First Light: Photography & Astronomy Introduction to Spectroscopy Christopher G. De Pree Agnes Scott College RARE CATS, June 20, 2002

Overview n Advent of Photography n Applications of photography in astronomy u Imaging u Spectroscopy n Early pioneers in astrophotography u Harvard College Observatory u Annie Cannon n Impact on our understanding of the universe

Advent of photography n “Camera” from camera obscura, Latin for “dark room” n Drawing by Gemma Frisius u De Radio Astronomica et Geometrica (1545) n Thomas Wedgwood ( ) and Sir Humphry Davis u Late 18th century first photographic images u Photosensitivity of silver nitrate and silver chloride

Daguerreotypes n 1831 French painter Louis Jacques Mandé Daguerre n Photographs on silver plates u Coated with a light-sensitive layer of silver iodide n Permanent photographs u Developed plate was coated with a strong solution of table salt n Daguerre method produced a single image on the silver plate

Evolution of the Form n British inventor William Henry Fox Talbot u photographic method involving the use of a paper negative n Calotype process (30 sec exp.)-- multiple prints n Daguerre and Talbot announced processes in 1839 u Within three years the exposure time in both processes was a few seconds

Applications in Astronomy n Imaging u Long exposure times--faint objects u Tracking single objects u Long time-scale monitoring n Spectroscopy u Emission mechanisms u Identifying elements F Helium in the solar spectrum u Relative motions (Doppler Shift)

Impact on our Understanding n What’s out there? n What is it made of? n How is it moving with respect to us?

Imaging n Development of “fast” film u Problem of first photographic techniques (no smiles) u Rotation of the Earth u Exposure times of a few minutes n Long exposure times u Detect faint sources u Tracking required u Telescope stability (mounts)

Tracking n Equatorial Mount u Moves to counteract Earth’s Rotation u Must be “polar aligned” n Altazimuth mount u Not a unique coordinate system u More difficult to track F Some modern telescopes use (e.g. VLA, VLBA)

Telescope Designs n Cassegrain n Coude n Herschelian (obsolete) n Newtonian

Beck Telescope n 30” diameter primary mirror n Cassegrain or Newtonian arrangement n German equatorial mount n Counterbalance n Tracking system (clock drive) u Mechanical (1930) u Electrical (1966) u Computer-controlled (1998)

Impact on astronomy (Imaging) n Images create archives u Classification of galaxies u Classification of stars (spectroscopy) n Deep images show detail

Images taken over time n Look for change in successive images u Variable stars u New planets u Asteroids and comets

Spectroscopy n Pass light through a prism n Elemental fingerprints u Why are spectra unique u Electrons “hopping”

Where do spectral lines come from? n Electrons “hopping around” n Molecules spinning and vibrating

Doppler Shift n Motion of the Source n Train whistle n Water waves and ducks n Light experiences the same effect u Redshift--longer wavelength u Blueshift--shorter wavelength

Planetary Detection

Known Extrasolar Planetary Systems

Use of Stellar Spectra n Harvard College Observatory u Edward Pickering u “Computers” n Antonia Maury( ) u Classification scheme u Position and width of absorption lines n Annie Cannon ( ) u Established the current stellar classification scheme

Luminosity Classification (Maury) n Absorption lines are pressure- sensitive u Lines get broader as the pressure increases. u Giant stars are puffier, which means lower pressure u Giant stars have narrower absorption lines u Supergiant stars have absorption lines that are even narrower n Used by Ejnar Hertsprung

Temperature Classification (Cannon) n Harvard system (HD catalog) u Classifying stars by their spectra u Spectral classification by surface temperature n Systematic work on all stars down to 9th magnitude u Used photographic plates (16x fainter than human eye sees) u Cannon could identify spectral patterns at 3 stars/minute

Cannon’s Contributions n Most stars grouped into a small number of classes n Personally classified spectra for more than 225,300 stars u Henry Draper Catalogue n Discovered more than 300 variable stars and five novae

Conclusions n Significant advances in astronomy followed the advent of photography u Deep imaging F Classification of galaxies u Archiving F Time variability of stars F Discovery of planets, comets, asteroids

Conclusions (cont.) u Stellar spectroscopy F Classification scheme F Understanding of how stars evolve F Motion (Doppler Shift) Extrasolar planets u Spectroscopy in general Rotation of galaxes Expansion of the universe n The next great leap u Charge Coupled Devices (CCDs)