## Monday, September 28, 2009

### How Much of the Human Body is Made Up of Stardust?

This is another of those questions that is part of the series of posters produced by the APS's Physics Central. The last time I highlighted one of these, it was a question on how long does one have to yell to heat up a cup of coffee. This time, the question is on how much of our body is made up of startdust.

I'll cut right to the chase and give you the answer from that page:

Now that we have established that every element in the periodic table aside from hydrogen is essentially stardust, we have to determine how much of our body is made up of this stardust. If we know how many hydrogen atoms are in our body, then we can say that the rest is stardust. Our body is composed of roughly 7x1027 atoms. That is a lot of atoms! Try writing that number out on a piece of paper: 7 with 27 zeros behind it. We say roughly because if you pluck a hair or pick your nose there might be slightly less. Now it turns out that of those billion billion billion atoms, 4.2x1027 of them are hydrogen. Remember that hydrogen is bigbang dust and not stardust. This leaves 2.8x1027 atoms of stardust. Thus the amount of stardust atoms in our body is 40%.

Since stardust atoms are the heavier elements, the percentage of star mass in our body is much more impressive. Most of the hydrogen in our body floats around in the form of water. The human body is about 60% water and hydrogen only accounts for 11% of that water mass. Even though water consists of two hydrogen atoms for every oxygen, hydrogen has much less mass. We can conclude that 93% of the mass in our body is stardust. Just think, long ago someone may have wished upon a star that you are made of.

As Carl Sagan used to say, we are star stuff!

Zz.

#### 1 comment:

Anonymous said...

I'd go further and say that we are more bigbangdust than stardust, since presumably every proton in every atom of stardust is leftover from before heavy nucleosynthesis.