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Astronomy 1 – Fall 2014

Astronomy 1 – Fall 2014. Lecture 6: October 21, 2014. Previously on Astro 1. Light can have particle- like properties. Particle energy: E = h  = hc /  Electronic energy levels are quantized (i.e., discrete) Every element (even every ion) has a unique spectral fingerprint.

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Astronomy 1 – Fall 2014

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  1. Astronomy 1 – Fall 2014 Lecture 6: October 21, 2014

  2. Previously on Astro 1 • Light can have particle-like properties. • Particle energy: E = h= hc/ • Electronic energy levels are quantized (i.e., discrete) • Every element (even every ion) has a unique spectral fingerprint. • Spectroscopy reveals the composition of distant objects • Geometrical optics • Reflection & Refraction • Focus, Spherical aberration, Chromatic aberration • Telescopes • Light gathering power • Magnification • Resolving power

  3. The Secondary Mirror Does Not Cause a Hole in the Image This illustration shows how even a small portion of the primary (objective) mirror of a reflecting telescope can make a complete image of the Moon. Thus, the secondary mirror does not cause a black spot or hole in the image. (It does, however, make the image a bit dimmer by reducing the total amount of light that reaches the primary mirror.)

  4. Telescopes:Light gathering power

  5. Light gathering power depends on size of objective lens or primary mirror

  6. Reflecting Telescopes This view of the Gemini North telescope shows its 8.1-meter objective mirror (1). Light incident on this mirror is reflected toward the 1.0-meter secondary mirror (2), then through the hole in the objective mirror (3) to the Cassegrain focus

  7. How Do You Make a Lightweight 10m Diameter Mirror?

  8. How much more light gathering power does a 10m telescope have than an 0.5 m telescope? Answer: The light gathering power is proportional to the square of the mirror’s diameter. (10m)2/(0.5m)2 = 100m / 0.25m = 400 So you can see objects about 400 times fainter with the 10m telescope in the same amount of time.

  9. Angular Resolution

  10. Angular resolution of the telescope • Limited by: • Blurring effects of the atmosphere (“seeing”), i.e. the twinking of stars • The quality of the optics and detector on the telescope. • The size of the telescope – the “diffraction limit.”

  11. The diffraction limit θ= diffraction-limited angular resolution of the telescope, in arcseconds λ= wavelength of light, in meters D = diameter of telescope objective, in meters Example: What is the diffraction limit for red light (640nm=6.4×10-7m) for a telescope with with a 0.5m objective/primary. So even if you had a perfect atmosphere and perfect optics, you couldn’t resolve details finer than 0.32” with a 0.5m telescope.

  12. Today astronomers build telescopes at the best sites in the world, then travel to the telescope to observe, or have someone else on-site observe for them, or observe remotely over the internet. Mauna Kea, an extinct volcano in Hawaii that reaches 13,400 feet, is the best site in the world for optical and infrared telescopes. It has mostly clear, dark skies, little atmospheric turbulence, and is above most of the water vapor in the Earth’s atmosphere. Notice the snow and lack of vegetation.

  13. Adaptive Optics Help Telescopes on Earth Remove the Blurring Caused by the Atmosphere

  14. Laser Beacon Makes an Artificial Star

  15. Adaptive Optics System

  16. Astronomy Uses the Entire EM Spectrum The percentage of radiation that can penetrate the Earth’s atmosphere at different wavelengths. Regions in which the curve is high are called “windows,” because the atmosphere is relatively transparent at those wavelengths. There are also three wavelength ranges in which the atmosphere is opaque and the curve is near zero: at wavelengths less than about 290 nm, which are absorbed by atmospheric oxygen and nitrogen; between the optical and radio windows, due to absorption by water vapor and carbon dioxide; and at wavelengths longer than about 20 m, which are reflected back into space by ionized gases in the upper atmosphere.

  17. Hubble Space Telescope

  18. James Webb Space Telescope(see movie on class website)

  19. Orion

  20. The Milky Way Galaxy

  21. Discovery Enabled by Year The heavens are not perfect and unchanging; (ultimately) the Earth is not the center of the universe. The telescope and Galileo’s observations. ~1609 The sun and stars are giant balls of hydrogen undergoing fusion. Fraunhofer’s invention of the spectrograph. 1814 Our galaxy is not the center of the universe, and the universe is expanding. Edwin Hubble and the giant Palomar 200-inch and large-format photographic plates. 1929 The universe started as a hot “Big Bang” Penzias and Wilson using a radio “telescope,” confirmed by satellites. 1965 Planets are common in the universe. Modern charge-couple-device detectors (CCD); Iodine cell for spectrograph. 1995 Dark Energy dominates the universe. Large-format CCD detectors; 10m Keck telescope. 1998 And there are many more involving infrared, x-ray, ultraviolet and gamma-ray discoveries.

  22. Today on Astro1 • Our Solar System • Terrestrial vs. Jovian Planets • Why do some planets have atmospheres? • Smaller chunks of rocks and ice also orbit the Sun. • What do craters tell us about the geological history of planets? • What do magnetic fields tell us about planetary interiors?

  23. All the planets orbit the Sun in the same direction, in nearly the same plane, and most also rotate in the direction of the orbit. • Any model for the origin of the solar system must explain this!

  24. Our Solar System Has Two Broad Categories of Planets This Diversity Results From Its Origin and Evolution • Terrestrial planets • Closest to Sun • Small, high density, rocky • Jovian planets • Furthest from Sun • Large, low density, gaseous

  25. Orbital Radii of Terrestrial and Jovian Planets

  26. Average Density of a Planet:A Clue About Its Composition • Measuring Density • Distance: From period via Kepler'sThird Law (P2 =a3) • Size: Observed angular size and distance (small angle formula) • Mass: satellite’s period & Newton’s Form of Kepler'sThird Law • Density: mass/volume (r(H2O) = 1000 kg/m3) Rocky!

