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Revision as of 18:41, 5 December 2006

An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)

hi A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties:

Spiral galaxies are also 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, has long been thought to be a spiral, with a Hubble sequence classification of Sbc (possibly SBb); recent research from the Spitzer Space Telescope, however, confirms that it is in fact a barred spiral.

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 center. 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.

Alternative hypotheses that have been proposed involve waves of star formation moving about the galaxy; the bright stars produced by the star formation would die out quickly, leaving darker regions behind the waves, and hence making the waves visible.

Structure

Spiral arms

Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name.

The existence of spiral arms once puzzled scientists, as the stars on the outermost edge of the galaxy would move faster than those near the center as the galaxy rotates. In fact, spiral arms are not a result of star motion, but density waves that induce star formation. That is, the arms appear brighter because there are more young stars (and hence more massive, bright stars that do not survive long), not because stars are moving in a way that produces arms.

Galactic bulge

A bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies.

The bulge in galaxy spirals is usually composed of Population II stars, small, red and old. This is because all stars were born together with the galaxy, thus at least several billion years ago. Only small and reddish stars can live for this long.

Most bulges are thought to host a supermassive black hole at their center. Such black holes have never been directly observed, but many indirect proofs exist.

Some galaxies have bulges with Population I blue, young stars, or a mix of the two populations. While far from clearly understood, this is usually taken as evidence of interaction with another galaxy (such as galaxy merging), that sends new gas to the center and promotes star formation.

Bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity).

Galactic spheroid

The bulk of the stars in a spiral galaxy are located either close to a single plane (the Galactic plane) in more or less conventional circular orbits around the center of the galaxy (the galactic centre), or in a spheroidal galactic bulge around the galactic core.

However, some stars inhabit a spheroidal halo or galactic spheroid concentrated towards the centre of the galaxy. The orbital behaviour of these stars is as yet disputed, but they may describe retrograde and/or highly inclined orbits, or not to move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Elliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.

Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.

The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the centre of the galaxy—if they do so at all—these stars often display unusually high proper motion.

Examples

See also

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