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Discover the Universe Week 12

by: Jocelyn

Discover the Universe Week 12 AST 1002

Marketplace > University of Florida > Science > AST 1002 > Discover the Universe Week 12

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Star Formation & Stellar Evolution – Discover how the stars form out of the ISM. Follow the timeline for a typical, Sun-like star from infancy to death
Discover the Universe
Reyes, Francisco J
Class Notes
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This 4 page Class Notes was uploaded by Jocelyn on Sunday November 15, 2015. The Class Notes belongs to AST 1002 at University of Florida taught by Reyes, Francisco J in Summer 2015. Since its upload, it has received 15 views. For similar materials see Discover the Universe in Science at University of Florida.


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Date Created: 11/15/15
Thursday, November 12, 2015 Discover the Universe Week 12 Chapter 10: Measuring the Stars - Parallax is the apparent shift of an object relative to some distant background as the observer’s point of view changes • It is the only direct way to measure distances to stars • It makes Earth orbit as a baseline • Parallactic angle = 1/2 angle shift • A new unit of distance: Parsec • By definition, parsec (pc) is the distance from the Sun to a star that has a parallax of 1” (1 arc sec) - Parallax Formula: • Distance (in pc) = 1/Parallax • One parsec = 206,265 AU or 3.3 light years - As the distance increases to a star, the parallax decreases • If the parallax measured is 1 arcsec, then the distance of the star is 1 pc • If the parallax measured is 0.5 arcsec, then the distance of the star is 2 pc - The Nearest Stars • The nearest star Proxima Centauri has a parallax of 0.76 arcsec • The next nearest star Barnard’s Star has a parallax of 0.55 - Star with the largest proper motion - From the ground, we can measure parallactic angles of 0.03 arc sec corresponding to 30 parsec • There are several thousand stars within that distance from the Sun - Using the stellar parallax, the distance to these stars can be determined directly - From space, parallax’s can be measured down to about 5/1000 arcsec which corresponds to 200 pc. 1 Thursday, November 12, 2015 • There are several million stars within that distance - Stellar Proper Motion • Parallax is an apparent motion of stars due to Earth orbiting the Sun • But stars do have real space motions • Space motion has 2 components: - 1) Line of sight or radial motion measured through Doppler shift of emission or absorption lines - 2) Transverse motion perpendicular to the line of sight - Use trig to calculate the transverse velocity - This method works for stars nearby so we can measure the proper motion - Total V = Square Root: (Radial V)2 + (Transverse V)2 - What does proper motion depend on? • Space velocity • Distance - Luminosity is the amount of radiation leaving a star per unit time. • Intrinsic property of a star • It is also referred as the star of absolute brightness • It doesn't depend on the distance or motion of the observer respect to the star • Apparent brightness: When we observer a star we see its apparent brightness, not its luminosity. The apparent brightness or flux is the amount of light striking the unit area of some light sensitive device such as the human eye or a CCD - Apparent brightness depends on the distance to the star - Flux: Luminosity/4pid^2 - Light spreads like the distance squared 2 Thursday, November 12, 2015 - Through a sphere twice as large, the light energy is spread out over four times the area (4pid^2) - The apparent brightness or Flux decreases with distance, it is inversely proportional to the square of the distance - The Magnitude Scale - Hipparchus ranked all visible stars with the brightest being a 1 and the faintest being a 6 - Corresponds to a change in brightness fo 100 - To our eyes, a change of one magnitude = factor of 100 ^1/5 = 2.5 - 5 magnitudes = factor of 100 - Absolute magnitude is the apparent magnitude of a star as measured from a distance of 10 pc • Sun’s absolute magnitude = 4.8 • It is the magnitude of the Sun if it is moved to a distance of 10 pc • Just slightly brighter than the faintest stars visible to the naked eye (magnitude = 6) - Stellar Temperature: Spectra • The spectra shows 7 stars with the same chemical composition but different temperatures • Different spectra result from different temperatures - Example: Hydrogen Absorption Lines are relatively weak in the hottest star because it is mostly ionized - Conversely, hotter temperatures are needed to excite and ionize Helium so these lines are strongest in the hottest star. - Molecular absorption lines (TiO) are present in low temperature stars. The low temperatures allow formation of molecules - Spectral Classification • Oh Be A Fine Girl/Guy Kiss Me (OBAFGGKM) • Each letter is divided into 10 smaller subdivisions from 0 to 9. The lower the number, the hotter the star. 3 Thursday, November 12, 2015 - Stellar Radii • Almost all stars are so distant that the image of their discs looks so small. Their images appear only as an unresolved point of light. (p. 103) • Diameter/2pi x distance = Angular diameter/360 • We can use radiation laws to make indirect determinations • Luminosity - Surface Area x T^4 4


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