Chapter 4
By : A E
Roy and D Clarce
From The
Book “ Astronomy Principles and Practice”
4.1 Introduction
Energy
is arriving from space in the form of microscopic bodies, atomic particles and
electromagnetic radiation. A great part of this energy is, however, absorbed by
the Earth’s atmosphere and cannot be observed directly by ground-based
observers. In some cases, the absorption processes give rise to re-emission of
the energy in a different form. Macroscopic bodies have kinetic energy which is
converted into heat; atomic particles interact with the gases in the higher
atmosphere and liberate their energy in the form of light, giving rise to such
phenomena as the aurorae. Electromagnetic energy of particular frequencies, say
in the ultraviolet or x-ray region, is absorbed and re-emitted at other
frequencies in the visible region. Thus, besides gaining knowledge about the
sources which give rise to the original energy, observation of the re-emitted
energy leads to a better understanding of the nature of our atmosphere.
However, by far the greater part of our knowledge of astronomical objects is
based on the observation of electromagnetic energy which is collected by
satellite instrumentation or transmitted directly through the atmosphere and
collected by telescope.
4.2 Macroscopic bodies
As
themacroscopic bodies penetrate the Earth’s atmosphere, the air resists their
motion and part of their energy is lost in the form of heat. The heat generated
causes the ablated material and the atmospheric path to become ionized and,
when the atoms recombine, light is emitted and the rapid progress of the body
through the upper atmosphere is seen as a flash of light along a line in the
sky. The flash might last for a few seconds. The event is known as a meteor (popularly known as a shooting
star). The rate of burning of the meteor is not constant and fluctuations in
brightness may be seen on its trail, usually with a brightening towards the end
of the path. Positional measurements can be made of the meteor and the event
can be timed. Simultaneous observations of a meteor at different sites allow
determination of its trajectory within the Earth’s atmosphere.
On
occasions, many meteors can be observed during a relatively short period of
time and, by observing their apparent paths across the sky, it is noted that
there is a point from which the shower of meteors seems to originate. This
point in the sky is known as the radiant
of the shower. Meteor showers are often annual events and can be seen in the
same part of the sky at the same time of the year, although the numbers counted
vary widely from year to year. The regular appearances of showers result from
the crossing of the Earth’s orbit of a fairly tight band of orbits followed by
a swarm of meteoritic material.
Meteors
can also be detected during the day by radar. As a meteor passes through the
upper atmosphere, as has already been mentioned, some of the gases there are
ionized. The ionized trail which persists for a short time acts as a good
reflector for a radar beam and the effect of any daytime meteor can be
displayed on a cathode ray tube. Several daytime showers have been discovered
by the use of this technique.
Some
of the larger meteors have such large masses that they are incompletely ablated
or destroyed in the atmosphere. In this case, the meteor suffers an impact on
the Earth’s surface. The solid body, or meteorite, is frequently available
either in the form of a large piece or as scattered fragments. The material can
be exposed to the usual analyses in the laboratory.
The
smaller meteors or micrometeorites can now also be collected above the Earth’s
atmosphere by rocket and analysed on return to Earth. It also appears probable
that some micrometeorites are continuously percolating through the atmosphere.
Because of their size, they attain a low terminal velocity such that any local
generated heat by air friction is radiated away at a rate which prevents melting
of the particle. Previous micrometeorite sedimentation can be explored by
obtaining cores from ancient ice-fields. It is now a difficult problem to
separate any fresh contribution from the general dust which is constantly being
stirred in the lower atmosphere of the Earth.
4.3 Atomic particles
The
atomic particles which arrive in the vicinity of the Earth range from nuclei of
atoms of high atomic weight down to individual nuclear particles such as
protons and neutrons. The study of these particles is known as cosmic ray physics. The analysis of the
arrival of such particles tells us about some of the energetic processes
occurring in the Universe but so far little has come from these observations in
us being able to pinpoint the exact sources which generate the energetic
particles. Because of the Earth’s magnetic field, any charged particle is
deflected greatly from its original direction of travel by the time it arrives
at the detector, making it exceedingly difficult to say from which direction in
space it originated. At the present time, the Sun is the only body which is definitely
known to be a source of particle energy.
