Spectra_and_the_lives_of_stars[1] - PHY217 Lecture 5...

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PHY217 Lecture 5 : SPECTRA AND THE LIVES OF STARS 4. Spectroscopy The wave nature of light: Unlike other branches of science, astronomers cannot touch or do field work on their samples. The only information that we have is from the light that is imaged by our telescopes. Fortunately, light contains a wealth of physical information about the Universe. Light can be viewed as a traveling wave (like an ocean wave). It is made up of electric & magnetic fields so light is referred to as electromagnetic radiation . The wavelength is the distance between corresponding on a successive cycles. (e.g. between wave crests). Measured in units of length. For optical light, a shorthand notation of angstroms (Å ) is used such that 1 Å= 1*10 -10 meters. So, visible wavelength light is about 5000 Å. The frequency is the number of wave crests that pass by a detector per second of time. The units are Hertz (Hz), where 1 Hz is 1 cycle/sec. A typical radio frequency is 100 MHz (where MHz is a million Hz). Light travels very fast at c=3 *10 5 km/sec. This speed is constant throughout space for all wavelengths. 4.1 Wavelengths of the electromagnetic radiation visible from the surface of thermonuclear Earth In the visible, the rainbow of colours make up the spectrum. Colour is the same as wavelength . Blue light has the shortest wavelength and red light has the longest. Light is with a specific range of wavelengths. Radio , microwave and infrared electromagnetic radiation has longer wavelengths than visible light . UltraViolet, X-rays and gamma rays have shorter wavelengths than visible light. Gamma-rays have wavelengths < 1 Å and radio waves have wavelengths of meters. The atmosphere is mostly opaque to the electromagnetic spectrum. Only visible and radio waves easily penetrate the atmosphere with a bit of ultraviolet & infrared as well. This means ground- based telescopes operate in the visible & the radio, whereas space-based telescopes (above the atmosphere) function at other wavelengths. By splitting light up into its component wavelengths, we can measure quantities such as temperature, chemical composition, velocity, magnetic field strengths and density in many different celestial objects. It is AMAZING that we can find out what the universe is like at such large distances and what it was like billions of years ago.
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Fig 4.1 Light that our eyes can detect is just a small part of the electromagnetic spectrum of light . Light of most other wavelengths is absorbed in the atmosphere so to observe stars and galaxies at, for example UV, X-ray and Gamma Ray wavelengths, we send satellite observatories into space. Optical, radio and some infra-red light is observable from the ground 4.2 The Doppler Effect - The Doppler Effect is the shift of the wavelength of light produced by motion of an object toward or away from us. We experience a similar effect with sound when a train approaches us & then moves away from us - the pitch or the wavelength of the sound changes. -
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Spectra_and_the_lives_of_stars[1] - PHY217 Lecture 5...

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