Observing Stars

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 (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.

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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 Earths 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 stars 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. Weins 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 emiss …