Irregular galaxy

Irregular Galaxy

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Irregular galaxy

Some galaxies do not have a regular shape, like a spiral

Spiral galaxy

A spiral galaxy is a galaxy belonging to one of the three main galaxy morphological classification originally described by Edwin Hubble in his 1936 work ?The Realm of the Nebulae? and, as such, forms part of the Hubble sequence....
or an elliptical galaxy

Elliptical galaxy

An elliptical galaxy is a galaxy having an approximately ellipsoid shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flattened and in size from hundreds of millions to over one trillion stars....
. Those galaxies are known as irregular galaxies. Their shape is uncommon. They do not fall into any of the regular classes of the Hubble sequence

Hubble sequence

The Hubble sequence is a galaxy morphological classification for galaxies invented by Edwin Hubble in 1927. It is often known colloquially as the Hubble tuning-fork diagram because of the shape in which it is traditionally represented....
, and they are often chaotic in appearance, with neither a nuclear bulge

Bulge (astronomy)

In astronomy, a bulge is a tightly packed group of stars. The term almost exclusively refers to the central group of stars found in most spiral galaxy....
nor any trace of spiral arm structure.

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How do irregular galaxies differ from spiral and eleptical galaxies?

Encyclopedia

Some galaxies do not have a regular shape, like a spiral

Spiral galaxy

Elliptical galaxy

Hubble sequence

Bulge (astronomy)

In astronomy, a bulge is a tightly packed group of stars. The term almost exclusively refers to the central group of stars found in most spiral galaxy....
nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by gravitational action. Irregular galaxies also contain abundant amounts of gas and dust.

There are two major Hubble types of irregular galaxies:

An Irr-I galaxy (Irr I) is an irregular galaxy that features some structure
Structure

Structure is a fundamental and sometimes intangible notion covering the recognition, observation, nature , and stability of patterns and relationships of entities....
but not enough to place it cleanly into the Hubble sequence. de Vaucouleurs subtypes this into galaxies that have some spiral structure Sm, and those that do not Im.

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 third classification of irregular galaxies are the dwarf irregulars, labelled as dI or dIrrs. 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

Metallicity

In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium....
and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local (and therefore more recent) version of the faint blue galaxies

Faint blue galaxy

A faint blue galaxy is a distant, irregularly shaped galaxy in which star formation occurs at a high rate. From the late 1970s it became apparent in Hubble Ultra Deep Fields that a population of these blue galaxies existed at vast distances....
known to exist in deep field galaxy surveys

Hubble Ultra Deep Field

The Hubble Ultra Deep Field, or HUDF, is an image of a small region of outer space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 24, 2003 through January 16, 2004....
.

Some of the irregular galaxies are small spiral galaxies that are being distorted by the gravity of a larger neighbor.

The Magellanic Cloud galaxies

Magellanic Clouds

The two Magellanic Clouds are irregular galaxy dwarf galaxy Galaxy morphological classification, which are members of our Local Group of galaxies....
were once classified as irregular galaxies, but have since been found to contain barred spiral structures, and have been since re-classified as "SBm", a fourth type of barred spiral galaxy

Barred spiral galaxy

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately half of all spiral galaxies....
, the barred Magellanic spiral

Magellanic spiral

Magellanic spiral galaxies are galaxies which are classified as the type Sm . They are galaxies with one spiral arm, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy....
type.

Irregular galaxy

See also