Please join 2 interesting discussions about historical astronomy, Saturday, October 6, 2001.  NCHALADA LVIII:  Northern California Historical Astronomy Luncheon and Discussion Association. www.nchalada.org

Chabot Space & Science Center, 10000 Skyline Blvd, Oakland. In the Board Room, west end, Dellums Bldg.  www.chabotspace. org/visit/directions.asp

Parking is free in the overflow lot or $4 in the structure. 

Morning discussion, 10-12:30:  How Far the Stars?  Chair:  John E. Westfall, Editor, Association of Lunar and Planetary Observers.

Lunch at a local (but not low-cal) restaurant, then a brief business meeting.

Afternoon discussion, 2-5 PM:  The History of the Discovery of Elementary Particles.  Chair:  Nancy K. Cox, San Francisco Amateur Astronomers.

Please, please bring munchies.

For further information, email nsperling@california.com.

Sessions are always discussions, never lectures.  Your contributions are eagerly welcomed.

NCHALADA LVIII

Disorganized by John Westfall, NCHALADA LVIII, AM session, October 6, 2001.

Notes: (1) This outline and bibliography grew pretty long; certainly too long to work through point-by-point when we meet to discuss the topic. Instead I recommend that we discuss issues that the outline raises, such as paradigm-shifts (from stellar sphere to infinite space, recognizing stars as suns, recognizing stars as unequal, etc.), the validity or non-validity of assumptions made (the order of the planetary spheres, relationship of proper motion to distance, etc.); good or bad use of evidence (naked-eye estimates of apparent diameters of stars, impact of telescope, etc.); acceptance, overconfidence, or hesitation in the use of new technologies (micrometer, heliometer, photography, etc.); difference between estimated and actual accuracy of results; merits of competing methods of finding distances; the sample of stars (i.e., volume of space) with accurate distances; impact of knowing distances (absolute magnitudes, H-R diagram, etc.).

(2) This would be yet longer did I not use some abbreviations: ER = earth radius (6378 km), AU = astronomical unit (149.6 million km), ly = light year (9.46 trillion km), pc = parsec (30.9 trillion km), kpc = kiloparsec (modern units are in parentheses, but contemporary units may differ considerably from them). The Greek letter is the standard symbol for parallax, ' = arc-minute, " = arc-second, mas = milliarcsecond (0".001), uas = microarcsecond (0".000001), deg = degree, mag = magnitude.

The Geocentric Era.

Geocentric system; stars on surface of earth-centered sphere; thus all stars same distance.

Naked-Eye Appearance:

Stars vary in brightness; must vary in absolute size or brilliance

Stars appear pretty small; Hipparchus (f. 140 B.C.) believed faintest ca. 3'; this, combined with lack of visible parallax, implied faintest at least size of earth (does he explicitly say the latter?).

Estimates of Radius of Stellar Sphere (thus Distance to Earth) and Star Sizes:

A common approach to estimating the distance was to assume the actual size of a star and compare this with an estimate (always vastly too large) of its angular diameter; distance must be greater than that of the Sphere of Saturn. (Note: Order of planets arbitrary.)

No apparent (diurnal) parallax; radius must be at least 1000-2000 ER.

Archimedes (ca. 267-212 B.C.) (according to Hippolytus) makes 248,264,780 stades (if based on Attic stadium, ca. 46.0 million km or 7200 ER). In Sand Reckoner, Archimedes gave maximum distance as 100 million ER (ca. 640 billion km).

Ptolemy (f. AD 130), starting with a maximum lunar distance of 64 ER, a solar distance of 1,260 ER, equally spaced planetary spheres, and a small spacing between the Spheres of Saturn (the largest planetary sphere) and the stars, arrived at a distance of 569,463,333 stades (ca. 105.5 million km or 20,000 of his ER); stellar diameters 4.55 ER with 1-1/4' apparent diameter.

Al-Farghani (ca. AD 800-870) gave distance as 20,100 ER; first magnitude appeared ca. 1'.5, with diameter 4.75 ER (fainter stars progressively smaller). Al-Farghani's values adjusted slightly by al-Battani (c. 850-929).

Al-Urdi (d. 1266) recalculating Hipparchus's values as used by Ptolemy, enlarged stellar sphere to 140,147 ER (894 million km).

Roger Bacon (1214-1292), in Opus Maius, adopted al-Farghani's values.

Nicholas of Cusa (1401-1464) perhaps first to reject stellar-sphere concept; stars were suns scattered through space; concept adopted by Thomas Digges (1543-95).

The Copernican Dilemma.

By having the earth move, Nicolas Copernicus (1473-1543) greatly extended our baseline. Thus, lack of observed parallaxes implied vast distances for stars.

Tycho Brahe (1546-1601) attempted and failed to measure parallax of Polaris (also for Nova 1572), concluded it was less than 1', implying an unacceptably-large distance of at least 7,000 AU in a Copernican universe. thus advocated his modified geocentric model, where Tycho placed the stellar sphere at 14,000 ER--closer than Ptolemy.

Giordano Bruno (1548-1600) rejected the stellar-sphere concept; advocated infinite cosmos with stars of same absolute brightness (that of sun) but at varying distances.

Johannes Kepler (1571-1630), following Copernicus in De Stella Nova in Pede Serpentarii (1606), retained stellar sphere but at 34,177,066-2/3 ER from sun. (sun at 1,432 ER, so stellar parallax would be 9"; enlarged this to 60 million ER in Epitome Astronomiae Copernicanae (1618-21).

Galileo Galilaei (1564-1642) compared apparent diameter of sun with estimated telescopic diameter of mag 6 star, assumed the same absolute size as the sun, concluded that the star was 2,160 AU distant. However, the implied parallax of over 3' would have been measurable by Tycho. Galileo also advocated varying distances of stars and proposed use of differential parallax for bright stars (assuming that they were closer than faint stars).

Note (1) assumption that stars are same intrinsic diameter (not necessarily brightness) as sun; (2) attempts to estimate stars' apparent diameter (all far too large even with telescope; e.g., Martinus Hortensius [1605-39] made Sirius 10", Kepler 240"[!], Jacques Cassini [1677-1756] in 1717 made it 5"-6").

