- Active galactic nucleus (AGN): It is a compact region at the centre of a galaxy which has a much higher than normal luminosity over some or all of the electromagnetic spectrum. A galaxy hosting an AGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion on to the supermassive black hole at the centre of the host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects.
- Aquarius Dwarf galaxy: It is a dwarf galaxy and an irregular galaxy. Its most distinctive characteristic is that it is one of the few galaxies known to display a blueshift, as it is travelling towards the Milky Way at 137 km/s. Aquarius dwarf is also part of the Aquarius constellation.
- Barred spiral galaxy: It is a spiral galaxy with a central bar-shaped structure comprised of stars. Bars are found in approximately half of all spiral galaxies.
- Blue compact dwarf galaxy: It is a small galaxy which contains large clusters of young, hot, massive stars. These stars cause the galaxy to appear blue in colour.
- Clusters and group Galaxy: they are the largest gravitationally-bound objects. They form the densest part of the large scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called superclusters.
- Elliptical galaxy: A galaxy which has an ellipse-shaped light profile.
- Galaxy mergers: It can occur when two (or more) galaxies collide. They
are the most violent type of galaxy interaction. Although galaxy mergers
do not involve stars or star systems actually colliding, due to the vast
distances between stars in most circumstances, the gravitational interactions
between galaxies and the friction between the gas and dust have major effects
on the galaxies involved.
" When one of the galaxies is significantly larger than the other,
the larger will often "eat" the smaller, absorbing most of its
gas and stars with little other major effect on the larger galaxy. The Milky
Way is thought to be currently absorbing smaller galaxies in this fashion,
such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds.
" If two spiral galaxies of approximately the same size collide at
appropriate angles and speeds, they will likely merge in a fashion that
drives away much of the dust and gas through a variety of feedback mechanisms
that often include a stage in which there are active galactic nuclei. This
is thought to be the driving force behind many quasars. The end result is
an elliptical galaxy.
- Irregular galaxies: Small galaxies that lack a coherent structure are also be referred to as irregular galaxies.
- Large Magellanic Cloud (LMC): It is a nearby satellite galaxy of the Milky Way. At a distance of slightly less than 50 kiloparsecs (?160,000 light-years), the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal (~ 16 kiloparsecs) and Canis Major Dwarf Galaxy (~ 12.9 kiloparsecs) lying closer to the centre of the Milky Way. It has a mass equivalent to approximately 10 billion times the mass of our Sun (1010 solar masses), making it roughly 1/10 as massive as the Milky Way. The LMC is the fourth largest galaxy in the local group, with the Andromeda Galaxy (M31) and Triangulum Galaxy (M33) also having more mass. It is visible as a faint 'cloud' in the night sky of the southern hemisphere, straddling the border between the constellations of Dorado and Mensa.
- Peculiar galaxies: These are galaxies with irregular or unusual shapes; they typically result from disruption by the gravitational pull of neighbouring galaxies.
- Quasar (contraction of QUASi-stellAR radio source): It is an extremely bright and distant active galactic nucleus. They were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light that were point-like, similar to stars, rather than extended sources similar to galaxies. While there was initially some controversy over the nature of these objects, there is now a scientific consensus that a quasar is a compact halo of matter surrounding the central supermassive black hole of a young galaxy.
Radio galaxies: They and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminous at radio wavelengths (up to 1038 W between 10 MHz and 100 GHz). The radio emission is due to the synchrotron process. The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming. Radio-loud active galaxies are interesting not only in themselves, but also because they can be detected at large distances, making them valuable tools for observational cosmology.
- Sagittarius Dwarf Elliptical Galaxy (SagDEG): It is an elliptically looped shaped satellite galaxy of the Milky Way Galaxy. The main cluster is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth and travelling in a polar orbit at a distance of about 50,000 light-years from the core of the Milky Way.
- Sagittarius Dwarf Irregular Galaxy or SagDIG: It is a dwarf galaxy in the constellation of Sagittarius. It lies about 3.4 million light-years away. SagDIG is a much more luminous galaxy than Aquarius Dwarf and it has been through a prolonged star formation.
- Spiral galaxies: Galaxies that are disk-shaped assemblages with curving, dusty arms.
- Spiral nebula: It is an old term for a spiral galaxy.
- Starburst galaxies: Interactions between nearby galaxies may ultimately result in galaxies merging and may induce star formation, producing what is called a starburst galaxy.
- Superclusters: They are large groupings of smaller galaxy groups and
clusters, and are among the largest structures of the cosmos. The existence
of superclusters indicates that the galaxies in our Universe are not uniformly
distributed; most of them are drawn together in groups and clusters, with
groups containing up to 50 galaxies and clusters up to several thousand.
Those groups and clusters and additional isolated galaxies in turn form
even larger structures called superclusters.