PHY121 Ch 8 Notes

# PHY121 Ch 8 Notes - Astronomy Chapter 8 – The Family of...

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Unformatted text preview: Astronomy Chapter 8 – The Family of Stars- Distance is critical in astronomy. Your goal in this chapter was to characterize the stars by finding their luminosities, diameters, and masses. Only by first knowing the distance to a star could you find its other properties. • The Surveyor’s Triangulation Method: to measure distance across a river Two stakes in the ground that is a known distance apart, on same side of river Choose a landmark on opposite side of river (i.e. tree), establishing a triangle Site the tree from the two ends of the baseline and measure the two angles on your side of the river By using trigonometry with the two angles, you can find the distance across the river • The Astronomers Triangulation Method: to find distance to star Baseline is the diameter of the Earth’s orbit Take a photo of a nearby star and then wait 6 months, Earth will have moved halfway around its orbit. Then take another photo, and the star will be 2AU from the original star. Baseline = 2AU, and lines to stars equal a long thin triangle.- Parallax: the apparent change in the position of an object (star) due to a change in the location of the observer. 1.1. Example: your thumb, held at arm’s length, appears to shift position against a distant background when you look first with one eye and then with the other. The baseline is the distance between your eyes, and the parallax is the angle through which your thumb appears to move when you change eyes. Farther away you hold your thumb, the smaller the parallaxes.- Astronomers can measure the distance to nearer stars by observing their stellar parallaxes. The most distant stars are so far away that their parallaxes are unmeasurably small (seconds of arc). Space telescopes above Earth’s atmosphere have measured the parallaxes of huge number of stars. Stellar Parallax: star appears slightly shifted from different positions of Earth on its orbit. The farther away the star is (larger d), the smaller the stellar parallax (angle p). d = (d in parsec (pc), p in arc seconds) 1.1. Example: The star Altair has a parallax of 0.20 second of arc. How far away is it? Answer: d= = 5pc- Stellar distances are commonly expressed in parsecs . 1pc = 206,265 AU = 3.26 LY- The amount of light received from a star, the light flux , is related to its distance by the inverse square law. Flux is the energy is joules (J) per second falling on 1 square meter....
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## This note was uploaded on 12/11/2011 for the course PHY 121 taught by Professor Weinstein during the Fall '08 term at SUNY Buffalo.

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PHY121 Ch 8 Notes - Astronomy Chapter 8 – The Family of...

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