HOW COME? Sun's gravity has a pull on us

How does the sun hold all of the planets in orbit, when it is made of such light elements? asks Caleb Hogan, of Lawrence, Kansas

If you've ever let go of a helium-filled balloon, you know what happens: it rises through the air, just as an inflated beach ball, pushed below the surface, rises through water.

The sun is made of gases, and mostly of two that are lighter than air. About 72 percent of the sun's mass is hydrogen; 26 percent is helium. (The other 2 percent is composed of heavier elements, from oxygen to neon to sulfur.)

But while the sun is made mainly of the universe's two lightest elements, its wispiness is deceiving. In the core, where our star's gravity is at its most crushing, compressed gases are 20 times denser than iron, and an inch-square cube of sun-stuff would weigh about 6 pounds.

Our star's enormous gravity is due to its enormous size. Rocky planet Earth has a mass of about 6.6 sextillion tons. The total mass of the gaseous sun? An astonishing 2.2 octillion tons, about 300,000 times that of Earth's.

And when it comes to gravity, mass rules.

If it weren't for gravity, planets would go flying away from their home suns like billiard balls. In fact, gravity is what shapes planets and stars into balls to begin with. And just as gravity holds the Earth together, it pulls raindrops down from clouds, and makes baseballs trace arcs across the sky.

Still, even as the Earth pins our feet to the ground, other bodies tug at us from the sky. Even if we don't notice the gravitational pull of the moon, the oceans do. Each bit of matter in the universe tugs on every other bit.

The more matter clumped together in one place, the more powerful the force of gravity. The sun and the Earth are each attracted to the other. But because the sun is 300,000 times more massive, it pulls harder. And while the force drops off over distance, the sun's gravity is strong enough to keep planets and moons, asteroids and comets, in orbit, over at least 65 billion miles of space.

Scientists think that gravity actually shapes space itself. Imagine holding a sheet with someone, pulled taut, and placing a heavy ball in the center. The sheet will curve around the ball, and other balls dropped on the sheet will tend to roll toward the depression. Likewise, a star's enormous mass distorts the fabric of space, forcing already-in-motion objects like planets to travel in roughly circular paths, rather than flying off at tangents into space.