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Old 07-01-2006, 11:51 AM   #1
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Is the sun growing or shrinking?

Simple question.

It's been a while since science class and I do remember towards the end of a star the size of the suns life it will grow into a red giant.

but right now, is it growing or is it shrinking?

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Old 07-01-2006, 12:38 PM   #2
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The mass of the sun is decreasing at a very small (relatively) rate.

The volume of the sun, while fluxuating, has remained essentially unchanged through recorded time.

The process of becoming a red-giant has to do with the interaction of the forces that control the size of the sun... it's not as simple as your "if it's shrinking now, how is it going to get bigger later".

People, for example, get bigger as they get older to a point then start getting smaller (usually).
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Old 07-02-2006, 04:36 PM   #3
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The mass of the sun is decreasing at a very small (relatively) rate.

The volume of the sun, while fluxuating, has remained essentially unchanged through recorded time.

The process of becoming a red-giant has to do with the interaction of the forces that control the size of the sun... it's not as simple as your "if it's shrinking now, how is it going to get bigger later".

People, for example, get bigger as they get older to a point then start getting smaller (usually).
Thanks Jerry. Oh, and I wasn't trying to dispute that one day the sun will become a red giant.

At what rate is the sun shrinking? and would there be a difference in size between the present day sun and the sun of a couple of million years ago?
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Old 07-02-2006, 04:43 PM   #4
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Hmm, that's interesting Jerry. Do you happen to know how they actually measure the mass of the sun?
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Old 07-02-2006, 09:20 PM   #5
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At what rate is the sun shrinking? and would there be a difference in size between the present day sun and the sun of a couple of million years ago?
The volume isn't shrinking. It simply fluxuates.

As to the question of the loss of mass, the rate is hard tobe certain on. We can estimate the mass lost through the fusion of hydrogen into helium by looking at the sun's energy output; but I don't think we have an accurat model of nutrino loss, nor are we certain how much mass the sun gains from consuming matter around it.

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Hmm, that's interesting Jerry. Do you happen to know how they actually measure the mass of the sun?
http://zebu.uoregon.edu/~soper/Sun/mass.html
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Old 07-02-2006, 09:34 PM   #6
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Thanks a bunch, but how do they know that it is changing in any way?
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Old 07-02-2006, 11:12 PM   #7
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Thanks a bunch, but how do they know that it is changing in any way?
Volume is measured directly.

Mass? I don't know that the change is observed vs estimated. I'd have to reasearch if there's any data that establishes the mass of the sun at some point in the distant past or if the change since people started doing the math has been enough to measure (I doubt the latter).
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Old 07-05-2006, 10:38 AM   #8
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well I was watching TV the other day and some guy was on talking about evolution. He said that the sun loses about 2 1/2 miles of diameter a year and that a million years ago the sun would have been 6 times it's current size thus making earth uninhabitable.

I've never heard of such an idea, so that's why I asked.
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Old 07-05-2006, 11:12 AM   #9
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It seems to me that that person assumes a constant rate of size loss. I don't know if I would be so bold as to make that assumption. Though if it has adequate backing it would indeed be an interesting point to make.
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Old 07-05-2006, 01:10 PM   #10
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well I was watching TV the other day and some guy was on talking about evolution. He said that the sun loses about 2 1/2 miles of diameter a year and that a million years ago the sun would have been 6 times it's current size thus making earth uninhabitable
On top of everything else: He's failed at the math.

870,000+2,500,000 = 3,370,000
3,370,000 / 870,000 = [about] 3.9

So the diameter (with those numbers) would have been 4, not 6 times. The volume would be considerably larger than six times.

observations carried out at the San Fernando Solar Observatory show the sun expanding and contracting significantly, over the roughly eleven year solar cycle. In any case, it is quite certain that today, no such systematic decrease in solar radius is happening.

http://solar-center.stanford.edu/FAQ/Qshrink.html
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Old 07-05-2006, 01:30 PM   #11
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Yeah, looks like I'll have to go with Jerry on this one.
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Old 07-05-2006, 01:32 PM   #12
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On top of everything else: He's failed at the math.

870,000+2,500,000 = 3,370,000
3,370,000 / 870,000 = [about] 3.9

So the diameter (with those numbers) would have been 4, not 6 times. The volume would be considerably larger than six times.

observations carried out at the San Fernando Solar Observatory show the sun expanding and contracting significantly, over the roughly eleven year solar cycle. In any case, it is quite certain that today, no such systematic decrease in solar radius is happening.

http://solar-center.stanford.edu/FAQ/Qshrink.html
well don't quote me on the numbers. and I remember now that he was talking about diameter, not volume.

but with that, what would our sun have been like 200 million years ago? Could it have supported life on a prehistoric earth? That's really what he was getting at.
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Old 07-05-2006, 03:49 PM   #13
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The sun's mass is 2x10^33g of mostly hydrogen.

