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Hang On!

Week of May 25, 1998

For some reason, a lot of people seem to be under the impression that the Sun is shrinking. The first I heard of this was an email I received a couple of months ago asking me that if the Sun is shrinking 5 meters a day, how can it be 4.5 billion years old? Mustn't it have been much larger in the past, so large that we must be wrong about its age?

I assume that this is some sort of Young Earth argument, favored by folks that want the Earth to be six or seven thousand years old due to religious beliefs. I won't go into any sort of religious argument here, but this particular argument is fallacious. The Sun, as far as I know (and I do have some contacts in the solar astronomy field) is not shrinking by such a large amount at all, so the whole line of argument is based on a false premise.

But then, I thought, there are two reasons the Sun might indeed be shrinking. One is the solar wind, the wind of particles blown off by the Sun that give us such a light show as the aurorae here on Earth. The other is that the Sun is losing energy which we see as visible light (and a lot of other types of light like infrared and ultraviolet that we cannot see). This energy, as I'll show in a moment, comes from a tiny loss of mass in the Sun. The big question is, how much mass does the Sun lose through these two processes?

Let's start with the energy loss. In the Sun's core, it converts hydrogen into helium by nuclear fusion. Basically, four hydrogen nuclei get converted into one helium nucleus, and a little bit of the mass of the hydrogen is converted into energy in the process (a la Einstein's famous formula E=mc2). Only a tiny bit of matter is converted per atom, but a whole lot of atoms are converted every second. The total mass converted to energy is a whopping 2 billion kilograms (4.4 billion pounds) every second! But, the Sun has a total of 2 x 1030 kilograms of mass, or a trillion billion times as much as it converts to energy every second. So compared to its entire mass, the Sun isn't losing much that way! Even over the past billion years, it only lost about one one-hundred-thousandth of its total mass! This is such a small amount we can safely ignore it.

But what about the solar wind? This may surprise you (it surprised me), but the solar wind - which creates the beautiful aurorae and has even been known to knock out satellites and cause blackouts here on Earth 150 million kilometers away from the Sun- represents a mass loss from the Sun one thousandth of the (negligible) mass loss from nuclear fusion! I would have guessed it was more, but the numbers are easy enough to find (or figure out).

So amazingly, the Sun has lost only a teeny fraction of its total mass over its 4.5 billion year lifetime. If someone comes up to you and argues otherwise, why, send them my way and we can talk it over!

©2008 Phil Plait. All Rights Reserved.

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