A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties:
Spiral Galaxy M74 presents a face-on view of its spiral arms. It contains about 100 billion stars, 30 million light-years away toward the constellation Pisces.
Spiral galaxies are so named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge. Though sometimes difficult to discern, such as in flocculent spirals, these arms distinguish spiral galaxies from their lenticular counterparts, which exhibit a disk structure but no evident spiral.
The disks of spiral galaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.
Our galaxy, the Milky Way, is a spiral, with a Hubble sequence classification of Sbc (possibly SBb).
Origin of the spiral structure
The early pioneer of studies on the formation of the spiral arms was Bertil Lindblad. He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is not what is observed.
Explanation of spiral galaxy arms.
The first acceptable theory was devised by C. C. Lin and Frank Shu in 1964. They suggested that the spiral arms were manifestations of spiral density waves. They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic centre. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.
External links
Spiral Galaxies @ SEDS Messier pages (http://www.seds.org/messier/spir.html)
SpiralGalaxy M74 presents a face-on view of its spiral arms.
Spiralgalaxies are so named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge.
The disks of spiralgalaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.
A barred spiralgalaxy is a spiralgalaxy with a band of bright stars emerging from the center and running across the middle of the galaxy.
Spiral arms appear to emerge from the ends of the "bar" in these galaxies, whereas they appear to emerge directly from the core in ordinary spiralgalaxies.
When observing a distant spiralgalaxy with a rotational axis perpendicular to the line of site, or one that appears "edge-on" to the observer, the shape of the bulge can be easily observed, and therefore quickly classified as either a barred spiral or a regular spiral.
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