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Reminders! Website: http://starsarestellar.blogspot.com/ Lectures 1-3 are available for download as study aids. Reading: You should have Chapters 1-3 read, and probably Chapters 4-7 by the end of this weekend. Homework: Save the chapter 4 problems for the next homework set. Just do the chapters 1, 2, and 3 problems on homework #1. Homework #1 is due this Wednesday at the BEGINNING of class. Discussion: Today from 1-2 pm is the last math review class. For Tuesday and Wednesday, we’ll have open office hours. Telescopes: Extending Our Senses Today’s Lecture: • Finish discussing light Why is the sky blue and sunsets red? • The tools of astronomers (Chapter 3, pages 38-63) Introduction to telescopes Different telescopes for different types of light • Phases of the Moon (Chapter 4, pages 64-68) Explain the phases as a geometrical effect Show that the phase is related to the position of the Moon in the sky Why blue sky and red sunsets? Blue light is preferentially scattered. Away from the Sun, the sky looks blue. As the light path through the atmosphere increases, the Sun looks progressively more yellow, orange, and red and it approaches the horizon. Telescopes • The primary purpose is to collect more light • Typical large telescope: D ≈ 6 m mirror • Dilated eye pupil: D ≈ 6 mm • A = area = πR2 = πD2/4, since R = D/2 • The ratio of areas for two telescopes is D1 D2 Telescopes (continued) If D2 = 6 m and D1 = 6 mm = 6 x 10-3, then the ratio of areas is (1000)2 = 106! • A 6 meter telescope can see a million times fainter than the human eye! • A large telescope also provides higher resolution -- the ability to see finer details. • But turbulence in the Earth’s atmosphere blurs the image, and limits the detail that can be seen (especially at optical wavelengths) Angular Resolution Resolution ≈ 0.002λ/D Resolution in arc seconds, λ in Å, and D in cm Refracting Telescopes • Invented in Holland in the 1600s • Light is bent (or “refracted”) by a lens and brought to a focus. Lens Light rays Focused to a point Light rays from a star are essentially parallel Stars are very, very far away, so the rays reaching us diverge only very slightly. They are PARALLEL for all practical purposes. Refracting Telescopes (cont.) • Simple to make, but they suffer from several problems, including chromatic aberration • This is a property of all lenses…including your eyes! • Lenses are also heavy and absorb light Reflecting Telescopes • Use a mirror to bring light to a focus. spherical mirror parabolic mirror • A spherical mirror does not focus the light to a single point, which leads to spherical aberration. • A parabolic mirror does focus the light to a single point. • This was in fact the problem that Hubble first had. Common Reflector Designs Telescopes in space • no distortion (blurring) by the atmosphere • darker sky (especially in the infrared) • be able to see ultraviolet, X-rays, gamma-rays, and infrared ozone (O3) blocks UV radiation at altitude of 20-40 km water vapor (H2O) blocks IR at altitude of 2-10 km other atoms/molecules block X-rays and gamma rays Telescopes in space (cont.) • IR: telescopes on top of high mountains, on balloons, in airplanes, and in space. • X-rays, gamma-rays: balloons, rockets, orbiting satellites • UV, optical: orbiting satellites (for example, the Hubble Space Telescope) Twinkling of Stars • Air bends (refracts) starlight. Due to the atmospheric turbulence, the bending varies with time, so the amount of light hitting your eyes varies. • Stars closer to the horizon => more air, more turbulence => more twinkling! • This causes blurring of the star’s image, so that it doesn’t look point-like. Twinkling of Stars (cont.) • Planets don’t twinkle as much. They are closer, and thus disks of light (not point-like) and the randomly varying points average out. Star (far away) Planet (much closer) • But planets DO twinkle when close to the horizon. • For a telescope larger than 20-30 cm, twinkling is the limiting factor for resolution (<1’’ resolution). • How do we get rid of the twinkling? Adaptive Optics • Used with ground-based telescopes • A method to correct for blurring of images due to turbulence in the Earth’s atmosphere over small areas of the sky • Monitor the light from a “guide star” or “laser guide star” • Make small changes to the mirror shape many times per second, and this cancels the smearing from the turbulence Adaptive Optics corrects the wavefront Makes for a MUCH clearer image. Phases of the Moon The position of the Moon in the sky is closely related to the phases! Can you see a Full Moon at 12 o’clock noon? What would someone see for the phases of the Earth from the Moon?