0 Members and 1 Guest are viewing this topic.
quote:Spiral and Elliptical GalaxiesThe classification of galaxies due to Hubble (1925) remains the most popular in use today. There are two main categories, the spiral and elliptical galaxies, but also lenticulars and irregulars. Spiralslike our own galaxy, fall into several classes depending on their shape and the relative size of the bulge: ordinary spirals are labelled either Sa-d,m while those which have developed a bar in the interior region of the spiral arms are SBa-d,m. Spiral galaxies are characterized by the presence of gas in the disk which means star formation remains active at the present time, hence the younger population of stars. Spirals are usually found in the low density galactic field where their delicate shape can avoid disruption by tidal forces from neighbouring galaxies. M100 and NGC1365 (AAO) Ellipticals are placed in the categories E0-7 depending on their degree of ellipticity. They have a uniform luminosity and are similar to the bulge in a spiral galaxy, but with no disk. The stars are old and there is no gas present. Ellipticals are usually found in the high density field, at the centre of clusters. The giant elliptical galaxy M87 (AAO)Lenticularsare labelled S0 and, although they possess both a bulge and a disk, they have no spiral arms. There is little or no gas and so all the stars are old. They appear to be an intermediate. Sombrero galaxy NGC4549 (AAO)Irregularsare small galaxies, labelled Irr, with no bulge and an ill-defined shape. The Magellenic clouds are examples. The Large Magellenic Cloud (AAO)
quote: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 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, has long been thought to be a spiral, with a Hubble sequence classification of Sbc (possibly SBb); recent research, however, suggests that it may in fact be a barred spiral.Origin of the spiral structureThe 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.[img=right]http://upload.wikimedia.org/wikipedia/commons/thumb/5/5c/Spiral_galaxy_arms_diagram.png/180px-Spiral_galaxy_arms_diagram.png[/img=right]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.Alternative hypotheses that have been proposed involve waves of star formation moving about the galaxy; the bright stars produced by the star formation die out quickly, leaving darker regions behind the waves, and hence making the waves visible.
quote:Originally posted by ukmickyThat's got to be the most gargantuan thing ever posted on TNSMichael
quote:Originally posted by neilepHey !!...WE were ALL attending to this thread at the same time !!..how's that for synergy ?
quote:Originally posted by another_someonequote:Originally posted by neilepHey !!...WE were ALL attending to this thread at the same time !!..how's that for synergy ?Indeed – and now all the other reader's will wonder what all the fuss was about size (maybe I should repost the big image, just to let them know)It is not easy when people start commenting one one's responses just as one is still making corrections to them  – I know, I should always get it right first time – but I never do – cela vie.George
quote:George..it just proves you're human like the rest of us !