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Added section on structure and caption to image
[[File:Spiral galaxy.jpg|right]]
[[File:Spiral galaxy.jpg|thumb|right|300px|The spiral galaxy NGC 6384]]
A '''spiral galaxy''' is a [[galaxy]] consisting of a rotating disk of [[star]]s, [[gas]] and [[dust]] which has a number of arms extending outwards from the galactic centre.<ref>[https://www.britannica.com/topic/galaxy/Types-of-galaxies#ref559197 Galaxies] from britannica.com</ref> Some spiral galaxies have a prominent bulge around the centre, while others have a bar crossing the centre with the spiral arms extending from the ends of the bar. They tend to be fairly large and blue in color. Approxmately 77% of all known galaxies are spiral galaxies.<ref>[http://cas.sdss.org/dr6/en/proj/basic/galaxies/spirals.asp Spiral galaxies] from cas.sdss.org, Sloan Digital Sky Survey</ref>
A '''spiral galaxy''' is a [[galaxy]] consisting of a rotating disk of [[star]]s, [[gas]] and [[dust]] which has a number of arms extending outwards from the galactic centre.<ref>[https://www.britannica.com/topic/galaxy/Types-of-galaxies#ref559197 Galaxies] from britannica.com</ref> Some spiral galaxies have a prominent bulge around the centre, while others have a bar crossing the centre with the spiral arms extending from the ends of the bar. They tend to be fairly large and blue in color. Approxmately 77% of all known galaxies are spiral galaxies.<ref>[http://cas.sdss.org/dr6/en/proj/basic/galaxies/spirals.asp Spiral galaxies] from cas.sdss.org, Sloan Digital Sky Survey</ref>
The [[Milky Way]] is a spiral galaxy, as is the [[Andromeda galaxy]], the closest large galaxy to the Milky Way.
The [[Milky Way]] is a spiral galaxy, as is the [[Andromeda galaxy]], the closest large galaxy to the Milky Way.
==Structure==
Spiral galaxies have three main components: the bulge, the [[galactic disk|disk]] and the [[galactic halo|halo]].
===Bulge===
The bulge is a round structure, located in the centre of the galaxy, and contains large amounts of [[dust]] and [[gas]].<ref name=hubbleSite>[http://hubblesite.org/reference_desk/faq/answer.php.id=39&cat=galaxies Galaxy components] from hubblesite.org</ref> It has an extremely high density of [[star]]s. The large amounts of dust and gas present in the bulge make it almost impossible to explore it in [[visible light]] and other wavelengths such as [[radio waves]] and [[infrared]] must be used. Many bulges are believed to contain a [[black hole|supermassive black hole]] at their centre. Stars in the bulge tend to be yellow/red.
===Disk===
The [[galactic disk|disk]] is a flat region, usually only a thousand light years thick but perhaps a hundred thousand light years across that surrounds the bulge. The spiral arms of the galaxy are located in the disk. Dust is also present in the disk, concentrated into the spiral arms. The spiral arms tend to be very bright as they contain large numbers of very luminous blue stars, with ongoing star formation.
===Halo===
The [[galactic halo|halo]] is a large, almost spherical structure that surrounds the galaxy, centred on the bulge. It contains stars and [[globular cluster]]s, but at a density much lower than either the disk or bulge.
==Classification==
==Classification==