Helping Aubie this week is Dr. Satoshi Hinata , Professor of Physics in AU’s College of Sciences and Mathematics.

Dear Lena,

Four and a half billion years ago, a dense cloud of gas in the location of the present solar system began to condense into an even denser ball of gas as the gravitational force pulled gas particles together. When a gas is compressed without losing heat, it becomes hotter. To see this, imagine you have a balloon that fits into your palm. Now try to squeeze it. As you squeeze it harder, the balloon gets smaller and it becomes harder for you to squeeze. This is because, when the gas is squeezed without losing heat, the particles in the gas begin to move with faster speeds and to bang on the balloon’s wall with more force. You will feel this as a higher pressure on your hands. The faster speed of particles tells you that the gas is hotter, as the energy of a particle is proportional to the square of its speed, and the heat is a form of energy.

In this way, the pre-solar gas was heated more as it collapsed more. Eventually, the gas became so hot that the particles in the gas were moving with such large speeds that a process called fusion took place and continues today. When particles (mostly hydrogen atoms) collide with each other with sufficient energy then they occasionally stick to each other and form into new particles (helium atoms). When four hydrogen atoms fuse into a single helium atom, a small fraction of the mass is lost; that is, the mass of the helium atom is less than the mass of four hydrogen atoms.

You might have heard about Einstein’s famous statement, E=mc², which means that the lost mass goes into another form of energy. That’s right, in the sun the lost mass in the fusion process is converted into heat. This fusion process has been taking place in the sun for the last four and a half billion years and will continue to do so for a similar number of years until all the hydrogen in the solar core is fused into helium. Although the surface temperature of the sun is about 5,000K, the core is 16 million K, because of the heating by the nuclear fusion processes. The heat generated in the core of the sun is carried by photons (light particles) to the surface and radiated away, which keeps the sun from over heating.

Thanks for your question,

Aubie and Dr. Hinata