- #1

redruM

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what do you think would happen to the colour of the sun if it suddenly shrank in size?

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- Thread starter redruM
- Start date

- #1

redruM

- 15

- 0

what do you think would happen to the colour of the sun if it suddenly shrank in size?

- #2

On Radioactive Waves

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- #3

Tail

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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...?

- #4

Tail

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Ooops, you said "colour". Missed that...

Another wild guess, but it might look "bluer"...

Another wild guess, but it might look "bluer"...

- #5

Integral

Staff Emeritus

Science Advisor

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Now, what do you mean by shrink?

- #6

Tail

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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...

- #7

russ_watters

Mentor

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...which is still meaningless. Volume or mass?Originally posted by Tail

I think he/she means "decrease in size"

- #8

redruM

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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...

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

- #9

On Radioactive Waves

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The color is basicly a function of mass and radius (or temperature, see later)

The mass-luminosity relationship states

L_{star}=M^{3.5}_{star} in units of L_{sun} and M_{sun}.

Now we find the flux**E** = L_{star}/area

**E** = L_{star}/4[pi]r^{2}

From there, the total radiation given off per meter^{2} = the Stephan-Boltzman constant times the themperature^{4} or

T_{star}= {E/[sig]}^{1/4}

now replacing E with L_{star}/4[pi]r^{2} we see

T_{star}= {L_{star}/4[pi]r^{2}[sig]}^{1/4}.

Next, we use Wien's law to see [lamb]_{max}=3,000,000/T_{star} in nm

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

[lamb]_{max}=3,000,000 / (L_{star}/4[pi]r^{2}[sig]}^{1/4}).

and recall that:L_{star}=M^{3.5}_{star}

and finally we obtain :

[lamb]_{max}=3,000,000 / (M^{3.5}_{star}/4[pi]r^{2}[sig]}^{1/4})

where we only need mass and radius and

where:

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.

The mass-luminosity relationship states

L

Now we find the flux

From there, the total radiation given off per meter

T

now replacing E with L

T

Next, we use Wien's law to see [lamb]

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

[lamb]

and recall that:L

and finally we obtain :

[lamb]

where we only need mass and radius and

where:

M= mass

L=luminosity (both in unts of the sun)

[sig]= Stephan-Boltzman constant

[lamb]

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

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:

- #10

Tail

- 208

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I'm sorry, I didn't mean to be impolite... just tired I guess...

- #11

redruM

- 15

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no need to apologise tail. i appreciate u giving up ur time to help individuals like meI'm sorry, I didn't mean to be impolite... just tired I guess...

radioactive waves, i have read through it twice...and unfortunately i don't 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 don't get. thanks for ur help.

- #12

redruM

- 15

- 0

what is flux E?

also how does the finding of (lambda)max help in predicting the colour?

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