Spiral galaxy

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A spiral galaxy is a galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae”^[1] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk of stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the bulge into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them. Roughly half of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. Our own Milky Way has long been believed to be a barred spiral, although the bar itself is difficult to observe from our position within the Galactic disk. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, of stars in the Galactic center.^[2]

Together with irregulars, spiral galaxies make up approximately 70% of galaxies in the local Universe.^[3] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.^[4]

Contents 1 Structure 1.1 Spiral arms 1.2 Galactic bulge 1.3 Galactic spheroid 2 Origin of the spiral structure 3 Spiral nebulae 4 Examples 5 See also 5.1 Components 5.2 Classification 5.3 Other 6 References 7 External links

[edit] Structure

Spiral galaxies consist of several distinct components:

• A flat, rotating disc of (mainly young) stars and interstellar matter
• A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
• A near-spherical halo of stars, including many in globular clusters
• A supermassive black hole at the very centre of the central bulge

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

[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. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sa and SBa galaxies, for instance, have tightly wrapped arms, whereas Sc and SBc galaxies have very "loose" arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.

[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.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that is old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are a great deal smaller, and are composed of young, blue, Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity), and others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many 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. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole.

[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. The orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not 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.

[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 angular 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. Bertil Lindblad proposed that 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.

This idea was developed into density wave theory by C. C. Lin and Frank Shu in 1964.^[5] 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.^[6] 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.^[7]

[edit] Spiral nebulae

“Spiral nebula” is an old term for a spiral galaxy. Until the early 20th century, most astronomers believed that objects like the Whirlpool Galaxy were just one more form of nebula that were within our own Galaxy. In 1926, Edwin Hubble^[8] finally proved that the spiral nebulae were, in fact, stellar systems independent of our own Milky Way and the term “spiral nebula” has since fallen into disuse.

[edit] Examples

• Triangulum
• Whirlpool
• Milky Way
• Andromeda
• Sunflower

[edit] See also

[edit] Components

• Galactic disk
• Bulge (astronomy)
• Galactic halo
• Galactic corona

[edit] Classification

Galaxy morphological classification Hubble sequence Disc galaxy Active galaxy Barred spiral galaxy Dwarf galaxy Dwarf elliptical galaxy Dwarf spheroidal galaxy

Elliptical galaxy Intermediate spiral galaxy Irregular galaxy Lenticular galaxy Ring galaxy Starburst galaxy Seyfert galaxy Unbarred spiral galaxy

[edit] Other

• Galactic coordinate system
• Galaxy formation and evolution
• Groups and clusters of galaxies
• List of galaxies
• List of nearest galaxies
• Timeline of galaxies, clusters of galaxies, and large scale structure
• Tully-Fisher relation

[edit] References

[edit] External links

• Giudice, G.F.; Mollerach, S.; Roulet, E. (1994). "Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo?". Physical Review D 50: 2406–2413. Retrieved on 2007-02-04.
• Stephens, Tim. "AEGIS survey reveals new principle governing galaxy formation and evolution", UC Santa Cruz, March 6, 2007. Retrieved on 2006-05-24. 
• On Spiral Nebulae, Adriaan van Maanen, et al This article deals with the view that the nebulae are associated with the Milky Way.
• Spiral Galaxies @ SEDS Messier pagesar:الهالة

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