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What is a Star?

What is a Star?. Wednesday, October 8 Next Planetarium Shows: Tonight 7 pm, Thurs 7 pm. Doppler shift review:. Light source moving toward you: wavelength is shorter ( blueshift ). Light source moving away from you: wavelength is longer ( redshift ).

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What is a Star?

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  1. What is a Star? Wednesday, October 8Next Planetarium Shows: Tonight 7 pm, Thurs 7 pm

  2. Doppler shift review: Light source moving toward you: wavelength is shorter (blueshift). Light source moving away from you: wavelength is longer (redshift).

  3. Size of Doppler shift is proportional to radial velocity: Δλ = observed wavelength shift = λ-λ0 λ0 = wavelength if source isn’t moving V = radial velocity of moving source c = speed of light = 300,000 km/sec

  4. Example: Hydrogen absorbs light with λ0 = 656.3 nm. You observe a star with a hydrogen absorption line at λ = 656.2 nm.

  5. What is a star? Examples of stars: Sun Betelgeuse

  6. What is a star? A large, hot, luminous ball of gas.

  7. “Why do stars shine?” Stars are dense (Sun is 40% denser than liquid water). Stars are opaque (you can’t see to the Sun’s center). Stars are hot.

  8. What happens when a dense, opaque object becomes hot? It emits light.

  9. What do I mean by “HOT”? 90°F 212°F 9980°F

  10. At the submicroscopic level: atoms in a gas

  11. Object is hot when the atoms of which it’s made are in rapid random motion. Temperature: measure of typical speed of the atoms. Random motions stop at absolute zero temperature.

  12. Kelvin = Celsius + 273 Water boils: 373 Kelvin (K) Water freezes: 273 K Absolute zero: 0 K Room temperature: ~300 K Surface of Sun: ~5800 K

  13. Different elements respond in different ways to changes in temperature.

  14. (This assumes there’s no oxygen for the hydrogen to bond with.) At high density & low temperature, hydrogen is a gas of molecules. Molecular hydrogen = H2 = two H atoms bonded together

  15. At low density & low temperature, hydrogen is a gas of atoms. Much of the interstellar gas in our Galaxy is atomic hydrogen. density ≈ 10 atoms/cm3 T ≈ 100 K

  16. At high density & high temperature, hydrogen is an ionized gas. Much of the Sun’s interior is ionized hydrogen. Sun’s center: density ≈ 150 tons/m3 T ≈ 15 million K

  17. A star is an approximate blackbody. A blackbody is an object that absorbs all the light that hits it. Heat a blackbody: it emits light of all wavelengths (a continuous spectrum). Wavelength at which spectrum peaks depends only on temperature.

  18. 21,100°F 10,300°F 4900°F Blackbody spectra

  19. Wavelength of peak emission for a blackbody is inversely related to temperature.

  20. Examples: You: 9400 nm (Infrared) Sun’s surface: 500 nm (Visible)

  21. 2 am tomorrow, looking east

  22. Another example: taking the temperature of a star! Betelgeuse is red. (Hard to see colors with the naked eye – binoculars help!) Rigel is blue.

  23. Betelgeuse: 2900 K (4800°F) Rigel: 14,500 K (26,000°F)

  24. The Sun’s actual spectrum: Perfect blackbody Intensity Close to a blackbody, but not perfect.

  25. Friday’s Lecture: What is a Galaxy? Reading: Chapter 3

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