Spiral Galaxies

Spiral galaxies, like irregulars, are sites of ongoing star-formation and therefore contain lots of young stars. They have a disk, formed of both stars and gas, surrounding a central bulge which is rather similar to an elliptical galaxy (just smaller). Waves propagating through the disk form the spiral arms that we see, and cause the gas clouds to collapse and form new stars. Because the spiral arms are regions of active star formation they contain many hot young blue and blue-white stars, making them extremely visible. The disk is thus rich in young stars. The bulge of a spiral galaxy is typically much more red in color, resembling elliptical galaxies; this indicates the presence of many old stars in the nucleus.

This sequence shows a galaxy changing its appearance from spiral to almost elliptical as the observed wavelength is varied from blue to red (infrared) light. The hot young stars are found predominantly in the spiral arms of the disk, while the cooler red star dominate the bulge. As we run through the different stellar populations, the appearance of the entire galaxy changes drastically.

Images of spiral galaxy M81. As the images shift from short wavelength blue light to longer and longer wavelengths, ending with red and infrared, there are clear trends. In blue light there are prominent bright regions along the spiral arms, and the central light distribution is subdued. At longer wavelengths the bright regions dull and the central bulge becomes brighter and brighter. At the longest wavelengths the spiral arms can barely be found within the disk.
Galaxy M81 [G. Bothun]

Spiral galaxies are rich in gas and dust, which is often visible as lanes of dust when viewed from the "top" or "bottom", and as layers of dust when viewed from the side. In our own Milky Way the center of the galaxy is invisible from our vantage point because the interstellar dust between us and the center is so thick (forming the zone of avoidance).

NGC 3184 NGC 1365 M100 NGC 6782
Image of face-on NGC 3184, showing clearly defined spiral arms with a multitude of bright violet regions and diffuse patchy gas, and a warm yellow central bulge. Image of barred NGC 1365, showing a warm yellow bar overlaid with brown dust and greatly extended spiral arms shooting off the ends of the bar. Image of M100, showing a gigantic fleecy blue-white disk rife with spiral structure, and a yellow central disk. It looks as though someone swept the floor and pushed a jumpy heap of dust bunnies onto the galaxy disk. Image of NGC 6782, an inclined buttery yellow disk with bluer star-forming regions along the outer disk, and a bright white ring of light surrounding the central bulge.
Image of the HCG 87 group of galaxies, showing several inclined spiral galaxies, one with prominent dust lanes. Though not massive enough to qualify as a cluster, this set of galaxies is gravitationally bound together. Image of M51 (aka the Whirlpool galaxy), a large face-on spiral galaxy with clearly defined spiral structure and extended spiral arms looping around the central bulge. Note the presence of a smaller yellow galaxy passing behind M51 and tugging on the stars along one spiral arm. Image of ESO 510-G13, showing a clear warp (distortion) in the disk shape. Image of NGC 4650A, showing a rare polar ring galaxy with an inner disk of older, reddish stars and a perpendicular larger ring of diffuse dust, gas, and stars. This is suspected to be the remnant of a galaxy-galaxy collision.
HCG 87 (group) M51 (Whirlpool) ESO 510-G13 (warped) NGC 4650A (polar ring)
A wide variety of spiral galaxies, from edge-on to face-on inclinations. [NASA/HST]