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Name: KELLY
Status: student
Age: 11
Location: N/A
Country: N/A
Date: 1999 - 2000


Question:
I checked all the answered questions and found none of them answered my question. Let me rephrase. Does the sun lose some mass every year ? If the answer is yes, how big would it have been 4.5 billion years ago . If it was to close to earth for life to exist, does this argue for a young earth ?


Replies:
Yes, surely the Sun loses mass each year. It sends off all that light energy, right? And you know Einstein showed us that energy is the same as mass. There is an easy way to figure out how much mass is being lost that way. The Sun sends out about 4 x 10^33 ergs/sec of energy. Einstein's mass-energy relation is E = m c^2. Thus the equivalent mass loss of all this energy loss is m = 4 x 10^33 ergs/sec divided by the speed of light squared, (3 x 10^10 cm/sec)^2. This gives a mass loss of 4 x 10^12 grams/sec or about 4 million metric tons per second.

Gosh, that sounds like a lot. Well, it is and it isn't. On an absolute scale it seems like a lot. It would impress us if we were talking about something human-scale, like the yearly production of steel in China or the rate at which marijuana is imported into San Diego from Tijuana. But on an astronomical scale this is piddling. Remember the Sun is, well, awfully awfully big. In fact it weighs about 2 x 10^33 grams today. So if the Sun lost mass by radiation at this rate it would lose 1% of its mass in (0.01)(2 x 10^33 grams)/(4 x 10^12 grams/sec) = 5 x 10^18 seconds, or about 160 billion years. The best estimate for the age of the Sun is presently about 5 billion years at most, so pretty clearly mass loss through radiation emission ain't going to compromise that limit in the slightest. In fact, you'd look pretty ridiculous even suggesting it, given the simple calculation above which rules it out utterly.

Now, the Sun *does* lose mass at a much higher rate through another mechanism, the Solar wind. This is the result of light pressure on the Sun's atmosphere blowing off particles at a high speed, so fast they escape the Sun's gravity field and fly off into space. Astrophysicists actually believe the Sun may have lost quite a big chunk of its mass (like 20%? I am unsure of the numbers) in its youth during the so-called ``T-Tauri'' phase, when the Solar wind was immense. Probably the Solar wind puts a more stringent limit on the Sun's age than radiation emission. But it is still not going to mean the Sun was more than a couple of percent bigger in its youth than it is now.

And now let's tackle the other problem here: how big must the Sun get until it becomes too close to the Earth? Well, the Sun is 93 million miles away. It's present radius is about a quarter of a million miles, I think (within a factor of two or so). So the Sun would need to increase its radius by a factor of 200 or so to become close to the Earth. That would correspond to increasing its volume and hence mass by a factor of 200^3 = 10,000,000. I can't think of any obvious mechanism whereby the Sun would have lost 99.99999% of its mass since it's birth, so that when it was young it was the size of Earth's orbit.

You should probably be aware that there is no evidence whatsoever that argues for a young Earth. All the evidence argues quite the other way. In fact, the thesis that the Earth had an origin at all -- that it has not always been here -- is what needs justification to the skeptical mind. I don't know about you, but when I look around me the Earth is the one thing that does not seem to change one iota over time. The mountains I climbed in my youth don't look one particle different today, 30 years later, which I wish I could say about the face in the mirror. The contours of the coast of England look precisely the same to me in photographs from orbit in A.D. 2000 as they do on the maps of William the Conqueror drawn in A.D. 1066. Why on Earth (so to speak) would I even conceive of the idea that the Earth has changed drastically over time, that in fact it was once not even here? The idea seems silly, on the face of it.

From this point of view, it would seem someone who argues that the Earth came into being relatively recently has A LOT to explain, and the person who thinks it has pretty much always been here has the luxury of relying on the fact that this tentative conclusion is supported by every fact our senses report to us.

Dr. Grayce



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