  27. Density of Jovian Planets • Gaseous • Visible surfaces show cloud formations

  28. - Seven large satellites almost as big as terrestrial planets • Comparable in size to Mercury • Only Titan has an atmosphere • - Remaining satellites (>140 known today!) much smaller Giant Satellites

  29. Classification of Extrasolar Planets (iclickers Question) • Suppose that in the near future a series of extrasolar planets are discovered with the following characteristics: spherical solid surfaces; mean densities about four times that of water; radii about 4000 km; low density atmospheres. How would these planets be classified in terms of our solar system • A) Jovian Planets • B) Cometary nuclei • C) Asteroids • D) Terrestrial Planets

  30. Spectroscopy of Titan (iclickers Question) • A ground based telescope is pointed at the atmosphere of Titan and a spectrum is obtained. The spectral lines observed in this spectrum: • A) Can only be features of Titan • B) can be characteristic of the Earth’s atmosphere as well as Titan’s atmosphere • C) Can be characteristic of the cooler outer layers of the Sun’s atmosphere as well as of Titan’s atmosphere • D) can be characteristic of the atmosphere of Titan and the Earth and also of the cooler outer layers of the Sun’s atmosphere.

  31. Spectroscopy:Chemical Composition of Atmosphere • Dips: due to absorption by hydrogen atoms (H), oxygen molecules (O2), and methane molecules (CH4) • - Only methane actually present in Titan’s atmosphere

  32. Europa: - No atmosphere - Sun light reflected from surface

  33. What are ices? • Hydrogen and Helium are gaseous except at extremely low temperature and high pressure. • Rock forming substances such as iron and silicon are solids except at temperatures well above 1000 K. • Between these two extremes are substances which solidify at low temperatures (from below 100 to 300 K) to solids called ices. • H20, CO2, CH4, NH3 • They are liquids or gases at somewhat higher T.

  34. Mars: - Composed mostly of heavy elements (iron, oxygen, silicon, magnesium, nickel, sulfur) → red surface - Atmosphere thin, nearly cloudless - Olympus Mons = extinct volcano, nearly 3 times height of Mount Everest

  35. Jupiter: - Composed mostly of lightest elements (hydrogen, helium), colorless - Colors: trace amounts of other substances - Giant storm = Great Red Spot, >300 years old

  36. What Determines Whether a Planet Has an Atmosphere? • Escape Speed • Vesc = (2GM/R)1/2 • Jupiter’s escape velocity is about 5.3x that on Earth (11.2 km/s). • The higher surface temperatures of terrestrial planets help to explain the absence of H and He. • Average speed of a gas atom or molecule • V = (3kT/m)1/2 • A planet can retain a gas if the escape speed is at least 6 times greater than the average speed of the molecules in the gas.

  37. Particle Velocities in an Atmosphere Most Probable Velocity Required Escape Velocity

  38. Average Speed of Molecules in an Atmosphere(iclicker Question) • The temperature of Earth’s atmosphere is roughly 20o C. Jupiter’s atmosphere is colder at -148o C. Compare the speed of nitrogen molecules in Earth’s atmosphere to that of He atoms in Jupiter’s atmosphere. • Nitrogen molecules move faster because Earth’s atmosphere is hotter. • Nitrogen molecules move slower because Earth’s atmosphere is colder on the Kelvin temperature scale. • Nitrogen molecules move slower because Nitrogen (atomic number 7) is heavier than Helium (atomic number 2). • They move at roughly the same speed. • Both B & C

  39. Small Chunks of Rock & Ice Also Orbit the Sun • Asteroids • An extension of planets to lower masses • Found in asteroid belt between Mars and Jupiter • Trans-Neptunian Objects • Pluto and Eris are the most massive • Over 900 identified at much lower masses • Orbits cross Neptune’s orbit • Comets

  40. Asteroids - Rocky objects between Mars and Jupiter in “asteroid belt” - Left-overs that did not form a planet - Combined mass < Moon

  41. Asteroids 433 Eros: - 33 km (21 mi) long and 13 km (8 mi) wide - Gravity too weak to have pulled it into a spherical shape - Image taken March 2000 by NEAR Shoemaker, first spacecraft to orbit around and land on an asteroid.

  42. Trans-Neptunian Objects (TNOs) = Kuiper Belt Objects (KBOs) Pluto and Eris (2003 UB313): - Two largest Trans-Neptunian Objects - Orbits steeply inclined to the ecliptic

  43. Trans-Neptunian Objects (TNOs) = Kuiper Belt Objects (KBOs) - Rocky & icy objects beyond Neptune (> 900 known; maybe up to 35,000?) - High eccentricities - Pluto is first discovered TNO (1930) and second biggest - Reside in Kuiper belt (30-50 AU from sun) - Debris left over from formation of solar system

  44. Comets (“Dirty Snowballs in Space”) - Rocky & icy objects on eccentric orbits that come close to sun. - Few tens of km in diameter - From Kuiper Belt or even further out (Oort Cloud; 50,000 AU) - e.g. if collision of two KBOs, a fragment can be knocked off and diverted into elongated object, brings it close to sun

  45. Comets Hale-Bopp: (April 1997) - Near Sun: solar radiation vaporizes some icy material - Bluish tail of gas, white tail of dust - Tails can extend for tens of millions of kilometers

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