Galaxies are vast cosmic systems bound by gravity, typically consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. Their structure is not uniform across all types, but it can generally be viewed as consisting of distinct components that give them their characteristic shapes and appearances.
According to a common view of galaxy structure, it comprises six separate parts:
- Nucleus: This is the innermost region of a galaxy, often containing a supermassive black hole at its very center. It is the dense core around which the rest of the galaxy is organized.
- Central Bulge: Surrounding the nucleus is the central bulge, a densely packed, spheroidal (or sometimes peanut-shaped) group of stars. Stars in the bulge are typically older and orbit randomly within the bulge.
- Disk (both a thin and a thick disk): This is a flattened region extending outwards from the bulge, prominent in spiral and lenticular galaxies. It contains most of the galaxy's gas, dust, and younger, metal-rich stars. It is further divided into a thin disk and a thicker disk, differing in star age, composition, and vertical extent. Stars in the disk generally orbit in the same direction.
- Spiral Arms: Found within the disk of spiral galaxies, these are elongated, swirling structures that are regions of higher density. They are often sites of active star formation, containing many bright, young stars, as well as gas and dust clouds.
- Spherical Component: While not always listed as a distinct part separate from the bulge and halo in all contexts, this term can encompass populations of stars (like globular clusters) and dark matter distributed spherically around the galaxy's center, contributing to the overall spherical distribution of matter beyond the disk.
- Massive Halo: This is a vast, roughly spherical region extending far beyond the visible components of the galaxy. It is thought to contain older stars (like those in globular clusters) and is dominated by dark matter, which provides much of the gravitational pull that holds the galaxy together.
These parts combine in varying degrees to form the different types of galaxies we observe, such as spirals, ellipticals, and irregulars, each exhibiting a unique overall structure dictated by the interplay and properties of these fundamental components.