Although rejecting Copernicus, telescopic measurements of planetary diameters forced Giovanni Battista Riccioli (1598-1671) to enlarge the Sphere of Saturn and move the fixed stars out to ca. 200,000 ER.

Orbit of Comet 1682, calculated by Edmund Halley (1656-1742) crossed paths of Venus-Saturn, demolishing theory of planetary spheres and thus weakening further concept of stellar sphere.

Halley (beginning in 1718) compared ancient stellar positions with those of Flamsteed, discovered proper motions for Sirius, Arcturus, Betelgeuse and Aldebaran; further invalidating sphere of "fixed" stars; also suggested that the nearer stars will tend to show the larger proper motions.

Christiaan Huygens (1629-95) tried photometric approach, estimated sun 27,664^2 as bright as Sirius; therefore Sirius at 27,664 AU distance.

Isaac Newton (1643-1727) also used photometric approach, comparing the sun with Saturn, using Saturn's apparent size and assumed albedo, then compared Saturn with 1st-magnitude star; concluded that star was at 100,000 times Saturn's distance (i.e., ca. 950,000 AU).

Robert Hooke (1635-1703) measured zenith distance of gamma Draconis July-Oct., 1669, attributed movement of 24-26" to parallax; however Jean Picard (1620-82) in 1675-81 measured Vega and reported no shift; John Flamsteed (1646-1720) in1689-96 found 42" annual shift for Polaris, interpreted as parallax. (Jacques Cassini [1677-1756] later pointed out that the seasonal pattern contradicted a parallactic cause but did not suggest an alternative explanation.)

James Gregory (1638-75), using photometric approach, in 1668 estimated Sirius as 83,190 AU distant.

James Bradley (1693-1762) and Samuel Molyneux (1689-1728) in 1725-26 used zenith telescope to attempt parallax of gamma Draconis; measured 20" amplitude, but seasonal pattern contradicted parallactic explanation; led Bradley in 1728 to propose aberration of starlight as the explanation--a significant spin-off from attempt to detect parallax. Also concluded that actual parallax less than 1", beyond precision of 18th-century measurement.

William Herschel (1738-1822), assuming that double stars are optical pairs, began in 1780s to apply Galileo's differential parallax method, dids not detect parallax but instead found orbital motion--the discovery of binary stars was the second major spin-off from unsuccessful parallax detection. Incidentally this proved, along with the evidence of star clusters, that stars varied enormously in brightness from each other. Herschel, in his "star gauging", estimated stars' relative distances with respect tothat of Sirius, 1st magnitude = distance of Sirius, 2nd magnitude = twice distance of Sirius, etc.

Perhaps the last photometric-method application was by William Wollaston (1766-1828), who compared Sirius with the sun's reflection on a glass bead, getting Sirius's distance as 141,421 AU (pub. in 1829).

There were several "false alarms"--premature announcements of trigonometric parallaxes: William Herschel, 0".3553 for Vega; Giuseppi Piazzi (1746-1826) (in 1805), parallaxes of 2" for Vega, Aldebaran 1".6, Procyon 5".7 and 4" for Sirius; Giuseppi Calandrelli (1749-1827), 4".4 for Vega; John Brinkley (1763-1835) (in 1814), 1".1 for Vega, 1".1 for Arcturus, 1".0 for Deneb and 2".75 for Altair. The variations among these values for the same stars, and their large sizes, cast doubts on the reliability of all stellar parallaxes; e.g., the John Brinkley-John Pond (1767-1836) debate from 1814-1824; in the 1820s-1830s Pond and George B. Airy (1801-1892) had tried to measure parallaxes and failed, generally concluding that they must be under 0".1.

The Prize is Won.

By ca. 1800 recognized that: (1) the stars were at varying distances; (2) the stars differed significantly in intrinsic brightness; (3) their parallaxes are going to be under 1"; (4) stars show proper motions, and the number of stars measured for proper motion was increasing; (5) the expected sub-arc-second parallaxes would be hard to measure, so effort should be concentrated on the most likely (closest) closest candidate stars; and (6) there had been several "false alarms" so credibility would be a major issue for any announcement of a parallax measurement.

The problem was how to select close stars when we didn't know their distances in the first place. To maximize the likelihood of success, Wilhelm Struve (1793-1864) recommended three criteria: (1) bright stars; (2) stars with large proper motions; and (3) binary stars with relatively wide separations yet with relatively short periods.

Struve used the Dorpat Observatory's Fraunhofer 9.5-inch refractor (then the world's largest) with an eyepiece micrometer on Vega, chosen because it was bright and had a large proper motion, implying closeness; also a faint reference star was nearby. Observing between 1835-36, he found = 0".125+-0".05, published in August, 1837; unfortunately when updated to 1838 (pub. in 1840), rose to 0".262. (Modern = 0".129.)

Friedrich Wilhelm Bessel (1784-1846) used the Koenigsburg 6.25-in Fraunhofer heliometer with 61 Cygni, which had the largest proper motion then known, with two faint reference stars within the field of the heliometer. Observing in 1834 and 1837-38, he found = 0".31 +- 0".02 (pub. in Dec., 1838). (Modern = 0".287.)

Thomas Henderson (1798-1844) used the position of alpha Centauri, chosen because it was a bright, rapidly- rotating binary with a large proper motion, as measured during 1832-33 with the Cape of Good Hope mural circle finding = 1".16 =- 0".11. [Modern = 0".742.] However, he did not publish his result until 1839.

The heliometer measurements gave the most consistent result (and the one closest to the modern value); also the heliometer could use comparison stars up to 2 deg from the target star. Heliometers were first made by John Dollond in 1754, but did not become routinely used until after Struve's time. From then through the rest of the nineteenth century, heliometers made by Repsold and Sons of Hamburg (firm: 1799-1919) and Merz and Mahler of Munich (firm: 1826-1903) were the preferred instruments for parallax determination. (Most heliometers were used in Germany; the only heliometer in the Western Hemisphere is the one installed at Yale in 1882, used for stellar parallaxes especially by William Lewis Elkin [1855-1933]).

