Photo electic Photometry Lab 1 Ob jectives To learn how to make precise measurements of stellar brightnesses, and how the Earth's atmo- sphere a ects our ability to measure brightness. 1. Learn how to measure stellar brightness with a photo electric photometer 2. Get go o d familiarity with the celestial sphere 3. Analysis of data, estimation of errors 4. Learn ab out Standards in measurement and calibration 5. Learn how the atmosphere e ects starlight that passes through it 2 Skills Required This is an advanced lab b ecause it requires some knowledge of observing so that the team is ecient enough to get all the required data, and will require extensive data analysis. The equipment used in this lab is very easy to use. Skills used: Polar Alignment of telescop e, use of setting circles Ability to nd things using star charts Statistical Analysis Spherical Trigonometry Understanding Sidereal time, Hour Angle, Airmass Linear Least Squares ts 3 Background Photometry is a quantitativeway to measure the brightness of a star. Photometry is imp ortant in photography, astronomy, and illumination engineering. Instruments used for photometry are called photometers. Lightwaves stimulate the human eye in di erent degrees, dep ending on the wavelength of the light. Because it is dicult to make an instrument with the same sensitivity for di erent wavelengths as the human eye, photometers need sp ecial colored lters to make them resp ond like the human eye. Photometry is very imp ortant in astronomy b ecause it gives the astronomer a direct measure of the energy output of stars, or of the amount of light re ected or scattered by surfaces of planets and other small b o dies. Colors, or measurements of the amount of light through lters centered at di erentwavelengths can give information on the temp eratures of stars. 1 3.1 The Photomultiplier The key to the op eration of the photomultiplier is called the photoelectric e ect, discovered in 1887 by H. Hertz. When light strikes a metal surface, electrons are released, the numb er released b eing prop ortional to the intensity of the light. Electrons are b ound to the metal by electric forces, and light with sucient energy can lib erate the electrons. The way a photometer works is that light enters the instrument and strikes the photo catho de made of a metal chosen so that optical light exceeds the threshold for release of the electrons. For typical materials the quantum eciency is ab out 10, meaning for every 100 incident photons, only 10 electrons are released. In order to get enough electrons to measure as a current, the photo catho de is places in a multiplier tub e. A series of dyno des are kept at electric p otentials less negative than the photo catho de, thus the released electrons are accelerated and travel toward the dyno de. The impacts of the electrons on the dyno de release ab out 4-5 times as many electrons, and these are accelerated to another dyno de at an even less negative p otential. This pro cess is rep eased many times until there is a large cascade of electrons which can b e measured at the last dyno de called the ano de. At the end of the multiplication chain, 1 initial electron can deliver ab out 4 10 10 electrons at the ano de! Figure 1: Diagram of a photomultiplier, from N. Gin 4 Exp eriment In this lab you will measure the brightness of some variable stars, and fully calibrate them. Figure 2 shows an example of an unusual typ e of variable star, called an R Coronis Borealis star. It varies irregularly in brightness { usually b eing very bright, but o ccasionally growning faint. For this typ e of star, dust in the star's atmosphere condenses out o ccasionally and blo cks the starlight. The star gets bright again when the star heats up and vap orizes the dust or blows it o . We will rep ort our brightnesses in the standard V visual and R red astronomical mag- nitude system. Magnitudes arose historically from the ancient greeks who listed the brightest stars in the sky as having \ rst imp ortance" or rst magnitude. The next brightest as having \second imp ortance" or second magnitude. The eye is actually a logarithmic detector, and this system as b een formalized such that each magnitude di erence is a factor of 2.5 in brightness. We will b e measuring an electric current or counts photons p er second, C , from the star, and this has to b e converted into a magnitude m system. This is done with the following equation: 2 Figure 2: Data on R Coronis Borealis from mid 1966 through mid 1996. m = 2:5 logC 1 4.1 Eclipsing Binaries Eclipsing Binaries are a typ e of variable star system which is not varying intrinsically. Instead the apparent brightness variation is caused by the geometry as viewed from earth of a pair of orbiting stars. As one star passes in front of another the starlight dims. There will b e 2 eclipses each p erio d, and the eclipse with the greatest light loss will b e called the primary eclipse. This o ccurs when the hotter, brighter star is blo cked from view. The light curves are imp ortant to study b ecuase they contain information ab out the star's sizes, their shap es, mass exchange, and star sp ots. Information ab out sp eci c eclipsing binary systems in Table 1 are listed b elow: The AW UMa system is probably either a triple or quadruple star system. The masses of the primary stars are M = 1.790.14M , and M = 0.1430.011M . The third star 1 2 has an apparent mass of M = 0.850.13M . 1 Lib is a chemically p eculiar close contact binary star system. The star 68 Herculis u Herculis, HD 156633, SAO 65913 is a Beta Lyrae typ e eclipsing binary. It was discovered to be variable by J. Schmidt in 1869, and was found to be an eclipsing binary in 1909 by Baker. The maximum magnitude of the system is ab out 4.7 and the minima alternate b etween ab out 5.0 and 5.4. Figure 3 shows a set of observations made by Phil McJunkins Texas A&M Univ. and Dan Bruton Austin State Univ.. 3 Figure 3: Lightcurve of eclipsing binary 68 u Her. 4.2 Intrinsic Variable Stars Intrinsic variables are stars whichvary in brightness b ecause of internal changes which can cause pulsations. Some of the pulsations can b e very long overayear while others can b e short. For this lab we will select only short-p erio d variables of the following typ es: Scuti stars {low amplitude, sinusoidal b ehavior with p erio ds < 0.3 dy. Dwarf Cepheids { Amplitudes < 1 mag, p erio ds < 0.3 day and can have asymmetric light curves. RR Lyrae stars { Similar the Dwarf Cepheids, with p erio ds b etween 0.3 to 1.0 day. Below is a description of the intrinsic variable stars which might be observed during the lab. Below are some brief descriptions of the intrinsic variable stars included in this lab. The Bo otes variable star is a rapidly rotating A-typ e dwarf star which has avery small p erio dicity 38 min and probable low amplitude variation thus is not our rst choice for a target. This star p ossesses a circumstellar dust disk. V703 Sco is a p ost-main sequence red giant star, adwarf Cepheid. X Sgr is a sp ectroscopic binary system with a cepheid variable star. The orbital p erio d of the companion star is 507.25 days. Note: the columns with 7UT, 8UT and 9UT show the airmass and altitude of the ob jects as a function of time. 4 Table 1: Candidate Variable Stars Name 2000 2000 V Typ e Per Epoch 7UT 8UT 9UT AW UMa 11:30:04 +29:57:53 WUMa 6.8-7.1 0.43 38044.782 1.4/46 1.8/33 2.9/20 Bo o 14:16:10 +51:22:02 Sct 6.5-7.1 0.267 39370.422 1.2/58 1.2/54 1.4/47 Lib 15:00:58 {08:31:08 EA/SD 4.9-5.9 2.33 22852.360 1.1/61 1.1/61 1.2/53 68 u Her 17:17:20 +33:06:00 Lyr 4.7-5.4 2.05 27640.654 1.3/52 1.1/64 1.0/73 V703 Sco 17:42:17 {32:31:23 RRLyr 7.8-8.6 0.115 37186.365 2.4/24 1.8/33 1.6/40 X Sgr 17:47:34 {27:49:50 Cep 4.2-4.9 7.01 35643.31 1.8/32 1.4/44 1.2/52 W Sgr 18:05:01 {29:34:48 Cep 1.58-3.98 7.59 34587.26 2.0/30.2 1.5/42 1.3/52 4.3 Op en Clusters Op en clusters are groupings of stars which physically reside in the same place in space i.e. they are all at approximately the same distance from the earth, and formed at the same time. Because they are at the same distance from us, their apparent relative brightnesses are the same as their absolute relative brightnesses. The one main di erence between the stars will b e their masses.