domingo, 8 de enero de 2012

Astrophysics - Enormous Comsic Ray Activity 1,000 Times Higher Than Center of Milky Way Detected in Closest Spiral Galaxy


The H.E.S.S. telescope system detectspotentially Earth-threatening  high-energy rays from the starburst region of a galactic system outside the Milky Way. Between 2005 and 2008, astrophysicists used the H.E.S.S. telescope system in Namibia over a total observation period of 119 hours to detect the expected gamma rays at energies exceeding 220 GeV (billion electronvolts). The source of these rays lies precisely at the optical center of NGC 253  one our closest spiral galaxies outside the so-called local group of our Milky Way and its companions at a distance of some twelve million light years away. 

The flux of radiation from the starburst region of NGC 253 measured by H.E.S.S. implies an enormous cosmic ray density - more than 1,000 times higher than at the center of the Milky Way.
Large numbers of massive stars are born in the hearts of these starburst galaxies, and later explode as supernovae. In the remnants they leave behind, particles are accelerated to very high energies.
Moreover, the high gas density makes the conversion of cosmic rays into gamma rays around one order of magnitude more efficient. Accordingly, the central region of NGC 253 shines around five times as brightly in the light of gamma rays as all the rest of the galaxy together.


Observations in the visible light as well as in the infrared and radio frequency ranges had already shown there was a small region at the center of NGC 253 which gave birth to a very high number of stars. This region exhibits a very high density of interstellar dust and gas.
The high-mass stars born in this region use up their nuclear fuel relatively quickly and stagger into an energy crisis at the end of their life. The nucleus collapses while the star destroys itself in one final explosion. Such a supernova suddenly flares up a million or even a billion times brighter than before. The charged particles accelerated to very high energies in the remnants of such explosions react with the surrounding medium or with electromagnetic fields to generate extremely high-energy gamma quanta.
The four H.E.S.S. telescopes, each with a mirror area of 108 square metres, observe weak bluish and extremely short flashes of light. This so called Cherenkov radiation is emitted by showers of particles created when high-energy gamma quanta collide with molecules in Earth's atmosphere. H.E.S.S. stands for High Energy Stereoscopic System and has been in operation since the beginning of 2004. 
Source: The Daily Galaxy -

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