Comet Diagram
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(Click on part of comet)
[Nucleus | Coma (Head) | Type I Tail | Type II Tail]
[Anomalous Tail | Sodium Tail | Hydrogen Tail]
What is the Nucleus?
The nucleus is a compact body of irregular form with a diameter between 1 and 100 km (from
40 to 50 km for Hale-Bopp). However it is not visible from the Earth. The American
astronomer Fred Whipple proposed the " dirty snow ball " model: the nucleus is a
solid amalgam of ices and dust which sublimates when it approaches the Sun.
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The Coma or the Head
What is the coma?
It is a very luminous halo which one observes all around the nucleus. It is about
spherical with an average diameter of 200 000 km at 1 AU of the Sun (100 000 km for
Hale-Bopp). Its size varies inversely with the square of the heliocentric distance.
Why is there a luminous halo?
Sublimation creates a gaseous environment that become excited under the influence of
sunlight and of the protons from the solar wind. The ensuing fluorescence results from a
subsequent transition of an electron between two energy levels.
Why does the coma vary in size?
When the nucleus approaches the Sun, the condensed gases evaporate. As the temperature
increases, so does the rate of sublimation and new species (which have a higher
temperature of sublimation) are set free. So, the size of the coma increases when it
approaches the Sun. The size of the coma can also vary from one comet to another depending
upon the materials it is made of (because they have different temperatures of
sublimation).
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The Type I Tail
(Ionic or Plasma Tail)
Physical Appearance
The ionic tail, known as type I or plasma, is made up of ions. This tail is straight and
is always opposed to the direction of the Sun. It is up to 100 000 km wide and of a few
tens of million km long. Blue is the colour which dominates its spectrum.
Why are there ions?
In order to understand that, it is important to know another phenomenon: solar
wind. This is a wind of charged particles wound in an helix around the solar magnetic
field. This wind moves at a speed of 400 km per s.
The gases are ionised either by photoionisation of the neutral molecules
under the action of the solar ultraviolet radiation, or under the action of the solar wind
by a phenomenon where a proton removes an electron from an atom.
Why is it straight and opposed to the direction of the Sun?
The initial velocity of the ions is provided by the reaction that produced them.
The ions are carried by the solar magnetic field at 10 to 100 km per s. The tail is thus
straight on account of the speed of the ions, and forms an angle of a few degrees with
direction of the Sun, in the direction opposed to the motion.
Where does the light come from?
It is emitted by fluorescence: a particle of solar wind excites an electron of
the atom or molecule concerned. This electron reaches a metastable level, goes down again
and releases its surplus energy in the form of a photon (particle of light) of a well
determined energy and thus, of a specific colour.
Why is it blue?
The blue colour is explained by the fact that the most abundant ion, CO+ emits around 400
to 460 nm, in the blue part of the spectrum.
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Physical Appearance
This tail is the most spectacular part: it extends over several millions km and is made up
of particles of dust whose size varies between 0.1 and 10 m m. It is very different from
the ion tail. Although remaining in the plane of the orbit, the dust trails behind the
motion of the comet giving this tail a curved appearance. The dominant colour of the dust
tail is yellowish.
Why is dust accelerated in the direction opposite to the Sun?
The motion of the particles leaving the coma is influenced by forces external to
the comet: the gravitational attraction of the Sun,
Fg = G M m/d2 (where M
is the mass of the Sun and m, that of dust), and also a repelling force which will project
them towards the tail; this force comes from the radiation pressure of the Sun (the
photons communicate to the particles an impulse directed opposite to the Sun; it is the
pressure of radiation, Fr = (pi) a2 sr(a) F /
c r2 (where a = the radius of the particle; s r(a) =
coefficient measuring the effectiveness of the radiation pressure; F = Solar constant; c =
the speed of the light and r = the heliocentric distance).
Why is it curved?
The speed of the dust which makes it up is about 0.1 to a few km per s (much
smaller than the orbital velocity of the comet around the Sun). Each grain of dust follows
a slightly different ellipse leaving a curved trail.
Why is it of yellowish colour?
The dust is mainly made up of silicates (principal component of sand). This dust
does not emit any light (like the ion tail does it by fluorescence) but only reflects that
of the Sun (which is of a yellowish white).
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Physical appearance
The anomalous tail or anti-tail, is observed when the Earth is about in the plane of the
cometary orbit; it is thus rather rare and of short duration. One then sees a tail
pointing towards the Sun, hence its name. Such anti-tails were observed in the comets
Arend-Roland (1957 III), Kohoutek (1973 XII) and Hale-Bopp (C/1995 O1).
Why does this tail point towards the Sun?
To explain the appearance of this tail, one can put forward two hypothesis:
1. It could be due to the accumulation of grains of large size
(approximately 50 m m) which escape very slowly from the nucleus and would be emitted at a
much earlier time. They need to be seen " on edge " so that the light that they
diffuse is sufficient to simulate a tail.
2. It could be due to a visual effect where the curve of the tail of dust
would reveal a small point opposed to the principal tail.
We think that the second hypothesis is more likely because the first one
does not explain why the anti-tail is so narrow.
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Physical Appearance
As the photograph of its discovery shows, its appearance is markedly different from the
other tails. It would be about 800 000 km broad and at least 12 000 000 km long. It is
observable only with a narrow band yellow filter because the concentration of neutral
sodium is very small.
Is this the first time this phenomenon is observed?
No, it has already been observed during a few hours on certain comets with a perihelion
lower than 0.05 AU. At that point, temperature is so high that metals like sodium can be
ionised. Hale-Bopp however did not reach that close to the Sun, and its sodium tail is
thus one of a kind.
Where does this tail come from?
We know that the neutral sodium is not directly emitted from the nucleus but is
ejected from grains of sand from the tail of dust.
How is this tail formed?
That remains a mystery because the magnetic field cannot carry molecules or atoms
which are not ionised and the pressure of radiation does not act on grains of diameter
lower than 0.1 m m.
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How was it discovered?
From spectroscopy carried out by satellites in 1970 hydrogen was discovered in the comets
Tago-Sato-Kosaka and Bennett from the Lyman a line at 121.6 nm. Ionised hydrogen forms in
fact an enormous halo (150 millions km for Hale-Bopp) which largely exceeds the limits of
the coma.
Why isn't it not visible since the Earth?
Atomic hydrogen emits in the ultraviolet (l < 300 nm) but
the ozone layer (O3) stop these wavelengths. The hydrogen cloud can only be
observed from space, with satellites, for example. It is also for this reason that the
hydrogen cloud was discovered so late.
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