Spiral galaxies

Spiral galaxies :
These galaxies are conspicuous for their spiral-shaped arms, which emanate from or near the
nucleus and gradually wind outward to the edge. There are usually two opposing arms arranged
symmetrically around the center. The nucleus of a spiral galaxy is a sharp-peaked area of smooth
texture, which can be quite small or, in some cases, can make up the bulk of the galaxy. The arms
are embedded in a thin disk of stars. Both the arms and the disk of a spiral system are blue in
color, whereas its central areas are red like an elliptical galaxy.
M100
Notice in the above picture of M100 from HST, that the center of the spiral is red/yellow and the
arms are blue. Hotter, younger stars are blue, older, cooler stars are red. Thus, the center of a spiral
is made of old stars, with young stars in the arms formed recently out of gas and dust.
NGC 4639
The bulge of NGC 4639 is quite distinct from the younger, bluer disk regions.
NGC 1365
NGC 1365 is a barred spiral galaxy. Note the distinct dark lanes of obscuring dust in the bar
pointing towards the bulge. A close-up of the spiral arms shows blue nebula, sites of current star
formation.
NGC 253 is a typical Sa type galaxy with very tight spiral arms. As spiral galaxies are seen
edge-on the large amount of gas and dust is visible as dark lanes and filaments crossing in front of
the bulge regions.
Irregular galaxies :
Most representatives of this class consist of grainy, highly irregular assemblages of luminous
areas. They have no noticeable symmetry nor obvious central nucleus, and they are generally bluer
in color than are the arms and disks of spiral galaxies.
NGC 2363
NGC 2363 is an example of a nearby irregular galaxy. There is no well defined shape to the
galaxy, nor are there spiral arms. A close-up of the bright region on the east side shows a cluster of
new stars embedded in the red glow of ionized hydrogen gas.