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Unformatted text preview: Astronomy Picture of the Day Sputnik 1 - the first artificial satellite to orbit the Earth Homework Turn in the homework in the P20A drop box Remember to show your work on mathematical questions Don't forget the units! Homework grading: (2 points per question) (5 questions) Emission lines from gas clouds
Emission lines Energy Wavelength A dilute (non-opaque) gas cloud is not a black-body emitter Atoms in a hot, dilute cloud of ionized gas will emit a characteristic pattern of spectral lines This is called an emission-line spectrum Absorption-line spectra
Absorption lines Energy Wavelength If there is a cloud of atoms in between a black-body emitter and you, you'll see absorption lines where some photons are "missing" from the black-body spectrum Absorption-line spectra Energy Wavelength Normal stars like the sun have absorption-line spectra Black-body radiation originates deep within the star It passes through cooler, less dense layers of gas on its way out The spectrum of a star The spectrum of a star Doppler effect demo:
http://www.astro.ubc.ca/~scharein/a311/Sim/doppler/Doppler.html The Doppler Effect Stationary source of waves The Doppler Effect Moving source of waves Person over here hears a lower pitch Person over here hears a higher pitch Doppler Effect: Summary If the source of waves is moving toward you, you'll see waves of shorter wavelength For light waves, this is called a blueshift If the source of waves is moving away from you, you'll see waves of longer wavelength For light waves, this is called a redshift By measuring the amount of blueshift or redshift, we can determine the object's velocity toward or away from us Note: the speed of the waves is not affected! Doppler shift of light The Doppler effect depends only on the object's motion along a direction toward or away from the observer
Star moving this way: Observer receives light that is not Doppler-shifted If star isn't moving relative to the observer, then the observed spectrum will not be Doppler-shifted Star moving this way: Observer sees redshifted spectrum Doppler shift of light Important note: Doppler effect depends only on the object's motion along a direction toward or away from the observer Star moving this way: The Doppler shift only depends on the component of the star's motion toward or away from the observer Star at rest relative to observer: No shift Star moving away from observer: Spectrum is redshifted Star moving toward observer: Spectrum is blueshifted Doppler shifts from rotating objects If an object is rotating, then light from different parts of the object will have different doppler shifts Blueshifted emission from this side of disk Redshifted emission from this side of disk Telescope: a device used to gather light and bring it to a focus. Two Main Varieties of Telescopes
Refracting telescopes: A lens is the primary light-gathering element Reflecting telescopes: A mirror is the primary light-gathering element Telescopes as "light buckets" The primary goal of an astronomical telescope is to collect as much light as possible and bring it to a focus We refer to the size of the telescope by the diameter of its primary mirror or lens A "2-meter" telescope means a telescope with a primary mirror diameter of 2 meters Telescopes as "light buckets" The ability of a telescope to collect light depends on the area of its primary mirror or lens D is the diameter of the primary mirror or lens Area = R2 = D2 / 4 1m 3m
A 3-meter telescope collects 9 times more light than a 1-meter telescope, in the same amount of time Light from very distant objects All stars (except the sun) are so far away from us that light rays from the stars arrive essentially on parallel paths Lenses and Refraction
Incoming parallel light rays from a distant star The path of a light ray will bend (or refract) at the interface between two materials such as air and glass This is how lenses focus light Lenses and Refraction
Incoming parallel light rays from a distant star The focal length of a lens is the distance from the center of the lens to the location where parallel rays are brought to a focus Refracting Telescopes
Objective lens Eyepiece A basic refracting telescope has 2 lenses, the objective and the eyepiece The magnification depends on the ratio of focal lengths of the two lenses Different wavelengths of light are refracted by different angles when passing through glass Limitations of Refracting Telescopes Refracting telescopes suffer from chromatic aberration The focal length of a lens will be different for different wavelengths of light An image that is in focus in one color will be out of focus in other colors white light Cameras use complex arrangements of multiple lenses to give good image quality and minimize the chromatic aberration Limitations of Refracting Telescopes A lens can only be supported by its edges, and big lenses are very heavy. Heavy lenses will sag in the middle too. Also, telescope lenses have very long focal lengths, which means you have to build huge domes to house the telescopes. The world's largest refracting telescope has an objective lens diameter of 40 inches (about 1 meter) Yerkes Observatory refractor in Wisconsin, built 1897 Reflecting Telescopes Reflecting telescopes use mirrors to gather and focus light Advantages: No chromatic aberration A mirror can be supported from underneath, not just from its edges- so it can be much larger than a lens Telescope mirrors are made from glass and are coated with a thin layer of aluminum to make them reflective Spherical mirrors A spherical mirror is the easiest to manufacture But spherical mirrors suffer from spherical aberration: Light rays incident on different radii in the mirror will not come to the same focus Parabolic mirrors A parabola is the curve defined by the function y = x2 y x Parabolic mirrors A parabolic mirror has the property that parallel incoming light rays all come to the same focus Reflecting telescopes commonly use parabolic mirrors One disadvantage: A parabolic mirror gives a very narrow field of view over which good images can be obtained Reflecting telescope designs ...
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This note was uploaded on 04/20/2008 for the course PHYS 20A taught by Professor Staff during the Fall '02 term at UC Irvine.
- Fall '02