![]() Most spiral galaxies in the Universe have a bar structure in their centre, and Hubble’s images of NGC 1073 and NGC 1300 offer particularly clear views of these. Particularly popular is the Andromeda Galaxy - a large spiral galaxy - which Hubble has observed in unprecedented detail, capturing over 100 million stars and representing a new benchmark for precision studies of this galaxy type. Hubble has captured beautiful imagery of the distinctive arms and spiral features of spiral galaxies throughout its more than 30-year history. A puzzle of modern astronomy is how spiral galaxies maintain their spiral arms. Spiral galaxies rotate, and their spiral shape is not stable. Figure 3.1 and Table 3.1 they are labeled as Bar + Ring galaxies. Spiral galaxies are populated by stars that are on average much younger than those that populate elliptical galaxies, and current thinking suggests that spiral galaxies may evolve into elliptical galaxies. A relatively small fraction of spiral galaxies has three spiral arms. Approximately 60% of all galaxies are thought to be spiral galaxies, making spiral galaxies the home of the majority of the stars in the Universe. The Milky Way is thought to be a barred spiral galaxy. The most challenging to identify are fully ‘side-on’, meaning that only the outer edge of one side of the galaxy’s arms is visible. The most visually spectacular spiral galaxies are ‘face-on’, meaning that their bulge and all their spiral arms are clearly visible. Classifying spiral galaxies is not always straightforward, as their appearance varies considerably depending on their orientation relative to Earth. A few disc galaxies (S0, SB0) do not have any spiral arms and these are called lenticular (or 'lens shaped') galaxies. The Milky Way is a barred spiral galaxy with a D 25 isophotal diameter estimated at 26.8 1.1 kiloparsecs (87,400 3,590 light-years), but only about 1,000 light-years thick at the spiral arms (more at the bulge). About half also have well-defined 'bars' near the center, and these are called barred spirals (SBa, SBb, SBc above). Approximately two-thirds of all spiral galaxies are thought to be barred spiral galaxies. Most disc galaxies (Sa, Sb, Sc above) also have spiral arms and are called spiral galaxies. Barred spirals differ from normal spiral galaxies in that the arms of the galaxy do not lead all the way into the centre, but are connected to the two ends of a straight bar of stars which contains the nucleus at its centre. An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy.It is designated as SAB in the galaxy morphological classification system devised by Gerard de Vaucouleurs. Spiral galaxies are surrounded by sparsely populated halos - roughly spherical regions above and below the plane of the discs. Spiral galaxies have a complex structure: a dense central bulge lies at the centre of a rotating disc, which features a spiral structure that originates at the bulge. The Milky Way is a spiral galaxy.įour classes are used to classify galaxies: spiral barred spiral elliptical and irregular. ![]() A spiral galaxy typically has a rotating disc with spiral ‘arms’ that curve out from a dense central region.
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