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Chapter 17

Chapter 17 Review: The Stars

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University of Toronto Scarborough
Brian Wilson

January 11 , 2011 ASTA02 Intro to Astronomy and Astrophysics Part II Chapter Seventeen: The Stars The Solar Neighbourhood Milky Way is our galaxy o Galaxy: Enormous collection of stars and interstellar matter held together by gravity As with the planets, knowing the distance to the stars within the Milky Way is essential to determining many of their other properties By observing other distant galaxies, we can statistically infer the properties of trillions more o The observable universe probably contains several tens of sextillions (1 sextillion = ) stars Yet remarkable, despite their incredible numbers, the essential properties of stars their interactions with their environment can be understood in terms of just a few physical stellar quantities: luminosity (brightness), temperature (colour), chemical composition, size and mass As more stellar distances become known, new insights into stellar properties are obtained, and these in turn present new techniques for distance measurement, applicable to even greater distances Stellar Parallax Parallax: Apparent shift of a foreground (essentially the closer one to the observer) object relative to some distant background as the observers point of view changes o To determine an objects parallax, we observe it from either end of some baseline and measure the angle through which the line of sight to the object shifts www.notesolution.com As the distance to the object increases the parallax becomes smaller and therefore harder to measure Any baseline on Earth is still too small to measure the closest stars; however, photographs taken at different times of the year are used instead o Using the diameter of Earths orbit around the Sun as a baseline (about 2 AU) Parsec: The distance at which a star must lie in order for its observed parallax to be exactly 1 (arc second); roughly 3.3 light-years o Because parallax decreases as distance increases, we can relate a stars parallax to its distance by the following formula: Parallax distance (in parsecs) = o Thus a star with a measured parallax of 1 lies at a distance of 1 parsec from the Sun Our Nearest Neighbours Proxima Centauri is the closest star to the Earth (besides the Sun obviously) o Member of a triple star system (three separate stars orbiting one another, bound together by gravity) known as the Alpha Centauri complex o Proxima Centauri displays the largest known stellar parallax, 0.77, which means that it is about 10.77 = 1.3 parsecs away approximately 270,000 AU, or 4.3 light-years away This is a fairly typical interstellar distance in the Milky Way Galaxy If Earth were a grain of sand orbiting a marble-sized Sun at a distance of 1 meter; the nearest star (Proxima Centauri), also a marble-sized object, is then more than 270 kilometers away o Nothing else of consequence (except the other planets of our solar system )exists in the 270 km separating the two stars Such is the void of interstellar space www.notesolution.com
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