Spiral galaxy

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

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

A considerable total angular momentum
Composed of a central bulge surrounded by a disk
- The bulge resembles an elliptical galaxy, containing many old, so-called "Population II" stars, and usually a supermassive black hole at its center.
- The disk is a flat, rotating assembly consisting of interstellar matter, young "Population I" stars and open star clusters.

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.

1 Origin of the spiral structure
2 Structure
3 Examples
4 See also
5 References
6 External links
7 External links

[edit] Origin of the spiral structure

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. 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. Or, the stars on the outermost edge of the galaxy would have to move faster than those near the center, as the galaxy rotates. Neither behaviour is observed. According to Bertil Lindblad, with the Density Wave Theory, the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.

Explanation of spiral galaxy arms.

Subsequent work was developed by C. C. Lin and Frank Shu in 1964. They suggested that the spiral arms were manifestations of spiral density waves, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms. They assumed that the stars travel in elliptical orbits and that the sizes as well as the orientations of their orbits are slightly-varying from each other, 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.

Alternative hypotheses that have been proposed involve waves of star formation moving about the galaxy, also called the stochastic self-propagating star formation model or SSPSF model. This model proposes that star formation propagates via the action of shock waves produced by stellar winds and supernovae that compose the interstellar medium. The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the (darker) background stellar distribution behind the waves, hence making the waves visible.

The different hypothesis do not have to be mutually-exclusive, as they may explain different types of spiral arms.

While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.

Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids.^[1] That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids.^[2]

[edit] Structure

[edit] 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. Nothing moves at their rotation speed.

[edit] 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 stars with low metal content which are now old and red. All such stars were born together with the galaxy at least several billion years ago. Only small 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).

[edit] 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.