Comets: history, names and models of the stars

Comets: history, life cycle and names of stars, with in-depth analysis on cometary models and on the most famous of periodic comets, Halley’s Comet

What you will learn

• The history of comets: from superstition to science
• The structure of comets and their classification
• What is the life cycle of comets
• How comets are named
• What are the different cometary patterns
• The history and characteristics of Halley’s Comet

INDEX

1. Comets: premises
2. The history of comets
3. What are comets
4. The life cycle of comets and their classification
5. The names of the comets
6. The cometary models
7. Halley’s Comet
8. Watch the video on comets
9. Key concepts

Infobox

1Comets: premises

The Hale-Bopp Comet – Source: Getty-Images

The comets are considered among the oldest objects in the solar system as only marginally involved in its formation process. The purpose of the numerous missions is precisely to have the opportunity to study the structure of comets , their origins, the nucleus and its composition.

2The history of comets

The first interpretationsThroughout history, the passage of spectacular light trails in the sky has contributed to numerous interpretations : astrologers and American Indians, for example, considered the passage of comets as a harbinger of misfortune , death, wars, plagues.
The Greek philosopher Aristotle , in the fourth century BC. C., argued that comets were not part of the Solar System, ordered and perfect, but were the manifestation of emissions emanating from the Earth and reached the sky.

Learn more

Aristotle’s Metaphysics

The historical disasters linked to the passage of cometsThe belief that comets could herald catastrophic events was born in 44 BC. C. when Julius Caesar was assassinated . On that occasion the manifestation of the soul of Julius Caesar was invoked which, according to the beliefs of the time, took place in the month of July with the appearance of a comet . It is precisely for this reason that the Roman Senate decided to have a coin minted in honor of Julius Caesar . The British, on the other hand, suffered a disastrous defeat by William the Conqueror and considered the appearance of Halley’s comet fatal. in 1066.

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Biography of Julius Caesar

From superstition to scienceThe examples that history provides us on the nefarious events “connected” to the spectacular light trails in the sky are manifold. Only around 1577 the astronomer Tycho Brahe , studying the motion of the planets and celestial bodies, was able to demonstrate that comets were celestial bodies while Newton deduced that the comet appeared in 1680 followed a parabolic orbit. In 1705, however, the astrologer Halley stated that the comets of 1531, 1607 and 1682 were apparitions of the same comet and predicted their return in 1758, a prediction confirmed by the appearance of thecomet on 25 Christmas 1758.

Curiosity

The first observation of a comet dates back to 1058 – 1059 BC. C. and was described by the Chinese as “a broom with a stick facing East”.

3What are comets

The structure of cometsThe comets are celestial bodies of small dimensions composed mainly of ice . They consist of a nucleus, a crown and a tail.

• The cometary nucleusis solid, several kilometers thick and is made up of frozen metals, methane, ammonia, water (about 80%), silicates and carbon dioxide. When comets reduce the distance with the Sun , the nucleus, due to the heat that the Sun emits, increases its temperature and the volatile materials present tend to evaporate, developing a coma of variable dimensions.
• The comaconsists of a dense cloud of water, carbon dioxide and various gases around the core. Under the effect of the solar wind , the crown moves to the opposite side to the Sun giving rise to a tail.
• The tailis several million kilometers long and is made up of two different types of materials: the dust tail and the ion tail.

The dust tail of comet Siding Spring, in the Andromeda galaxy – Source: Istock

The tail of dustThe dust tail is the predominant part of the comet and is the one visible even to the naked eye . It extends for several million kilometers and is composed of dust particles, mainly silicates and carbon compounds, coming from the core. Usually the dust tail has a curved and open fan shape and is distinguished from that of ions for its orange or red color .

The ion tailThe ion tail , on the other hand, extends for over 100 million kilometers and is characterized by a blue or azure color due to the presence of carbon dioxide. The ion tail is characterized by its tubular shape, with the axis in the opposite direction to that of the Sun, and consists mainly of ionized gases such as carbon dioxide, ammonia, methane, formaldehyde, etc.

Curiosity

On July 16, 1994, man, for the first time in history, witnessed the effects of a cometary impact as twenty nuclei of the comet fell on Jupiter.

