Spiral Galaxy

Galaxy with spiral arms. These are classified based on their appearance in optical light, into those in which the arms radiated from a central bulge (classic spirals, S), and those where the arms radiated from a central bar (barred spirals, SB). Both types of spiral galaxies have a central bulge of old stars surrounded by a flattened disk of young stars, gas and dust. These regions are obvious in color images of face-on spirals: the central bulge or bar is yellow (older stars), whereas bright nebulae and young blue stars trace out the spiral arms within the disk. Dust is also visible in edge-on spirals as dark lanes, similar to the dark lanes we see in our own Milky Way when we observe the night sky.

Supernova in M100 (arrow). Image source: http://zuserver2.star.ucl.ac.uk/~idh/apod/ap060307.html.

Spiral galaxies are classified using the Hubble Classification Scheme (q.v.). Classes include Sa/SBa, Sb/SBb or Sc/SBc (classic/barred) according to the tightness of their spiral, the clumpiness of their spiral arms, and the size of their central bulge. These differences reflect the relative amounts of gas and dust contained within them. Only ~2% of the mass of Sa spiral galaxies is in the form of gas and dust. Since these are essential ingredients in the formation of new stars, this means that a relatively small proportion of Sa galaxies are have active star formation. Consequently, these galaxies are dominated by large bulges of old stars and relatively small disks containing faint, smooth, tightly wound arms. In contrast, Sc spirals contain ~15% gas and dust; a relatively high proportion of the galactic mass is involved in star formation. Sc galaxies have small bulges and are dominated by loosely wound arms that are often resolved into clumps of stars and HII regions. The proportion of young stars increases from Sa to Sc galaxies.

Spiral galaxies range in size from 5–100 kpc across, have masses of 109-1012 Msun, and luminosities of 108-1011 Lsun. Most spiral galaxies rotate in the sense that the arms trail the direction of the spin. The measuring rotation curves of spiral galaxies indicate that the orbital speed of material in the disk does not decline as expected were most of a galaxy’s mass concentrated near its center. This indicates the visible portion of spiral galaxies contains only a small fraction of the total mass of the galaxy, and that spiral galaxies are surrounded by an extensive halo consisting mostly of dark matter.

Image source: http://w3.iihe.ac.be/icecube/3_Activities/1_WIMPs%20Analysis/.

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