Glossario - O

OB association is a group of massive stars that are not gravitationally bound to each other, but move together through space in a loose association. The OB in the name is a reference to stars of stellar classification O and B.

Opacity is a measure of the resistance of a medium to the radiative transmission of energy. Within a star, it is an important factor in determining whether convection occurs.

Open cluster is a gravitationally-bound group of up to a thousand stars that formed together in the same molecular cloud.

Opposition occurs when two celestial objects are on opposite sides of the sky. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun.

Orbit plot or orbital plot is a schematic diagram of a complete orbit. For a binary system, it is typically presented from the primary component's frame of reference.[11]

Orbital elements are parameters that uniquely define an orbit.