PKS 2155-304
Encyclopedia
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
in the sky.
It is a strong emitter from radio to high energy frequencies.
PKS 2155-305 is at redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
z = 0.116 (Falomo, Pesce & Treves 1993) and it is one of the brightest and most studied BL Lacs and is often considered the prototype of X-ray selected BL Lacs.