Jody Dyer, age 9, of Rochelle, Illinois, for her question:
Can the stars be solid?
In our everyday world we expect the seas to be watery liquid and the air to be made of filmy gases. The dry land is crowded with a multitude of metals and minerals and other solid substances. This is the way things are on our planet. But on a seething star things are very different.
It is extra heat that changes solid ice to liquid water and more heat changes the water to steamy gas. If the temperature then starts to drop, the steamy water vapor changes to liquid water. If the water loses still more heat, it freezes to solid ice. Every solid substance melts to its liquid form at its own special temperature. At a certain higher temperature, it boils and changes to a gaseous vapor. Compared to other parts of the universe, the range of everyday temperatures on our planet is not vary great. A scorching summer day may seem like the hottest event in the whole universe, but it is not.
The blazing faraway stars are hotter by far than things ever get on our moderately cool planet. Some are hotter than others, but the coolest star is about twice as hot as our steel melting blast furnaces. It is hot enough to boil iron and lead and all the other solid substances of our world and change them into gaseous vapors.
The differences between solids, liquids and gases take us down to the miniature world of atoms and molecules. Stars, planets and all other objects are made from these same bitsy particles of matter. Atoms and molecules get their energy from heat. Loss of heat makes them slow down and stand more or less still in rigid formations. Extra heat gives them extra energy to move around as liquids, and still more heat gives them enough zip to zoom off as separate particles of gas. In a star, even the coolest star, the atoms, molecules and smaller particles of matter have enough heat to zoom around as free and separate particles of gaseous material.
Gases spread out as far as they can. In a star they are hugged down by the mighty force of gravity. This force is strongest at the center of a heavenly body, and most of its gaseous particles are crammed in the core. The outer layers of a big, fairly
cool star may be 25 times thinner than our ordinary air, while the crowded core is heavier than lead. But its densely packed particles do not act like our solid lead. They are not locked together. Each has enough heat energy to zoom around on its own at a terrific speed. Certain small stars called dwarfs are very dense, perhaps 5,000 times heavier than our solid planet. Even so, its tremendous heat keeps its small particles of matter separated into seething gases.
It is, naturally, too hot to stand on the surface of any star, but even if we could withstand the heat, we could not stand on a star. Its peppy, high speed particles do not cling or lock together to make a solid surface. On earth, the normal gases are airy and too thin to hold our weight. On a dense and heavy star, the hot gases are more restless than boiling soup. You could not stand or build a house, even on a star that is thousands of times heavier than our ground.