astro 120 lab #6 - Lab #6 Galaxies Objective: The objective...

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Lab #6 – Galaxies Objective: The objective of this lab is to learn how to find the mass of galaxies (especially the Milky Way) through Kepler’s Laws and approximate calculation determined by observations from the astronomy computer program. We are expected to find the apparent magnitudes and distances to the well-known globular cluster, M15, by graphing data obtained from variable stars. We are required to be proficient in using the distance formula to calculate the distance (from the Earth) to center of the Milky Way. Also the sizes, or diameters of various globular clusters need to be obtained. Introduction: Our solar system is within the spiral galaxy known as the Milky Way. We are located on one of the spiral arms of this galaxy. The globular clusters within our galaxy are concentrated around the galactic center; these clusters would generally be visible in the constellation Sagittarius if looking into the sky. Globular clusters are groups or very old stars that can have collection of tens of millions stars just within one cluster. At the galactic center of our galaxy, globular clusters are arranged in a sphere configuration. In the 1600, it was noticed that some of the nearby clusters contained stars with varying brightness. Variable stars are those in which their brightness changes with time as there surface layers contract, and expand repeatedly or in which exhibit change from earth’s perspective because of their motions. Variable stars can be classified into two groups, extrinsic or intrinsic. Extrinsic variable stars are those in which their brightness changes are due to external properties such as eclipsing or rotating. Extrinsic stars can be further sub-classified into two more groups, rotating variables and eclipsing binaries stars. Rotating variable stars show light variation because of strange effects induced by their rotation. Eclipsing binary stars occur with a two-star/double star system in which one star passes in front of the other, blocking out some of its light or completely eclipsing it, causing an apparent change in brightness, from Earth’s perspective. Intrinsic stars on the other hand vary in luminosity because of changes in their physical components. There are three sub-classes of intrinsic stars, which are, eruptive, pulsating and cataclysmic variable stars. Eruptive variable stars have eruption on their surfaces that cause flashes, or variations in their light-energy output. Cataclysmic variable stars have explosive properties similar to supernovae and finally pulsating variable stars change size, and in consequence there luminosity as they age, getting bigger then smaller repeatedly. In the lab, intrinsic variable RR Lyrae star will be studied. RR Lyrae stars are
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This note was uploaded on 10/14/2008 for the course ASTRO 120 taught by Professor Robb during the Spring '08 term at University of Victoria.

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astro 120 lab #6 - Lab #6 Galaxies Objective: The objective...

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