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An Irr-II galaxy ( Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence. A dI-galaxy (or dIrr) is a dwarf irregular galaxy. [7] This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and ...
Tuning-fork-style diagram of the Hubble sequence. Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin ...
Magellanic spiral. A Magellanic spiral galaxy is a spiral galaxy with only one spiral arm. Magellanic spiral galaxies are classified as the type Sm (with sub-categories SAm, SBm, SABm ); the prototype galaxy and namesake for Magellanic spirals is the Large Magellanic Cloud, an SBm galaxy. They are usually smaller dwarf galaxies and can be ...
The percentage of galaxies with a particular axis ratio (minor/major) for a sample of lenticular and spiral galaxies. The inset is a visual representation of the profile of either at the specified minor (b) to major (a) axis ratios. Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component.
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to ...
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge.
Dwarf galaxy. A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way 's 200–400 billion stars. [1] The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, [2] is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy.
Size (left) and distance (right) of a few well-known galaxies put to scale. The following is a list of notable galaxies.. There are about 51 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in the Local Supercluster, and an estimated 100 billion in all of the observable universe.