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Why do things collide?

Stars in a globular cluster collide.
(click to enlarge) ©AMNH

When two things collide, the energy of the objects' motions is released — sometimes with destructive outcomes. Think of what happens when you drop a watermelon off the roof of a ten-story building, or when two cars collide! The energy of motion goes into rearranging the matter of the objects. And sometimes the outcome is a mess! But as Cosmic Collisions illustrates, the forces of nature can also transform that mess into new and beautiful forms: galaxies re-shape themselves, new stars are born from old ones, and even life on Earth has been made possible by collisions. Cosmic collisions drive change on cosmic scales, and thus the evolution of the universe.

The universe is constantly in motion, and things in motion will maintain that same motion...unless acted upon by an outside force. This is Isaac Newton's 1st law of motion, the law of inertia. Newton's 2nd law of motion, F=ma, conversely states that a force is something that changes an object's motion. When a driver steps on the brakes, the car slows; when a comet passes by the Sun, its orbit is deflected by the Sun's gravitational pull. If this sounds rather circular, it is! Scientists use the concept of force to describe how things behave, not what a force is. Investigating what a force is remains a fundamental goal of science.

Gravity — the force of attraction between any two bodies with mass — is by far the most important force in setting the objects in the universe in motion. The more mass an object has, the greater the gravitational pull it exerts. And the closer the two objects are, the stronger the pull of
gravity they exert on each other. That's why we feel more gravity on Earth than on the Moon.

Gravity is responsible for the fall of that proverbial apple onto Newton's head, as well as the inexorable attraction between the Milky Way and our nearest neighbor, the Andromeda Galaxy. Although it is much weaker than the other forces of nature, gravity acts over enormous distances.

Things are also in motion because they are attracted or repelled by electromagnetism. Like gravity, this is a fundamental force at work in the universe — and it's much stronger than gravity. It's driven by the interaction of electrical charges (such as protons or electrons), causing attraction or repulsion between them. It is the repulsion between the electrons in a baseball and the electrons in a bat (helped by the momentum of the swing) that sends the ball flying when the two collide. Another manifestation of electromagnetism is the magnetic field generated by the movement of electrical charges in the outer core of the Earth as it rotates, which protects us from collisions with solar particles.

Since everything is in motion in our dynamic universe, and the forces of gravity and electromagnetism operate throughout, collisions are inevitable.

Suggested activity: Ask students to name things in motion on Earth and what put them in motion. (For example, things fall because of gravity, cars move because of the friction of their wheels, and electromagnetic repulsion between shoes and sidewalk keep a jump-roper from falling through the concrete.) Ask students to distinguish between the different kinds of forces at work, both natural and mediated by humans. What is the source of the energy?

What's the evidence?

Molten rocks collide, fuse together, and become our Moon - Click to enlarge

How do we know the Moon formed from a collision with Earth?
Rocks gathered during the Apollo missions suggest that the Moon formed from the same reservoir of materials as the early Earth. In particular, rocks from both bodies share the same oxygen-isotope ratio, pointing to a common origin. Another clue is the relative size of the Moon's iron core. Since the Moon is less dense than the Earth, it is thought to have a very small iron core. In order to explain these phenomena, scientists theorize that an object the size of Mars smashed into the early Earth (after most of the Earth's iron had settled into its core), ripping off only our planet's outer layer. Some of this churned-up material fell back to Earth, and some moved away to form the Moon. Strong support for this theory has come from scientists' ability to successfully model the Moon-forming collision; if the physics of the collision could not be correctly modeled, the theory would have been discarded.

How do we know the K-T meteorite impact occurred?
Wherever scientists have studied the geological record from 65 million years ago, they've encountered a layer rich in iridium and full of shocked quartz. Iridium is a chemical element abundant in meteorites, and the crystalline structure observed in shocked quartz only forms when the quartz crystals are super-heated and squeezed by a tremendous impact. Found all over the Earth, this quartz-iridium layer also marks a clear change in the fossil record. Below it we find the remains of many life forms, including all non-avian dinosaurs ever identified; above the layer we see fossils of an abundance of new species. The "smoking gun" was discovered in 1991: a huge impact crater in the Yucatan that's also 65 million years old — the date of the great extinction.

How do we know that stars collide in globular clusters?
Globular clusters are groups of hundreds of thousands of stars almost as old as the universe itself. In these ancient environments the gas and dust from which new stars form were long ago used up or blown away. Yet at their very center, we see very, very young blue stars. Stars are crammed together a million times more densely in these clusters than in our Solar System — so scientists infer that they must be colliding. Numerical simulations show that this has an effect similar to tossing new logs on a fire. When two old stars collide, new hydrogen gets mixed into the core of the larger of the two, rejuvenating it.

How do we know that the Milky Way and the Andromeda Galaxies are going to collide?
Calculating the probability of an impact between two objects involves knowing how far apart they are and how they are moving relative to one another. Scientists can then apply Newton's laws of motion and gravity to predict what will happen in the future. Most scenarios indicate that the Milky Way and the Andromeda Galaxy will collide in several billion years. The details are uncertain, however, because although we know how far away Andromeda is and how fast it's moving toward us, scientists don't yet have the technology to directly measure its side-to-side motion across such a vast distance, although we can infer it from the motions of other nearby galaxies. More sophisticated instruments will provide a more precise picture in the years to come.