Astro Glossary

Omega Nebula, M17

A nebula about 5,500 light-years away that is giving birth to new stars.

Oort Cloud

The Oort Cloud is a hypothetical shell of icy bodies that completely surrounds the solar system. It is expected to begin about 2,000 astronomical units (AU) out and extend to 100,000 AU or farther (one AU is the average Earth-Sun distance, about 93 million miles/149 million km). Estimates say it could contain 100 billion bodies to one trillion or more. Most of its members are no more than a few miles in diameter. Passing stars may occasionally knock some of these blobs of ice and rock out of their orbits, either expelling them into interstellar space or sending them hurtling toward the inner solar system. Such bodies may account for long-period comets, which take more than 200 years to orbit the Sun. The Oort Cloud is named for Dutch astronomer Jan Oort, who first proposed its existence in 1950.

Ophiuchus, the Serpent Bearer

Orbits

The motion of a massive body around another body, governed by the force of gravity. Planets in our solar system follow an orbit around the Sun, as first noted by Johannes Kepler, in the shape of an ellipse. An orbit is actually composed of two motions: one directly toward the other body (planet or star...) and the another that points away. In the case of a circular orbit, these two component motions are orthogonal (90 degrees apart, or at right angles). Einstein would say that an orbit is actually a geodisc in space — time, formed by the interaction between space and the mass of the two bodies.

Origins of Life

Origins of the Solar System

Origins of the Universe

Orion Nebula, M42

A nursery for newborn stars about 1,270 light-years away in the constellation Orion.

Orion, the Hunter

A large, well-known constellation visible high in winter’s south-southeastern sky.

Orion's Belt