# 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: – Kepler’s 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 object’s apparent shift relative to some distant background object as the observer’s 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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## This note was uploaded on 03/01/2011 for the course ASTRO 1a03 taught by Professor Samantha during the Spring '11 term at McMaster University.

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

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