What are the types of galaxies: and clusters?
The Universe what we understand today is infinite and expanding day by day. However, by 1923, it was an established notion that the universe was limited only to the milky way galaxy. Many astronomers including the famous Harlow Shapley, who calculated the actual distance of the milky way were ardent advocates of this notion. They assumed that Andromeda was a nebula within the milky way, the lone galaxy of the popular universe.
What are galaxies:
Galaxies are bigger collections of gas, and dust,(very important elements for the formation of new stars and planets) stars and their planetary systems, are held together by gravity, dark matter, and dark energy. Every galaxy has some shapes like a spiral, elliptical and irregular, etc.
Smaller galaxies may host a few hundred million stars, whereas the largest galaxies contain up to one hundred trillion stars. ” Galaxies are the representation of how matter in the Universe was organized -at last, on the large scale,” says NASA. Astronomers and researchers have estimated very huge numbers, such as two trillion galaxies in our known universe.
Search for galaxies:
Since time immemorial the sky (when the term space was not coined) was an amazing, & curious horizon for philosophers, researchers, and inquisitors. Stars fitted clear sky made many poets and philosophers. The milky way was popular even in the early 4th-5th century BC as per available records.
Democritus, an ancient Greek pre-Socratic philosopher during the 5th century BC proposed that the milky way consists of stars. In the 4th century, BC Aristotle also observed the milky way. However major breakthrough in astronomy came in 2nd half of the 10th century, when Abd al-Rahman al -Sufi (Azophi), a Persian astronomer made the first recording of his observations about the Andromeda Galaxy and the Large Magellanic Cloud in his Book of Fixed Stars.
It was a historic observation made beyond the milky way by Persian scholars in the ancient time when no modern instruments were available to see distant objects. During the medieval period, Persian and Muslim astronomers and scholars continued their studies on stars, planets, and the sky.
In 1610, Galileo Galilei used a telescope to observe the milky way and he discovered that a galaxy was composed of a huge number of faint stars. French astronomer Charles Messier contributed a lot to space research. He was an expert in comet observation, but in 1774 he released a list of Messier objects where Andromeda and Triangulum galaxies were included.
Charles Messier’s discoveries were finally published as Messier objects, whereas 34 objects were later declared as galaxies. Further, no considerable progress was witnessed in the search for new galaxies and almost all astronomers have concluded that the milky way was a lone galaxy where the entire universe exists.
The shocking history was waiting to be created in near future. Edwin Hubble and a small team of scientists headed by Heber Curtis of Lick Observatory at the University of California started observing Andromeda in 1923. Edwin Hubble started observing Andromeda at the Mount Wilson Observatory, with the world’s largest Hooker telescope.
Finally, he got a Cepheid variable star blinking in one of the spiral arms of Andromeda, called “Variable number one” or VI, which revolutionized the understanding of the universe. The light from VI made it clear that Andromeda was an incredibly old galaxy, not a nebula, and not included in the milky way. Hubble presented his findings at a conference of the American Astronomical Society held in January 1925, where everybody was stunned and the world shocked. The new findings paved the way for further research beyond the milky way in space.
To date, more than 30,000 galaxies and galaxy clusters have been identified and cataloged, out of which more than 10,000 have been discovered by NASA.
Types of galaxies:
The galaxies have been classified on the basis of their shapes and size captured in telescopes and observatories, such as;
The spiral galaxies look like flat disk shapes of rotating stars, of gas and dust. Such types of galaxies were more popular and constitute 3/4th of the total galaxies discovered to date. They are mostly found in low-density regions of the galaxy clusters. Spiral galaxies were first described in detail by Edwin Hubble in 1936.
Further, the spiral galaxies are divided into a normal spiral and barred spiral groups. In the case of normal spirals, the arms appear to emanate from the nucleus, whereas in barred spiral contains a bright linear structure known as a bar that straddles the nucleus. Almost all spiral galaxies’ centers of their bulge host a supermassive black hole.
Researchers estimated that spiral galaxies are formed about 1.4 billion years after the Big Bang. Some examples of this type of galaxy are our own galaxy milky way, M 74/NGC 628, and neighbor Andromeda galaxy.
