14. Stars - Measuring the Stars Outline Our goals for this...

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Unformatted text preview: Measuring the Stars Outline Our goals for this section: To understand the way in which we rank the relative brightnesses of stars on the magnitude system To learn how to determine the basic properties of stars (radius, luminosity, distance, etc.) To introduce the concept of spectral types, and to learn the basic properties of stars belonging to each type To introduce the all-important HR diagram , which helps us organize all of the important information about stars Within the solar system, distances can usually be determined accurately using one or more of: Keplers laws radar ranging laser ranging These techniques are not useful in determining the distances to stars (why not?) Instead, for relatively nearby stars, we use the method of trigonometric parallax to determine distances Parallax is an objects apparent shift relative to some distant background object as the observers point of view changes The Distances to the Stars One parsec ( par allax arc sec ond) or pc, is the distance at which 1 AU subtends 1 1 AU 1 AU p p = parallax d = distance to star nearby star distant stars By the small angle formula: p d 1 pc = 3.26 ly = 206265 AU=3.09x10 16 m The apparent brightness of a star is measured in terms of its apparent magnitude, m The magnitude scale was developed by the Greek astronomer Hipparchus, who devised it in the 2nd century B.C. as a means of ranking the naked-eye brightnesses of stars The magnitude scale is logarithmic and backwards !!!! Hipparchus original magnitude scale went from 1 (bright star) to 6 (faint star), but the use of telescopes and the extension of the scale to include other objects (such as the Sun) has necessitated some adjustment Luminosity & Apparent Brightness large magnitude = small brightness = faint star Sample objects on the magnitude scale: Apparent Magnitude Object-26.8 the Sun-4.4 Venus (brightest planet)-1.4 Sirius (brightest star) 1 bright naked-eye star 6 faintest naked-eye star 28 faintest object visible to ground-based telescopes 30 faintest object visible to Hubble increasing brightness Which of the stars in the constellation Orion has the lowest apparent magnitude? Example The apparent magnitude of a star only tells us how bright a star appears to us on Earth; it does not tell us what the intrinsic brightness of the star is Two stars can appear equally bright at Earth even if they have different intrinsic brightnesses--how? The brighter star need only be farther away: bright, distant star dim, nearby star same apparent magnitude In order to be able to compare the intrinsic brightnesses of stars, we use their absolute magnitudes, M The absolute magnitude, M , of a star is the apparent magnitude it would have if it were situated 10 pc from Earth In terms of its actual distance, d, and its apparent magnitude, m, the absolute magnitude of a star is: Example Betelgeuse has an apparent magnitude of +0.41, and it is 150 pc away. What is its absolute magnitude?...
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14. Stars - Measuring the Stars Outline Our goals for this...

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