Blazar << BLAY zahr >> is an extremely luminous (bright) object at the center of some distant galaxies . Astronomers once thought that blazars were irregular variable stars within our own galaxy. But they have since discovered that blazars are actually among the most distant objects detected in the universe .
Scientists determine the distance to a blazar by measuring its redshift . Redshift is a stretching of the wavelengths of light or other electromagnetic waves given off by an astronomical object as it moves away from Earth. Like ocean waves, light waves have crests. Wavelength is the distance from one crest to the next. Astronomers often measure an object’s redshift to determine its speed or distance relative to Earth. The more distant an object is, the greater its redshift. By measuring the redshift, astronomers have determined that the nearest blazar lies roughly 400 million light-years from Earth. A light-year is the distance light travels in a year, about 5.88 trillion miles (9.46 trillion kilometers).
Characteristics.
Blazars can vary wildly in apparent magnitude (brightness as seen from Earth) over the course of years, days, or even hours. They can emit (give off) electromagnetic waves that range from low-frequency radio waves, through visible light, to extremely energetic gamma rays. The electromagnetic waves emitted by blazars are polarized—that is, they vibrate in a simple, regular pattern. Some blazars also lack emission lines. These are bright lines that appear in the electromagnetic spectrum where the energy at specific wavelengths is especially strong.
Scientists think that these features may result from high-speed collimated outflows (parallel streams) of particles and energy emitted by the blazar in the direction toward Earth. They think such outflows are emitted by a supermassive black hole that lies at the center of the blazar. A black hole is an object with a gravitational pull so strong that nothing—not even light—can escape it. As matter falls toward the black hole, it releases enormous amounts of energy and particles. The black hole’s rotation and its magnetic field channel the high-energy particles into enormous bright jets shooting away from the black hole at nearly the speed of light.
History.
As radio astronomy developed in the 1960’s, astronomers discovered that powerful beams of radio waves were being emitted by objects previously classified as variable stars. They also learned that these objects were millions of light-years away, rather than within our own galaxy, the Milky Way . An astronomical object designated BL Lacertae was one of the first to be linked to a powerful burst of radio waves detected by radio telescopes. Astronomers named this group of objects blazars for BL Lacertae and because they shared many characteristics with objects called quasars .
Blazars, like quasars and Seyfert galaxies , are a type of active galactic nucleus (AGN). An AGN is the extremely bright center of a galaxy that astronomers believe is powered at its center by a supermassive black hole. Many astronomers have proposed that the different types of AGN’s may not actually be different types of objects. Rather, they believe that the observed differences among AGN types may simply be due to variations in the angle and distance at which they can be seen from Earth. For example, a blazar may be an AGN streaming a jet of charged particles pointed directly toward Earth. Another type of AGN may simply be a similar object with jets visible from the side or at an angle when viewed from Earth.
