Observing Stars Essay

This essay has a total of 2832 words and 15 pages.

Observing Stars



Observing Stars

Our view of the sky at night is possible because of the emission and reflection of light.
'Light' is the better-known term for the electromagnetic spectrum, which includes waves in
the visible, ultra-violet, infra-red, microwave, radio, X-ray and gamma-ray regions. The
scale of the spectrum is so large that no region is distinct, several overlap each other.


Each of these regions in the electromagnetic spectrum represent transverse waves,
travelling as electrical and magnetic fields which interact perpendicularly to each other,
with different ranges of wavelength. The magnetic field oscillates vertically and the
electric field horizontally, and each field induces the other.


By the end of the nineteenth century, Maxwell gave a realistic value for c, the speed of light:

c = __1__ = 3 x 108 ms-1
Ö(mo eo)

The relationship between the speed of all electromagnetic radiation, wavelength (l) and
frequency (f) is shown to be c = l f.


Because the Universe is so vast, interstellar distances are so great that light emitted
can take upwards of millions of years to reach us. Such large distances are often
measured in ‘light-years’; one light-year (ly) is the distance travelled by a wave of
light in a year. Because of the massive speed of light and distances, the light arriving
at us would have left the object many years ago, so that looking at a far away star is
much like looking back in time.


Scientific observation of the stars is difficult because of the distorting effect of the
Earth's atmosphere. One problem is atmospheric refraction-where light is bent. Turbulent
air currents cause varying refractive indices, as there is no uniform air density. This
causes an effect called scintillation, where stars appear to twinkle. The effect on
regions of the electromagnetic spectrum other than the visible part, such as the
absorption of certain frequencies by atmospheric chemicals, and the reflection of waves by
charged molecules in the ionosphere, means that some spectral data is simply invisible to
us on Earth.


The Earth receives electromagnetic radiation of all wavelengths from all directions in
space, but most of the electromagnetic spectrum is blocked out by the atmosphere well
above the Earth's surface, where our eyes and instruments are mostly based. However,
wavelengths from only two regions of the electromagnetic spectrum are able to penetrate
the atmosphere. These two spectral windows in the atmosphere through which we can observe
the Universe are called the optical window-which allows the visible wavelength region
through; and the radio window-which includes the wavelength region from about 1 mm to 30
m. The telescopes used by astronomers on the ground are therefore classed as optical and
radio telescopes. Optical telescopes work by either reflecting or refracting light, using
lenses or curved mirrors to focus the light from a subject to form an image. Radio
telescopes consist of a parabolic reflector and receiver on which the waves are focused.
The gathering and resolving power depend on the diameter of the antenna. Radio
observations are unaffected by the weather or time of day, and because of the larger
wavelength of radio waves, dust in space and atmospheric convection currents are not a
problem. Radio astronomy is used in the chemical analysis of elements (by emission and
absorption spectra); to detect the motion of bodies due to the Doppler effect; and in
investigation into the early Universe and the Big Bang. We can analyse radio waves from
the centres of galaxies, including our own.


Despite the radio window, there are still wavelengths that do not penetrate the
atmosphere. Some radio waves are reflected from the ionosphere, part of the thermosphere,
where streams of charged particles from the sun ionise gas molecules: this is
photo-ionisation. Ultra-violet radiation, X-rays and gamma-rays are also absorbed at this
layer.


Absorption of the electromagnetic spectrum at various altitudes above Earth occurs to
varying degrees. Much infra-red radiation does not reach ground level because of
absorption in the upper atmosphere by water, and some carbon dioxide and oxygen molecules
that lie between the ground and about 15 km of altitude (the troposphere). Ozone
(tri-oxygen) and di-oxygen in the stratosphere absorbs much of the ultra-violet radiation
(hence the ‘ozone layer’ at about 30km). A side effect of the ozone layer is that
molecules re-radiate the energy in a few wavelengths of the green, red, and infrared
regions, causing ‘airglow’.


