Irregular Galaxy
Irregular Galaxy
As the name suggests, "irregular" galaxies have no specific form, and so
the group contains a very diverse selection of objects. In fact, there
are two types of irregular galaxy. Type I's are usually single galaxies
of peculiar appearance. They contain a large fraction of young stars,
and show the luminous nebulae
that are also visible in spiral galaxies. Type II irregulars include
the group known as interacting or disrupting galaxies, in which the
strange appearance is due to two or more galaxies colliding, merging or
otherwise interacting gravitationally. Type II's appear to contain a
large amount of dust.
Most irregulars are small and faint. The
dwarf irregulars may be the most common type of galaxy in the universe (or maybe
the dwarf ellipticals are). The estimates of the number of dwarf irregulars
and dwarf ellipticals are based on the proportions of these types of galaxies
in nearby groups. The dwarf galaxies far away are too faint to be seen and
are, therefore, overlooked in surveys of the sky. Perhaps if the dwarf
galaxies were brighter, Hubble would have arranged the galaxies in a different
sequence instead of the two-pronged sequence. Examples of irregular galaxies
are the Large and Small Magellanic Clouds (two small irregulars that orbit the
Milky Way).
Type 1 Irregular Galaxy :
( ngc55)
In the light of the name "irregular", it is perhaps surprising that some systematic structure is
observed in the type I galaxies. In fact, they are most closely related
to spirals, and this is a relation which is alluded to in the presence
of disks and bulges like their more orderly counterparts. However, it is
here that the similarity ends, since the disks of irregular galaxies
certainly show no sign of the spiral structure, and the galactic bulges
are not even located at the centre of the object .These galaxies may in some ways be regarded as "primitive". They are relatively poor in "heavy" elements
Type II irregulars galaxy :
Type II irregular galaxies are
remarkable, and often very spectacular, objects. They can be formed by
several mechanisms, and one of the most common is a gravitational
interaction with another nearby galaxy.
The idea of
collisions between galaxies might at first seem unlikely. In general,
galaxies are widely scattered, and there exists so much space between
one and the next, that only one collision would occur in around 100
times the age of the universe, so that we would not expect to see any of
these objects. However, it must be remembered that galaxies often exist
in clusters, where the average spacing between cluster members is much
smaller.
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