I've spent a lot of time in the forest lately watching leaves fall. When the wind blows they spiral, seesaw, twirl and drop to the ground. Leaves also fall when there's no wind. On an utterly still day this September I listened to the scratch and clatter as one leaf here and one leaf there broke free and tumbled earthward, guided by gravity.

We know that leaves change color because the chlorophyll that drives photosynthesis breaks down and exposes other pigments that have been there all along. But why exactly do leaves fall? It turns out that trees simply get rid of them. They're baggage that might otherwise drain resources from the plant when the weather turns cold.

In response to a decrease in sunlight a tree makes special abscission cells that form along the juncture of the leaf stem and branch and slowly push the leaf away, cell by cell, until either the wind or gravity finishes the job off.

Leaves abandoned by their trees and then felled by gravity pile into a colorful heap. (Bob King for the Duluth News Tribune)
Leaves abandoned by their trees and then felled by gravity pile into a colorful heap. (Bob King for the Duluth News Tribune)

On that windless day, the maple leaves literally hung by a molecular thread. Gravity's delicate tug was all that was needed to break the remaining chemical bonds and set the leaf free. Now on its own, it fell irresistibly to the forest floor

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There are four fundamental forces — electromagnetism (chemical interactions), the strong and the weak forces, both which operate on the atomic level, and gravity. That day in the forest, gravitational force seemed almost tender, tugging gently at each leaf like a lover's kiss. Gravity is the weakest force but operates across vast distances; under the right circumstances it can wield terrifying power. Tapping into its hidden potential is as simple as packing a lot a material into a small space.

This montage of images taken by the Voyager spacecraft of the planets and four of Jupiter's moons includes Earth's moon in the foreground. All the planets and many moons are massive enough to have self-gravitated into delightful spheres. NASA
This montage of images taken by the Voyager spacecraft of the planets and four of Jupiter's moons includes Earth's moon in the foreground. All the planets and many moons are massive enough to have self-gravitated into delightful spheres. NASA

As a boy I wondered why all the stars and planets in the universe were spherical. Later I learned that if enough material gathers together in one place self-attraction of the pieces will crush and meld them into a ball. In essence, a sphere is the most efficient way to package large amounts of matter.

I can't help but think of playing with Play-Doh as a kid, taking all the odd-shaped bits of dough and rolling them into one neat ball again. Gravity does something similar except that it works from the center to the edges, pulling equally from all sides to draw the "clay" into a sphere. For rocky bodies the minimum size to self-gravitate into a sphere is around 370 miles (600 km). For bodies made of ice it's 250 miles (400 km). Ice is easier to crush than rock.

A selection of small comets and asteroids photographed during spacecraft flybys reveals their irregular shapes. The largest is the asteroid Lutetia with a diameter of 62 miles (100 km). NASA / ESA
A selection of small comets and asteroids photographed during spacecraft flybys reveals their irregular shapes. The largest is the asteroid Lutetia with a diameter of 62 miles (100 km). NASA / ESA

Smaller asteroids and comets don't have enough mass to do this so their shapes are irregular or at best roughly spherical. The approximately spherical Earth you're living on got that way when smaller asteroids and dust bumped and ultimately stuck together in the disk of dust and rock that circled the nascent sun. The piling-on increased the proto-Earth's gravitational power even more, allowing it to capture ever more material and grow into the familiar orb we know today.

Planets orbit the sun, captive to its gravitational pull. On a larger scale, stars join together by the billions in galaxies, captive to their own self-gravity as well as the massive amount of the still mysterious dark matter that makes up the bulk of the matter in the universe. We have no idea what this invisible material is because it emits and absorbs no light. But we know it's there because we can directly measure the gravitational effect it has on stars in galaxies.

Gravity finds its ultimate expression in black holes which form inside enormous stars called supergiants that are anywhere from 10 to 70 times as massive as the sun. All that material wants to pile into the center of the star, but heat and pressure generated from nuclear fusion pushes back and keeps gravity at bay.

This artist conception shows a black hole that may lurk in the center of the globular star cluster 47 Tucanae. In a black hole gravity is so strong that even light can't escape. B. Kiziltan /  T. Karacan
This artist conception shows a black hole that may lurk in the center of the globular star cluster 47 Tucanae. In a black hole gravity is so strong that even light can't escape. B. Kiziltan / T. Karacan

A supergiant is incredibly hot inside and races through its energy reserves until there's literally nothing left to "burn." When the massive star runs out of elements to fuse and its internal "fires" are quenched, it offers no resistance to gravity and suddenly collapses. Matter is crushed to incredible density in the star's heart to form a sphere just a few miles across with a gravitational field strong enough to prevent light itself from escaping. We call this a black hole because no light from it can reach your eyes. It's invisible. Meanwhile, the shockwave caused by the collapse rips apart the outer layers of the star in a massive explosion visible across tens of millions of light years.

Gravity achieves this incredible feat because matter has a lot of empty space. A favorite analogy is to imagine a single atom magnified to the size of a football stadium. The nucleus, which contains most of the mass, would be the size of a blueberry on the 50-yard line with the electrons whizzing around the uppermost deck. If you get rid of the space twixt the two you could pack atoms much closer together, greatly increasing the strength of gravity at that body.

NASA's Gravity Recovery and Climate Experiment or GRACE mission uses two spacecraft to create maps of gravity variations on Earth. Areas in blue have slightly weaker gravity and areas in red have slightly stronger gravity. NASA/University of Texas Center for Space Research
NASA's Gravity Recovery and Climate Experiment or GRACE mission uses two spacecraft to create maps of gravity variations on Earth. Areas in blue have slightly weaker gravity and areas in red have slightly stronger gravity. NASA/University of Texas Center for Space Research

Massive stars do this naturally once their nuclear furnaces shut down and gravity takes over, either crushing the star's core into a ball of neutrons about 15 miles (24 km) across (called a neutron star) or going even further and forming a black hole.

For a fun mental exercise, let's crush the sun. If you could pack its nuclei together cheek to jowl into a ball 4 miles (6 km) wide it would collapse into a black hole. Crush the Earth to the size of a ping pong ball and the same would happen. So much wasted space, right? Not really. Those electrons keep things apart and provide solidity to our world. When you pick up a cup of hot coffee you don't want your hand to suddenly blend into the cup!

Gravity gently tugs on a raindrop, stretching it to the point of falling to the ground. (Bob King for the Duluth News Tribune)
Gravity gently tugs on a raindrop, stretching it to the point of falling to the ground. (Bob King for the Duluth News Tribune)

Gravity works in ways small and big. Here on Earth it keeps our feet on the ground and the air over our heads. It plucks raindrops from twigs and let's us flop into bed. Elsewhere, notably inside black holes, its potent power can crush stars to microscopic points.