العربية
中国
Français
Deutsch
Ελληνική
हिन्दी
Italiano
日本語
Português
Русский
EspañolSPIRAL GALAXY
An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)
A 'spiral galaxy' is a type of late galaxy in the Hubble sequence which has a basic physical characterization of:
★ A central 'bulge' surrounded by a 'disk' and encased by a spherical 'halo'.
★
★ 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 (which contributes to a large angular momentum) consisting of interstellar matter, young "Population I" stars and open star clusters, which contain stars.
★
★ The halo is a spherical region containing Population II stars, often in large globular clusters consisting of stars, that orbit the galactic center.
Spiral galaxies are named after the arms that extend—roughly logarithmically—from the bulge. These arms are bright from the high rate of star formation (and hence the OB stars that inhabit the arms). These arms can be difficult to discern, but distinguish spiral galaxies from their lenticular counterparts, which have a disk, but no spiral arms. Within the overarching term of "spiral galaxy", there have been numerous classifications to distinguish between particular types of spiral galaxies, beginning with 'Hubble's Tuning Fork', which was augmented over the years by astronomers such as Gerard de Vaucouleurs. Sidney van den Bergh introduced a 'luminosity class' for spirals, where those spirals with the most distinct arms have class I, and class V represents spirals with the least distinct arms. It should be noted that the name is rather misleading, as "luminosity class" does not necessarily correspond to the absolute magnitude of the galaxy.[1]
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.
| Contents |
| Origin of the spiral structure |
| Structure |
| Spiral arms |
| Galactic bulge |
| Galactic spheroid |
| Examples |
| See also |
| References |
| External links |
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. 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.[2] 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.[3]
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. 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.
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. Bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity).
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.
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
★ Triangulum
★ Whirlpool
★ Milky Way
★ Andromeda
★ Sunflower
See also
'Components:'
★ Galactic halo
★ Galactic corona
'Classification:'
'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
References
1. An Introduction to Modern Astrophysics, Carroll, Bradley W. and Dale A. Ostlie, , , Addison Wesley, 2007,
2. Detection of the Effect of Cosmological Large-Scale Structure on the Orientation of Galaxies, I. Trujillo, C. Carretero, S. G. Patiri, , , The Astrophysical Journal, 2006
3. Galaxies like necklace beads
External links
★ Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo?, Giudice, G.F.; Mollerach, S.; Roulet, E., , , Physical Review D, 1994
★ AEGIS survey reveals new principle governing galaxy formation and evolution Tim Stephens
★ Spiral Galaxies @ SEDS Messier pages
This article provided by Wikipedia. To edit the contents of this article, click here for original source.
psst.. try this: add to faves