To put out energy at the current rate: the sun converts 600,000,000 tons of (6x10^14g) hydrogen per second.

That would mean that it would take a bit more than 3x10^18 seconds to consume it all.

3x10^18 seconds = 5*10^16minutes = 8*10^14 hours = 4x10^12 days = 1x10^10 years (10,000,000,000 or 10 Billion) all in very rough numbers (ones I can easily show).

This is, in fact, about the expected life-span of our currently 4.5-billion-year-old star.
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Old 07-10-2006, 10:02 AM   #14
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The sun's mass is 2x10^33g of mostly hydrogen.

To put out energy at the current rate: the sun converts 600,000,000 tons of (6x10^14g) hydrogen per second.

That would mean that it would take a bit more than 3x10^18 seconds to consume it all.

3x10^18 seconds = 5*10^16minutes = 8*10^14 hours = 4x10^12 days = 1x10^10 years (10,000,000,000 or 10 Billion) all in very rough numbers (ones I can easily show).

This is, in fact, about the expected life-span of our currently 4.5-billion-year-old star.
Perhaps you should be a bit more explanatory to her. I feel like her question was not directly answered.

Now, assuming that you are correct in regards to the amount of hydrogen consumed, how large would the sun have been in prehistoric Earth's time? Even if the consumption rate remains the same at any size (the larger it is, the more that should be consumed at a time), the sun has twindled to about 2/3 it's original size (the full size would have undoubtedly been too much). Now, looking at things realistically, when it was at it's largest size, it was most definately consuming hydrogen at a much faster pace. I also suggest that the sun will last even longer than you claim it will because the rate will continue to decrease. So the sun was probably much larger than we imagine. The final piece to my puzzle would be on finding out the difference between earth's current orbit and the orbit claimed to be had during prehistoric earth. Though it doesn't really matter all that much, because at some point it would have undoubtedly been too close to a sun that was too large.

It is a really interesting point, now that I think about it. Perhaps more research should be conducted along those lines of thinking.
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Old 07-10-2006, 11:41 AM   #15
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Now, assuming that you are correct in regards to the amount of hydrogen consumed, how large would the sun have been in prehistoric Earth's time?
Same size, presumably more mass (how much more depends on how long back).

Sun's weight = 2x10^33g
Mass lost = 4x10^9g/s

So if you want to know the mass of the sun at a given point in time (realizing this only works forwards and backwards about 5 billion years), simply multiply the number of seconds you want to add or subtract by 4x10^9g.

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Even if the consumption rate remains the same at any size (the larger it is, the more that should be consumed at a time),
On what physics do you base this? There's no actual support for this claim.

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the sun has twindled to about 2/3 it's original size (the full size would have undoubtedly been too much).
Why do dolphins live in igloos?

The sun's volume has not changed in more than 4 billion years. The sun's mass has decreased only a small fraction in that time.

In the entire life of the sun, only 10% of it's hydrogen will be turned into helium. Of that 10%, 0.7% will be converted to energy.

So assuming no matter gets directly added nor removed from the sun, in 10,000,000,000 years, the sun will loose 0.07% of its mass. (seven ten-thousandths).

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I also suggest that the sun will last even longer than you claim it will because the rate will continue to decrease. So the sun was probably much larger than we imagine.
The first fundamental flaw is the false assertion that the rate of consumption is changing. I have no idea where you came up with this, but it's not accurate.

Your second flaw is that the sun will burn until there is no more hydrogen. There are quite a few factors (not the least of which is temperature) that will stop the sun from consuming hydrogen at around 10% converted.

Your third fundamental flaw is that the size and mass of the sun are related.

I'm sure there are more, but that should be a good start.

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The final piece to my puzzle would be on finding out the difference between earth's current orbit and the orbit claimed to be had during prehistoric earth. Though it doesn't really matter all that much, because at some point it would have undoubtedly been too close to a sun that was too large.
It's not as simple as you believe it to be. The orbit of the Earth varies over time. Sometimes more circular, sometimes more elliptica, sometimes farther out, sometimes farther in. In addition to the inherent issues with any orbiting body, the effects of other gravitic sources (other planets for example) cause both cyclial and non-cyclical changes in orbit.

The Earth's axis rotates in 41,000 year cycles, the obliquity cycles in 1,250,000 year cycles, orbital eccentricity every 100,000 years or so. Then there are the several cycles in solar activity to deal with.

Reality is far more complicated than you give it credti for.

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It is a really interesting point, now that I think about it. Perhaps more research should be conducted along those lines of thinking.
As a far more efficient idea: you should familiarize yourself with the already conducted reasearch that answers your questions.

I'm reminded of a quote from Microsoft that 90% of features that users requested they add to Word were already in the product. The users were simply unaware of them.
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