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Understanding the Stars

Understanding the Stars. How can we learn about the lives of stars, which last millions to billions of years? Consider the story of the Ephemera Image from: http://www.startunzflutes.com/graphics. Classification of Stars. Stars were originally classified based on: their brightness

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Understanding the Stars

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  1. Understanding the Stars • How can we learn about the lives of stars, which last millions to billions of years? • Consider the story of the Ephemera • Image from: http://www.startunzflutes.com/graphics

  2. Classification of Stars • Stars were originally classified based on: • their brightness • their location in the sky • This classification is still reflected in the names of the brightest stars…those we can see with our eyes: Order of brightness within a constellation Latin Genitive of the constellation  Orionis  Geminorum

  3. Summer Triangle • Which of these three Stars in the Summer Triangle is the brightest (to the naked eye) • Which is the most Luminous? • Which of these three are the closest?

  4. Dark Sky image of Summer Triangle

  5. Don’t always trust your eyes! • Note that Deneb is a bit larger than the other two • Its also a bit farther away—2250 light years! • Vega is 22 light years away—its in the neighborhood • Note that Altair (about 30 lyrs) is probably flattened..its rotating very fast! • Apparent brightness depends on size, temp, and distance!

  6. Luminosity of Stars Luminosity – the total amount of power radiated by a star into space. • Apparent brightness (I) refers to the amount of a star’s light which reaches us per unit area. • its apparent brightness decreases as the (distance)2 • Apprent brightness is also known as the Intensity • The apparent brightness of a star depends on two things: • How much light is it emitting: luminosity (L) [watts] • How far away is it: distance (d) [meters] I = L / 4d2

  7. Apparent Brightness The Inverse Square Law for Light What Determines Apparent Brightness?

  8. Measuring Distances to Stars parallax – apparent wobble of a star due to the Earth’s orbiting of the Sun. Check out annimation at: http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html

  9. Measuring Distances to Stars p = parallax angle d = 1 AU/ p Gives distance in Parsecs. convert p into arcsec d = 206,265 AU/ p

  10. Measuring Distances to Stars let’s define 1 parsec  206,265 A.U. = 3.26 light years d = 1 / p If p is in arcsec and d is in parsecs A star with a parallax of 1 arcsec is 1 parsec distant

  11. The Brightness of Stars Astronomers still use an ancient method for measuring stellar brightness which was proposed by the Greek astronomer Hipparchus (c. 190 – 120 B.C.) Magnitude Scale This scale runs backwards: The bigger the number, the fainter the star Brightest stars are #1, next brightest are #2, etc.

  12. The Modern Magnitude System apparent magnitude = -2.5 log (app bright) • brightness of a star as it appears from Earth • each step in magnitude is 2.5 times in brightness absolute magnitude • the apparent magnitude a star would have if it were 10 pc away

  13. What good is this? • If you know apparent brightness, you can find magnitude. • If you know magnitude, you can use another relationship to find distance • M – m = 5 – 5log(d) • M= Absolute magitude • M = m when distance is ten parsecs.

  14. An example of how this works! • Deneb has an apparent visual magnitude of 1.26 (see chart of brightest stars at end of your text) Deneb has an Absolute visual magnitude of -8.73 (this is about the same brightness as the quarter moon---but at 32.6 light years away!) Using the weird equation, the distance to deneb can be calculated: 2500 light years (M – m = 5 – 5log(d)) One last obvious question: How did we ever know the Absolute visual magnitude to Deneb without knowing its distance in the first place?

  15. 16.3 Classifying Stars • Hypothysis: • the Luminoisty and Abolsute Magnitude of Stars can be known if we know their Classification. • There Classification is completely revealed in their Spectra. • Comparing the spectra of nearby stars allows us to test this hypothis. • The Classification is known as Spectral Type

  16. Spectral type is revealed in the Colors of Stars Stars come in many different colors. The color tells us the star’s temperature according to Wien’s Law. Bluer means hotter!

  17. Spectral Type Classification System O BAFGKM (L) Oh Be A Fine Girl/Guy, Kiss Me! 50,000 K 3,000 K Temperature

  18. Spectral Types of Stars

  19. Spectral Types of Stars

  20. Spectroscopic parllax • If we know distance (d) from parallax measurements and.. • If we know apparent visual magnitude (m) from photometry or image size (apparent brightness is also measured in this way). • Then we can calculate Absolute visual magniutde (M). Luminosity is also measured in this way. • At the same time, we can obtain spectra and spectral type for all these nearby stars (about 10,000!) • Now we know M from looking at the spectra!

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