The Spectrum, 23 September 2013 !Opening question: What is black body radiation? !1.A black body is an object that absorbs all the energy that hits it; it doesn't A.reflect any. However, when an object absorbs energy, it changes that energy to heat and this heat radiates in a pattern that will be discussed when we talk about stars. !The term contradicts itself by the third word. !a.A more apt phrase would be "thermal radiation" because it depends 1.on the temperature of the object. !The hotter an object is, the shorter the wavelength of its dominant A.emission. !This is a theoretical concept and doesn't exist in reality. !b.Stars are assumed to be black bodies at first approximation until 1.astronomers find detail in their spectrum, like a color from a chemical atmosphere. !Stars are closest to being black bodies: They get only a little light from 2.outside of themselves, and they radiate thermal energy in all directions from their cores.!Spectroscopy started out in physics and chemistry, but was later used in 2.astronomy to learn about the composition, orientation, speed, direction, and direction of rotation of a celestial body.: !Physicists developed the first spectroscopy by using prisms and lenses to A.try figure out of light worked.!Prisms are a triangular piece of optical glass which split up light into its a.component colors. !For white light, this is the colors of the rainbow: red, orange, yellow, 1.green, blue, indigo, violet. (ROY G BIV)!NOTE: There are no scientific boundaries between these colors; A.the distinction between them is based on the distinguisher's culture. !Both Galileo and Newton worked on this issue.!b.Spectroscopy using a spectroscope goes back about 150 years, but no one B.really understood the results from it until the mid 1800s.!How spectroscopes work: !a.Light enters a spectroscope through a slit (so the output will be in a 1.straight, easy to read line). !The light then enters a diffraction grating or a prism. !2.The resulting spectrum is beamed at photo film or ccds (charge-3.coupled devices). !Results of early spectroscopy:!b.The usual red, orange, yellow, green, blue, indigo, and violet, but also 1.black lines.!These black lines signify the absence of light. !A.In the early 1800s, Joseph von Fraunhofer catalogued and labeled a 2.dozen of these, and they were then called "fraunhofer lines."!No one understood what they were, including Fraunhofer. !A.Near the end of the 1850s, two German chemists Kirchhoff and Bunsen C.figured out that the lines were due to different chemicals absorbing specific types of light. This was valuable to astronomers, and inspired further developments in astronomical spectroscopy. !These lines are now called "absorption" lines.!a.Using this concept, you could tell what chemicals were in the source of a b.spectrum by looking at a spectrum's absorption lines. This meant that astronomers could see what stars, planets, and comets were made of without having to go to them. !Although absorption lines are sometimes caused by gases in the 1.earth's atmosphere, they are also usually caused by gases in the atmosphere of the star, planet, or nebula.!In any case, people learned how to tell the difference between A.absorption lines caused by the earth's atmosphere and those caused by and object's atmosphere. !The absorption lines also changed when you varied temperature and c.pressure, so it was possible to tell the temperature and pressure of the spectrum's source. !Later research showed that you could also tell how fast the object was d.moving, whether it was moving towards the earth or away from the earth, how it was rotating, and what orientation it was rotating at. !All of this information came from applying the Doppler effect to the 1.spectra of stars:!Doppler effect: the frequency of a wave appears to decrease as its A.emitter moves away from you and increase and its emitter moves towards you.!This applies to the wavelength of light, which appears more red a.(red shifted) as its source moves away from you and more blue (blue shifted) as the source moves towards you.!Light wavelengths with a lower frequency look red, whereas 1.light wavelengths with a higher frequency look blue. !If you get the spectrum of a nonmoving chemical in a laboratory, B.you can compare it to the spectra of stars composed of the same chemical to see if the stars are moving away from you ( the spectrum is more red than the resting spectrum) or towards you (the spectrum is more blue that the resting spectrum)!Almost all objects in astronomy are shifted towards the red end a.of the spectrum (they are moving away). !Stars tend to be blue shifted more than galaxies tend to be 1.blue shifted, but a blue shifted object is almost always noteworthy. !The orientation of an object's rotation can be found with the C.doppler effect by looking at the spectral lines from various diagonal angles: the angle which has the least amount of smear between red and blue is the object's pole, and the angle with the most smear is the angle of the equator. !Astronomers could also tell how strong the magnetic field of an object.!e.People noticed early in the 1800s that there were three different classes of D.spectrum: absorption, continuous, and emission line.!Absorption line spectra (discussed above) !a.These were prevalent in astronomy.!1.Continuous spectra:!b.These only appear in laboratories 1.where there is no gas between a source of energy and a spectroscope.!It never occurred in astronomy. !2.Emission line spectra:!c.Only a few objects in astronomy have 1.emission lines.!The reason for these three classes of spectra puzzled people for 75 years E.before they realized that they resulted from our point of view from earth.!If you look at an object through a gas,the gas will absorb part of that a.light and reradiate it weakly in all directions.!Since the radiation is in all directions where the original light was in 1.one direction, the radiation is so diluted that it looks like there is no light there at all. This causes absorption lines to appear.!Because there is at least the earth's atmosphere and usually the 2.object's atmosphere between the spectroscope and the star, most objects in space have these types of spectra.!Emission lines are rare because of how weak reradiation is:!b.They are generated by a thin cloud of hot gas between the source and 1.the spectroscope. !This circumstance is so rare, that it is specifically labeled "emission 2.spectra." It is safe to assume
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