What is Spiral galaxy?
[[Image:M101 hires STScI-PRC2006-10a.jpg|thumb|A famous spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)]] [[File:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg|thumb|A barred spiral galaxy, NGC 1300]] A '''spiral galaxy''' is a kind of [[galaxy]] that looks like a flat, slowly [[rotate|rotating]] [[disk]] with a [[bulge]] in the centre, and spiral patterns extending outward from the bulge. It contains stars, gas, dust, [[dark matter]] and a [[supermassive black hole]] at its centre.<ref>'Supermassive' means not just large size and huge mass, but ''huge'' gravitational effect.</ref> Galaxies were long thought to be [[nebula|nebulae]]. The spiral nebula as a type was first described by [[Edwin Hubble]] in his 1936 work ''The Realm of the Nebulae''.<ref>{{cite book |last=Hubble |first=E.P. |authorlink=Edwin Hubble |title=The Realm of the Nebulae |url=https://archive.org/details/in.ernet.dli.2015.212163 |year=1936 |publisher=Yale University Press |location=New Haven |isbn= 0300025009}}</ref> They are now listed as galaxies, and are named after their shape. The '''barred spiral galaxy''' is an important and common type of spiral galaxy. There are three other kinds of spiral galaxies. ''Grand-design'' spiral galaxies have two well-shaped and well-defined arms. ''Multiple-arm'' spiral galaxies have more arms. In ''flocculent'' spiral galaxies, it is hard to see the arms at all, as they are choppy and are ill-defined.<ref>{{Cite web |url=http://burro.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html |title=Spiral galaxies |work=burro.cwru.edu |accessdate=2 October 2010 |archive-date=8 June 2019 |archive-url=https://web.archive.org/web/20190608213202/http://burro.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html |url-status=live }}</ref> About 60% of galaxies in the universe near us (the 'local universe') are spiral and irregular galaxies.<ref>{{cite journal |author=Loveday, J. |date=February 1996 |title=The APM Bright Galaxy Catalogue. |journal=Monthly Notices of the Royal As