(n.) Gaseous mass enveloping a planet or star.

(n.) A cosmological model, in which the universe was once compressed to infinite density and has been expanding ever since. Originally a term used by unbelieving scoffers, now as widely accepted as th [..]

(n.) The ability of a telescope to collect light from an astronomical source. The light-gathering power is directly related to the area of the primary mirror or lens.

(n.) The magnitude that a star would appear to have if it were at a distance of ten parsecs from us.

(n.) A rotating disk of gas surrounding a compact object (such as a neutron star or black hole), formed by material falling inward.

(n.) Fraction of incident electromagnetic radiation reflected by a body such as a planet, star, or cloud.

(n.) A cluster of two protons and two neutrons; a helium nucleus.

(n.) The large spiral galaxy located some 700,000 parsecs from the sun; the most distant object visible to the unaided eye.

(n.) Unit of length convenient for measuring wavelengths of electromagnetic radiation: 1 A = 10-10m.

(n.) A measure of the mass, radius, and rotational velocity of a rotating or orbiting body. In the simple case of an object in circular orbit, the angular momentum is equal to the mass of the object t [..]