To the planets, animals, and peoples of the Earth, the Sun is a unique and vital star. Every living thing on the Earth owes its existence to the fact that the Sun is nearby and keeps shining and has done so for about 5 billion years. The energy that comes from the burning of coal, oil and natural gas was once sun-energy. These fuels are the remains of plants and animals that grew in the warmth of sun-energy millions of years ago. The nearest star, apart from the Sun, is 300,000 times farther away and the weak star energy we receive from it cannot possibly replace sun-energy.
The Sun is far larger than the Earth – one hundred and nine Earth-planets placed side by side would stretch from one side of the Sun to the other. Its volume is 1.3 million times greater than the Earth and its mass 330,000 times as much. The Earth is about 150 million kilometers or 93 million miles from Sun. Light and heat take eight minutes and twenty seconds to race across interplanetary space and reach the Earth from this distance.
The Sun’s gravity pulls much harder than the Earth’s. A person who could venture to the surface of the Sun would weigh about one and a half tons. However, this is an impossible adventure since the Sun has no solid surface and the temperature there is about 10,000°F or 5,537°C. This exceeds the melting temperature of every known substance. The temperature of the surface seems high, but inside the Sun, it is much hotter. Its entire globe is a glowing mass of gas. At the center, the temperature is about 27 million degrees Fahrenheit or 15 million degrees Celsius.
The gas inside the Sun is three-quarters hydrogen, the lightest gas. Deep inside the Sun, hydrogen atoms crowd together, a group of them collides so violently with another group that they fuse and make a completely different substance: helium. Each second, 650 million tons of hydrogen become helium. A small part of this mass of material is transformed in the process and reappears as pure energy, as Einstein has predicted would be the case. In one second, the Sun’s mass falls by 4 million tons. In 50 million years, the lost mass is equal to the mass of the Earth.
Flashes of energy burst forth as the hydrogen turns to helium. The great density of matter traps the energy flashes inside the Sun. They wander through the interior for a million years or so before reaching the surface and streaming off into space.
Along with heat and light, the Sun emits radiation that can be harmful to live creatures. Ultraviolet rays and X-rays damage the cells in plants and animals. The Earth’s blanket of atmosphere soaks up almost all of this radiation although the small amount that reaches the ground on a fine day will make fair skin tan or cause painful sunburn if exposure is too long. Astronauts journeying into space have to be protected from the Sun’s harmful rays.