Thus, the technology and methodology of parallax measurement remained basically unchanged for about 60 years (ca. 1840-1900). Despite the rapid initial success, the number of stars with reliable parallaxes grew slowly, and is hard to calculate because of squabbles over which were reliable. Arguable numbers of well-measured stars by (year) are: (1839) 3, (1850) 6, (1862) 10, (1888) 25, (1901) 38.

A 1912 catalogue listed the parallaxes of 244 stars, determined as follows: 8 with filar micrometers, 83 with meridian transits, 39 by photography, 3 by spectroscopy, and 111 with heliometers.

The Photographic Era

Using dry plates and a reflecting telescope, Charles Pritchard (1808-93) photographed 61 Cygni 200 times in 1886, obtaining the first photographic parallax of 0".438. Doubts about the new method were initially expressed, but J.C. Kapteyn (1851-1922) in 1900 measured photographic parallaxes for 248 stars.

Frank Schlesinger (1871-1943) applied the photographic method extensively with the Yerkes 40-in refractor and the Alleghany 30-in Thaw refractor and also recruited and coordinated observers at other observatories, particularly those with large, long-focus refractors. Another pioneer was Henry Norris Russell (1877-1957), working in 1905-06.

Photography with large refractors remained the dominant method of measuring stellar parallaxes through the 1980s. Again, the technology changed little for a long period, although plate measuring became automatic in the 1960s. The Yale parallax catalog began in 1924. The number of stars measured for parallax grew as follows: (1906) 263, (1924) 1670, (1949) 5751, (1962) 6607, (1992) 7874; note slowdown after Schlesinger's retirement in 1942.

The observatories involved (with telescope statistics, refractors unless otherwise indicated and percentage of stars by 1992) were: Yale (Johannesburg station, 26-in f/16.6) 15.5%, Leander McCormick (26-in f/15) 15.4%, Alleghany (30-in f/18.4) 15.1%, Royal Ob. Cape of Good Hope (now SAAO; 24-in f/11) 13.9%, Sproul (24-in f/17.9) 10.4%, U.S. Naval Ob. (Flagstaff, 61-in f/10 reflector) 6.6%, Royal Ob. Greenwich (26-in f/10.2) 6.1%, Van Vleck (20-in f/16.5) 4.7%, Yerkes (40-in f/18.9) 3.6%, Mount Wilson (60-in f/20 Cass. reflector) 3.5%, Dearborn (18.5-in f/15.0) 1.4%, Bosscha (Java, 23.6-in f/17.9) 1.1%, Lick (36-in f/19.3) 0.9%, U.S. Naval Ob. (Flagstaff, CCD with 61-in f/10) 0.5%, Upsala (13-in f/13) 0.4%, Stockholm (Saltsjoebaden, 338-in FL), 0.3%, Univ. of London (Mill Hill, 24-in f/11.5) 0.3%, Leander McCormick South (Mt. Stromlo, 26-in), 0.2%. [Note the similarity of the Van Vleck refractor to Rachel, although the Van Vleck is by Alvan Clark & Sons.]

Just prior to WWI the Astronomical and Astrophysical Society of America established a "Committee on Stellar Parallaxes to coordinate the often-counterproductive competition among parallax-measuring institutions; apparently not very successfully.

Accuracy: Limitations and Implications.

Photographic accuracies improved slowly, 14 mas in 1957, 11 mas in 1969, 8 mas in 1991. With 10 mas accuracy, stars to 10 pc located to 10% distance accuracy; stars to 25 pc to 25%.

Note that doubling the accuracy doubles the distance reachable, but increases the accessible volume of space by a factor of eight.

The "classical" photographic method used a network of nearby reference stars, whose distances (i.e., parallaxes) were usually estimated based on their magnitudes and spectral classes. It was possible to occasionally obtain negative parallaxes due to measurement errors, incorrect reference star distances, or both.

Statistical biases could occur. Rejecting the obviously wrong negative parallaxes caused a non-normal error distribution. Also, even normally-distributed parallax errors resulted in non-normal distance errors. Another, less obvious, problem is caused by the fact that the number of stars increases with the volume of space within measurement, not linearly with distance; faraway stars estimated as too close outweighed nearby stars estimated as too far (the "Lutz-Kelker bias"). Rare types of stars only recently have had reasonably accurate distances. Two significant distance-indicator stars are delta Cepheus (ca. 300 pc) and RR Lyra (ca. 230 pc), both too far for accurate measurement by classical methods. Novae, so far, have been too distant to measure.

The USNO 61-in Astrometric Reflector, beginning in 1964 reduced the photographic method's uncertainty to 2 mas, moving the 10%-accuracy limit to 50 pc, and the 25%-limit to 125 pc.

Alternative "Parallaxes."

It is recognized that trigonometric annual parallaxes are fundamental and form the basis of all other interstellar and intergalactic distances. However, because of the limited volume of space reachable by the classical method, alternatives have been applied.

Photometric parallaxes are found by estimating a star's absolute magnitude (M), based on its two-dimensional spectral classification (the "MK system"; e.g., G2 V), and comparing this with its apparent magnitude, m. Then d, the distance in pc is found by: d = 10^((m - M + 5)/5). (Photometric parallaxes are also called "spectroscopic parallaxes" but this term is also used for the solar parallax found from stars' annual variations in radial velocity.) Of course, the luminosity-spectral class relationship has to be calibrated by using stars with trigonometric parallaxes. Also, despite the fact that distance is found, the term "parallax" is used for the method. Walter S. Adams (1876-1956) and Arnold Kohlschuetter (1883-1969) pioneered this method in 1914-16. Note that, before trigonometric parallaxes were actually measured, photometric parallaxes were found by assuming the intrinsic brightness of stars.

Statistical and secular parallaxes can go out to perhaps 500 pc, but are applicable only to groups of stars, not individual stars. These methods were pioneered by J.C. Kapteyn (1851-1922), although W. Herschel had proposed secular parallax method in 1783.

