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Micro-quasars, Quasars and Radio Galaxies |
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Super-massive black holes at the heart of galaxies with a mass of up to a billion suns appear to be the cause of the most energetic sources of emission in the Universe. As matter, in the form of gas, dust and stars, comes under the massive gravitational attraction of the black hole, it spirals in towards it and over 10% of its rest mass is converted into energy ( a process >10 times more efficient than that in an H bomb). This energy accelerates beams of particles to speeds close to that of light; these particles then radiate intense beams of energy which can be observed in the x-ray, visible and radio parts of the spectrum. Quasars and Radio Galaxies The centres of such galaxies are called Active Galactic Nuclei and when they emit a beam of radiation that points towards us, the intense, often point like objects that we observe are called Quasars. When the beams lie nearer to the plane of the sky, we see the emission concentrated into two opposing jets; they are then termed Radio Galaxies. Micro-quasars Micro-quasars are objects in our own galaxy which also contain black holes, but here the black hole results from the collapse of the core of a massive star at the end of its life. One example, GRS1915+105, is believed to be a black hole several times more massive than our Sun in orbit around a normal star. Matter from the star is heated to intense temperatures and radiates X-Rays as it spirals in towards the black hole. The process is so violent that unpredictable explosions occur producing spectacular jets - like the beams produced in a quasar but on a much smaller scale; hence the name micro-quasar. A sequence of images (left) made by MERLIN following an outburst in 1997 shows two bullets of ultra-hot gas being shot out in opposite directions with an apparent speed of more than twice the speed of light! This apparent faster-than-light motion is, in fact, an illusion, and can be explained if we are seeing jets moving at about 90% of the speed of light - one jet moving towards us and the other one moving away from us.
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