Galaxy:

Introduction:

A galaxy is a collection of stars, stellar remnants, interstellar gas, dust, and dark matter that is gravitationally bound. The name comes from the Greek word galaxias, which means "milky" and refers to the Milky Way galaxy, which houses the Solar System. Galaxies range in size from dwarfs with fewer than one hundred million (108) stars to the largest galaxies known as supergiants with one hundred trillion (1014) stars, each orbiting its galaxy's centre of mass. Galaxies are thought to contain an average of 100 million stars.

 

Galaxy

According on their optical shape, galaxies are classified as elliptical,[5] spiral, or irregular. It's believed that the centres of several of them contain supermassive black holes. Sagittarius A*, the name of the Milky Way's primary black hole, is four million times more massive than the Sun. GN-z11 is the oldest and furthest away galaxy as of March 2016. It can be seen as having existed only 400 million years after the Big Bang and is located 32 billion light-years away from Earth.

 

Following a 2016 estimate that there were two trillion (21012) or more galaxies in the observable universe overall and as many as an estimated 11024 stars, the estimate for the number of galaxies in the observable universe was revised in 2021 to approximately 200 billion galaxies (21011). (more stars than all the grains of sand on all beaches of the planet Earth).The majority of galaxies have diameters between 1,000 and 100,000 parsecs (or around 3,000 and 300,000 light years) and are spaced apart by millions of parsecs or more (or megaparsecs). For comparison, the distance between the Milky Way and its nearest massive neighbour, the Andromeda Galaxy, which has a diameter of around 152,000 ly, is 780,000 parsecs, is at least 26,800 parsecs (87,400 ly) (2.5 million ly.)

 

The intergalactic medium, a thin gas with an average density of fewer than one atom per cubic metre, fills the space between galaxies. Most galaxies form groups, clusters, and superclusters through gravitational organisation. The Milky Way is a member of the Local Group, which it and the Andromeda Galaxy dominate. A member of the Virgo Supercluster, the group. These relationships are typically organised into sheets and filaments at the greatest scale, encircled by enormous spaces. The Laniakea structure, a much bigger cosmic structure, contains both the Local Group and the Virgo Supercluster.

 

Historical observations:

Major discoveries regarding the Milky Way and other nebulae are analogous to the revelation that our galaxy is one of many.

 

Galaxy

A. Milky Way:

The 450–370 BCE Greek philosopher Democritus theorised that the Milky Way, a luminous band in the night sky, might actually be made up of far-off stars. The Milky Way, according to Aristotle (384–322 BCE), is the result of "the ignition of the fiery exhalation of some stars that were large, numerous, and close together," and the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions."Olympiodorus the Younger, a Neoplatonist philosopher who lived from 495 to 570 CE, disagreed with this theory, claiming that the Milky Way should have parallax and appear differently depending on the time of day and location on Earth if it were sublunary (located between Earth and the Moon). He considered the Milky Way to be celestial.

 

The earliest attempt to see and measure the Milky Way's parallax was performed by Arabian astronomer Alhazen (965–1037), who, according to Mohani Mohamed, "decided that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." Al-Brn, a Persian astronomer, argued that the Milky Way galaxy was "a collection of endless shards of the nature of hazy stars" (973–1048).Ibn Bâjjah (also known as Avempace, who died in 1138) of Andalusia proposed that it was made up of numerous stars that nearly touched one another and that its appearance as a continuous image resulted from refraction from sublunary material. He used his observation of the conjunction of Jupiter and Mars as proof that this happens when two objects are close to one another. Ibn Qayyim, a Syrian-born scientist, argued in the 14th century that the Milky Way galaxy is made up of "a myriad of little stars crowded together in the sphere of the fixed stars."

 

Galileo Galilei, an Italian astronomer, used a telescope to investigate the Milky Way in 1610 and found it was made up of a great number of dim stars. This was the first concrete evidence that the Milky Way is made up of numerous stars.

 

In his 1750 book An Original Theory or New Hypothesis of the Universe, English astronomer Thomas Wright correctly hypothesised that the universe might actually be a rotating body made up of an enormous number of stars held together by gravitational forces, similar to the Solar System but on a much larger scale, and that the resulting disc of stars could be seen as a band on the sky from a perspective inside it. Immanuel Kant expanded on Wright's theory of the Milky Way's structure in his 1755 treatise.

 

William Herschel began his first endeavour in 1785 to count the number of stars in various parts of the sky in order to explain the Milky Way's form and the Sun's position. He drew a picture of the galaxy's form, with the Solar System located in its centre. Kapteyn in 1920 came to the image of a small (diameter roughly 15 kiloparsecs) ellipsoid galaxy with the Sun near to the centre using a revised methodology.Harlow Shapley's alternative approach, which was based on the cataloguing of globular clusters, produced an entirely opposite result: a flat disc with a diameter of about 70 kiloparsecs and the Sun far from its core. Both analyses neglected to account for the galactic plane's interstellar dust, but the current picture of the Milky Way galaxy only became clear until Robert Julius Trumpler measured this effect in 1930 by looking at open clusters.