Statistical parallaxes estimate the mean parallax of groups of stars, assuming that their proper motions are random, by comparing their radial velocities with their proper motions.

Secular parallaxes are found, again as the means for groups of stars, but using the baseline created by the sun's spatial motion (2.8 AU per year).

The moving-cluster method starts by finding the convergence point of the proper motions of its members; the angular distance of the cluster from this point gives the angle between the stars' spatial motions and the line of sight. Then the stars' radial velocities can be converted to the line-or-sight velocities and the distance calculated. Lewis Boss (1846-1912) pioneered this method, using the Hyades; it has also been applied to the Ursa Major and Scorpio-Centaurus clusters.

Dynamical parallaxes are found for binary stars where the orbital semimajor axis in arc-seconds (a), the orbital period in years (T), and the masses of the two stars in solar masses (m1, m2) is known. Then: = a*T^(-2/3)*(m1 + m2)^(-1/3). Usually m1 and m2 are initially assumed equal to one, or the masses are estimated from their spectral classes, but the result is sensitive only to the cube root of the mass sum; iteration is used to refine the masses and the parallax. This method was first suggested by F.G.W. Struve in 1837, obtaining 0".24 for 70 Oph and 0".25 for 61 Cygni. Sometimes called "orbital parallaxes."

Hipparcos.

ESA's Hipparcos satellite is the only (so far) example of satellite astrometry. Launched in 1989, it operated until 1993 doing astrometry of unprecedented accuracy. The Hipparcos Catalogue (1997) included 118,218 stars to mag 12 (complete to mag 9), with parallaxes accurate to a mean of 0.97 mas. The 10% accuracy limit extends to 200-300 ly (depending on magnitude), including 22,396 stars. The associated Tycho Catalogue contains about 1 million stars with 20-30 mas accuracy.

Hipparcos raises the question of the future of ground-based parallaxes. However: (1) Even classical methods can reach fainter stars than Hipparcos; (2) Use of CCD astrometry gives accuracies comparable to Hipparcos; (3) Interferometers may give yet better earthbased accuracies.

Surprises generated by Hipparcos include:

The "runaway stars" 53 Arietis, AE Aurigae and mu Columbae, all receding from us, were previously thought to have originated in the Orion Nebula, but Hipparcos indicates that they are closer than the nebula.

Deneb may be almost twice as far as previously thought. This, and other revised distances, may change some details of the H-R diagram.

Cepheids are about 10% more luminous (5% farther) than previously thought.

Children of Hipparcos.

HST--The Hubble Space Telescope Fine Guidance Sensor 3 has been used for some parallax projects, such as for Proxima Centauri, selected Hyades stars, delta Cephei and RR Lyrae. Accuracy 2-4 mas for single observations; 0.5-0.6 mas for repeated observations.

FAME--USNO Full-Sky Astrometric Mapping Explorer. Launch planned in 2004 with 2.5- year mission, possibly extended to 5 years. FAME will measure 40 million stars of mag 5-15, accuracy to 50 uas at mag 9 and 500 uas at mag 15. At mag 9 about 20X the accuracy of Hipparcos, giving 10% distance accuracy to 2500 pc (8150 ly).

DIVA--Funded by the Deutsches Zentrum fur Luft- und Raumfahrt for a 2004 launch. DIVA will measure 35 million stars to mag 17. Parallax accuracy of 0.19 mas at mag 10 and 5 mas at mag 15, giving 10% distance accuracy at 530 pc.

GAIA--ESA 5-year mission to operate at L2 point (1.5 million km anti-sunward from earth). Will measure positions of 1.3 billion stars to mag 20-21 to 160 uas for mag 20, 11 uas for mag 15, and 4 uas for mag 10). It would give distances for sun-type stars to 10% accuracy at 10 kpc. Launch ca. 2010- 2012.

SIM--JPL Space Interferometry Mission, scheduled for launch in 2009. Two telescopes 10 m apart and 95 million km from earth will have 4 uas parallax accuracy (10% distance accuracy at 2500 pc), limiting mag 20. Will look for extrasolar planets and also target Cepheid and OB stars. (Prev. called OSI.)

DARWIN--ESA Infra-Red Space Interferometer, also called "IRSI." Emphasis is on imaging extrasolar planets, down to Mars-size at 10 pc.

StarLight--JPL's "formation flying space interferometer"; two orbiting telescopes. A prototype multi- spacecraft interferometer with launch planned for 2005. Planned to measure stellar diameters to <1 uas.

LIGHT--Japanese "Light Interferometer satellite for the study of Galactic Halo Tracers", with launch in 2007-2010. Plans to observe 100 million stars to mag 18 and 10-15 kpc distances with parallaxes accurate to 50 uas in V-band.

POINTS--SAO/Itek/JPL proposal, "Precision Optical INTerferometer in Space." Ten-year mission to observe 300 stars with 0.4 uas parallax accuracy and 0.6 uas position accuracy in order to create "superbly accurate" reference frame. Implies 10% distance accuracy to 170,000 pc (all of galaxy plus Magellanic Clouds). (Note: POINTS has been discontinued; "Newcomb" is proposed as a lower-cost fallback.)

OSIRIS--Russian project--Space Optical Interferometer for Astrometry, to be attached to Space Station, possibly in 2003. Will measure arcs of 30-100 deg between stars with final accuracy of 10 uas. A 3-year mission to measure about 5000 stars to distances of 4000 pc (13,000 ly).

For all these missions, a strong motivation for their high positional accuracies is the detection of extrasolar planets, ideally down to terrestrial size. Some of them are clearly pipedreams, but I have listed the more probable (in my estimation) or the sooner first.

Mundane Advances--CCD Astrometry.

Pioneered by USNO Flagstaff Station, which began using CCD images with the 61-in astrometric reflector in 1983, reducing uncertainties to 0.5-0.7 mas. With 0.5-mas accuracy, 10% distance accuracy to 200 pc, 25% to 500 pc.

USNO CCD Astrographic Catalog (UCAC), in progress and about 2/3 done, with planned publication in 2004. Stars to 16th mag with 20 mas accuracy for 10th-14th mag. Potential for parallaxes for close, 2005. faint stars and for detecting faint stars with rapid proper motions.

