We start our tour at the Sun, the central object of our solar system, which is about the size of a grapefruit in our model. The Sun is by far the largest object in our solar system and the only object easily visible anywhere within the model. In fact, the Sun contains more than 99.8% of the solar system’s total mass, making it nearly 1000 times as massive as everything else in the solar system combined.

Although the Sun’s surface looks solid in photographs, it is a roiling sea of hot (about 5500°C, or 10,000°F) hydrogen and helium gas. The surface is speckled with sun spots that appear dark in photographs only because they are slightly cooler than their surroundings. Solar storms sometimes send streamers of hot gas soaring far above the surface and can disrupt radio communications on Earth and disable orbiting satellites.

The Sun is gaseous throughout. If you could plunge beneath the Sun’s surface, you’d find ever higher temperatures as you went deeper. You’d find the ultimate source of the Sun’s energy deep in it’s core, where the temperatures are so high that the Sun becomes a natural nuclear-fusion power plant. Each second, fusion transforms about 600 million tons of the Sun’s hydrogen into 596 million tons of helium; the “missing” 4 million tons becomes energy in accord with Einstein’s famous equation E = mc². Despite losing 4 million tons of mass each second, the Sun contains so much hydrogen that it has shone steadily for almost 5 billion years already and will continue to shine for some 5 billion years to come.