Galaxies - Celestial Objects, Cosmic Structures, Star Systems - A galaxy is a vast system of stars, gas, dust, dark matter, and planetary systems bound together by gravity. Galaxies vary in size from dwarf galaxies with a few billion stars to giant galaxies containing trillions of stars. They are classified into three main types: spiral galaxies (such as the Milky Way, with distinct arms and a central bulge), elliptical galaxies (which are smooth, featureless, and range from spherical to elongated), and irregular galaxies (lacking a defined shape, often due to gravitational interactions). Galaxies are the building blocks of the universe, forming cosmic structures such as clusters and superclusters. The Milky Way Galaxy, home to the Solar System, is part of the Local Group, a collection of more than 50 galaxies, including the Andromeda Galaxy (M31), which is on a collision course with the Milky Way in about 4.5 billion years. Observations from telescopes such as the Hubble Space Telescope and the James Webb Space Telescope provide insights into galaxy formation, revealing that the earliest galaxies formed over 13 billion years ago, shortly after the Big Bang. The study of galaxies helps astronomers understand the evolution of the universe, the distribution of dark matter, and the role of supermassive black holes at galactic centers.
