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: L = d-2 E.g., As Saturn is ten
times the distance from the Sun as Earth, the intensity of radiation is receives
is 1/100 th of that for Earth. The light reaching Earth from the sun can be
analysed using a technique called spectroscopy. It is used to identify the
chemical composition of stars (which is mostly hydrogen and helium), and their
surface temperature. Once these are known, stars can be classified accurately.
An emission spectrum is the spectrum of wavelengths of light emitted from atoms
or molecules. They do this when they lose energy, which corresponds to a
specific frequency of the electromagnetic spectrum.
