Natural History of theHeavens in the Eighteenth

Century

Last time … Newtonianastronomy in the 18c The tasks

– 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

The tools– 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 toolsLarge light-gathering telescopes to see

farther into sky (rather than to measureangles)

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 yardsticksJan

Jun

Sun

Parallaxangle=1”

1 AU

Distance

Parallax of 1” gives distanceof 200,000 AU = 1 parsec(parallax in arc seconds) =3.3 light years

Stellar parallax in parsecsParsec = distance of an object with 1 arcsec parallax

Hence, from parallax triangle:

Distance (in parsecs) = 1/parallax (in arcsecs)

Measured parallax Distance1 arcsec 1 parsec0.5 arcsec 2 parsecs0.005 arcsec* 200 parsecs (360 light years)

*Smallest parallax angle that can be measured todaywith 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

d d d

Area of beam

x

x2

4 x29 x2

Photometric yardsticks 18th century photometry

Assume all stars are identical (sameintrinsic brightness as Sun)

Assume starlight passes unimpededthrough space

Apparent Brightness-Distance Relation:

b = 1/d2

Distance to any star (*)relative to known star (s):bs/b

* = (d

*/ ds)2

or d* = ds (bs/b

* )1/2

Photometric yardsticks Huyghens, Cosmotheores, 1698

View Sun through tiny hole so that appears asbright as the star Sirius

Finds bs/b* = 7.8 x 108, 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 reflectivityof 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 stableworlds

– 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 “naturalhistory” going beyond planets, stars, comets

Star clusters

Open cluster “Pleiades,” M45Found in plane of Milky Way

Globular cluster, “Omega Centauri,” foundoutside 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’slargest 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

Motion of Sun

Proper motionsof stars

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, propermotion, star clusters Planets becoming uninteresting after 2000 years

of dominating Western astronomy

Distances to stars very large (depending onassumptions)

Models of physical shape of the cosmosproposed (assumption-dependent)