Name:Partner(s):Lab #6 The Spectral Classification of StarsWhy is Classification Important?Classification lies at the foundation of nearly every science. In many natural sciences, oneis faced with a bewildering degree of complexity in the subjects o f one’s study. However, bysorting objects into distinct classes, patterns and relationships can be revealed, suggesting anunderlying order. For example, biologists have classified plants and animals into genus andspecies and through these classifications have discovered evolutionary connections amongdifferent life forms. Geologists likewise have an elaborate system of classification fo r rocksand minerals, which helps them to constrain t he for matio n mechanisms for various materials.Astronomers are no exception. They classify planets as terrestrial or Jovian, g alaxies a sspiral, elliptical or irregular, and stars according to the appearance of their spectra.In this exercise, you will study the method that astronomers use to classify stars by theirspectra. The resulting classification was a key step in elucidating the underlying physicsthat produced stellar sp ectra. Thus, in astronomy as well as biology, the relatively mundanestep of classification eventually yields the critical insights which lead to breakthroughs inunderstanding.The Spectra of StarsA spectrum of a star is produced by photons emitted from its very outermost layers. Photonsemitted from further within the star interact frequently with the dense stellar material, andcan not “escape” before being absorbed or redirected. Thus, we never see these inner photons,and only observe photons emitted from the star’s more tenuous “atmosphere”. This is similarto what would happen to someone trying to observe the Earth from outer space. They couldnever “see” below the ground, because the Earth was too dense, and could only observelight emitted in the at mosphere1. Technically, we say that the inner parts of a star (or theEarth) are “ opaque”, or “optically thick”, and that only the atmophere is “transparent” or“optically thin”.Almost all stellar spectra consist of a broad, smooth distribution of photons of differentwavelengths, known as “continuum emission”, with many absorption lines superimposed,changing the featureless smooth continuum spectrum int o a much more complex one. Thecontinuum emission is dominated by thermal radiatio n, which you have studied in a previouslab. Thermal radiation is produced by the interactions between the moving particles in astar’s atmosphere and photons. The exact shape a nd amplitude of the thermal spectrum1Note that this is not an exact analogy, since the Earth is solid, and stars remain gaseous all the way tothe center.Astronomy 101 6 – 1 Introduction to Astronomydepends on the temperature of the star’s a tmosphere (i.e. how fast the particles in theatmosphere are moving).While the spectrum of black body radiation is smooth, the actual spectrum of a star ismuch more complicated. Many absorption lines remove light from the underlying blackbody spectrum at specific wavelengths. These wavelengths depend on the exact ions, atoms,and molecules that exist in a star’s atmophere, and thus the pattern of absorption linesreveals the chemical content and physical state of the star’s atmosphere (e.g. are there manyheavy elements in the star? is the atmosphere hot and mostly ionized, or cool and mostlymolecular?). The absorption lines are produced when electrons in atoms and/o r ions absorbindividual photons from the smooth continuum that happen t o have exactly the energyneeded to boost a bound electron to a higher energy level. This removes light from thecontinuum, and leaves behind a dark region in the spectrum. These absorption lines arevery narrow in wavelength, meaning they only remove photons at very specific wavelengths.However, sometimes there are so many absorption lines in a spectrum that it is difficultto see individual lines, and instead the spectrum has large jagged dark r egions made fromthe superposition of many many different absorption lines from many different elements andions. In some very cool stars (cooler than 3,000 K), the absorption lines are produced byent ire molecules absorbing photons, rather than electrons in atoms.Why do Spectra of Stars Vary?Stars come in a wide range of sizes a nd temperatures. The ho ttest stars in the sky havetemperatures in excess of 40,000 K, whereas the coolest stars that we can detect opticallyhave temperatures of only 1,000-1,500 K. The appearance of the spectrum of a star is verystrongly dependent on its temperature. The temperature changes the shape of the underlyingthermal continuum as well as the kinds of ions, atoms, a nd molecules which can exist in astar’s atmosphore. For example, the very hottest stars (called O-type stars) show absorptionlines due to ionized helium (He II) and doubly or even triply ionized carbon, oxygen or silicon.On the other hand, the coolest stars (M- , L-, and T-type) show lines produced by moleculeslike Titanium Oxide (TiO).ProcedureThis lab teaches the basic techniques and criteria of the Morgan-Keenan system of spectralclassification. You will put several stars in a temperature sequence, first using the shap e ofthe continuum, and then using the strength of the absorption lines. You will then use theseobservations to classify the stars.Astronomy 101 6 – 2 Introduction to AstronomyEstimating Temperature from the Shape of the Thermal RadiationSpectrumThe plots below show how the spectra of objects of different temperatures would appear atoptical wavelengths. Each curve shows a “thermal” or “black-body” radiation spectrum, fordifferent temperatures.1 (1pt). A thermal spectrum should always have a peak. Why don’t you see a peak in allof these plots?2 (2 pts). Rank these spectra from hottest to coolest. Label the spectrum with the highesttemperature with a “1”, the next hottest with a “2”, etc.Astronomy 101 6 – 3 Introduction to AstronomyEstimating Stellar Temperature from the Shape of the ContinuumThe plots above show spectra for actual stars. Although there are many complicated featuresin the spectra, their overall shapes follow thermal radiation spectra much like you saw onthe previous page. Because these stars have different temperatures, t he shape o f the spectravary substantially among the different stars.3 (1 pt). Based on your work in question 2 , rank the stars from hott est to coolest.
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