Mundane Advances--Ground-Based Interferometers.

Projects with countries, dates and baselines in meters:

GI2T/REGAIN (France, Grand Interferometre a 2 Telescopes, 1985-96, 65m).

IONIC (France, Integrated Optics Near-infrared Interferometric Camera).

Keck Interferometer (USA, operational).

PTI (USA, Palomar Testbed Interferometer, 1994-).

ISI (USA, Infrared Spatial Interferometer, operational, 35m).

COAST (UK, Cambridge Optical Aperture Synthesis Telescope, 1991-, 100m, resolving to 1 uas).

FLOUR (France, Fiber Linked Unit for Optical Recombination, 1992-93).

SUSI (Australia, Sydney University Stellar Interferometer, 1991-, 640m).

IOTA (USA, Infrared-Optical Telescope Array, 1994-, 45m).

NPOI (USA, Navy Prototype Optical Interferometer, 1994-, 250m, accuracy to 1 mas).

ASEPS (USA, operational, 100m).

CHARA (USA, Center for High Angular Resolution Astronomy, 1999-, 350m, resolving to 0.2

mas).

KIIA (USA, 2001?, 75/180m).

VLTI (Europe, Very Large Telescope Interferometer 2001?, 128/200m).

LBT 9USA/I/D (2005, 20m).

MAGELLAN (USA, >2005, 20m).

Under development: MIDI, AMBER, PRIMA, OHANA, OVLA, SI, SPECS, SPIRIT, Magdalena Ridge Observatory.

The question is how adaptable interferometers are to parallax, or distance, measurement. One way is by measuring disk diameters of pulsating variables, combined with doppler shifts, as COAST did for chi Cygni in 1997-1999. Another method is to resolve and compute the orbital elements for spectroscopic binaries (e.g., Capella, resolved by COAST in 1995), which allows the dynamical-parallax method to be applied. (CHARA plans to apply this method out to distances of 10 kpc). Systems that can accurately measure long arcs between stars can provide parallaxes directly.

Why Stellar Parallaxes?

Trigonometric parallaxes:

(1) are the basis for the cosmic distance scale--the distances to star clusters in our own galaxy and to other galaxies.

(2) allow the absolute magnitudes of stars to be calculated, necessary for knowing the spectrum-luminosity relationship, necessary for constructing the H-R diagram. We still need good distances (and hence absolute magnitudes) for a large sample of Cepheid, RR Lyra, Mira and delta Scuti variables; O, B and A stars; Population II subdwarfs; and star-forming regions.

(3) when applied to binaries, allows determination of stellar masses.

Units of Measure.

When people first began to guess (and later to measure) interstellar distances, they had to express distances far greater than ever before, raising the question of what units would be appropriate. Some units used were:

The parallax itself--although is inversely proportional to distance, has a one-to-one correspondence with it and the technical literature has often simply listed (and still lists) parallax values by themselves.

Terrestrial Units--probably the first used, with two advantages (1) already familiar, (2) impressively large. Both kilometers and miles were used, but the numbers were so large as to be awkward to visualize and work with (e.g., alpha Cen at 41,660,000,000,000 km).

Astronomical Unit (AU)--If you express in radians (or use sin ), its reciprocal directly gives the distance in AU. For nearby stars the numbers are more reasonable in size; e.g., alpha Cen at 278,500 AU)

Light-time--Pre-20th-century literature often quoted light passage times (e.g., "four years and three months") prior to the adoption of "light-year." One early example was by Francis Roberts in 1694, with the nearest stars 6 light-weeks away.

Light Year--The distance light travels in vacuum in one year (9,460,536,000,000 km).

Parsec, macron, astron, astrometer, sternweite, siriometer--all proposed for the distance corresponding to = 1".0; of these only the parsec (short for PARallax-SECond) survived.

Siriometer--also used to mean the distance to Sirius; alternatively 1 million AU (4.848 pc). (Possibly coined by W. Herschel, but original reference needs to be found.)

Andromede--distance of Andromeda Nebula; at the time (sometime before 1913) thought to be 1600-8000 ly!

Siriusweite--a German term for a distance corresponding to = 0".2 (apparently the distance to Sirius was once thought to be this, which would be 5 pc or 16.3 ly).

Distance Modulus--The difference, (m - M), where m is a star's apparent magnitude and M is its absolute magnitude (i.e., magnitude at a distance of 10 pc). Thus, (m - M) = 5 log(0".1/) = 5 log (distance/10 pc).

Only parsec and light-year have survived for use for interstellar distances. There has been some debate as to (1) which term came first, or (2) for that matter, when either began to be used. The debate appears unresolved, although the Oxford English Dictionary gives 1913 (F.W. Dyson, M.N.R.A.S., 73: 342) as the earliest usage of parsec and C.A. Young's General Astronomy (article 814) in 1888 refers to the "light year". It is also sometimes debated which term is the "better"--professional astronomers prefer "parsec," science fiction prefers "light-year." Parsec distances, of course, are directly derived from parallaxes, but to convert them to any other unit (including light-years) requires knowledge of the value of the AU. Another question is why do we use "kiloparsec" and "megaparsec" far more often than we use "light- century," "light-millennium", or "light-eon"?

STELLAR DISTANCES--REFERENCES

Adams, Walter Sydney. 1922. "Stellar Distances and Stellar Motions." Scientia, 32: 189-300.

"American astronomers report/Improving the accuracy of stellar parallaxes." 1969 Mar. Sky and Telescope, 37, No. 3: 148.

Arenou, Frederic & Luri, Xavier. 1999. "Distances and absolute magnitudes from trigonometric parallaxes." In Egret & Heck, 1999: 13-32.

Ashford, Adrian R. 2001 Jul. "Star catalogs for the 21st century." Sky & Telescope, 102, No. 1: 65-67.

Batten, Alan H. 1988. Resolute and undertaking characters: the lives of Wilhelm and Otto Struve. Dordrecht: D. Reidel.

