If it is true that all heavier substances move towards the center, then why is it that the sun is the center of our solar system when it is made of hydrogen?

If you had ten cotton balls they would not weigh very much, but if you had 10,000 they would. Each cotton ball is very light, but if you have enough of them together they are very heavy.

Hydrogen atoms are like the cotton balls. Each individual hydrogen atom is very light, but if you have enough of them together yo can still get something, like the Sun, which has a lot of mass and a lot of gravitational pull.

Another thing to mention is that it is the density of an object that makes it sink rather than their total weight. Density is how much stuff is put in a particular volume. That is why a giant iceberg the size of a mountain will float, but a rock the size of your hand will sink. The rock weighs less, but is denser than the iceberg.

Check out this web site for more on the Sun:
http://solar-heliospheric.engin.umich.edu/hjenning/Core.html
(University of Michigan Solar site)

Our thanks this week to the SOHO website (http://soho.nascom.nasa.gov) for the question this week. Our scientists were all busy digging out of the big snowstorm this week.