Stelsels

Stelsels are gravitationally bound systems composed primarily of stars, interstellar gas and dust, and dark matter. They span a wide range of masses and sizes, from dwarf systems with only millions of solar masses to giant galaxies containing hundreds of billions. The luminous matter traces the structure of the stelsel, while a substantial dark matter halo extends beyond the visible edge and influences its dynamics.

Common categories of stelsels include spiral, elliptical, irregular, and lenticular. Spiral stelsels feature a rotating disk with spiral arms, a central bulge, and substantial amounts of gas and young stars. Elliptical stelsels have ellipsoidal shapes and tend to be older, with little gas and low star formation. Irregular stelsels lack a coherent shape and often arise from interactions or mergers. Lenticular stelsels combine a disk-like structure with a reduced interstellar medium, serving as an intermediate class.

Formation and evolution are driven by cosmological growth and interactions. Stelsels assemble through accretion of gas and mergers with other systems, a process influenced by the surrounding environment. Star formation proceeds in dense gas regions, regulated by feedback from supernovae and active nuclei. Many stelsels host supermassive black holes at their centers, whose activity can heat or expel gas and affect future star formation.

Observations of stelsels rely on multiwavelength astronomy and distance indicators. Redshift measurements reveal cosmic expansion and help place stelsels in a large-scale structure. Rotation curves and velocity dispersions provide evidence for dark matter. Proximity examples include the Milky Way and the Andromeda Galaxy, both part of the Local Group. Large surveys and space missions catalog thousands of stelsels, enabling statistical studies of their properties and evolution.