Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Gravitational red shift

  1. May 11, 2010 #1
    1. The problem statement, all variables and given/known data

    After a star like the Sun has exhausted most of the hydrogen in its core it expands and cools to form a red giant. Eventually, when it has exhausted all its nuclear fuel, it sheds its outer layers and contracts and becomes a white dwarf of similar size to the Earth as shown below. Note that the mass of the sun is 2 × 1030 kg, the radius of the Earth is 6,380 km and Newton's gravitational constant G is 6.67 × 10–11 Nm2 kg–2.

    Light leaving the surface of a star of mass M and radius R is stretched in wavelength (i.e. "gravitationally red-shifted") by an amount Δλ/λ = GM/(Rc2) where c is the speed of light.

    Calculate the gravitational red shift for light leaving the surface of the white dwarf


    3. The attempt at a solution

    I tried substituting the values into the formula Δλ/λ = GM/(Rc2) but that didn't produce the right answer...

    Then I tried using the "scape speed" equation is given by

    [tex]v_{esc}=\sqrt{\frac{2GM}{R}}[/tex]

    Where G is the gravitational constant. M is the mass of the white dwarf and R is its radius converted to meters.

    But this didn't work either. The right answer has to be 0.000279. Can anyone explain to me how the got this answer? I have an exam tomorrow...

    Thanks in advance
     
  2. jcsd
  3. May 11, 2010 #2
    Any help guys?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook