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Astronomy & Astrophysics
Michael Reid

- A Watt is an unit of energy per unit time. So, in a 100 W rating on a light bulb tells us how much energy it is using per unit time. - The Suns luminosity tells us how bright the Sun is intrinsically, just like the wattage of the light bulb. - They are blackbodies, which means their colours are determined entirely by their temperatures. Every star emits light of all wavelengths. Their colours are determined by the colour of light they emit the most of (Colour Surface Temperature). - For nearby stars, we use parallax. As Earth orbits the Sun, the position of a nearby star appears to shift against the background of more distant stars. (Parallax Distance for nearby stars) - Of all the light a star emits, Earth receives only a small fraction. This small fraction determines the stars apparent brightness or apparent magnitude as seen from Earth. - The amount of light received from a star falls with the square of our distance from it. This is called the inverse square law of light. 2 - So, a stars luminosity and apparent brightness are related as: I = L4pid - Therefore, Distance + Apparent Brightness equals Luminosity. (Luminosity Distance and Apparent Brightness) - Pickering and his associates collected stellar spectra, naming each new type alphabetically: A, B, C, D etc. But, these types of stellar spectra didnt make any sense. Annie Jump Cannon, made sense of the vast catalog of stellar spectra. She realized that you could get rid of most of the spectral categories keeping only O, B, A, F, G, K, and M. This order formed a pattern. - Each stellar spectrum has absorption lines corresponding to the chemical elements in the stars atmosphere. The strengths of the lines depend on the temperature of the star. This is a second, more precise way to measure a stars temperature. - Example: Hydrogen is weak in cool stars because they are not hot enough to excite it, strong in warm stars because they are warm enough to excite it, weak again in hot stars because all the hydrogen is ionized. - Presence of absorption lines composition, strength of particular absorption lines temperature (method 2) - The composition of stars is determined by measuring the spectral emission and absorption lines. www.notesolution.com
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