The History and Philosophyof Astronomy (Lecture 16: Birth of Astrophysics I)Instructor: Volker BrommTA: Jarrett JohnsonThe University of Texas at AustinAstronomy 350L (Fall 2006)Big Q: What is the Nature of the Stars?Can we ever know the `Physics of the Stars’? • Auguste Comte (1798-1857)• founder of `Positivism’- real knowledge only due to hard facts,e.g., laboratory science, measurements• claimed that we will never knowthe nature of the stars- distant stars are forever out ofour reach- we cannot conduct laboratory experiments with them(= astrophysics)• Which questions would an astronomer haveasked about the stars in the early 1800s? • How far away are they (stellar distance scale)?• What are the stars made of (stellar composition)?• How long do they live (stellar lifetimes)?• By what mechanism do they shine?• Is the Sun just a (nearby) star?A: Yes, already widely believed (Descartes, Newton)• How massive are they?The Hunt for Stellar Parallax!The Hunt for Stellar Parallax! ResolutionMeasuring the Distance to the Stars • Friedrich Wilhelm Bessel (1784-1864)• highly talented in astronomyand mathematics (“Bessel functions”)•director of Konigsberg Observatory• 1838: First stellar parallax..Q: How to select promising candidates? • possible criteria:1) brightest stars2) most rapid proper motionBessel’s choice!Stellar Motions on the Sky • proper motion 1/ddQ: How to select promising candidates? • 61 Cygni: the `flying star’ (5 arcsec per year)Great Success: First Stellar Parallax (1838)! Bessel’s heliometer• Bessel: 61 Cygni- 1/3 arcsec 10.3 Lightyears• almost simultaneously:-Wilhelm Struve: Vega- Thomas Henderson: Alpha CentauriThe True Brightness of the Stars • what we measure: flux = energy/area(`apparent brightness’)• if distance (d) to staris known, can figureout true (intrinsic)brightness = Luminosity (L)• L = 4 x pi x d2x flux (“inverse-square law”)What stuff are the Sun and stars made out of? • scrutinize the light that we receive!The Message of Starlight (Newton 1666) • white light is composed of different colors!The Spectrum of the Sun • Joseph Fraunhofer(1787-1826)• master optician and telescope builder• 1814: Discovery of SpectralLines in Solar Light(= `Fraunhofer lines’)The Spectrum of the Sun (1814) • Fraunhofer lines: dark lines SunprismWhat are the Spectral Lines? • Heidelberg in 1850s and 60s:• Gustav Kirchhoff(1824-87) and Robert Bunsen (1811-99) - discover the `Laws of Spectral Analysis’• Robert Bunsen (`Bunsen burner’)What are the Spectral Lines? • `Flame test’ (Spectral Analysis):- each chemical element has a distinct fingerprint!What are the Spectral Lines? • `Flame test’ (Spectral Analysis):- each chemical element has a distinct fingerprint!What are the Spectral Lines? • `Flame test’ (Spectral Analysis):- Kirchhoff/Bunsen discover new elements(Rubidium, Caesium)Periodic Table: Dmitri Mendeleyev(1869)What are the Spectral Lines? • The Laws of Spectroscopy:- dark lines = absorption lines- bright lines = emission linesWhat are the Spectral Lines? Niels Bohr (1885-1962)Bohr’s quantum modelof the atom (1913)What are the Spectral Lines? • Bohr’s quantum model of the atom (1913):- emission and absorption lines!Classifying the Spectra of the StarsClassifying the Spectra of the Stars • Father Angelo Secchi(Jesuit, 1818-78)• first scheme to classifystellar spectraClassifying the Spectra of the Stars • great classification effort at Harvard College Observatory, beginning in 1880sClassifying the Spectra of the Stars • the `women computers’ of HarvardClassifying the Spectra of the Stars • Annie Jump Cannon(1863-1941)• master classifyer• instrumental in publishingthe Henry Draper Catalogue- 1918-24, ~ 225,000 stars:- each with spectral type andbrightnessThe Harvard Sequence of Spectral Types Traditional mnemonic: “Oh, Be A Fine Girl, Kiss Me!”• arranged in order of decreasing temperatureon the surface of the starSpectral Type and Temperature • Red  lower Temperature, blue  higher TClassifying the Spectra of the StarsChemical Composition of the Stars • Cecilia Payne-Gaposhkin(1900-79)• Harvard PhD 1925• hydrogen and helium aremost abundant elements inthe universe!Chemical Composition of the Stars • measured strength of spectral line (S) =abundance (A) x transition probability (P)-Metal lines (e.g., Ca):S= a x P-hydrogen lines: S= a x P• Hydrogen is most abundant element!!!Birth of Astrophysics (part 1)• Measuring the Distance to the Stars: - Friedrich Wilhelm Bessel - 1838: First successful stellar parallax- 61 Cygni: 1/3 seconds of arc  10 lightyears- “the greatest triumph which astronomy has ever witnessed”(John Herschel)• Figuring out the composition of the stars:- spectral analysis (absorption and emission lines)- Harvard classification: stars can be grouped accordingto spectral type (and thus surface temperature)- OBAFGKM- Hydrogen and Helium are most abundant elements in theSun and the stars (Cecilia