Batten, Alan H. 2000. "The nearest stars." Observer's Handbook 2001 (Royal Astronomical Society of Canada): 231- 235. (Gives Hipparcos parallaxes to a precision of 0.1 mas for closer than ca. 17 light years.)

Benedict, G.F. et al. 1999 Aug. "Interferometric astrometry of Proxima Centauri and Barnard's Star using Hubble Space Telescope Fine Guidance Sensor 3: detection limits for substellar companions." Astron. J., 118, No. 2: 1086-1100.

Bessel, Friedrich Wilhelm. 1838. "A letter from Professor Bessel to Sir J. Herschel, Bart., dated Konigsberg, Oct. 23, 1838 [the parallax of 61 Cygni)." Monthly Notices, Royal Astronomical Society, 4, No. 17: 152-161. (Reprinted in Shapley and Howarth, 1929, pp. 216-220.)

Both, Ernest E. 1977 July. "Joseph von Fraunhofer." Sky & Telescope, 54: 49-50.

Bradley, James. 1727-1728. "An account of a new-discovered motion of the fixed stars [aberration]." Philosophical Transactions (London), 35: 636-661. (Reprinted in Shapley & Howarth, 1929, pp. 103-108, with excerpt on nutation on pp. 108-112.)

Brosche, P.; Dick, W.R.; Schwartz, O. & Wielen, R., eds. 1998. The message of the angles-- astrometry from 1798 to 1998. Acta Hist. Astron., Vol. 3. Frankfurt am Main: Verlag Harri Deutsch.

Chapman, Alan. 1995. Dividing the circle: the development of critical angular measurement in astronomy, 1500-1850. New York: Ellis Horwood. 2nd ed.

Clerke, Agnes. 1902. A popular history of astronomy during the nineteenth century. London: Adam and Charles Black. 4th ed.

Dahn, C.C. 1992. "Stars, distances and parallaxes." In: Maran, 1992.

________. 1994. "USNO parallax program: directions, results and applications." In Morrison & Gilmore, 1994, pp. 55-60.

Davis, Herman Stearns. 1895a. "The parallax of 61 Cygni, deduced from the Rutherfurd photographic measures." Annals, New York Academy of Sciences, 8, No. 10: 123-160.

________. 1895b. "The parallax of n Cassiopeia, deduced from the Rutherfurd photographic measures." Annals, New York Academy of Sciences, 8, No. 11: 301-321.

Debarbat, S. et al. (eds.). 1988. Mapping the sky: past heritage and future directions: proceedings of the 133rd symposium of the International Astronomical Union, held in Paris, France, June 1-5, 1987. Dordrecht: Kluwer Academic Publishers.

Dick, W.R. & Ruben, G. 1988. "The first successful attempts to determine stellar parallaxes in the light of the Bessel/Struve correspondence." In: Debarat, S., et al., 1988, pp. 119-121.

Dyson, F.W. 1915. "Measurement of the distances of the stars." Observatory, 38: 249-254, 292-299.

Egret, Daniel & Heck, Andre (eds.). 1999. Harmonizing cosmic distance scales in a post-Hipparcos era. Proceedings of a symposium held at Haguenau, France 14-16 September, 1998. Astronomical Society of the Pacific Conference Series. San Francisco: Astronomical Society of the Pacitic.

Eichorn, Heinrich. 1974. Astronomy of star positions. New York: Frederick Ungar Pub. Co.

European Space Agency. 1997. Hipparcos Venice '97. ESA SP-402. Proc. of conference, Venice, 13-16 May, 1997. Noordwijk: ESA Publications Division.

Ferguson, Kitty. 1999. Measuring the universe: Our historic quest to chart the horizons of space and time. New York: Walker and Co.

Fernie, J.D. 1975. "The historic search for stellar parallax." Journal, Royal Astronomical Society of Canada, 69: 153-161, 222-239.

Flamsteed, John. 1701. "On the distance of the fixed stars." Philosophical Transactions (London): 815.

Galewood, G. & de Jonge, J.K. 1995 Sep 1. "MAP-based trigonometric parallaxes of Altair and Vega." Astrophys. J., 450, No. 1: 364-368.

Garrison, Robert E. & Beattie, Brian. 1996. "The brightest stars." Observer's Handbook 1997 (Royal Astronomical Society of Canada): 199-210. (For 314 stars brighter than V = +3.55 gives pre-Hipparcos parallaxes to 1-mas precision.)

Garrison, Robert E. & Beattie, Brian. 1999. "The brightest stars." Observer's Handbook 2000. (Royal Astronomical Society of Canada): 219-230. (For stars brighter than V = +3.55 gives Hipparcos parallaxes to .01-mas precision; the Observer's Handbook 2001 rounds these to 1 mas.)

"Getting acquainted with astronomy/distances of the stars." 1964 Nov. Sky and Telescope, 28, No. 5: 280-281.

Gill, David, Sir. 1893. Heliometer observations for determination of stellar parallax made at the Royal Observatory, Cape of Good Hope. London: Eyre and Spottiswoode.

Gill, David, Sir and Elkin, William Lewis. 1884. Heliometer determinations of stellar parallax in the southern hemisphere. London: The Society.

Green, Robin M. 1985. Spherical astronomy. Cambridge: Cambridge University Press. (See Cpt. 14, "Stellar distances and movements," pp. 333-361.)

Hajian, A.R. & Armstrong, J.T. 2001 Mar. "A sharper view of the stars." Scientific American.

Harris, H.C.; Dahn, C.C. & Monet, D.G. 1997. "Accurate ground-based parallaxes to compare with Hipparcos." In European Space Agency, 1997, pp. 105-107.

Harrison, T.E. et al. 1999 Apr 20. "Hubble Space Telescope fine guidance sensor astrometric parallaxes for three dwarf novae: SS Aurigae, SS Cygni and U Geminorum. Astrophys J., Lett., 515, No. 2: L93-L96.

Hemenway, P.D. et al. 1994. "Status of Hubble Space Telescope fine guidance sensor astrometry." In Morrison & Gilmore, 1994, pp. 80-88.

Henderson, Thomas. 1838 Dec 14. "On the parallax of alpha Centauri." Monthly Noices, Royal Astronomical Society, 4, No. 18: 168-169.

