Galaxies

The word galaxy comes from the Greek word galaxias, which means milky’ in English, and refers to the Milky Way galaxy, which contains the Solar System. The vast majority of galaxies are organised gravitationally into groups, clusters, and superclusters. The Milky Way is a member of the Local Group.

Galaxy

Galaxies are classified into three types based on their optical morphology: elliptical, spiral, and irregular galaxies. A supermassive black hole is believed to be present at the centre of many of them. Sagittarius is the name given to the Milky Way’s core black hole, which has a mass four million times greater than that of the Sun. As of March 2016, GN-z11 is the galaxy that has been observed to be the oldest and farthest distant. It is located 32 billion light-years away from Earth and is seen as it existed only 400 million years after the Big Bang, according to current estimates.

A new estimate based on data from the NASA New Horizons space probe was released in 2021, reducing the previous estimate to approximately 200 billion galaxies. This was in response to a 2016 estimate that there were two trillion or more galaxies in the observable universe overall, as well as as many as 1 1024 stars, according to the International Astronomical Union (more stars than all the grains of sand on all beaches of the planet Earth). Comparatively, the Milky Way is around 100,000 light-years across and is separated from the Andromeda Galaxy (which has a diameter of approximately 220,000 light-years), which is its nearest large neighbour, by 780,000 light-years (2.5 million ly.)

The vast majority of galaxies are organised gravitationally into groups, clusters, and superclusters. The Milky Way is a member of the Local Group, which shares dominance with the Andromeda Galaxy and is the largest galaxy in the universe. The organisation is a member of the Virgo Supercluster. Generally speaking, these linkages are structured as sheets and filaments that are surrounded by vast spaces when viewed at the biggest scale. Both the Local Group and the Virgo Supercluster are included within a much bigger cosmic structure known as Laniakea, which is located in the constellation Virgo.

The Milky Way

Democritus (450–370 BCE), a Greek philosopher, hypothesised that the Milky Way, a dazzling strip of light visible in the night sky, might actually be made up of distant stars. To the contrary, according to Aristotle (ca. 384–322 BCE), the Milky Way was created by the “ignition of the fiery exhalation of some stars that were large, numerous, and close together,” and that the “ignition takes place in the upper part of the atmosphere, in this region of the World which is continuous with the heavenly motions.” According to the Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 CE), if the Milky Way was sublunary (located between the Earth and the Moon), it should show differently at different times and places on Earth, and it should have parallax, both of which were not the case at the time. According to him, the Milky Way was a celestial object.

William Herschel embarked on the first endeavour to define the structure of the Milky Way and the location of the Sun in 1785, which involved measuring the number of stars in different parts of the sky. This was the beginning of the modern astronomy era. A diagram of the galaxy’s form, with the Solar System located towards the centre, was created by him and his team. Kapteyn arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the centre of the galaxy in 1920 by employing a refined method of observation. A different method developed by Harlow Shapley, which was based on the cataloguing of globular clusters, produced a drastically different picture: a flat disc with a diameter of around 70 kiloparsecs with the Sun located distant from the centre of the disc. However, once Robert Julius Trumpler measured the absorption of light by interstellar dust in the galactic plane in 1930 through the study of open clusters, the modern picture of our host galaxy began to emerge.

Conclusion

This is a typical galaxy: it contains hundreds of billions of stars, enough gas and dust to generate billions of additional stars, and at least ten times as much dark matter as all of the stars and gas combined. And gravity is the glue that holds it all together.

The Milky Way galaxy, like more than two-thirds of all known galaxies, is shaped like a spiral. A great deal of energy is being generated at the centre of the spiral, which is occasionally accompanied by bright flares. The researchers infer that the centre of the Milky Way is a supermassive black hole based on the enormous gravity that would be necessary to explain the movement of the stars and the energy emitted.