Editing Hubble sequence (section)

===Spirals===
[[Image:M101 hires STScI-PRC2006-10a.jpg|thumb|The [[Pinwheel Galaxy]] (Messier 101/NGC 5457): a spiral galaxy classified as type Scd on the Hubble sequence]]
[[Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg|thumb|right|The barred spiral galaxy [[NGC 1300]]: a type SBbc]]
{{see also|Spiral galaxy}}
On the right of the Hubble sequence diagram are two parallel branches encompassing the [[spiral galaxy|spiral galaxies]]. A spiral galaxy consists of a flattened [[galaxy disk|disk]], with [[star]]s forming a (usually two-armed) [[spiral]] structure, and a central concentration of stars known as the [[bulge (astronomy)|bulge]]. Roughly half of all spirals are also observed to have a bar-like structure, with the bar extending from the central bulge, and the arms begin at the ends of the bar. 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. Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows:
* Sa (SBa) – tightly wound, smooth arms; large, bright central bulge
* Sb (SBb) – less tightly wound spiral arms than Sa (SBa); somewhat fainter bulge
* Sc (SBc) – loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge

Hubble originally described three classes of spiral galaxy. This was extended by [[Gérard de Vaucouleurs]]<ref name="devaucouleurs">{{cite book |last1=de Vaucouleurs |first1=G. |author-link1=Gérard de Vaucouleurs |last2=Oemler |first2=Augustus Jr. |last3=Butcher |first3=Harvey R. |last4=Gunn |first4=James E. |chapter=Classification and Morphology of External Galaxies |date=1959 |title=Astrophysik IV: Sternsysteme / Astrophysics IV: Stellar Systems |series=Handbuch der Physik |volume=53 |pages=275–310 |bibcode=1959HDP....53..275D |doi=10.1007/978-3-642-45932-0_7 |isbn=978-3-642-45934-4 }}</ref> to include a fourth class:
* Sd (SBd) – very loosely wound, fragmentary arms; most of the luminosity is in the arms and not the bulge

Although strictly part of the [[Galaxy morphological classification#De Vaucouleurs system|de Vaucouleurs system]] of classification, the Sd class is often included in the Hubble sequence. The basic spiral types can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending two lower-case letters to the main galaxy type (for example, Sbc for a galaxy that is intermediate between an Sb and an Sc).

Our own [[Milky Way]] is generally classed as Sc or SBc,<ref>{{cite journal |first=Paul |last=Hodge |year=1983 |title=The Hubble type of the Milky Way galaxy |journal=Publications of the Astronomical Society of the Pacific|volume=95 |pages=721–723 |doi=10.1086/131243 |bibcode=1983PASP...95..721H |s2cid=121754114 |doi-access=free }}</ref> making it a barred spiral with well-defined arms.

Examples of regular spiral galaxies: ([[Andromeda_Galaxy#Structure|visually]]) [[Andromeda Galaxy|M31]] (Andromeda Galaxy), [[Messier 74|M74]], [[Messier 81|M81]], [[Sombrero Galaxy|M104]] (Sombrero Galaxy), [[Whirlpool Galaxy|M51a]] (Whirlpool Galaxy), [[NGC 300]], [[NGC 772]].

Examples of barred spiral galaxies: [[Messier 91|M91]], [[Messier 95|M95]], [[NGC 1097]], [[NGC 1300]], [[NGC1672]], [[NGC 2536]], [[NGC 2903]].