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CORNELL ASTRO 1102 - Exam 2 Study Guide
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ASTRO 1102 1st EditionExam # 2 Study Guide Lectures: 9, 11 -19Lecture 9 (February 9)Telescopes and Instruments- Combinations of lenses or mirrors are used to focus light onto a detector.- The eye is, in its most basic form, made up a pupil, a lens, and a retina.  Pupil: controls how much light enters the eye Lens: bends light to form an image in the retina Retina: where the focal plane is ideally located (where the image appears in focus)- The eye was the first astronomical detector, but does not store reliable records ofobservations. only detect visible wavelengths long-term records are often faulty/inaccurate short integration times- If we want to study an image, we record it with a camera. The camera is similar to the eye in a few ways. has an opening that lets light in, like the pupil bends light through a lens, bringing it to focus on a detector (analogous tothe retina) which makes a permanent record of the image- Longer exposure timeon the detector (controlled by the shutter)  more photons reach the detector  more detail recorded that might be too faint to beseen in shorter exposures.- Astronomers use electronic detectors, most commonly CCDs (charge-coupled devices). Millions of CCDs are chemically etched into an array on a single chip of silicon to construct images; each detector is a picture element or pixel.- Angular resolution:The smallest angle over which we can tell that two dots – or two stars – are distinct. The human eye has an angular resolution of about 1 arcminute.- Diffraction Limit:The angular resolution that a telescope could achieve if it were limited only by the interference of light.- Refracting Telescopes:Operates much like an eye, using transparent glass lenses to collect, refract (bend) and focus light. These were the first telescopes. limitations: long and cumbersome tubes, hard to polish both surfaces of large lenses, some color distortion- Reflecting Telescopes:Uses a curved primary mirror to gather light, which is then reflected by a secondary mirror that lies in front of it. This is the design of modern large telescopes. advantages: easier to polish, larger collecting area = we are able to study fainter objects, no color distortion- Resolution:A measure of the smallest details that can be seen in an image. spatial resolution: a measure of the smallest details that can be identified by an instrument RESOLUTION IS NOT MAGNIFICATION- Resolution is proportional to wavelength andinversely proportional to aperture diameter. - The resolution limit of a telescope is often expressed in terms of a small angle θ. Objects with angular separation α < θ are blurred together Objects with angular separation α > θ can be seen as two objects- Both α and θ can be expressed in terms of length ratios. The angular separation α is equal to the ratio s/r  The diffraction resolution limit θ is set by the ratio λ/D- Resolution is limited by “seeing,” or the turbulence of out atmosphere. can be surmounted by: placing telescopes in space, speckle imaging, and adaptive optics- Adaptive Optics:A technique in which telescope mirrors flex rapidly to compensate for the bending of starlight caused by atmospheric turbulence. allows large telescopes to take images at close to the diffraction limit- Speckle Imaging: A technique in which many images are taken quickly, and then shifted back and forth in software applications to produce a sharp combined image.- Light-Collecting Area: The area of the primary mirror that collects light in a telescope. It tells us how much total light a telescope can collect at one time.  The light-collecting area is proportional to the square of the diameter of the primary mirror (L= d2). We characterize atelescope’s size bythe diameter of itslight-collectingarea.- Radio telescopes focus andcollect radio waves.  Because radiowaves have longwavelengths, verylarge radiotelescopes (and thus a large diameter) are necessary to achieve reasonable angular resolution.- Observational sites must be placed in isolated, high, and dry observing sites – or in space. Putting telescopes in space, however, can be risky and expensive.Lectures 11 and 12(February 13 and 18)Dating Extraterrestrial Materials I and II- The Main Point:Material from other parts of the SolarSystem is sufficiently close that it canbe sampled directly by spacecraft, oreven delivered to Earth. This permitsmuch more detailed studies of the materials’ compositions and ages thanis usually possible for other fields ofastronomy.- Mass Spectrum:Shows the number of atoms or molecules with different masses. masses are measured in atomic mass units (amu) which is roughly the mass of a proton or a neutron; thus the mass tells us how many protons and neutrons are in each atom or molecule a mass spectrum is formed by: - 1) breaking a sample up into its components atoms/molecules- 2) sorting atoms by mass using electric and magnetic fields- 3) counting number of atoms/molecules with different masses- Simple mass spectrometers have beencarried on spacecraft and have measuredthe compositions of various materials in the solar system.- Elements:Characterized by their unique number of protons.- Isotopes:Can belong to a given element, but differ in their number of neutrons.- Three Types of Nuclear Decay Alpha Decay (α):Duringalpha decay, the nucleusemits an alpha particle, ora particle containing twoprotons and two neutrons.The nucleus is said todecay, or change into onethat is a little lighter, onewith four lessparticles.Alpha decayoccurs because the nucleushas too many protons. Beta Decay (β):During beta decay, the number of neutrons in the atom decreases by one, and the number of protons increases by one. Since the number of protons before and after the decay is different, the atom has changed into a different element. The beta particles released during this process are high-energy and can pass through thicker materials than alpha particles can. Beta decay occurs because the nucleus has too many neutrons.  Gamma Decay (γ): During gamma decay, particles inside the nucleus collide during radioactive decay, and energy is released. This energy can leave the nucleus in the form of waves of electromagnetic energy called gamma rays.A photon is released, but atomic number and atomic mass remain unchanged. This type of radiation is the strongest, and is able to penetrate most common substances, including


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CORNELL ASTRO 1102 - Exam 2 Study Guide

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