A satellite galaxy orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

s a larger galaxy due to gravitational attraction. Although a galaxy

Galaxy

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 is made of a large number of objects (such as star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, planets, and nebulae) which are not connected to each other, it has a center of mass

Center of mass

In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...

, which represents a weighted average (by mass) of the positions of each component object. This is similar to how an everyday object has a center of mass which is the weighted average of the positions of all its component atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s.

In a pair of orbiting galaxies, if one is considerably larger than the other, then the larger is the "primary" and the smaller is the satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

. If two orbiting galaxies are about the same size, then they are said to form a binary system

Binary system (astronomy)

A binary system is an astronomical term referring to two objects in space which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions A binary system is an astronomical term referring to two objects in space (usually stars, but also...

.

Galaxies which encounter one another from certain directions may collide, merge, rip each other apart, or transfer some member objects. In these situations, it can be difficult to tell where one galaxy ends and where another begins. "Collisions" between galaxies do not necessarily involve collisions between objects from one galaxy and objects from the other, since galaxies are mostly empty space.