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...careful when talking about 'the size' of an AGN. Question 7 An AGN at 50 Mpc appears smaller than 0.1 arcsec in an optical observation made by the Hubble Space Telescope, and shows variability on a timescale of one week. Calculate the upper limit placed on its size by (a) the angular diameter observation, and (b) the variability observation. Answer (a) An angular size...

...parenthesis full stop This is a substantial force, to put it mildly. However you may care to compare it to the buoyant force exerted by the sea on the same length of tunnel when the centreline is submerged by only 20 m. The buoyant force cap f sub b on an immersed object is equal to the weight of fluid displaced by it (Archimedes’ principle), so cap f sub b equals rho times g times cap v cap f sub b equals 1020 times ......

...careful when you hear someone talking of ‘ice’ in the Solar System; they do not necessarily mean just frozen water. Generally, a moon’s composition is related to where in the Solar System it formed and how far it is from the primary planet. The moons that formed within accretionary discs around planets show a grading of density and ice : rock ratio. This can be seen...