spiral galaxy. This was extended by Gérard de Vaucouleurs 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 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, making it a barred spiral with well-defined arms. Examples of regular spiral galaxies: (visually) M31 (Andromeda Galaxy), M74, M81, M104 (Sombrero Galaxy), M51a (Whirlpool Galaxy), NGC 300, NGC 772. Examples of barred spiral galaxies: M91, M95, NGC 1097, NGC 1300, NGC1672, NGC 2536, NGC 2903. Irregulars Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed irregular galaxies. Hubble defined two classes of irregular galaxy: Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the Magellanic Clouds – two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure in the Large Magellanic Cloud led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the Small Magellanic Cloud (denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork. Examples of irregular galaxies: M82, NGC 1427A, Large Magellanic Cloud, Small Magellanic Cloud. Physical significance Elliptical and lenticular galaxies are commonly referred to together as "early-type" galaxies, while spirals and irregular galaxies are referred to as "late types". This nomenclature is the source of the common, but erroneous, belief that the Hubble