Define what is meant by!!?

Define what is meant by!!?


(i) the unit one parsec, and give it s value in metres





(ii) the luminosity of a star or galaxy. How does luminosity differ from the brightness of the same object as seen at the earth!





(iii)A galaxy has a luminosity of 6.6 x 10^39 W and its brightness is measured as 1.25 x 10~11 W m~2. How far away is the galaxy? express your answer in megaparsecs.





(iv)Describe what is meant by red shift when applied to the motion of a galaxy moving away from the observer. An absorbtion line of the element beryllium is measured in an Earth based laboratory as being at 488.2 nm. The same line is observed in the spectrum of the galaxy aslying at 509.4nm. What is the speed of recession of the galaxy in kms-1? You may assume the speed of light c=3.00x 10~5 km s~1.





(v) Using your answers to already discussed calculate the value of the hubble constant!

Define what is meant by!!?
1) Parsec is short for Parallax Arc Second, the distance away something has to be so that there's a parallax (shift in apparent position) of one arc second between observations taken 6 months apart (i.e. from opposite sides of the Sun).





http://en.wikipedia.org/wiki/Parsec also gives diagrams and the value in meters.





2) Luminosity is how much light is put out, brightness is how much light reaches a square meter here.





3) To get brightness from luminosity, divide the luminosity by the area of a sphere with radius equal to the distance between source and observer. In this case, you have brightness and luminosity, so solve to find the area, and hence the distance.





4) You have "proper" and redshifted wavelengths, this is just a matter of manipulating the doppler shift equation from your textbook.





5) The Hubble constant is just a ratio of how fast and how far away, both of which you should know from 3 and 4.
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