Galaxies
Galaxies
Galaxies are vast systems composed of stars, stellar remnants, interstellar gas, dust, and dark matter, all bound together by gravity. These massive entities are the fundamental building blocks of the universe. The observable universe is estimated to contain between 200 billion to 2 trillion galaxies, each varying significantly in size, structure, and composition.
Classification
Galaxies are primarily classified based on their visual morphology into three main types: elliptical galaxies, spiral galaxies, and irregular galaxies.
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Elliptical Galaxies: These are characterized by their elongated spherical shapes and lack of structure. They contain older stars and less interstellar matter, making star formation processes minimal.
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Spiral Galaxies: These galaxies, like our own Milky Way, have a flat, rotating disk with spiral arms extending from a central bulge. The arms are sites of active star formation and are rich in gas and dust.
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Irregular Galaxies: As the name suggests, these galaxies do not have a distinct shape and do not fit into the categories of elliptical or spiral galaxies. They often show chaotic appearances due to gravitational interactions or collisions with other galaxies.
Structure and Composition
The structure of a galaxy is typically organized into a galactic core, disk, and halo. The core is densely packed with stars, often dominated by a supermassive black hole. The disk contains stars, planets, and nebulae, and it is the site of most of the galaxy's star formation activity. The halo surrounds the galaxy and is composed of older stars and globular clusters.
Dark matter plays a crucial role in a galaxy’s mass and dynamics. Though it does not emit or absorb light, it exerts gravitational forces that influence the motion of stars and gas within galaxies.
The Local Group and Superclusters
Galaxies are not isolated; they are often found in groups, clusters, and superclusters. The Local Group is the galaxy group that includes the Milky Way, the Andromeda Galaxy, and about 54 other smaller galaxies. This group is part of the Virgo Supercluster, which itself is a segment of the Laniakea Supercluster.
Intergalactic Medium
The space between galaxies is known as the intergalactic medium. It consists of a very low-density gas, primarily hydrogen, with an average density of less than one atom per cubic meter. This medium can be a crucial factor in the evolutionary processes of galaxies.
Galaxy Filaments
On a grander scale, galaxies are part of enormous structures called galaxy filaments, which are the largest known structures in the universe. These filaments consist of threads of galaxies and galaxy clusters and are separated by vast voids.