Seeing comets is always a significant event whether comets are known as a part of the solar system, which orbits the Sun or not, in the history of humanity. In early times of our history, humans have thought comets as a messenger of gods or an expression from which are angry to them due to their insufficiency. Also, humans believed that plagues and natural disasters, such as earthquakes and storms, were foreshadowing by comets. Comets had remained to be mysterious objects created by gods for humans until some brave astronomers observed the sky without using the perspective of myths and beliefs.
For simulating the former conditions of the solar system by trying to dissolve the collapsing interstellar cloud of dust and gas which the solar system formed, astronomers and physicists scrutinize comets by using the electromagnetic spectrum of the comet nucleus and tails, because comets were formed by the gravitational force produced by the outer regions of the same collapsing cloud. In the outer regions of the solar system where it was cold enough to condense, the comet nucleus was formed and began to move and orbit. Comets have four visible parts affected by the orientation of the comet nucleus according to the line of sight, for instance when a comet gets closer to the Sun, the volatile ices in its nucleus sublimate, which are the nucleus, the coma, the blue tail and the white tail. The transformation of a comet; “The coma changes from a diffuse round mass to sharply defined layers, called envelopes. Nebulous matter streams away from the comet’s head in the direction of the sun and forms an immense tail. Most comets of this type have only one tail. A very few have two or more. The bright comet of 1744 had six in all. Some comets occasionally also have forward spikes. As a comet recedes from the sun, the tail(or two tails) can no longer be seen, the coma become diffuse again, and in the great majority of cases the comet itself disappears from view(1)."
Comets are a mix of dust, rock, and water, which has been called dirty snowballs due to their nucleus that is a giant ice ball full of dust and gases. Chemical structure of the comet nucleus; ”The spectrograph has revealed that comets contain various gases, including cyanogen(CN), carbon(C2), carbon monoxide(CO), nitrogen(N2), hydroxyl(OH) and nitrogen hydride(NH). The known presence of highly poisonous gases, such as cyanogen and carbon monoxide, in comets gave rise to a certain uneasiness in 1910, when Halley’s comet passed between the earth and the sun(2).", the chemical structure of the tails, “Comets normally have not one tail, but two, a white one and a blue one. The white tail is caused by dust particles evaporating from the comet’s nucleus due to solar heating and radiation pressure. The dust reflects sunlight and the tail looks white. The blue tail is caused by the solar wind; the protons of the “wind” strike the atoms and molecules of the comet’s nucleus and ionize them(knock out of their electrons). This tail is self-luminous, as the ionized material fluoresces(3)."
Depending on its period(the time it takes for it to orbit the Sun), a comet is called a short-period comet or a long-period comet; “The origins of the two types are different. Short-period comets originate in the Kuiper Belt: Neptune pulls them out of the belt as it passes through. Long-period comets, which have extremely elliptical orbits, come from a vast reservoir of icy bodies that extends in all directions as far out as 100 000 AU, half the distance to the nearest star(Proxima Centauri). This region, which has never been directly observed because it is too distant, is named the Oort Cloud after the Dutch astronomer, Jan Oort, who proposed its existence in 1950(4)."
Also, according to a few theories on the origin of life, comets are the basis of the life on Earth; “We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as "dirty snowballs." They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system(5)."
(1) Bobrovnikoff, Nicholas T. "Comets." The New Book Of Popular Science Vol-1. Grolier Incorporated, 1988. 149-150. Print.
(2) Bobrovnikoff, Nicholas T. "Comets." The New Book Of Popular Science Vol-1. Grolier Incorporated, 1988. 151. Print.
(3) Bely, Pierre-Yves, Christian, Carol, and Roy, Jean-René. "What is a comet’s tail made of?." A Question and Answer Guide to Astronomy. Cambridge: Cambridge University Press, 2010. 66-67. Print.
(4) Bely, Pierre-Yves, Christian, Carol, and Roy, Jean-René. "Where do comets come from?." A Question and Answer Guide to Astronomy. Cambridge: Cambridge University Press, 2010. 65. Print.
(5) Shekhtman, Lonnie, Landau, Elizabeth, and Hoang, Celeste. “Comets.” Solar System Exploration: NASA Science. 7 December 2017. Web. 19 June 2018. https://solarsystem.nasa.gov/small-bodies/comets/in-depth/