Galaxies are vast, gravitationally bound collections of dust, gas, dark matter, and anything from a million to a trillion stars. Almost every giant galaxy is believed to have a supermassive black hole in its core. The sun is only one of about 100 to 400 billion stars that revolve around Sagittarius A*, a supermassive black hole with a mass of four million suns.

The farther into the universe we gaze, the more galaxies we observe. According to one 2016 research, the visible universe includes two trillion galaxies—or two million million galaxies. While some of those remote systems have a striking resemblance to our own Milky Way galaxy, others are very unlike.


Galaxies are classified according to their types.

Before the twentieth century, we were unaware that galaxies other than the Milky Way existed; older astronomers referred to them as "nebulae" due to their appearance as fuzzy clouds. However, astronomer Edwin Hubble demonstrated in the 1920s that the Andromeda "nebula" was a galaxy in its own right. Due to its great distance from us, it takes more than 2.5 million years for light from Andromeda to cross the chasm. Despite its enormous length, Andromeda is the nearest giant galaxy to our Milky Way, and it is bright enough in the night sky in the Northern Hemisphere to be seen to the naked eye.


Hubble introduced a classification system for galaxies in 1936, categorizing them as spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.


Spiral galaxies account for more than two-thirds of all observable galaxies. A spiral galaxy is characterized by a flat, rotating disk with a centre bulge encircled by spiral arms. At hundreds of kilometres per second, this spinning action may lead the disk's matter to take on a unique spiral form, similar to a cosmic pinwheel. Our Milky Way, like other spiral galaxies, is centred on a linear, starry bar.


Elliptical galaxies are spherical but may extend further along one axis than the other, to the point that some take on a cigar-like appearance. The enormous known galaxies in the cosmos are called gigantic elliptical galaxies. They may contain up to a trillion stars and span two million light-years in diameter. Elliptical galaxies may also be very tiny, referred to as dwarf elliptical galaxies.


A high concentration of older stars characterizes elliptical galaxies but a shortage of dust and other interstellar debris. Like those in spiral galaxies' disks, their stars circle the galactic centre but more random paths. In elliptical galaxies, few new stars are known to develop. They are often found in galaxy clusters.


Lenticular galaxies, such as the eponymous Sombrero Galaxy, are intermediate in elliptical and spiral galaxies. They are dubbed "lenticular" because they resemble lenses: They are similar to spiral galaxies in that they contain a tiny, spinning disk of stars and a central bulge but lack spiral arms. They, like elliptical galaxies, have a low amount of dust and interstellar matter and seem to develop more often in highly inhabited areas of space.


Inspiral, lenticular, or elliptical galaxies are referred to as irregular galaxies. Irregular galaxies, such as the Large and Small Magellanic Clouds that border our Milky Way seem deformed and devoid of distinctive form, often due to their proximity to neighbouring galaxies. They are dense in gas and dust, making them excellent nurseries for the formation of young stars.



Clusters and mergers of galaxies

While some galaxies exist alone or in pairs, they are more often associated with giant groupings, clusters, and superclusters. Our Milky Way, for example, is a member of the Local Group, a galaxy cluster approximately ten million light-years in diameter that also contains the Andromeda galaxy and its satellites. The Local Group and its neighbour galaxy cluster, the Virgo Cluster, are included inside the larger Virgo Supercluster, a collection of galaxies measuring about 100 million light-years in diameter. The Virgo Supercluster, in turn, is a limb of Laniakea, a much larger supercluster of 100,000 galaxies identified in 2014 by scientists.


Clusters of galaxies often interact and merge in a dynamic cosmic dance of interacting gravity. When two galaxies collide and merge, gases may flow toward the galactic centre, triggering phenomena such as rapid star formation. In about 4.5 billion years, our own Milky Way galaxy will join with the Andromeda galaxy.


Because elliptical galaxies contain more mature stars and less gas than spiral galaxies, it seems as if the galaxy types reflect a natural evolution. When spiral galaxies age, interact and merge, they lose their recognizable forms and transform into elliptical. However, scientists are still deciphering the details, such as why elliptical galaxies exhibit certain brightness, size, and chemical composition patterns.


Origins of the galaxy

The universe's earliest stars formed about 180 million years after the big bang, the explosive event that occurred 13.8 billion years ago and marked the beginning of the cosmos as we know it. By the time the universe reached 400 million years old or less than 3% of its present age, gravity had shaped the first galaxies.


Astronomers currently believe that almost all galaxies – with potential exceptions – are surrounded by massive haloes of dark matter. Additionally, theoretical models indicate that enormous tendrils of dark matter supplied the gravitational scaffolding necessary for a standard topic to form into the earliest galaxies in the early universe.


However, there are still unanswered issues regarding the formation of galaxies. According to some, galaxies originated from smaller groups of approximately one million stars called globular clusters, while others think that galaxies formed first and then gave birth to globular clusters. Additionally, it is challenging to determine how many stars in a galaxy originated in situ from its gas vs developing in another universe and joining the party later.