It
turns out that the basic processes of nuclear (hydrogen) burning within stellar
interiors such as the Sun produces the enigmatic neutrino particle. The
neutrino has very little interaction with other material and can penetrate
great distances through matter. For this reason, the neutrinos generated in the
depth of the Sun at a rate ∼1038s−1
pass from the centre to the surface, escaping very readily outwards. At the
distance of the Earth, their flux is ≈1014 s−1 m−2,
this same number (i.e. ∼1014)
passing through each person’s body per second. Their very low cross section for
interaction with other material makes them difficult to detect but some
large-scale experiments have been established for this purpose. It must be
mentioned that through ‘neutrino’ observatories, astronomy has helped greatly in
our understanding of this particle, particularly in relation to the issue of
its mass. Although the general flux from other stars is too low for detection,
some 10 neutrinos were detected in 1987 from a supernova in the Large
Magellanic Cloud. It is estimated that about 109 neutrinos passed
through each human being as a result of the event.
4.4 Electromagnetic radiation
4.4.1 The wave nature
of radiation
The
greatest quantity of information, by far, comes from the analysis of electromagnetic radiation. The word
describing the quality of the radiation indicates that it has both electric and
magnetic properties. As the radiation travels, it sets up electric and magnetic
disturbances, which may be revealed by an interaction with materials on which
the radiation impinges. In fact, some of the interactions are utilized in
detector systems to record and measure the strength of the radiation. For these
particular interactions, the energy present in the radiation is transformed
into another form which is then suitable for a quantitative assessment.
Thus,
any radiation has a strength which can be measured. Quantitative observations
of this property can give us information about the source or about the medium
through which the radiation has travelled after leaving the source. Experiments
in the laboratory have shown that all electromagnetic radiations have the same
type of wave nature. When any radiation passes through a medium, its velocity
is reduced by a certain fraction and the wavelength as measured within the
medium also reduces by the same fraction. If v is the measured velocity and λ
the measured wavelength, their relationship may be written as
v
= νλ (4.1)
where
ν is a constant of the particular radiation and is known as its frequency.
Thus,
the electromagnetic spectrum covers an extremely wide range of frequencies.
According to the value of the frequency of the radiation, it is convenient to
classify it under broad spectral zones, these covering γ -rays, x-rays,
ultraviolet light, visible light, infrared radiation, microwaves and radio waves.
The spectrum of electromagnetic radiation is illustrated in figure 4.1.
The
velocity of any electromagnetic disturbance in free space (vacuum) is the same
for radiations of all frequencies. In free space, the fundamental parameter
frequency, ν, is related to the wavelength, λc, of the radiation and its
velocity, c, by the expression:
c
= νλc.
(4.2)
The
velocity of electromagnetic radiation in free space has been measured in the
laboratory over a wide range of frequencies and, in all cases, the result is
close to c = 3 × 108 m s−1.
Wavelengths
of electromagnetic radiation range from 10−14 m for γ -rays to
thousands of metres in the radio region. At the centre of the visual spectrum,
the wavelength is close to 5 × 10−4 mm or 500 nm. In the optical
region, the wavelength is frequently expressed in Ángstro¨m units (Á ) where 1 Á = 10−7 mm. Thus, the
centre of the visual spectrum is close to 5000 Á.
If
the strength of any radiation can be measured in different zones of the
spectrum, much information may be gleaned about the nature of the source. In fact,
it may not be necessary for measurements to be made over very wide spectrum
ranges for the observations to be extremely informative. For example, as we
shall see later, measurements of stellar radiation across the visual part of
the spectrum can provide accurate values for the temperatures of stars.
Partly
for historic reasons, experimenters working in different spectral zones tend to
use different terms to specify the exact positions within the spectrum. In the
optical region the spectral features are invariably described in terms of
wavelength; for radio astronomers, selected parts of the spectrum are normally
identified by using frequency, usually of the order of several hundred MHz. By
using equation (4.1), it is a simple procedure to convert from wavelength to
frequency and vice versa
If,
for example, the wavelength of 1 m is involved, then its associated frequency
is given by
4.4.2 The photon nature
of radiation
There
is another aspect to the description of electromagnetic radiation that is
important in terms of the atomic processes occurring in astronomical sources
and in the process of detection by observational equipment.
