The Sun: Why Doesn't It Shrink?

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The Sun: Why Doesn't It Shrink?

Postby david48 » Sun Sep 06, 2015 10:16 pm

The Sun is supposed to be a giant ball of compressed hydrogen gas. The compression makes it operate as a kind of thermonuclear reactor, continually using up the hydrogen as fuel.

If this is true, the hydrogen in the Sun must be getting "used up" all the time. So shouldn't we see some sign of this, for instance by observing a shrinkage of the Sun? Yet we don't see this. The Sun seems to stay the same size all the time.

This seems contrary to common sense. I mean, suppose the Sun is made of hydrogen. And it keeps losing hydrogen, by turning the hydrogen into energy, which then gets blown away into space. Shouldn't this continual loss of hydrogen, make the Sun shrink?
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Re: The Sun: Why Doesn't It Shrink?

Postby Gfamily2 » Sun Sep 06, 2015 10:57 pm

The sun contains enough hydrogen to burn for billions of years - we've been observing it for only a few hundred.

In figures; the sun burns about 620 million of hydrogen tonnes per second, but it contains
about 1,500 million million million million tonnes of hydrogen. So we won't have noticed much change in the time we've been observing it.
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Re: The Sun: Why Doesn't It Shrink?

Postby david48 » Sun Sep 06, 2015 11:18 pm

Gfamily2 wrote:The sun contains enough hydrogen to burn for billions of years - we've been observing it for only a few hundred.

In figures; the sun burns about 620 million of hydrogen tonnes per second, but it contains
about 1,500 million million million million tonnes of hydrogen. So we won't have noticed much change in the time we've been observing it.


When you say we won't have noticed much change
We haven't noticed any change. Surely if we applied a very delicate micrometer to the solar disc, we should seem some change?

I mean how can the Sun be losing the staggering amount of 620 million tonnes of mass per second, to quote your figure, while staying the same size?
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Re: The Sun: Why Doesn't It Shrink?

Postby Gfamily2 » Mon Sep 07, 2015 12:11 am

david48 wrote:
Gfamily2 wrote:The sun contains enough hydrogen to burn for billions of years - we've been observing it for only a few hundred.

In figures; the sun burns about 620 million of hydrogen tonnes per second, but it contains
about 1,500 million million million million tonnes of hydrogen. So we won't have noticed much change in the time we've been observing it.


When you say we won't have noticed much change
We haven't noticed any change. Surely if we applied a very delicate micrometer to the solar disc, we should seem some change?

I mean how can the Sun be losing the staggering amount of 620 million tonnes of mass per second, to quote your figure, while staying the same size?


Divide 620 million by 1500 million million million million and see by how much the volume of the sun will change per second. Multiply that by 31 million (seconds) and 200 (years) and what's your answer? Is it something we could reasonably have measured?
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Re: The Sun: Why Doesn't It Shrink?

Postby david48 » Mon Sep 07, 2015 12:21 am

Well, as we can apparently measure the size of planets going round stars hundreds of light-years away, wouldn't you think that a slight diminution of the Sun's diameter, might be detectable?
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Re: The Sun: Why Doesn't It Shrink?

Postby Gfamily2 » Mon Sep 07, 2015 12:36 am

david48 wrote:Well, as we can apparently measure the size of planets going round stars hundreds of light-years away, wouldn't you think that a slight diminution of the Sun's diameter, might be detectable?

Have you done the maths I suggested? What is the difference in volume? Do you think that is reasonable to expect people 200 years ago to have made the initial measurements? Really?



Oh, and how do YOU think we measure the size of those very distant planets? Do tell...
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Re: The Sun: Why Doesn't It Shrink?

Postby david48 » Tue Sep 08, 2015 6:02 pm

Gfamily2 wrote:
david48 wrote:Well, as we can apparently measure the size of planets going round stars hundreds of light-years away, wouldn't you think that a slight diminution of the Sun's diameter, might be detectable?

Have you done the maths I suggested? What is the difference in volume? Do you think that is reasonable to expect people 200 years ago to have made the initial measurements? Really?



Oh, and how do YOU think we measure the size of those very distant planets? Do tell...


