Kierrepyörregalaksit

Kierrepyörregalaksit, known in English as spiral galaxies, are a type of galaxy that features a flattened, rotating disk with a prominent spiral structure. These arms typically extend outward from a central bulge. The spiral arms are regions of active star formation, containing young, hot, blue stars, as well as gas and dust clouds. The central bulge is usually composed of older, redder stars.

The classification of spiral galaxies was first proposed by Edwin Hubble in his 1926 paper, "The Realm of the Nebulae." He categorized them based on the tightness of their spiral arms and the size of their central bulge. Galaxies with tightly wound arms and a large bulge are classified as Sa, while those with looser arms and smaller bulges are classified as Sc. Intermediate types, Sb, also exist.

Our own Milky Way galaxy is a prime example of a spiral galaxy, though it is specifically classified as a barred spiral galaxy due to a bar-shaped structure of stars passing through the central bulge. Barred spiral galaxies are a common subtype, with the bar thought to play a role in channeling gas towards the galactic center, potentially fueling star formation or a central supermassive black hole.

Spiral galaxies are dynamic systems, with stars and gas orbiting the galactic center. The spiral arms themselves are not static structures but rather density waves that propagate through the galactic disk, compressing gas and triggering star birth.