David Smith, age 11, of Tucson, Arizona, for his question:

Why does hydrogen give heat when it changes to helium?

This is nuclear fusion, the dynamic activity that ignites the stars. Physicists believe that the sun's energy is created as small hydrogen atoms fuse to form larger atoms of helium. However, there is a catch. The surface of the sun is not nearly hot enough to ignite such a fiery furnace. But after fusion is started, fragments of matter are converted into energy. The heat energy from converted fragments of hydrogen is thought to be enough to keep the furnace going.

A 50 megaton H bomb demonstrates tihat happens when hydrogen atoms fuse to create atoms of helium. In a split second, it explodes with fiery fury equal to about 50 million tons of WIT. However, this is but a small sample of what goes on continuously in the sun. Physicists assume that it converts hydrogen to helium because these are the sun's main oases and also because it can generate enough heat to start such a furnace.

Tremendous heat is involved in fusing small atomic nuclei and the process is called thermonuclear reaction. The hydrogen to helium seems logical because, though more than 70 elements exist in the sun, virtually all of it is hydrogen and helium. The sun contains 99.8 per cent of the total matter in the Solar System, or enough to make 333,000 earth type planets.

However, the big, blazing ball is made of gaseous material called plasma. Enormous pressures, gravity and magnetic fields, strip electrons and other particles from normal atoms. Hence, the plasma is infused with charged nuclei and sundry ions. The sun's core is estimated to be about 15 million degrees centigrade. This, plus its enormous pressure, could be enough to fuse hydrogen nuclei to form helium nuclei. But the process is very, very slow.

Physicists suspect that the process is speeded up by stray neutrons in the solar plasma. The nucleus of ordinary hydrogen contains only a proton. If it absorbs a neu¬tron it becomes hydrogen two, alias deuterium. If it absorbs two neutrons. it becomes hydrogen three, alias tritium. It is thought that the fusion of these hydrogen isotopes requires less heat to create nuclei of helium four.

This is what most experts think happens in the sun. In any case, the hydrogen pairs contain more material than is needed to build a single helium nucleus. During the dynamic fusion process, this surplus matter is converted into energy. This energy pro¬vides the heat when hydrogen changes into helium. When the solar furnace was ignited, matter was converted to energy and provided enough heat to keep it going.

The sun has an abundant supply of hydrogen and this nuclear fuel is being consumed at a fantastic rate. It is estimated that every second, day and night, 650 million tons of hydrogen become helium. During the process, 4,600,000 tons of matter are con¬verted into energy. This is the source of the sun's light, heat and other forms of radiant energy.