Galaxys

A galaxy is a gravitationally bound system that houses stars, stellar remnants, gas, dust, and dark matter. They range from dwarf galaxies with only a few million stars to giants containing trillions. In the observable universe there are hundreds of billions to trillions of galaxies, distributed throughout the cosmic web. Stellar masses typically range from about 10^7 to 10^12 solar masses, with total mass including dark matter being larger.

Galaxies come in several broad morphologies. Spiral galaxies feature a rotating disk with spiral arms and a central bulge; elliptical galaxies are more rounded and lack a disk; lenticular galaxies sit between these, with a disk but little gas; irregular galaxies lack a defined structure. Classification schemes, such as the Hubble sequence, categorize galaxies by shape and structure, though many systems overlap and evolve over time.

Formation and evolution depend on dark matter halos and gas dynamics. Galaxies grow by accreting gas and by merging with others; star formation converts gas into stars, producing diverse stellar populations. Interactions and mergers can reshape morphology, trigger starbursts, and influence the distribution of stars and gas. The large-scale distribution of galaxies traces the cosmic web of dark matter and the history of cosmic structure formation.

Observational studies use multi-wavelength data and redshift measurements to determine distances, motions, and composition. Our Milky Way is a barred spiral galaxy, with satellite systems including the Large and Small Magellanic Clouds. Andromeda (M31) and Triangulum (M33) are the nearest large galaxies and serve as important laboratories for understanding galactic evolution.

Note on terminology: the standard plural is galaxies; "galaxys" is a nonstandard spelling and is generally considered incorrect in scientific writing.