Herschel, John F.W. 1827/1828. "On the parallax of the fixed stars." Philosophical Transactions (London): 1826: 266; 1827: 126.

Herschel, William. 1782. On the parallax of the fixed stars. London. (See also Philosophical Transactions, 1782: 82.)

Hinks, Arthur R. & Russell, Henry Norris. 1905 Jun 9. "Determination of stellar parallax from photographs made at The Cambridge Observatory. Introductory paper." Monthly Notices, Royal Astronommical Society, 65, No. 8: 775-787.

Hirshfeld, Alan W. 2001. Parallax: the race to measure the cosmos. New York: W.H. Freeman and Co.

Hoffleit, E. Dorrit. 1949. "The quest for stellar parallax." Popular Astronomy, 57: 259-273.

Hoffleit, E. Dorrit & van Altena, William. 1988. "Astrometry with the Yale heliometer, 1882-1910." In Debarbat, S. et al. (eds.). 1988, pp. 123-128.

Hog, E.; Makarov, V.V. & Pedersen, H. 1994. "Tycho astrometry of one million stars." In Morrison & Gilmore, 1994, pp. 71-73.

Hog, E. et al. 1997. "The Tycho catalog." Astron. Astrophys., 323: L57-L60.

Hooke, R. 1674. An attempt to prove the motion of the Earth. London.

Hunter, A. & Martin, E.G. 1956. "Fifty years of trigonometrical parallaxes." Vistas in Astronomy, 2: 1023-1030.

Hyneck, J.A. (ed.) 1951. Astrophysics. New York.

Jackson, John. 1956."The distances of the stars: a historical review." Vistas in Astronomy, 2: 1018-1022.

Jahreiss, H. 1994. "Ground-based parallaxes for nearby stars." In Morrison & Gilmore, 1994, pp. 44-49.

Jenkins, Louise F. 1963. General catalogue of trigonometric stellar parallaxes. New Haven: Yale University Observatory.

Johnston, Kenneth & Mozurkewich, David. 1995 May. "Stellar optical interferometry in the 1990s." Physics Today, 48, No. 5: 42-49.

Knobel, E.B. 1876. "Reference catalogue of astronomical papers and researchs." Monthly Notices, Royal Astronomical Society, 36, No. 9: 365-392. [See "Parallax and distance of stars," pp. 384-388, 392.]

Kovalevsky, J. 1998 Dec. "On the uncertainty of distances derived from parallax measurements." Astron. Astrophys., 340, No. 2: L35-L38.

Kurth, Rudolf. 1967. Introduction to stellar statistics. Oxford and New York: Pergamon Press. (See Cpt. VI, "Stellar Distances," pp. 116-129.)

Lankford, John. 1997. American astronomy: community careers and power, 1859-1940. Chicago; University of Chicago Press.

Lindgren, L.; Kovalevsky, J. & Perryman, M.A.C. 1994. "Hipparcos proper motions and parallaxes." In Morrison & Gilmore, 1994, pp. 66-70.

Lovi, George. 1985 Jan. "The distance dilemma." Sky & Telescope, 69: 45-46.

________. 1988 Sep. "An anniversary for a special star [61 Cygni]." Sky & Telescope, 76: 275-276.

Lutz, Thomas Edward & Kelker, Douglas Herson. 1973. "On the use of trigonometric parallaxes for the calibration of luminosity systems: theory." Pub. Astron. Soc. Pacific, 85: 573-578.

Makarov, V.V. 1998 Dec. "Absolute measurements of trigonometric parallaxes with astrometric satellites." Astron. Astrophys., 340: 309-314.

Maran, S.P. 1992. The Astronomy and astrophysics encyclopedia. New York: Van Nostrand Reinhold.

Mariotti, J.M. et al. 1996. "Interferometric connection of large ground-based telescopes." Astron. Astrophys. Suppl. Ser., 116: 381-393.

Maskelyne, Neville. 1760. "A proposal for discovering the annual parallax of Sirius." Philosophical Transactions (London): 889.

Michell, John. 1768. An inquiry into the probable parallax, and magnitude of the fixed stars from the quantity of light which they afford us, and the particular circumstances of their situation. London. (See also Philosophical Transactions, 1767: 234.)

Mignard, F. 1999. "GAIA: design and science." In Egret & Heck, 1999: 44-51.

Monet, D.G. et al. 1992. "U.S. Naval Observatory CCD parallaxes of faint stars. I. Program description and first results." Astron. J., 103: 638-664.

Morrison, L.V. & Gilmore, G.F. eds. 1994. Galactic and Solar System Optical Astrometry. Proceedings of the Royal Greenwich Observatory and the Institute of Astronomy Workshop, held in Cambridge, June 21-24, 1993. Cambridge: University Press.

Murray, C.A. 1988. "The distances to the stars." Observatory, 108: 199-217.

Newcomb, Simon. 1895. Popular astronomy. New York: Harper & Bros. [See "Appendix VII. Determinations of stellar parallax", pp. 548-550.]

Newton, Isaac. 1726. Principia. (Section, "Concerning the distance of the stars", from first American edition, 1848, translated by Andrew Motte in 1729, reprinted in Shapley & Howarth, 1929, pp. 86, 88. )

"Parallaxes of faint stars." 1971 Oct. Sky and Telescope, 42, No. 4: 212.

Paresce, F. 2001 Jun. "Scientific objectives of the LVT interferometer." The ESO Messenger, No. 104: 5-7.

Perryman, Michael A.C. et al. 1997. "The Hipparcos catalogue." Astron. Astrophys., 223: L49-L52.

Perryman, Richard. 1999 Jun. "Hipparcos: the stars in three dimensions." Sky & Telescope, 97: 40-50.

Pritchard, Charles. 1886 Jun 11. "On a remarkable instance of the detection of distortion in a photographic film, measured for the purpose of stellar parallax." Monthly Notices, Royal Astronomical Society, 46, No. 8: 442- 444.

________. 1887 Jan 14. "Note on the application of photography to the determination of stellar parallax." Monthly Notices, Royal Astronomical Society, 47, No. 3: 87-89.

