Galaxy - An enormous gravitationally bound stellar system, containing tens to hundreds of millions of stars, and huge clouds of dust and gas. Galaxies are divided into three basic categories: Spiral, Elliptical and Irregular.
Milky Way - The galaxy in which our solar system resides; it is a spiral galaxy containing approximately 100 billion stars, has a diameter of 100,000 light years and a thickness of 10,000 light years; visible to the naked eye as a broad, diffuse band running across the sky through some of the brightest constellations (e.g. - Taurus, Orion, Carina, Crux, Scorpius, Sagittarius, Cygnus, Cassiopeia).
Open Cluster - A loose collection of stars that are gravitationally interacting; the stars in an open cluster form together from the same interstellar cloud so they all have similar ages, distances, and initial elemental compositions (although the masses, temperatures and the stage of stellar evolution can vary considerably from star to star within the cluster); examples of open clusters include the Pleiades (M45) in Taurus and M41 in Canis Major.
Globular Cluster - A tight, gravitationally bound collection of stars having a strong spherical symmetry; star density rapidly increases towards the center of the cluster; the stars in an globular cluster form together from the same interstellar cloud so they all have similar ages, distances, and initial elemental compositions (although the masses, temperatures and the stage of stellar evolution can vary considerably from star to star within the cluster); the typical globular is 100 light years in diameter and contains tens of thousands of stars; globular clusters are more than 10 billion years in age making them among the oldest objects in the Milky Way; examples of open clusters include the M13 in Hercules and M22 in Sagittarius.
Constellation - in ancient astronomy, a pattern formed by prominent stars in the night sky associated with a cultural or mythological person or object; the Greek astronomer Ptolemy identified 48 constellations in his Almagest (2nd century); in modern astronomy, one of 88 internationally defined areas of the celestial sphere.
Diffuse Nebula - An interstellar cloud of dust and gas consisting mostly of hydrogen and helium, but other ionized gasses may be present; diffuse nebulae are often associated with star forming regions in which the force of gravity collapses the cloud into clumps and knots, and densities eventually grow high enough to initiate nuclear fusion in the cores of new stars; some of the remaining dust and gas are believed to form planets and solar systems; newly formed open clusters are often found still embedded within diffuse nebulae.
Planetary Nebula - An expanding shell of ionized gas surrounding a star late in its life; most stars between 0.8 and 10 solar masses eventually exhaust their hydrogen fuel, expand enormously to become cool red giants, undergo helium fusion, become unstable and eject their outer atmospheres in the form of a planetary nebula; the lifetime of a planetary nebula is quite short (10 000 years)!
Supernova - The cataclysmic explosion of a highly evolved star; the star's luminosity increases by as much as 20 magnitudes and most of the star's mass is blown away at very high velocity, sometimes leaving behind an extremely dense core; the sudden burst of radiation often outshines the host galaxy, before fading from view over several weeks or months. Supernovae are divided into two categories: Type Ia and Type II.
Light Year (ly) - The distance light travels in a vacuum in a period of 1 Julian year; a unit of distance equal to about 9.5 trillion kilometers (5.9 trillion miles).
Magnitude - The a measure of the brightness of a celestial body (usually as seen by an observer on Earth). modern astronomy defines that magnitude 1 is exactly 100 times brighter than magnitude 6; Sirius is the brightest naked eye star at magnitude -1.46.
Apparent magnitude (app. mag.) - magnitudes measure brightness of a celestial body.