It is because of the limitations of Earth’s atmosphere, that astronomers learnt the
benefits of observing from beyond it. Placing telescopes and instruments of mountain
tops-to avoid clouds, bad weather and turbulence-or using balloons or aircraft, are
useful, but satellites are far more so. All electromagnetic radiation can be detected,
unaffected by absorption, reflection or refraction, dust, atmospheric haze, airglow,
weather, light pollution or the time of day.


The Hubble Space Telescope is probably the most famous astronomical satellite in orbit
around Earth. Photographs taken by it have far improved detail than an Earth-based
telescope. We have greater knowledge of elements and compounds present thanks to emission
and absorption spectroscopy. The 1983 NASA Infra-Red Astronomical Satellite (IRAS) has
been successful in infra-red observations across the sky, detecting nuclear and chemical
reactions by spectrometry, and hot clusters where stars are born. The 1989 NASA Cosmic
Background Explorer (COBE) satellite undertook a detailed study of background radiation:
the ‘echo’ of the Big Bang. Low frequency microwaves present today are the result of the
red-shift over a long time of the original, high-energy electromagnetic radiation from the
time of the birth of the Universe. The future of satellite observations lies with X-ray
and gamma-ray astronomy. X-ray images show where high-energy events occur, such as
nuclear processes and matter entering a black hole. Gamma-rays are emitted from only the
hottest and most violent bodies, and although difficult to detect, telescopes are used to
map the Universe.


Most observations surround the light from stars. There are billions of them in the
Universe; we classify stars by their various characteristics. The properties of stars can
be determined by the application of principles explained below.


All stars visible to us must have surface temperatures high enough to emit light which we
can see from so far away. Some appear brighter than others. The difficulty is in
determining weather a star is very hot and bright, or not as bright but just much closer
to us. We know that very hot things appear ‘red hot’ or even ‘white hot’, that the
temperature of an object relates to the colour of light it radiates. The electromagnetic
radiation emitted by any object (whatever its temperature) is known as thermal radiation.
Hot objects such as stars emit high energy, high frequency radiation. At about 1000oc,
thermal radiation falls in the visible region of the electromagnetic spectrum.


To find out the temperature of a star, measurements need to be relative rather than
absolute, as there is no possible way of measuring a star’s surface temperature
physically! No object can perfectly emit (or absorb) light in practice, but it is useful
to imagine such a body to make comparisons with: a ‘black body’. A black body is a
perfect absorber of light; it follows therefore that it is also a perfect emitter of
light. A perfect absorber would appear totally black; a perfect emitter would emit all
radiation, including visible light, and would appear bright white. We know that a black
body therefore emits a broad range of the electromagnetic spectrum. The most intense
emission will peak at a particular wavelength. The hotter the body, the shorter the peak
wavelength, but the higher the peak. Wein’s displacement law states that the peak
wavelength, lmax , is inversely proportional to absolute (actual) temperature of an
object. We assume that a star behaves as a black body. The relationship is shown below:


lmax T = 2.898 x 10-3 m K

Hence, we can relate the colour of a star to estimate its temperature, depending on where
in the electromagnetic spectrum lmax lies. Astronomical objects have peak wavelengths
ranging from radio to X-rays, i.e. surface temperatures from absolute zero to 107 K.


It is apparent that the hotter an object is, the more intense the emission of radiation
is. Luminosity (L) is the total power emitted by a body. The Stefan-Boltzmann law states
that ‘the total energy radiated per unit time by a black body is proportional to the
fourth power of its absolute temperature’; it also depends on the surface area (A):


L = s A T4

Stefan’s constant (s) = 5.67 x 10-8 W m-2 K-4

The amount of power received per unit area is flux (equal to power / area). Light emitted
from an object spreads out in all directions, the further away it gets the less intense it
becomes according to the inverse square law:
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