![]() This was extended by de Vaucouleurs to include a fourth class: Hubble originally described three classes of spiral galaxy. Sc (SBc) - loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae smaller, fainter bulge.Sb (SBb) - less tightly-wound spiral arms than Sa (SBa) somewhat fainter bulge.Sa (SBa) - tightly-wound, smooth arms large, bright central bulge.Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows: Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, given the symbol SB. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. A spiral galaxy consists of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge. On the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies. The barred spiral galaxy NGC 1300: a type SBbc This is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles.Įxamples of elliptical galaxies: M49, M59, M60, M87, NGC 4125. Observationally, the most flattened elliptical galaxies have ellipticities e=0.7 (denoted E7). It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle). The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. They are denoted by the letter E, followed by an integer n respectively. Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. On the left (in the sense that the sequence is usually drawn) lie the ellipticals. The giant elliptical galaxy ESO 325-G004.
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