Natural History of the Heavens in the Eighteenth Century

Natural History of the Heavens in the Eighteenth Century

Last time Newtonian astronomy in the 18c The tasks The tools Measure planetary motions Explain motions with Newton s laws Prove long-term stability of solar system Advance of Mercury s perihelion (extra 88 secs/century)? Develop cosmogonical models Precise positional measurements of stars and planets Newly invented mathematical techniques of approximation

Rise of natural history New questions, mostly about stars Do stars move with respect to each other? How far away are the stars? How are stars arranged in 3-d space? What other species populate the cosmos? New tools Large light-gathering telescopes to see farther into sky (rather than to measure angles)

Stellar distances Ptolemy s Planetary Hypotheses Plenum spacing of planetary models Stellar sphere at 9 AU from Earth Post-Copernican stellar parallax Tycho found no stellar parallax Could measure 2 arcmins (implies 3500 AU) John Flamsteed, 18c, found no parallax Could measure 1 arcsec (implies 200,000 AU) Stellar parallax first measured 1830s Friedrich W. Bessel measured 0.31 arcsecs for 61 Cygni (implies 665,000 AU or about 10 light years)

Stellar parallax yardsticks Jan 1 AU Sun Parallax angle=1 Distance Jun Parallax of 1 gives distance of 200,000 AU = 1 parsec (parallax in arc seconds) = 3.3 light years

Stellar parallax in parsecs Parsec = distance of an object with 1 arcsec parallax Hence, from parallax triangle: Distance (in parsecs) = 1/parallax (in arcsecs) Measured parallax Distance 1 arcsec 1 parsec 0.5 arcsec 2 parsecs 0.005 arcsec* 200 parsecs (360 light years) *Smallest parallax angle that can be measured today with satellite-based telescopes (ca. 1 million stars)

Photometric yardsticks Problems of distance triangulation Proper motion of stars found (Halley, 1718) Maximal distance only 200 parsecs Faintness means farness Apparent brightness of any light source decreases with square of distance 9 x 2 Area of beam 4 x 2 x 2 x d d d

Photometric yardsticks 18th century photometry Assume all stars are identical (same intrinsic brightness as Sun) Assume starlight passes unimpeded through space Apparent Brightness- Distance Relation: b = 1/d 2 Distance to any star (*) relative to known star (s): b s /b * = (d * / d s ) 2 or d * = d s (b s /b * ) 1/2

Photometric yardsticks Huyghens, Cosmotheores, 1698 View Sun through tiny hole so that appears as bright as the star Sirius Finds b s /b * = 7.8 x 10 8, or d * = 28,000 AU Newton s study of Jupiter (published 1728) Jupiter appears as bright as Sirius Compute intrinsic brightness of Jupiter From diameter, distance from sun, assumed reflectivity of surface Computed d (Sirius) = 1,000,000 AU But are all stars identical to our Sun?

Ordering the stellar cosmos Newton s infinite universe--is it stable? Are stars uniformly distributed across sky? Thomas Wright, Original Theory, 1750 Speculative, natural theology Stars in thin spherical shells (like Aristotle, but multiple worlds) Saves appearances of Milky Way Proposed stars orbit centers (using Halley s 1718 discovery of proper motions of 3 stars), making stable worlds Plurality of inhabited worlds!

Messier s non-comet objects First French observer to find return of Halley s Comet Used small (3.5 ) refracting telescope Leading comet observer Discovered 21 new comets, observed total of 41 List of 109 non-comets, 1774-83 Nebulae (Latin for clouds) = M1, M31, M51 Open star clusters (up to several thousand) = M45 Globular star clusters (hundreds of thousands) = M13 Messier catalogue as first celestial natural history going beyond planets, stars, comets

Star clusters Open cluster Pleiades, M45 Found in plane of Milky Way Globular cluster, Omega Centauri, found outside plane of Milky Way

Herschels stellar cosmos William & Caroline, German emigrées to England (1752, 1772 respectively) Search for double stars (parallax) Assumes most doubles are optical, not binary Found star with disk (Uranus), 1781 =instant fame Moved to Windsor Castle, 1782, with 200 pound salary Royal grants to build large reflectors Forty-footer had 4 dia mirror, weighed > 1 ton, world s largest telescope Continued Messier s sweeps 800 new double stars 2500 nebulae, sorted into types Some binary stars had different brightnesses[!]

Herschels on motion of Sun Sought direction of solar apex by mapping proper motions Proper motions of stars Motion of Sun

Herschel on shape of cosmos Star guaging (# stars proportional to distance to edge of cosmos) Assume can see all [!] stars in cosmos with 20 footer Assume stars regularly distributed Cloven Disk proposed, 1785 Later refutes his own theory Some nebulae are stars Not all nebulae resolved into stars (beyond Milky Way or different type of nebulae?) Found many additional stars with 40 footer (the farther you look, the more stars you see)

Natural history by 1800 Cosmic objects now include planets, stars, comets, nebulae, binary stars, proper motion, star clusters Planets becoming uninteresting after 2000 years of dominating Western astronomy Distances to stars very large (depending on assumptions) Models of physical shape of the cosmos proposed (assumption-dependent)