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~ By Suheda Falay

Many of us have heard the term "Galaxy." It exists in the universe, and it includes our solar system, planets, stars, black holes, and other things. Do we also know how these galaxies formed, how long they've been around, and what types they are? If not, you will have discovered it after reading this article.

A galaxy is a massive collection of gas, dust, and billions of stars and solar systems held together by gravity. Again, under the influence of gravity, they can collide and form larger galaxy clusters. The universe's first stars are thought to have formed around 180 million years after the big bang. The formation of galaxies after the formation of the first stars occurs approximately 400 million years after the big bang. Although 400 million years may appear to be a long time, it is only 3% of the universe's current age (13.8 billion years).


We did not even know galaxies existed outside of the Milky Way until the 20th century; astronomers previously classified them as "nebulae" because they resembled fuzzy clouds. However, astronomer Edwin Hubble demonstrated in the 1920s that the Andromeda "nebula" was a galaxy in its own right. Despite its vast distance, Andromeda is the closest large galaxy to the Milky Way and is visible with the naked eye in the night sky.

Galaxy types are classified based on their morphological structures. The first scheme proposed by Edwin Hubble in 1926 is the basis for almost all existing galaxy classification systems. This is known as the Hubble tuning fork diagram. According to their morphological structures, galaxies are divided into four major groups in the Hubble tuning fork diagram: spiral galaxies, elliptical galaxies, lenticular galaxies, and irregular galaxies.


Spiral galaxies have a brighter and bulging region in the center due to the density of stars, and spiral arms surround this region. The helical arms are arranged to form a relatively flat disk with a slight thickness. In addition, spiral galaxies are surrounded by a halo. The galactic halo is a low-density structure made mostly of gas and dust that surrounds the galaxy in an approximately spherical shape. While the halo and central bulge we just mentioned are primarily composed of older stars, the spiral arms are typically composed of dust, gas, and relatively younger stars.

Spiral galaxies account for more than two-thirds of all observed galaxies. A spiral galaxy is characterized by a flat, rotating disk containing a central bulge surrounded by spiral arms. This rotational motion, which occurs at hundreds of kilometers per second, can cause matter in the disk to take on a distinctive spiral shape, similar to a cosmic pinwheel. Like other spiral galaxies, our Milky Way has a linear starry bar in the center.


Elliptical galaxies can have either a round or oval shape. The stars in this type of galaxy are generally evenly distributed throughout the galaxy, implying that the stellar density of the galaxy is roughly the same throughout (except at its centre). They have a halo and a bulging region with denser stars at their centers, just like spiral galaxies. However, unlike spiral galaxies, they do not appear as a flat disk. This type of galaxy contains stars that are relatively older.

The universe's largest known elliptical galaxies can contain up to a trillion stars and extend two million light-years. These are known as giant elliptical galaxies. Of course, not all elliptical galaxies are giant elliptical galaxies. There are also much smaller ones, and depending on their size, some are referred to as dwarf elliptical galaxies.


Because they resemble lenses, they are called "lenticular." Lenticular galaxies are intermediate in shape between spiral and elliptical galaxies. They have a thin, rotating disk and a bulging region at their center, just like spiral galaxies, but no spiral arms; they also contain small amounts of dust and interstellar material, just like elliptical galaxies. These galaxies form primarily in densely packed regions of the universe. The Sombrero galaxy is a well-known example of a lenticular galaxy. It also clearly depicts the appearance of such galaxies.


Irregular galaxies, such as the Large and Small Magellanic Clouds that surround the Milky Way, are amorphous galaxies that have no distinct shape but can be found in a variety of shapes. Gravitational perturbations or collisions are usually the causes of such deformities. Those that were originally spiral or elliptical can become irregular as a result of such interaction, as one might expect. Different processes may be activated as a result of the collision.


The universe was covered in a dense fog of primordial atomic particles for the first few hundred million years, during which time everything was hot and opaque. The universe then started to grow and cool over time. The first stars appeared in newborn galaxies 400 million years after the Big Bang. These early galaxies collided, creating star clusters that were bigger than they had been before.

Two significant theories help to explain how galaxies form. These models are top-down and bottom-up. While the first model argues that galaxies are generated by the collision and merging of smaller gas clouds, the second model claims that galaxies are formed by the collision and merger of larger gas clouds.

Although galaxies form in a similar way, their appearance after formation is different. The angular momentum of the gas cloud or clouds that created the galaxy is assumed to be responsible for the many forms of galaxy formation. The resultant galaxy tends to create a fast rotating system (spiral galaxy) with a flat structure when the original galaxy formed following the collision or collapse has significant angular momentum; when it has a relatively lower angular momentum, it tends to form a more round system (elliptical galaxy).

The question of galaxy formation is still debatable today, and there are still a lot of unanswered problems and models.



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