When two or more galaxies are close enough to each other, gravitational forces will pull the galaxies toward each other. This gravitational attraction increases as the galaxies travel toward each other. The galaxies may pass by each other or collide. Two galaxies that are interacting or colliding may be referred to as a pair, or one galaxy may be referred to as a companion of the other.

The Hubble images below show how different colliding galaxies can look. The appearance of an interacting system of galaxies depends on many factors, including the stage of the interaction, the number of galaxies involved in the interaction, their masses and types, how close they are, and how they approach each other.

Examples of Colliding Galaxies
The Antennae This is a close-up view of two galaxies, probably originally spirals, in the process of merging. The collision has triggered the birth of over 1000 clusters of young stars.	The Cartwheel A smaller galaxy (not visible here) has passed through the Cartwheel. A "ripple" of star formation (the blue ring) resulted from the collision.
The Polar Ring Galaxy This unusual structure may be the result of a collision between a large and a small galaxy. Young stars have formed in the ring of gas and dust stripped away from the smaller galaxy. This ring orbits the remains of the larger galaxy.	Stephan's Quintet Gravitational forces between galaxies in this group have altered some of the galaxies' shapes. The area is littered with stars and gas ripped from the galaxies, as well as new clusters of stars that formed as a result of the interactions.

The Antennae galaxies (upper left) are an example of two spirals that are in the process of colliding. We will not see the end result during our lifetimes because this process takes hundreds of millions of years. Sometimes smaller galaxies plunge into larger galaxies. This type of collision produces a ripple effect, like a rock thrown into a pond. The Cartwheel galaxy (upper right) is an example of this type of collision. The outer ring of blue stars in this galaxy indicates a ripple of star formation resulting from the collision.

Our Milky Way and Andromeda are two spiral galaxies that may eventually collide (about 5 billion years in the future).

When galaxies collide, they experience a gravitational pull toward each other. This gravitational pull distorts the shapes of the galaxies, and can pull material from one galaxy to the other. In many cases, the pull of gravity may result in the galaxies merging. The individual stars within the galaxies do not collide since they are so far apart, relative to their size.

Clouds of gas within the galaxies do collide, however. As a result, large amounts of gas become concentrated in one or more areas of the system. As the collision compresses the gas, the gas becomes dense. The clouds of gas collapse under gravitational forces and form large numbers of new stars. This rapid, short-lived episode of star formation activity is referred to as a starburst. Intense starbursts can use up nearly all of the available gas.

Interacting galaxies do not always merge together to form a single object. Scientists think that some galaxies (such as M51 and its companion, NGC 5195) will simply pass by each other without merging. Galaxies can also pass through each other without merging. The Cartwheel galaxy is believed to be the aftermath of such a pass-through. Some galaxies do merge. The Antennae are thought to represent a merger between two gas-rich spiral galaxies. When the merger is complete, the Antennae may end up looking like a single elliptical galaxy.

When we study astronomical objects, we are actually looking back in time. Light from the Sun takes eight minutes to reach Earth. The light we see today from the next nearest star was emitted about four years ago. Light from the nearest galaxy like our own, Andromeda, takes over 2 million years to reach us. That is, we see Andromeda as it appeared more than 2 million years ago. Observations of distant galaxies show us what the universe looked like at an earlier time in the history of the universe. By studying the properties of galaxies at different epochs, we can map the evolution of the universe.

They observe many properties of each galaxy, including size, shape, brightness, color, amount of star formation, and distance from Earth. This information helps astronomers to determine how these structures may have formed and evolved.

In astronomical terms, a deep field is a long-exposure observation taken to view very faint objects. Light from these objects is collected over a large period of time, so the detectors have a chance to gather as much light as possible. Objects can be very far away and appear faint to us due to the vast distances over which the light must travel; and/or objects can lie close to us and be faint because they don't give off much light. So "deep" doesn't necessarily mean far. However, in the case of the Hubble Deep Fields (HDFs) and the Hubble Ultra Deep Field (HUDF), deep does mean far away since the images were taken in areas that we know have few nearby stars.