Thanks Gfamily. I'd reply as follows:

1. On the maths, well, I agree, obviously if the volume of the Sun were diminishing, the diameter would diminish in proportion to the cube-root, and the visible area in proportion to the square root. But given the colossal size of the Sun, 886,000 thousand miles across, such changes ought to be detectable with modern techniques. After all, such techniques, when applied on a microscope scale, enable us nowadays to image individual atoms. That might have been dismissed as impossible 100 years ago, yet we can do it now.

2. On measuring the so-called "extra-solar" planets, personally I retain some scepticism about these "planets". They might turn out to be illusory, and only the product of some kind of instrumental or observational artefact. But if they are real, and these relatively "tiny" planets - which are only a few thousand miles in diameter- can be detected and measured across a distance of hundreds of light-years, then I don't see why "tiny" changes in the Sun's diameter couldn't be detected. I mean, isn't the Sun only 8 light-minutes away?
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Re: The Sun: Why Doesn't It Shrink?

Postby Gfamily2 » Tue Sep 08, 2015 7:07 pm

david48 wrote:
Thanks Gfamily. I'd reply as follows:

1. On the maths, well, I agree, obviously if the volume of the Sun were diminishing, the diameter would diminish in proportion to the cube-root, and the visible area in proportion to the square root. But given the colossal size of the Sun, 886,000 thousand miles across, such changes ought to be detectable with modern techniques. After all, such techniques, when applied on a microscope scale, enable us nowadays to image individual atoms. That might have been dismissed as impossible 100 years ago, yet we can do it now.

Would it harm you to calculate the actual amount of diameter change you are asking them to measure? At a distance of 150 million km.

david48 wrote:2. On measuring the so-called "extra-solar" planets, personally I retain some scepticism about these "planets". They might turn out to be illusory, and only the product of some kind of instrumental or observational artefact. But if they are real, and these relatively "tiny" planets - which are only a few thousand miles in diameter- can be detected and measured across a distance of hundreds of light-years, then I don't see why "tiny" changes in the Sun's diameter couldn't be detected. I mean, isn't the Sun only 8 light-minutes away?

You could usefully find out how they are detected, then ask if that has any real relevance to the measurement of the Sun's diameter
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Re: The Sun: Why Doesn't It Shrink?

Postby david48 » Tue Sep 08, 2015 8:09 pm

Thanks Glamily.

In answer to your questions,

1.After doing some mathematical calculations, I think that a high-resolution telescope, such as Hubble, would be capable in theory of detecting changes in solar diameter of about 100 miles. However Hubble couldn't achieve this in practice. Because if it was pointed directly at the Sun, the intense solar heat concentrated by its big 2.4 metre primary mirror, would quickly fry the telescope's delicate electronic chips, and shut it down.

2.As regards the "extra-solar" planets, I'm familiar with the "methods" used to detect them, and am still sceptical, they might be caused by sunspots on the stars, or some other effect. What makes them most suspicious, is their very close orbits. I mean planets the size of Jupiter, supposedly orbiting the star in a few days? Is that credible - we don't see anything like that in our Solar System!
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Re: The Sun: Why Doesn't It Shrink?

Postby Gfamily2 » Tue Sep 08, 2015 8:40 pm

david48 wrote:Thanks Glamily.
In answer to your questions,
1.After doing some mathematical calculations, I think that a high-resolution telescope, such as Hubble, would be capable in theory of detecting changes in solar diameter of about 100 miles.

However, the expected reduction in the diameter of the Sun over 200 years is about four foot by my calculation.

david48 wrote:2.As regards the "extra-solar" planets, I'm familiar with the "methods" used to detect them, and am still sceptical, they might be caused by sunspots on the stars, or some other effect. What makes them most suspicious, is their very close orbits. I mean planets the size of Jupiter, supposedly orbiting the star in a few days? Is that credible - we don't see anything like that in our Solar System!

Well - regardless of your scepticism, you're prepared to accept that the measurement of the sizes of these extra-solar planets has nothing to do with any direct measurement of the diameter?
Yes?
So your supposition that it has anything to do with our capability for measuring a reduction in the diameter of the Sun is misplaced. Yes?
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