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: 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 li
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-2E. Despite the radio window, there are still wavelengths that do not penetrate the atmosphere. Star populations are mapped on the Hertzsprung-Russell diagram, basically a graph of luminosity against surface temperature:From it we can examine the life sequences of a star, deduce a star's absolute magnitude, and then their spectral class according to their surface temperature and other properties. They vary in brightness as their surface temperature rises and falls. All stars visible to us must have surface temperatures high enough to emit light which we can see from so far away. Cepheid variable stars have luminosity which varies periodically. We can only estimate the distances of more distant objects such as supernovae. At about 1000oc, thermal radiation falls in the visible region of the electromagnetic spectrum. 67 x 10-8 W m-2 K-4The amount of power received per unit area is flux (equal to power / area). One method is called spectroscopic parallax, where we can make the assumption that all stars are equally bright (although we know of course that they are not), and so the brighter a star the closer it is. This causes an effect called scintillation, where stars appear to twinkle. A black body is a perfect absorber of light; it follows therefore that it is also a perfect emitter of light. This is where a distant object will appear at a different spot when viewed from a different angle. The telescopes used by astronomers on the ground are therefore classed as optical and radio telescopes.
Common topics in this essay:
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Sun Earth,
ESA Hipparcus,
Observing Stars,
Universe Bang,
Mpc-1 Doppler,
Earth Earth,
Earth Unsurprisingly,
Tan Distance,
T4 Stefan's,
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surface temperature,
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regions electromagnetic spectrum,
doppler effect,
radio waves,
binary stars,
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inverse square law,
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Acceptance Essays