At
the turn of the twentieth century, it was demonstrated that light also had a
particulate nature. Experiments at that time showed that radiation could be
considered as being made up of wave packets or photons. The energy associated
with each photon can be expressed in the form
E
= hν (4.3)
where
h is Planck’s constant and equal to 6·625 × 10−34 J s. Thus, it can
be seen that the photons carrying the most energy are associated with the high
frequency end of the spectrum, i.e. the γ -rays— photons associated with the
radio spectrum have very low energy.
For
many observational circumstances, the flux of energy arriving from faint
sources is such that it is the statistical random nature in the arrival of the
photons that limits the quality of the measurement. In observations where the
source of experimental noise errors is very small, it is perhaps the random arrival
of photons that constitute the noise on the measurements. The accuracy of data
recorded under such a circumstance is said to be limited by photon counting
statistics or by photon shot noise.
In order to be able to estimate the accuracy to which measurements of
brightness or details within the spectrum can be obtained, it is necessary to
know the photon arrival rate associated with the generated signal. For this
reason, the strengths of observed sources are sometimes referred to in terms of
photons s−1 rather than in watts. Equation (4.3) is all that is
needed to relate the two ways of expressing the amount of energy which is
received by the observing equipment. More detail of this topic will be
presented in Part 3.
It
may also be noted that in the zones covering the high energy end of the
spectrum, neither wavelength nor frequency is used to describe the radiation.
The more usual units used are those of the energy of the recorded photons.
Thus, for example, features occurring in x-ray radiation are normally described
in terms of photon energies of order 10 keV.
Equation
(4.3) describes the energy of a photon and this can be re-written as
By
remembering the conversion of units such that 1 eV = 1·6 × 10−19 J,
the photon energy expressed in eV units is
In
order to determine the wavelength associated with a photon of some given
energy, consolidation of the numerical parts leads to
4.4.3 Polarization
In
addition to the strength of any radiation and the variation with frequency, the
radiation may have another property. Two apparently identical beams of
radiation having the same frequency spread and intensity distribution within
that spectral range may interact differently with certain materials or devices.
From this we may conclude that radiation has another characteristic. This
quality is known as polarization. It manifests itself as an orientational
quality within the radiation.
The
usefulness of polarization as a means of carrying information about a radiating
source is sometimes ignored, perhaps as a result of the eye not being directly
sensitive to it. However, the simple use of a pair of Polaroid sunglasses
reveals that much of the light in nature is polarized to some degree. Rotation
of the lenses in front of the eye will demonstrate that the light of the blue sky,
light reflected by the sea and light scattered by rough surfaces are all
polarized. Measurement of the polarization of the radiation coming from
astronomical sources holds much information about the natures of those sources.
Its generation may result from scattering processes in a source or by the
radiating atoms being in the presence of a magnetic field. Because polarization
is essentially an orientational property, its measurement sometimes provides
‘geometric’ information which could not be ascertained by other observational
analyses. In stellar measurements, for example, knowledge of the orientations
of magnetic fields may be gleaned.
In
the optical region, the simplest polarimetric measurements can be made by
placing a plastic sheet polarizer (similar to that comprising the lenses of
Polaroid sunglasses) in the beam and measuring the transmitted intensity as the
polarizer is rotated. The larger the relative changes in intensity are, the greater
the degree of polarization is. If a wholly polarized beam is generated
artificially by using a polarizer (see figure 4.2) and this beam is then
analysed by a rotating polarizer in the usualway, then the measured intensity
will fall to zero at a particular orientation of the analysing polarizer.
Although the polarization of the radiation coming from astronomical sources is
usually very small, its measurement holds much information about the nature of
those sources.
All
the parameters which are used to describe radiation, i.e. its strength and its
variation across the spectrum, together with any polarization properties, carry
information about the condition of the source or about the material which
scatters the radiation in the direction of the observer or about the matter
which is in a direct line between the original source and the observer. If the
observer wishes to gain as much knowledge as possible of the outside universe,
measurements must be made of all of the properties associated with the
electromagnetic radiation.
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