The gorgeous tails of the “long-haired star.”
The long-haired star. That is how the word “comet” is translated from Greek. Indeed, our beauty has two strands of hair: the white and the blue ones. Sometimes we can even see the third tail of a comet, but it happens very rarely.
For a long time, most people believed a comet to be an ominous token. However, some of them tried to understand and explain this colorful phenomenon. Thus, in about the 6th century B.C., the ones who followed Pythagoreanism (the movement which was based on the Pythagoras’s beliefs and ideas) considered comets as a kind of planets which appeared on the horizon in a morning or evening sky infrequently. On the other hand, according to the Meteorology, written by Aristotle in 350 B.C., comets were assumed to be an atmospheric phenomenon — the exhalations of the Earth (other planets or stars) that caught fire high in the atmosphere.
It was only in 1577 when the Danish astronomer Tycho Brahe shattered Aristotle’s description of the comets. Tycho Brahe, failing to use parallax (a method used to estimate the distance of astronomical objects in space) to triangulate the distance to a bright comet, concluded that the comet was at least four times farther than the Moon, crossing out the idea of the comet’s atmospheric origin.
Anyway, if the comet’s tails are not the result of a comet burning in the atmosphere, then what are they?
The main tail or the dust tail is something that distinguishes comets from other cosmic objects. There are several steps in creating a dust tail. When a comet approaches the Sun, frozen volatiles (such as H2O, C2O) on the surface of the comet start to heat up and sublimate (change from solid to a gaseous state), carrying mixed with them dust particles away. It causes the formation of a temporary atmosphere around the comet called the coma.
When, finally, dust particles acquire freedom, they start feeling three forces that are acted upon them as if in punishment for leaving the comet’s parent body. Those three forces are the gravitational force from the Sun, the gravitational force from the main body of the comet, and the force from the Sun’s radiation or, in other words, the radiation pressure force which is responsible for pushing the dust particles from the coma, creating the dust tail. Indeed, electromagnetic radiation (visible light is a portion of electromagnetic radiation) does have the ability to create pressure. Light can behave as a wave and as a particle. When it behaves as a particle, we call it a photon. Having particular momentum, those photons can exert pressure (even now, you are experiencing this pressure, but it is so low that you cannot feel it).
These forces are also responsible for the path of the main tail — it always moves away from the Sun even if the position of the comet changes over time. Moreover, they have a different impact, basing on the comet’s motion and its distance from the Sun. Thus, for instance, the dust tail points away from our parental star since solar radiation pressure, acting on a comet, is bigger than the force of gravity from the Sun.
Interestingly, because of the irregular shape and rotation, a comet is heated up by sunlight unevenly, which may create additional dust tails.
The translation of the word “comet” — “long-haired star” — probably comes from the fact that its dust tail is extremely diffuse — taking up a very large volume in space. Dust grains of the tail come in various sizes and shapes. Each of the three forces acts on each grain of dust differently. Gravitational acceleration, caused by two gravitational forces, is relatively the same for each dust particle since it depends on the mass of the Sun and one of the comet but not on the mass of each dust particle. However, the amount of solar radiation pressure each grain experiences greatly depends on its size and shape (because the pressure is proportional to a surface area of an object).
The color of the dust tail is either white or gray. Light wavelength (the distance between two successive points of a wave) or its frequency (which is the number of waves that pass by a point per second) determine its color. Since the main tail reflects sunlight of all wavelengths, colors of different wavelengths are mixed up and perceived as sole white light. From Newton’s experiment with a prism, we know that white light is not light of some particular wavelength. It is an accumulation of light of different wavelengths.
As has already been mentioned, each wavelength corresponds to the particular color that we see. However, when there are too many various wavelengths, our brain stops deciphering discrete colors of light and perceive them all as white light. In other words, the white color of light is composed of all miscellaneous colors of light, blended.
We cannot forget about the last but not least tail of a comet — the one which glows blue. The ion tail.
