galaxy

A galaxy is a group of millions or billions of stars gravitationally bound, such as the Milky Way. Other galaxies are observed as relatively-fuzzy objects, some showing a spiral pattern. Before this structure was worked out (which was in the first half of the twentieth century), the terms the galaxy and Milky Way both indicated the glowing portion of the night sky now known to be our view of the bulk of the Milky Way galaxy, and what we observe as other galaxies were then considered a type of nebula. The question of whether these nebulae were beyond the night sky's discernible stars arose centuries ago, was of research interest for considerable time, and even after that fact was established, galaxies were still often referred to as extragalactic nebulae. Galaxy classification based upon its observed shape (morphology):

• elliptical galaxy - come in all sizes, the largest as much as a hundreds of thousands of parsecs in length.
• disk galaxy (including spiral galaxies) - Milky Way is typical, ~50,000 parsecs across.
• lenticular galaxy - disk galaxy with bright central bulge but no visible spiral.
• irregular galaxy - doesn't fit any of these descriptions.

Elliptical, lenticular, and possibly irregular galaxies are termed early-type galaxies (ETGs) and disk galaxies are termed late-type galaxies (LTGs), with the most fully-formed spirals being "latest": this does not match the early/late terminology of the stars in the galaxy: the galaxies labeled "early-type" generally have fewer early-type stars. Edwin Hubble introduced the early/late galaxy terms because folks were used to the terms for stars, but the early/late implication regarding stellar evolution was being (properly) dismissed at the time, and while Hubble's use of the terms for galaxies implicitly suggest they form disk and spiral structures as they age, he explicitly said he adopted the terms merely as a convenience.

Dwarf galaxies are small galaxies, as small as 100 parsecs across, which may be elliptical or irregular or some types specific to dwarfs, e.g., dwarf spheroidal galaxies. Some dwarf galaxies are satellite galaxies.

Star formation is most seen in spiral/disk galaxies. Elliptical galaxies have been theorized to be the results of mergers that have lost their gas, e.g., by going through a starburst stage. Molecular gas depletion time is the time it takes for the gas in a star-forming galaxy to be used. The term blue galaxy designates a galaxy with early-type (hot, blue) stars, indicating recent stellar formation, and red galaxy indicates the opposite, with late-type, red stars, whose age-range suggests they are older. This represents a galaxy bimodality of galaxies forming stars and not.

The movement of a galaxy's stars as compared to the mass revealed by the stars and gas indicates there is hidden mass in galaxies, which has been termed dark matter. In some galaxies, including the Milky Way, a central supermassive black hole (SMBH) has been detected, and it is suspected this is normal or universal for substantial galaxies, but they are of far too little mass to explain the stars' motion. To explain it, the dark matter must be spread throughout the galaxy, extending beyond the visible galaxy in a larger, somewhat-spherical shape that is termed the galaxy's dark matter halo, which is considered an invisible part of the galaxy. Dynamics further demands the dark matter constitute on the order of 80-90% of a galaxy's total mass and galaxies are thought to have formed after gravity produced regions of high dark-matter density, which drew together the gas to form the stars.

For the purposes of modeling, such as processes within galaxies, galaxies are often modeled with a basic/simplified shape, such as spherical (for the simplest models), axisymmetric, or triaxial.