4The life cycle of comets and their classification

Learn more

The Sun: structure and explanation of the star of the Solar System

The origin of cometsThe comets , according to current assumptions, come from an area at the edge of the solar system , called Cloud Oort , characterized by billions of celestial bodies. The cometary nuclei of these bodies, as a result of gravitational perturbations generated by the Milky Way or the passage of stars near the solar zone, move in the direction of the Sun and generate long-term comets .

The long-term cometsThe long-period comets have an elongated shape and are visible only once in the Solar System because of the excessive slowness of movement (about a thousand years) around the sun . They differ from short-term ones both in terms of characteristics and origin.

The short period cometsIn fact, short-period comets are presumed to originate in an area beyond Neptune and known as the Kuiper

 belt , characterized by the presence of thousands of icy bodies and billions of comets . The comets short period revolve around the Sun in much less time (about 200 years) compared to those long-term and, because of the disturbing effects of planets such as Jupiter , Neptune, Saturn and Uranus, through trajectories much shorter than of origin and describe parabolic or elliptical orbits .

Shooting Stars – Source: Getty-Images

Non-periodic cometsFinally, there is a further classification of comets into non-periodic or extrasolar comets : they follow parabolic or hyperbolic orbits, they are beyond Jupiter and cross the area close to the Sun but are not part of the Solar System.

From comets to shooting starsRegardless of the type, comets are characterized by a life cycle : they have an origin and an end.
The end of comets is often caused by the solar wind and the high temperatures to which they are subject when they orbit the Sun: this causes the dispersion of volatile materials and dust and transforms the comets into asteroids , difficult to identify from the Earth . The nucleus of these comets , made up exclusively of non-volatile materials, will continue its motion until its disintegration into small fragments. The dust that comets emit are arranged along an orbit and, once in contact with theEarth’s atmosphere , are burned causing a luminous trail in the sky . This is the phenomenon of the shooting stars .

Curiosity

The shooting stars that can be observed on the famous night of San Lorenzo (10 August), according to popular tradition, are none other than the “tears” shed by San Lorenzo during his torture.

5The names of the comets

The rule for naming cometsIn the past, astronomers studied celestial bodies without any rules and any kind of national or international agreement while nowadays all astronomers refer to the UAI (International Astronomical Union).
According to the instructions suggested by the UAI, each comet is characterized by the names of its discoverers (no more than three) or by a group and by an acronym consisting of a number and a lowercase letter that identifies it in the tables and catalogs.
The same symbology is used for comets that “return” after more or less long periods (for this reason they are definedperiodic comets ), treating each return as their first appearance.

The classification of comets by type – Source: Editorial Staff

Comets visible to the naked eyeAn exception to these rules concerns the large comets visible to the naked eye which are classified with a proper name or with that of the discoverer or with a short expression. An example is provided by Kohoutek , known as the “ Christmas comet ”, visible in Christmas 1973.
In the catalogs, in addition to the identification codes of the comets , the useful data for determining the orbits and calculating the ephemeris of the comets are reported . Ephemeris are tables of numbers that provide the coordinates of all celestial bodies for set time intervals.

6The cometary models

Comet immortalized by NASA – Source: Ansa

There are basically two cometary models known today: the abolished gravel pile model and the dirty snowball model , which is believed to be reliable.

The gravel pile comet modelAt the beginning of the century, comets were considered celestial bodies made up of a set of particles of porous meteoritic material in which there was a considerable amount of gas which, released from the heat of the Sun, formed the coma . This model was known as the gravel pile model. Following extensive studies, Whipple realized that this model could not be trusted because the canopy could only be fed for one day due to the limited gases.

Learn more

Astronomy: meaning and history of science that studies the stars

The dirty snowball comet modelIn 1950 he decided to propose a new model called a dirty snowball considering the core as a compact center of non-volatile materials and ice . In describing his model, Whipple specified the melting temperature of the non-volatile materials (ammonia, methane, carbon dioxide, water, etc.) that would make up the canopy.

The expulsions of materialIn this model, when the distance from the Sun became minimal ( perihelion ), the ice tended to vaporize in the surrounding space, the core decreased its temperature and the volatile material was expelled forming a tail of dust . Sometimes, however, phenomena characterized by a “rocket effect” could occur, a phenomenon in which very fast expulsions of nuclear material occurred due to the sublimation of the ice present.

