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

Sun shrank in size

  1. Aug 14, 2003 #1
    what do you think would happen to the colour of the sun if it suddenly shrank in size?
  2. jcsd
  3. Aug 15, 2003 #2
    I seriously doubt it would happen, but with a smaller surface area to radiate energy, and assuming energy is still being produced, it would look more blue or white.
  4. Aug 15, 2003 #3
    Would its mass remain the same?

    You said "shrunk", so I'm assuming yes.

    I'm guessing it would get brighter because the fuel would be burning more rapidly to resist gravitation...?
  5. Aug 15, 2003 #4
    Ooops, you said "colour". Missed that...

    Another wild guess, but it might look "bluer"...
  6. Aug 15, 2003 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It is is nearly impossible to give a meaningful answer to this question. The word shrink is not well defined in the world of Physics so it is not clear what you mean. There are several ways the sun could shrink. As it burns the supply of Hydorgen the nuclear fires will cool, this means that gravitation will gain the upper hand and cause the sun to "shrink" the cooler temperatures will mean that the sun will be a bit redder.

    Now, what do you mean by shrink?
  7. Aug 15, 2003 #6
    I think he/she means "decrease in size" and not by any natural methods, but just... well, just "shrink".

    Can't say I like this because he/she is asking us to drop some important physical laws, exactly the ones that are needed for the correct answer...
  8. Aug 15, 2003 #7


    User Avatar

    Staff: Mentor

    ...which is still meaningless. Volume or mass?
  9. Aug 15, 2003 #8
    first of all, tail, may i tell you that this was my first post. it should have gone in the homework help section. if you think i am looking for an answer you are correct, but not directly from you. i'm sorry if you think i am just looking for a quick answer. perhaps i should have written up what i think about the question so far too.....

    sorry about the ambiguity of the word 'shrink', but what i think it is referring to is a sudden lessening of the radius due to reason 'x'. think of it just becoming more dense. russ_watters, decrease in volume.

    so by using that approach, what i have thought up so far is that energy consumption would increase, there fore as tail mentioned, the colour would be 'bluer'.
    could i also include the red giant stage of a star in its life as an argument. as the star increases in size, less denser, it appears 'redder'?

    sorry again if this may appear to be an attempt to get quick answers, hope you understand:smile:
  10. Aug 16, 2003 #9
    The color is basicly a function of mass and radius (or temperature, see later)

    The mass-luminosity relationship states

    Lstar=M3.5star in units of Lsun and Msun.

    Now we find the flux E = Lstar/area

    E = Lstar/4[pi]r2

    From there, the total radiation given off per meter2 = the Stephan-Boltzman constant times the themperature4 or

    Tstar= {E/[sig]}1/4

    now replacing E with Lstar/4[pi]r2 we see

    Tstar= {Lstar/4[pi]r2[sig]}1/4.

    Next, we use Wien's law to see [lamb]max=3,000,000/Tstar in nm

    So, given mass and radius we can calculate the wavelength of maximum emission of the star by

    [lamb]max=3,000,000 / (Lstar/4[pi]r2[sig]}1/4).

    and recall that:Lstar=M3.5star

    and finally we obtain :

    [lamb]max=3,000,000 / (M3.5star/4[pi]r2[sig]}1/4)

    where we only need mass and radius and


    M= mass
    L=luminosity (both in unts of the sun)
    [sig]= Stephan-Boltzman constant
    [lamb]max= wavelength of maximum emission in nano meters.

    We find that the reddest stars are cool and big, large hot stars are blue, and small hot stars are white.

    Of course, astronomers are lazy and would just look at a http://www.astro.ubc.ca/~scharein/a311/Sim/hr/HRdiagram.html [Broken]

    I also need to point out that there are different ways to go about finding the answer depending on what information is given.. Note that mass is the most important measurment we can get. If we know the mass or the (absolute bolometric) luminosity then we can find the radius if we know temperature and vica versa. To know the mass or luminosity we really need a spectroscopic binary or an eclipsling binary, respectivly. But our most accurate measurment of luminosity comes from mass, as it is not possiblt to know exactly how much extinction occurs. We do know stars gain about 1.9 magnitudes per 1000 parsecs distance. I say gain beacuse the lower the magnitude a star is, the brighter it is. Maybe I should say they look 1.9 magnitudes fainter.

    Does that clarify things a bit? or did I just confuse you even more ?

    I think this is correct, or I hope someone will at least notice and correct me. I may be missing somthing about some of this only applying to main sequence stars. This was a nice refresher, so I'm sure I'm still forgetting much right now.
    Last edited by a moderator: May 1, 2017
  11. Aug 16, 2003 #10
    I'm sorry, I didn't mean to be impolite... just tired I guess...
  12. Aug 17, 2003 #11
    no need to apologise tail. i appreciate u giving up ur time to help individuals like me

    radioactive waves, i have read through it twice....and unfortunately i dont get it. its late night. i'll go over it tomorrow hopefully and then i'll let u know if there were any certain bits i dont get. thanks for ur help.:smile:
  13. Aug 31, 2003 #12
    hi radioactive :smile:

    what is flux E?

    also how does the finding of (lambda)max help in predicting the colour?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook