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The Classification Of Stellar Spectra Physics 210 – Laboratory 2 Richard Laugesen, 2/9/99 Physics 210 The Classification Of Stellar Spectra Lab 2 Introduction Stellar classification is a significant tool of astronomy if not the most significant tool. From the stellar spectral lines we can determine which elements are prominent on the surface of the star, and from that information extrapolate a model of stellar evolution. This was first done by Josef Fraunhofer in the early 1800’s, where he catalogued the spectral lines of our star, the sun. It was discovered that the dark bands (absorption lines) in the spectra of the sun lined up exactly with the absorption lines of the spectra of certain elements analysed in the laboratory. Stellar classification soon followed, which is the classification of a star based upon its spectral ‘signature’. It was discovered that some stars shared a common spectral shape, this gave rise to an ability to classify stars. Stars of a certain classification (spectral type) share a common temperature range and from the laws of continuos radiation, the amount of light they emit or absolute magnitude can be determined. From a comparison of a stars apparent magnitude and its absolute magnitude, which is known from its spectral type, the distance to the star may be determined. Stellar classification is the only way to determine the distance to stars beyond the limit of parallax. Definitions Apparent Magnitude The observed magnitude of a star or other object as seen from earth. Absolute Magnitude The magnitude a star would have if it were precisely 10 parsecs away from the sun. Parallax Any apparent shift in position caused by an actual motion of the observer. Parsec The distance to a star if it has a parallax of 1 arcsecond. 3.03 x 1013 km. Wavelength The distance between wavecrests in any type of wave. Richard Laugesen, Page 2 Physics 210 The Classification Of Stellar Spectra Lab 2 Intensity Angstrom The intensity at a certain wavelength in a spectrum The unit normally used to measure wavelengths of visible and ultraviolet light. One angstrom is 10-8 cm Kelvin Declination Unit of temperature. Coordinate in the equatorial system that measures positions in the north-south direction. Measured in units of degrees, minutes and seconds of arc Right Ascension The east-west coordinate in the equatorial coordinate system. Measured in units of hours, minutes and seconds to the east. Luminosity The total energy emitted by an object per second. For Stars usually measured in ergs per second. Signal-to-noise Is an indication of the amount of noise relative to the signal you want to measure. A value higher than 100 gives a fairly precise value when measuring stellar spectra. Doppler Shift The observer shift in wavelength (and frequency) of a wave due to the relative motion between the source of the wave and the observer Spectrograph An instrument for recording the spectra of astronomical bodies or other sources of light Spectra An arrangement of electromagnetic radiation according to wavelength. Richard Laugesen, Page 3 Physics 210 The Classification Of Stellar Spectra Lab 2 Results Spectral Classification Of Main-Sequence Stars I chose any point on the spectrum of HD124320 and recorded its wavelength and intensity. Wavelength: Intensity: 4038.00 0.990 Then measured the deepest point of the most prominent absorption line. Wavelength: Intensity: 4341.41 0.280 The spectral types in the atlas are: O5, B0, B6, A1, A5, F0, F5, G0, G6, K0, K5, M0 & M5 Next I looked at Wein’s Law and how it applied to the temperature of different classifications of stars. This is described mathematically as λmax = 2.9 x 107 / T Where λmax is the wavelength of maximum intensity in Angstroms (Å) and T is the temperature in Kelvin (K). I found that the hottest spectral type is M5 because its highest intensity is at a relatively high wavelength. I found that the spectral type with a peak intensity at 4200 Å is F5 and its temperature would be 6904.76 K. Now I estimated the spectral type of HD124320 to be an A3 type star. The absorption line at 3933 Å is due to Ca II, from the spectral line identification window. On a photographic spectrum the absorption line appear as dark bands, in a graphical trace they appear as dips or valleys in the trace. Next are my estimates of the classification of various stars. Included is a brief description of my educated guesses. Richard Laugesen, Page 4 Physics 210 The Classification Of Stellar Spectra Lab 2 Characteristic Absorption Lines For The Spectral Classes Spectral Type O5 B0 B6 A1 A5 F0 F5 G0 G6 K0 K5 M0 M5 Wavelength of Prominent Lines 3970.07, 4101.75, 4340.48 3968.49, 4104.75, 4340.48 3970.07, 4100.04, 4640.48 3933.68, 3970.07, 4101.75, 4340.48 3933.68, 3970.07, 4100.04, 4340.48 3933.68, 3968.44, 4100.04, 4340.48 3933.68, 3968.49, 4101.75, 4340.48 3933.68, 3970.07, 4101.75 3933.68, 3964.73 3933.68, 3964.73 3933.68, 3970.07 4226.74 4226.74 Ion or Atom Producing Line HI Ca II, H I, H I H I, He II, H I Ca II, H I, H I, H I Ca II, H I, He II, H I Ca II, Ca II, He I, H I Ca II, Ca II, H I, H I Ca II, H I, H I Ca II, Ca II Ca II, He I Ca II, H I Ca I Ca I The most prominent absorption lines were found then the wavelength of that line was found and from the spectral line identification window the atom producing the line was found. Richard Laugesen, Page 5 Physics 210 The Classification Of Stellar Spectra Lab 2 Taking Spectra Using A Simulated Telescope And Digital Spectrograph I located a star using the 0.4 metre telescope and took its spectra using the digital spectrograph. The details of the star: Right Ascension: Declination: Apparent Magnitude: File of spectrum: 6h 12m 37s 32d 59’ 49’’ +7.34 RLAUG111.CSP Richard Laugesen, Page 6 Physics 210 The Classification Of Stellar Spectra Lab 2 I then located a fainter star using the 4 metre telescope and also took its spectra using the digital spectrograph. Right Ascension: Declination: Apparent Magnitude: File of spectrum: 6h 10m 15s 33d 40’ 03’’ +11.08 RLAUG222.CSP Spectral Type Of Two Unknown Stars Star # 1 2 Object Name 246 437 Spectral Type G0 K0 Reasons Perfect Match* Perfect Match* *the trace of the unknown star matched perfectly with the trace of the spectral type from the atlas Richard Laugesen, Page 7 Physics 210 The Classification Of Stellar Spectra Lab 2 Distance To Two Unknown Stars Star 246 437 Spectral Type G0 K0 Absolute Magnitude +4.4 +5.9 Apparent Magnitude +7.34 +11.08 Distance in Parsecs 38.7 108.6 Having both the spectral classification and the apparent magnitude of the star, one is able to calculate the distance to that star. Since the absolute magnitude is known from the spectral type assigned to the star the distance in parsecs can be determined mathematically by the following relationship. m−M +5 5 log D = Where D is the distance in parsecs, m is the apparent magnitude and M is the absolute magnitude. Both these stars are members of the Milky Way Galaxy. If the Diameter of the Galaxy is 30000 parsecs then the two stars are well within the galaxy. The solar system would have to be a very close to the edge of the galaxy for our distance from the galactic centre to be taken into account. Richard Laugesen, Page 8 Physics 210 The Classification Of Stellar Spectra Lab 2 Discussion During this experiment I have gained a new perspective on the realistic experience of an astronomer and what it involves. The concept of signal-to-noise and having to wait for enough photons to be gathered by the telescope to get a reliable trace from the spectrograph, although obvious was a little surprising. This gives new meaning to the term aperture and its impact on real stellar observation. Since measuring the distance to stars using the parallax caused by the motion of earth around the sun during the course of the year is limited. The technique of classifying stars and using their apparent magnitudes to determine distance is a much more appealing alternative. To take the spectra of a star the only limiting factor is time. Time to build up a reliable signal-to-noise ratio of the faintest stars, and the time needed can be reduced by a larger aperture (or synthesised aperture). The other major use of stellar classification or more suitably stellar spectra is its ability to find which elements exist on the surface of a star. This has great significance to models of stellar evolution and determining the mass of the universe (another necessity to finding the age of the universe) To rephrase what was said in the introduction, the techniques of stellar classification are very important to determining the distance to stars and therefore the age of the universe (from Doppler shifts of absorption lines) which is a fundamental question of humankind. The ability to measure the distance to stars just from their luminosity really is incredible no matter how much it is hidden behind mathematics. Richard Laugesen, Page 9