7Halley’s Comet

Halley’s Comet – Source: Loop

Halley’s comet: birth and durationThe most famous of the periodic comets is Halley’s Comet , also known as 1P / Halley . It comes from the Kuiper Belt, and takes its name from the English astronomer Edmond Halley who observed it in 1682.
The Halley’s comet passes near the Sun every 76 years losing, at every passage, one ten-thousandth of its mass is then estimated that its life should last less than a million years before the comet dissolves or burns out.

The characteristics and orbit of the cometThe last passage that the comet made near the Sun was in 1986 and was observed and studied by the Giotto probe launched by the European Space Agency. From the data collected it emerged that the comet has a diameter of a few kilometers , is composed of rock, ice and carbon compounds. It has a well-defined and regular orbit and is characterized by a very dark and not very dense core, probably porous.
The Comet Halley gives rise to two swarms meteorite : the Eta Aquarids , visible from April to May, and Orionids, visible from October to November. Halley’s Comet is expected to return to the Solar System in 2061 .

5 photos

Hopping, planes and comets

8Watch the video on comets

Key concepts

·                                 History of comets

• Astrologers and American Indians: cometswere considered harbingers of misfortune and death
• Aristotle(4th century BC): comets were not part of the Solar System, they were manifestations of emissions emanating from the Earth
• 44 BC: killing of Julius Caesar. A manifestation of his soul was invoked which took place with the appearance of a comet
• 1066: defeat of the English by William the Conqueror. The appearance of Halley’s cometwas fatal
• 1577: Tycho Braheproved that comets were celestial bodies
• 1680: Cometappeared, according to Newton, following a parabolic orbit
• 1705: Halley predicted manifestation in 1758 of the comet thatappeared in 1531, 1607 and 1682. This prediction proved to be true.

·                                 The structure of comets

The comets are of the heavenly bodies formed by :

• a cometary nucleus: solid, made up of frozen metals and carbon dioxide
• a crown: dense cloud of water, carbon dioxide and gas, moves to the opposite side to the Sun, originating at the tail
• a tail: it can be of dust or ions
• dusttail : visible to the naked eye, orange – reddish color, curved and open fan shape, composed of dust particles
• iontail : extends over 100 km, bluish in color, tubular shape, consisting of carbon dioxide, methane, ammonia, etc.

·                                 The life cycle of comets and their classification

Origin of comets : Oort cloud.

Classification :

• Long-period comets: originating from cometary nuclei moving towards the Sun, characterized by elongated shapes, visible to the naked eye, slow movement around the Sun.
• Short period comets: origin in the Kuiper belt, reduced rotation times around the Sun, describe parabolic orbits
• Non-periodic comets: they follow parabolic or hyperbolic orbits, they are not part of the Solar System, they are beyond Jupiter

End of comets : caused by the solar wind and high temperatures that causes dispersion of volatile materials and transforms the comets into asteroids, the nucleus breaks down into small fragments.

·                                 Comet names and cometary models

Names of comets

• Names of its discoverers (maximum 3) or name of the discovering group
• Abbreviation consisting of a number and a lowercase letter

Large comets exception:

• Proper name or name of the discoverer or short expression

Cometary models

• Gravel pile model: abolished due to limited canopy feeding.
• Dirty Snowball Model: Reliable. When distance from the Sun was minimal, the ice vaporized leading to a decrease in the core temperature and the volatile material was expelled.
• Effect – rocket: very fast ejections of nuclear material caused by ice sublimation.

·                                 Halley’s Comet

• also known as 1P / Halley
• comes from the Kuiper Belt
• characterized by a diameter of a few km, composed of rock, ice and carbon compounds
• very dark, not very dense, probably porous core
• passes close to the Sun every 76 years
• estimated life time: less than one million years
• last passage around the Sun: 1986
• next passage around the Sun: estimated in 2061

Questions answers

• What is a comet?

It is a small celestial body made up of ice, rock and metals.

• Why does a comet become visible?

Visible comets are long-term comets that have an elongated shape and are characterized by excessive slowness of movement around the sun .

• Where do comets come from?

Long-term comets come from an area on the edge of the solar system called the Oort Cloud, which is characterized by billions of celestial bodies. Short-period comets, on the other hand, come from an area beyond Neptune and known as the Kuiper belt.