The ion tail becomes prominent even sooner than our shaggy one. So, how comes that the tail is blue? At some particular distance from the Sun — a critical threshold — there is the right amount of ultraviolet light striking our comet, which leads to ionization of the weakest ice-based molecule — carbon monoxide (CO), producing a positive carbon monoxide ion (CO+). Then, the interaction of ions from the tail and from the solar wind — which is consisted of charged particles — creates the blue light we see.
It is all fancy words, but what is happening in this process? What is ionization? How is blue light created? Let us back up a little to understand this fascinating process.
Firstly, we are going to make our life easier by looking at atoms, using the Bohr model: every atom has energy levels with electrons that occupy them. This concept is simplified for comprehension of the mechanism behind ionization. The actual structure of an atom is much more complicated.
Moreover, we have another component of our process — light. Each energy level of an atom has its value. The difference between the two energy levels may correspond to the specific energy value of light. Each photon of light has energy (E), which is quantized (has a discrete value) and equal to E=h*f, where h — Planck’s constant and f — frequency of light. You can view a photon as a little package of energy. Electrons can absorb or emit a photon which energy is equal to the difference between two energy levels — the difference between the energy level where an electron was and the energy level where the electron is after absorbing/emitting a photon. Electrons neither absorb nor emit photons which do not correspond to the difference of any energy levels — it would be the violation of the law of conservation of energy.
Blue light has a higher frequency (f) than red one does (gamma rays have the highest frequency of all spectrum of electromagnetic waves, whereas radio waves have the lowest one). Since blue light has a higher frequency, it has higher energy (remember E=h*f). When an electron absorbs a photon of blue light, it “jumps” to a higher energy level.
The interesting case is when a photon has energy that is higher than the difference between the first and the last energy levels. In that case, an electron which has “swallowed” this high-energy photon leaves an atom. It may even acquire kinetic energy. The energy, which is needed for an electron to abandon its atom, is called the work function. Hence, if an electron absorbs energy which is greater than the work function for this particular material (mostly, it is a solid material), it gains kinetic energy, which is proportional to speed squared.
In addition to absorbing a photon, an electron can emit one. If by absorbing a photon, an electron acquires necessary energy to “jump” to a higher energy level, it would be consequent to think that, by emitting a photon, an electron is going to lose energy, “falling” to a lower energy level. Again, the difference between energy levels of where an electron was and where it has fallen to will determine the energy of the emitted photon. If the difference is high, then the energy of the photon will be high as well. As we remember, blue light has higher energy than red or green ones do. Hence, we may conclude that the ion tail has its blue light because electrons emit photons by falling to lower energy levels.
We would think that we have found the origin of blue light, wouldn’t we? Unfortunately, we have to explain some rising questions — why do electrons decide to fall exactly in this tail or to fall to the lower energy level and emit photons of blue light at all?
It was said earlier in the article that ultraviolet light strikes comet, and something fascinating happens. By looking at the spectrum of electromagnetic waves, we can say, with confidence, that ultraviolet light has an enormous frequency. Thereby, it also posses great energy. When it hits a comet, inner electrons which are hidden in atoms of ice-based molecules absorb photons of higher energy. Since energy is so cumbersome, they get enough of it to be able to leave an atom (for inner electrons it is much harder to accomplish than for electrons which are located in an outer shell because inner negative electrons are strongly attracted by a positive nucleus). As an inner electron is gone, the poor atom (which has become an ion) is left with an electron vacancy (an “empty” place for another electron to take it. It is like filling a gap in a broken small atom’s heart). This atom is quite unstable. However, there is the solar wind consisting of ions and electrons. Thereby, another electron from the solar wind fills up this vacancy. Moreover, the gap-filling electron may come from an adjacent energy level of this atom. By falling to the inner (lower) energy level, the electron will, for sure, emit a photon with high energy.
Why is it blue light? No ultraviolet or green light? They also have great energy. The thing is that the energy difference between energy levels is unique for each element in the periodic table. Ice-based molecules mostly consist of oxygen and carbon, which energy level differences correspond to the energy of blue light.
That is a secret behind the marvelous color of the ion tail.
Through our magnificent journey, we have revealed the process behind the origin of comets’ tails. It has turned out that comets are not simple “ice-rocks,” which foreshadow a horrific event, but a remarkable part of the Universe.
