidt's calculations were startling: Athough 3C273 produced as much energy as a trillion suns, it was barely the size of the solar system.
What is a Quasar?
Today, astronomers refer to quasars as Quasi-Stellar "Objects" (QSO) rather than Quasi-Stellar radio sources, because most quasars are actually poor radio sources. Quasars are giant, distant, active galaxies, emitting powerful bursts of radiation that are most easily detected in the X-ray and Gamma-Ray spectrum. Because their energy took billions of years to get to Earth, Quasars are like astronomical fossils, giving astronomers a unique picture of the Cosmos when it was a much younger, hotter, denser place.
Active Galaxies
An active galaxy has a highly energetic core which is for some reason producing far more energy than a normal galaxy. This excess energy is detected on Earth as X-rays, gamma rays or sometimes radio or infrared radiation. To explain this, the best model so far developed has a supermassive black hole residing in the hot, dense, galactic core. As the powerful gravitational forces accelerates this material, it superheats the particles to millions of degrees, releasing an explosion of radiation into space. Astronomers consider quasars to be the most intense example of an active galaxy, but blazars and Seyfert galaxies both produce more energy than normal galaxies. Many astronomers think these are all examples of a similar phenomena, observed under different conditions. For example, a blazar appears to be a quasar with its radiation jets pointed in Earth's direction, while Seyfert galaxies, which look completely different and are considerably younger and closer, might be a type of "low-energy" quasar.
Studying Quasars
Astronomers have come a long way toward understanding quasars, but there is much left to learn. It's not widely understood why some galaxies are active and others aren't, or why quasars, which are only found at vast distances from Earth, were so apparently abundant in the early Universe and not today. Astronomers will never be able to study a quasar up close, but they can observe nearby Seyfert galaxies to develop new insights into the processes involved inside the brightest objects in the Universe.