1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Will a cloud collapse?

  1. Mar 6, 2013 #1
    1. The problem statement, all variables and given/known data
    A cloud consists of 8 solar masses of hydrogen gas (H2). Its temperature is 20 K and its radius is 0.6 light years. Will it collapse?

    2. Relevant equations
    jeans mass

    3. The attempt at a solution[/b
    I've tried to use jeans mass to solve this however I am finding the formula difficult to understand
  2. jcsd
  3. Mar 6, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Show your work.
  4. Mar 6, 2013 #3
    So far I'm using the formula
    3/2kT <GMm/R
    And (3KT/2Gm) x k >mj

    I'm substituting m=2.167x10^-27

    However I'm not sure what it is exactly that I'm gaining from using these formulae?
    I'm getting that (3kT/2Gm) x R = 1.115011357 x 10^15 ???
    I'm aiming to get an answer of
    Jeans mass = 1.05 x 10^31 kg
    Which is less than 8 solar masses (1.6 x 10 ^31) so it will collapse?
  5. Mar 6, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I struggled to follow your logic above. Seem to be a few typos:
    (3kT/2Gm) x R = Mj ?
    √((3kT/2Gm) x R) = 1.115011357 x 10^15 ?
    Also, I don't see online a formula with that 3/2 factor. I see one with 15/4pi, another that approximates that to 1. Anyway, using your 3/2 I get that the Jeans length/radius = 0.84, which suggests to me it won't collapse. (I'm a bit confused over whether the Jeans length is a radius or a diameter, but if it's a diameter then the actual radius is much too large to collapse.)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted