AST207 10/4/20101Bessel, Henderson, & Struve—4 Oct• Preclass Q for today were mislabeled. Extra credit.• Homework 4 is due on 11th• You are a young astronomer in 1814. What problem do you want to study?– In 1814, Madison evacuated Washington, DC before the British attack.– Napoleonic Wars• Measuring distances to stars enabled astronomers to study the physics of stars.Friedrich Bessel1784-1846hsci.cas.ou.edu/images/thumbnail/19thCentury/Bessel/F G Wilhelm Struve1793-1864www.obs.ee/ obs/pildid.htmlThomas Henderson (1798-1844). Bessel, Henderson, & Struve Measure Distances of Nearest Stars• Parallactic angleangle = baseline / distance• For the great distances of the stars, the angles are small and difficult to measure1. You are a young astronomer in 1825. What baseline should you choose?BaselineaStarDistanceTo distant starStation AStation Ba = B / DAST207 10/4/20102Bessel, Henderson, & Struve Measure Distances of Nearest Stars• Parallactic angleangle = baseline / distance• For the great distances of the stars, the angles are small and difficult to measure1. You are a young astronomer in 1825. What baseline should you choose?A. London to South AfricaB. Paris to CayenneC. Earth and Earth 6 months laterBaselineaStarDistanceTo distant starStation AStation Ba = B / DDefinitionsBaselineaStarDistanceTo distant stara = B / DEarthin 3moa = 1 / Da in arcsecD in parsecB is 1 AU1pc=200,000AUBaselineaStarDistanceTo distant stara = B / DEarthin 3moa = 1 / Da in arcsecD in parsecB is 1 AU1pc=200,000AUEarthSunAST207 10/4/20103Picking a star2. You are a young astronomer in 1825. Measuring the distance to one star requires years of work. What type of star should you choose to likely get a close one? Bright/faint? Fast/slow? (A fast star is one that moves across the sky a few arc per year. Motion across the sky is called “proper motion.”)A. Bright or fastB. Bright or slowC. Faint or fastD. Faint or slowFirst distances of stars 1837• Thomas Henderson in South Africa• Wilhelm Struve in Dorpat, now in Estonia• Frederich Bessel in Königsberg, now in Russia1. Which is the most distant star?A. 61 CygniB. VegaC. CentauriStar Char Parallactic angle [arcsec]Dist [pc]Bessel 61 CygProper motion 5arcsec/yr0.29Struve Vega Bright 0.12Henderson Cen Bright 0.75Star Char Parallactic angle [arcsec]Dist [pc]Bessel 61 CygProper motion 5arcsec/yr0.29 3.4Struve Vega Bright 0.12 8.3Henderson Cen Bright 0.75 1.3AST207 10/4/20104F G Wilhelm Struve• Born in Altona (Denmark, now Germany)• Educated at U Dorpat (Russia, now Estonia)• Petition to Prince Lieven, Chancellor of University of Dorpat– “The opportunity to acquire this instrument, the possession of which would raise our observatory to one of the first in Europe, perhaps will never return.”• The Great Refractor built by Fraunhofer arrived in 22 crates in 1824, and the city of Dorpat celebrated.• Czar Alexander I sent diamond rings to Struve & Fraunhofer.Friedrich Bessel• 61 Cyg was difficult because there is no nearby reference star. • Fraunhofer built a telescope with a split lens to show two separate images.– By moving the two lens pieces, Bessel could offset the two images and bring 61 Cyg close to reference star. pluslucis.univie.ac.at/ FBA/FBA99/Neho/3_1_1_.htmFraunhofer’s heliometer with split lensLens pieces movedslightlyLens pieces movedto align 61 Cyg & ref.AST207 10/4/20105Questions1. The parallactic angle a changes over the year becauseA. the distance to the star changes.B. the length of the baseline changes.C. both A and B are important.2. You want to measure the distance of a star at right ascension 0hr. When are two times of the year between which the change in the parallactic angle is the greatest?A. March & JuneB. March & SeptC. June & SeptD. June & DecBaselineaStarDistanceTo distant stara = B / DEarthin 3moa = 1 / Da in arcsecD in parsecB is 1 AU1pc=200,000AUBaselineaStarDistanceTo distant stara = B / DEarthin 3moa = 1 / Da in arcsecD in parsecB is 1 AU1pc=200,000AUEarthSunKnowing distance1. Without knowing the distance, you can measure the period of a binary star system and the distance between the two stars.A. True for period. False for distanceB. False for period. True for distanceC. True for bothD. False for both2. Without knowing the distance, you can measure the mass of a binary star system. (K’s 3rd: /A. TrueB. FalseAST207 10/4/20106Knowing distance1. Without knowing the distance, you can measure the period of a binary star system and the distance between the two stars.A. True for period. False for distanceB. False for period. True for distanceC. True for bothD. False for both2. Without knowing the distance, you can measure the mass of a binary star system. (K’s 3rd: /A. TrueB. False• Measuring distances to stars enabled astronomers to study the physics of
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