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Chapter 11 Surveying the Stars.doc

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Natural Science
NATS 1740
Randy Hoffman

Chapter 11 Surveying the Stars 11.1 Properties of stars How do we measure stellar luminosities? - 2 similar looking stars can be generating very different amounts of light - Apparent brightness: how bright stars look in our sky (amount of power reaching us per unit area) - Luminosity: the total amount of power that a star emits into space - A stars apparent brightness in the sky depends on both its true light output or luminosity and its distance from us The inverse square law for light - The apparent brightness of a star or any other light source obeys an inverse square law - Doubling the distance to a star would decrease is apparent brightness by a factor of 2 or 2 2 2 - Inverse square law: apparent brightness = luminosity/ 4π x (distance) Measuring distance through stellar parallax - We can measure the distance to a nearby star by observing how its apparent location shifts as earth orbits the sun - Stars parallax angle is smaller the further away it is The luminosity range of stars - Stars come in a wide range of luminosities with our sun sometwhere in the middle - Dim stars are far more common than bright stars The magnitude system - Apparent magnitude is used instead of apparent brightness - Absolute magnitude is used instead of luminosity How do we measure stellar temperatures? - Stars distance doesn’t affect temperature Colour and temperature - Stars come in different colours because they emit thermal radiation - Astronomers can measure surface temperature fairly precisely by comparing a stars apparent brightness in 2 different colours of light Spectral Type and Temperature - Stars spectral lines provide a 2 way to measure its surface temperature - Spectral type: determined from spectral lies present in star system (OBAFGKM) - Spectra of stars show that their surface temperatures range from more than 40,000K to less than 3000K, corresponding to the sequence of spectral types of OBAFGKM How do we measure stellar masses - Binary star systems: systems in which 2 stars continually orbit one another - Mass is harder to measure than luminosity and temperature  Most dependable way is to use Newtons version of Keplars third law Types of Binary Star Systems - Visual binary: pair of stars that we can see distinctly as the stars orbit each other (With a telescope) - Eclipsing binary: pair of stars that orbit in the plane of our line of sight  If a binary system is neither visual nor eclipsing, we may be able to detect its binary nature by observing Doppler shifts in its spectral lines  Some stars combine 2 or more binary types (Ex: Mizar) Measuring masses in binary systems - We can determine the masses of stars in binary systems if we can measure both their orbital period and the separation between them  Only exception is eclipsing binary stars 11.2 Patterns Among Stars - Most of the very brightest stars are reddish in colour - If you ignore those relatively few bright red stars, there’s a general trend to the luminosities and colours among all the rest of the stars  Brighter ones are white with a little blue, more modest ones are similar to our sun in colour with yellow-white and dimmest ones are barely visible specks of red What is a Hertzsprung-Russell Diagram? Basics of HR diagrams - Horizontal axis represents stellar surface temp
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