ASTRO 101 Lectures 11-14

# ASTRO 101 Lectures 11-14 - Parallax and Flux Now that we...

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Parallax and Flux Now that we have finished discussing the Solar System, including the Sun, we can turn our attention to the next nearest objects, which are stars in the solar "neighborhood". The sun and stars near to the sun all circulate around a huge galaxy (~100,000 light years across) at velocities of about 200 km/second. How we know this is a subject we will save for future discussion. However, we are ready to understand how we know which of the stars are nearby and which are far away. If all stars were exactly like the sun, we could simply infer that the brightest ones were closest. This was the distance method method attempted by Hipparcus, and it relies upon the inverse square law for how flux diminishes with distance. This is written: 2 4 d L F π = The book uses b d L 2 4 =

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Parallax and Flux The F stands for flux, which in this formula is the apparent brightness we measure (the energy falling upon a fixed surface area), L is the luminosity or total energy output of the star, and d is the distance to the star. The exponent on distance is a simple consequence of living in a 3 dimensional space, as the picture below from the text shows: The same light-energy passes through a larger area as the distance from the source increases. Thus the light becomes more dilute according to a 1/d 2 law. 2 4 d L F π =
Parallax and Flux Even naked eye observations show that not all stars are alike (and therefore cannot all be identical to the sun); some appear red in color and others blue. So we need another method for distance finding that does not depend on assumptions about luminosity. For nearby stars, we can use parallax, which relies upon an apparent shift in the position of a star due to the motion of the earth around the sun. Stellar parallax can be converted into distance by trigonometry. This figure shows how it works. Reminder: It was a failure to measure parallax that convinced some ancient astronomers that the Earth must be immobile, but Copernicus's alternative explanation, that the stars are simply too far away for the parallax to be detected by the naked eye, is the correct one. We now have instruments capable of measuring stellar parallax for the nearby stars.

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ASTRO 101 Lectures 11-14 - Parallax and Flux Now that we...

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