Olber's paradox A thought experiment suggesting that if the universe were homogeneous, infinite, and unchanging, the entire night sky would be as bright as the surface of the Sun.

Oort Cloud Spherical halo of material surrounding the solar system, out to a distance of about 50,000 A.U., where most comets originate.

opacity A quantity that measures a material's ability to block electromagnetic radiation. Opacity is the opposite of transparency.

open cluster Loosely bound collection of tens to hundreds of stars, a few parsecs across, generally found in the plane of the Milky Way.

open universe Geometry that the universe would have if the density of matter were less than the critical value. In an open universe there is not enough matter to halt the expansion of the universe. An open universe is infinite in extent.

outer core The outermost part of Earth's core, believed to be liquid, and composed mainly of nickel and iron.

outflow channel Surface features on Mars, evidence that liquid water once existed there in great quantity, believed to be the relics of catastrophic flooding about 3 billion years ago. Found only in the equatorial regions of the planet.

ozone layer Layer of the Earth's atmosphere at an altitude of 20 to 50 km where incoming ultraviolet solar radiation is absorbed by oxygen, ozone, and nitrogen in the atmosphere.