Galaxies: structure, classification, dark matter, and cosmic distribution
Overview of galaxy structure and morphology, the roles of stars, gas and dominant dark matter, central black holes, and how galaxies are organized from dwarf systems to clusters across the cosmic web.
Galaxy describes a vast system made up of hundreds of millions to trillions of stars together with interstellar gas, cosmic dust and a dominant component known as dark matter. These components are bound by gravity into a single, coherent object that can range in size from small dwarf systems to giant galaxies containing over a trillion stars. The English word derives from the Greek term galaxia, literally meaning milky, a reference to our own home galaxy, the Milky Way.
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Structure and components
Stars are the most visible constituents of a galaxy, but much of a galaxy's mass is in diffuse gas and invisible dark matter. Interstellar dust obscures and reddens starlight in some regions and contributes to the formation of new stars. Many galaxies also host a compact, very massive object at their centres—often a supermassive black hole—which can influence the motions and evolution of the central regions.
Gravity and large-scale behavior
Gravity is the force that keeps individual galaxies and collections of galaxies from dispersing, counteracting the general expansion of the cosmos. The expansion of space acts primarily on very large scales between separated groups and clusters; inside a galaxy, or inside a gravitationally bound group, gravity dominates and holds systems together. The motions of stars and gas in a galaxy are determined by the combined gravitational pull of its visible matter and its dark matter halo, so that nearly everything in a galaxy orbits around the system's centre of mass.
Galaxies themselves are often found in associations. For example, our Milky Way and its neighbors belong to the Local Group, while larger concentrations such as the Virgo Cluster contain many hundreds to over a thousand member galaxies. These clusters and superclusters are the largest bound structures in the observable cosmos.
Morphological classes
Astronomers sort galaxies by their shapes and internal features. Major categories include:
- Elliptical galaxies — smooth, ellipsoidal systems with relatively little gas and star formation.
- Spiral galaxies — flattened disks with spiral arms; some spirals have a central bar structure.
- Lenticular galaxies — intermediate systems with a disk but little spiral structure, which may or may not show a bar.
- Irregular galaxies — systems without a regular shape, often the result of interactions or other disturbances.
Numbers and cosmic context
Every galaxy exists within the larger universe. Deep observations and statistical studies indicate the observable Universe contains more than two trillion (2 × 10^12) galaxies across all sizes and types. The total number of stars in those galaxies is extremely large; estimates put the count at on the order of 1 × 10^24 stars—far more than the number of grains of sand on planet Earth. These figures are continually refined as surveys improve and new telescopes extend our reach.
General
Galaxies vary greatly in appearance (morphology), size and composition. The Milky Way has a diameter of up to 200,000 light years and thus belongs to the larger galaxies. Its nearest neighbour galaxy of comparable size is the Andromeda Galaxy at a distance of about 2.5 million light years. Together with other galaxies of smaller mass, both galaxies form the Local Group. Galaxies often appear in groups or clusters with up to several thousand members.
Exploration history
Before the power of astronomical telescopes was sufficient to resolve distant galaxies into individual stars, they appeared as "nebulae". For a long time it was unclear whether these "spiral nebulae" belonged to the Galaxy or formed their own star systems. Immanuel Kant already suspected milky way-like star systems in the "nebulous stars", and in 1923 Edwin Hubble succeeded in clarifying this question. He determined the distance to the Andromeda Nebula and found that it is much too far away to belong to the Milky Way, i.e. it represents a galaxy of its own.
Questions and answers
Q: What is a galaxy?
A: A galaxy is a group of many stars, gas, dust and dark matter held together by gravity.
Q: Where does the word 'galaxy' come from?
A: The name 'galaxy' comes from the Greek word galaxia meaning milky, which is a reference to our own galaxy, the Milky Way.
Q: How are galaxies held together?
A: Galaxies are held together against the general expansion of the universe by gravity. Expansion of the universe takes place between groups of galaxies rather than inside them.
Q: What types of galaxies exist?
A: There are various types of galaxies including elliptical, spiral and lenticular galaxies with or without bars as well as irregular galaxies.
Q: How many stars are in an estimated 1x1024 galaxies?
A: An estimated 1x1024 galaxies contain more stars than all the grains of sand on planet Earth.
Q: How many observable universes exist? A: The observable Universe contains more than 2 trillion (1012) galaxies.
Related articles
Author
AlegsaOnline.com Galaxies: structure, classification, dark matter, and cosmic distribution Leandro Alegsa
URL: https://en.alegsaonline.com/art/37265
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