 

B. Differentiating From Other Nebulas:

The Andromeda Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, and Triangulum Galaxy are some of the extragalactic objects that can be seen with the unassisted eye on a dark night. The Andromeda Galaxy was first named in the 10th century by the Persian astronomer Al-Sufi, who called it a "small cloud." The Large Magellanic Cloud was not generally known to Europeans until Magellan's journey in the 16th century; he likely referenced it in his Book of Fixed Stars in 964, referring to "Al Bakr of the southern Arabs" because it was not visible where he lived at a declination of about 70° south.

 

Simon Marius made a further independent observation of the Andromeda Galaxy in 1612. Emanuel Swedenborg, a philosopher, suggested in his 1734 book Principia that there might be further galaxies outside that were organised into galactic clusters and that these little regions of the universe went far beyond what could be seen. These perspectives "are strikingly similar to current ideas of the cosmos." Pierre Louis Maupertuis repeated Johannes Hevelius's theory that the bright spots were massive and flattened as a result of their rotation when he hypothesised in 1745 that some nebula-like objects were collections of stars with peculiar properties, such as a glow exceeding the light its stars produced on their own.Thomas Wright accurately hypothesised in 1750 that the Milky Way was a flattened disc of stars and that some of the nebulae that could be seen in the night sky might be distinct Milky Ways.

 

Isaac Roberts took a photo of the "Great Andromeda Nebula," which was eventually known as the Andromeda Galaxy, in 1899.

 

Charles Messier prepared a list of the 109 brightest celestial objects with hazy appearance toward the close of the 18th century. William Herschel then put together a list of 5,000 nebulae. By building a new telescope in 1845, Lord Rosse was able to discern between spiral and elliptical nebulae. In several of these nebulae, he was also able to distinguish distinct point sources, supporting Kant's prior theory.

 

Vesto Slipher conducted spectrographic analyses of the brightest spiral nebulae in 1912 to ascertain their chemical makeup. Slipher found that the spiral nebulae exhibit strong Doppler shifts, which suggests that they are moving faster than the stars he had previously detected. Most of these nebulae, he discovered, are relocating away from Earth.

 

The "Great Andromeda Nebula" included nova S Andromedae, which Heber Curtis discovered in 1917. (as the Andromeda Galaxy, Messier object M31, was then known). He discovered 11 additional novae by searching the image archive. These novae were, on average, 10 magnitudes fainter than those that happened in this galaxy, Curtis observed. He was able to estimate the distance at 150,000 parsecs as a result. He started to support the "island universes" theory, which claims that spiral nebulae are truly separate galaxies.

 

The Milky Way, spiral nebulae, and the proportions of the cosmos were the topics of the 1920 "Great Debate" between Harlow Shapley and Heber Curtis. Curtis highlighted the appearance of black lanes matching the dust clouds in the Milky Way as well as the large Doppler shift as evidence that the Great Andromeda Nebula is an external galaxy.

 

The distance estimate provided by Estonian astronomer Ernst Pik in 1922 confirmed the Andromeda Nebula's status as a far-off extragalactic object. Edwin Hubble was able to estimate the distance to the nebulae using the new 100-inch Mt. Wilson telescope because he was able to resolve the outer portions of some spiral nebulae as collections of individual stars and identify some Cepheid variables. These nebulae were too far away to be a component of the Milky Way. Hubble created a classification of galactic morphology in 1936 that is still in use today.

 

C. Current Research:

Hendrik van de Hulst predicted in 1944 that interstellar atomic hydrogen gas would emit microwave radiation with a wavelength of 21 cm; this was confirmed in 1951. Since the Doppler shift of this radiation is unaffected by dust absorption, it can be used to track the motion of the gas in this galaxy. These results supported the idea that the galaxy's centre is made up of a spinning bar structure.Hydrogen gas could be found in distant galaxies with the help of more advanced radio telescopes. Vera Rubin discovered a disparity between the known galactic rotation speed and the speed that would be expected given the observable mass of stars and gas in the 1970s. Today, it is believed that the existence of significant amounts of invisible dark matter explains the galaxy rotation issue.

 

The Hubble Space Telescope produced better observations starting in the 1990s. Among other things, its data helped prove that the galaxy's missing dark matter could not be made up entirely of stars that are naturally faint and tiny. There are approximately 125 billion (1.251011) galaxies in the observable universe, according to data from the Hubble Deep Field, which was a very long exposure of a mostly empty region of the sky.Improved instruments for detecting spectra that are invisible to humans, like as radio telescopes, infrared cameras, and x-ray telescopes, enable the discovery of other galaxies that Hubble has not yet identified. In particular, investigations in the Zone of Avoidance (the area of the sky that the Milky Way blocks at visible-light wavelengths) have found a number of new galaxies.

 

A 2016 study conducted by Christopher Conselice of the University of Nottingham analysed 20 years of Hubble photos to calculate that there were at least two trillion (21012) galaxies in the observable universe. The work was published in The Astrophysical Journal. Later observations made with the spacecraft New Horizons outside the zodiacal light, however, decreased this to around 200 billion (2 1011).