Milky Way is a barred spiral type, has four principal arms and a number of minor arms referred to as spurs. The four arms are the Norma and Cygnus arm, Sagittarius, Scutum-Crux, and Perseus. The Sun is placed in a minor arm called the Orion spur. The galactic disk of the milky way is estimated as about 100,000 light-years across and the bar at the center is about 27,000 light-years.
Andromeda ( Messier-31/M-31) is a barred spiral galaxy that is included in a local group of galaxies. It is our neighbor & closest big galaxy and is expected to collide with the Milky way around 4.5 billion years from now. Andromeda is about 2,500,000 light years away from earth.
Elliptical galaxies appear like large spherical or elliptical patterns of stars and some are cigar-shaped. They have little gas and dust consisting of low-mass old red stars, having no nucleus or bulge at the center. It is not clear how they are formed. However, scientists believe that they have been formed due to galactic collisions.
Further, it was apprehended that they were once spiral and reduced to such forms. At the center of the galaxy, it appears very bright and gradually fades to the outer edges of galaxies. Example-K87, Hercules A, ESO 325-G004, etc.
Peculiar galaxies have no specific shape or composition and are formed due to collisions of two or more galaxies as suggested by scientists. Five to ten percent of known galaxies are included in this category. Further such types of galaxies have a more active nucleus at the center which signals the presence of massive black holes in the centers.
They are two types, including interacting and active galactic nuclei (AGN). In interacting galaxies, the gravity of a galaxy intervenes with another and vice versa. Normally a satellite galaxy may disturb any of the arms of the main galaxy.
An active galactic nucleus is very bright at the central region of a galaxy where electromagnetic radiation dominates, due to the presence of a supermassive black hole in the center of the galaxy. Peculiar galaxies are treasures of very useful information regarding galactic evolution and formations. Example-ESO 162-17.
The galaxies which are not coming under any of the above categories are called irregular galaxies. They are small and dwarf, contain fewer stars in comparison to other galaxies, and have no nuclear or bulge at the center.
Mostly contain hydrogen gas and scientists thought they were once spiral or elliptical and not classified in the Hubble classification system. May be formed due to the collision of galaxies. Many of these galaxies are satellites or companions to larger galaxies. Example- Large Magellanic cloud, the satellite galaxy of the milky way.
Clusters and Superclusters:
A cluster is a group of galaxies identified to monitor or for understanding by scientists and astronomers. Superclusters contain 6 to 10 clusters. Wikipedia has divided the superclusters into four broad categories, such as galaxy supercluster, nearby supercluster, distance supercluster, and extremely distance supercluster.
Galaxy Super Cluster:
Laniakea superclusters, Virgo superclusters, Hydra Centaurus superclusters, Pavo-Indus supercluster, Southern supercluster, and Saraswati supercluster are included in Galaxy superclusters. A local group of galaxies like the milky way, Andromeda, Triangulum, Large Magellanic Cloud, Small Magellanic Cloud, and around other seventy dwarf galaxies are coming under Laniakea Superclusters.
Nearby Super Cluster:
Coma supercluster, Sculptor Supercluster, Hercules Super Cluster, Leo Super Cluster, Ophiuchus supercluster, Shapley supercluster, etc are included in the Nearby Super Cluster.
Distance Super Cluster: Distance Super Clusters include Pisces-Cetus Super Cluster, Horologium Reticulum Supercluster, Corona Borealis Super Cluster, and other twenty super clusters included in this group.
Extremely Distance Supercluster: Hyperion proto Super cluster discovered in 2018 was the largest and earliest proto-super cluster found to date included in this supercluster. Lynx supercluster, SCL @ 1338+27 at z=1.1, SCL@1604+43 at z=0.9, SCL@ 0018+16at z=0.54 in SA 26 and MS 0302+17 are included in Extremely Distance Superclusters.
It is estimated by astronomers and scientists that there are two trillion galaxies in the known universe. Since its lunch into low Earth orbit in 1990, Hubble Space Telescope has done a tremendous job in search of galaxies.
It has discovered around 10,000 galaxies in a small patch of sky as per NASA. Hubble can see back at galaxies formed 450 million years after Big Bang but James Webb Telescope(on 25 December 2021 launched to space) has the capability to look back at around 200 million years after Big Bang, which means new discoveries are waiting for James Webb Telescope. Pictures of galaxies and other objects taken by James Webb were definitely spectacular and amazing.