________. 1887 Jun 10. "On the parallax of 61-1 and 61-2 Cygni, as obtained by the aid of photography." Monthly Notices, Royal Astronomical Society, 47, No. 8: 444-446.

________. 1889. Researches in stellar parallax by the aid of photography, from observations made at the Oxford University Observatory. Oxford: Clarendon Press.

Rambaut, Arthur A. 1890 Mar 14. "On the parallax of double stars." Monthly Notices, Royal Astronomical Society, 50, No. 5: 302-310.

Rowan-Robinson, Michael. 1985. The cosmological distance ladder. New York: W.H. Freeman and Company.

Russell, Henry Norris. 1911. Determination of stellar parallax. Washington: Carnegie Institution of Washington.

Russell, Henry Norris and Moore, Charlotte E. 1940. The masses of the stars, with a general catalogue of dynamical parallaxes. Chicago: The University of Chicago Press.

Schlesinger, Frank. 1924. General catalogue of stellar parallaxes. New Haven: Yale University Observatory. [First edition; second edition in 1935]

________. 1927. "Some aspects of astronomical photography of precision." Monthly Notices, Royal Astronomical Society, 87: 506-523. (Reprinted in Shapley, 1960, pp. 112-115.)

Shapley, Harlow. 1960. Source book in astronomy 1900-1950. Cambridge, MA: Harvard Univ. Press.

Shapley, Harlow & Howarth, Helen E. 1929. A source book in astronomy. New York: McGraw- Hill.

Sinnott, Roger W. 2001 Jul. "The best star catalog ever/The stage is set for FAME." Sky & Telescope, 102, No. 1: 22-23.

Smart, R.L.; Lattanzi, M.G. and Drimmel, R. 1997. "Spectroscopic parallaxes in determining the correction of relative to absolute parallax. Astrophys. Space Sci. Libr., 212: 195-199.

Smart, W.M. 1949. Text-book on spherical astronomy. Cambridge: University Press. (See sections 125-130 [re. stellar parallaxes, pp. 217-224]; 152, "Secular parallax" [pp. 262-263 and Appendix E, p. 424], 175, "The measurement of stellar parallaxes" [pp. 309-312]; and 194, "Dynamical parallaxes" [pp. 356-357].)

Smith, H., Jr. & Eichorn, Heinrich. 1996. "On the estimation of distances from trigonometric parallaxes." Monthly Notices, Royal Astronomical Society, 281: 211-218.

Stewart, Albert B. 1964 Mar. "The discovery of stellar aberration." Scientific American. 210: 100-108.

Stone, E.J. 1977 Mar 9. "On apparent brightness as an indication of distance in stellar masses." Monthly Notices, Royal Astronomical Society, 37, No. 5: 232-237.

Strand, Kaj Aage Gunnar. 1960. "Determination of stellar distances." Science, 144, No. 3624: 1299-1309.

________. 1964. "The new 61-inch astrometric reflector." Sky and Telescope, 27, No. 4: front cover, 204-209, 232- 233.

________. 1994. "An analysis of the accuracy of the parallaxes in the 1952 Yale General Catalogue." In Morrison & Gilmore, 1994, pp. 61-65.

Struve, Friedrich Georg Wilhelm. 1837. Stellarum duplicium et multiplicium mensurae micrometricae. St. Petersburg. (Section on "Concerning the parallax of the fixed stars" translated by J. Jackson and printed in The Observatory, 1922, then reprinted in Shapley and Howarth, 1929, pp. 208-211.)

Struve, Otto. 1956 Nov./Dec. "The first determination of stellar parallax." Sky and Telescope, 16: 9-12, 69-72.

Tinney, C.G. 1993. "The faintest stars: trigonometric CCD parallaxes." Astron. J., 105: 1169-1178.

Tinney, C.G.; Reid, I.N.; Gizis, J. & Moula, J.R. 1995 Dec. "Trigonometric parallaxes and the diagram at the bottom of the main sequence." Astron. J., 110, No. 6: 3014-3034.

Turon, Catherine. 1997 July. "From Hipparchus to Hipparcos." Sky & Telescope, 94: 28-34.

________. (ed.) 1998 "The first results of Hipparcos and Tycho." Highlights of Astronomy, 11A: 531-587.

Turon, Catherine & Perryman, Michael A.C. 1999. "Global space astrometry: impact on cosmic distance scale." In Egret & Heck, 1999, pp. 1-12.

Unwin, S.; Pitesky, J. & Shao. M. 1999. "Science with the Space Interferometry Mission." In Egret & Heck, 1999, pp. 38-43.

Upgren, Arthur R. 1994. "The solar neighborhood." In Morrison & Gilmore, 1994, pp. 165-173.

van Altena, William F. & Lee, John T. 1988. "The Yale parallax catalog." In Debarbat, S. et al. (eds.). 1988, pp. 269-.

van Altena, William F.; Lee, John T. & Hoffleit, E. Dorrit. 1994. "The new edition of the Yale parallax catalogue: a look at systematic errors." In Morrison & Gilmore, 1994, pp. 50-54.

van Altena, William F.; Lee, John T. & Hoffleit, E. Dorrit. 1995. The general catalogue of trigonometric parallaxes. 4th ed. New Haven: Yale University Observatory.

van Biesbroeck, G. 1951. "Visual binary stars and stellar parallaxes." In Hyneck, 1951, pp. 425- 427.

van de Camp, Peter. 1967. Principles of Astrometry. San Francisco & London: W.H. Freeman and Co.

________. 1978 Nov. "The distances of VV Cephi and Epsilon Aurigae." Sky and Telescope, 56, No. 5: 397-399.

van Helden, Albert. 1985. Measuring the Universe. Chicago: University of Chicago Press.

Webb, Stephen. 1999. Measuring the Universe. The cosmological distance ladder. Chichester: Praxis Publishing.

Wielen, R. et al. 1994. "A test of preliminary Hipparcos parallaxes using photometric parallaxes of distant stars." In Morrison & Gilmore, 1994, pp. 74-79.

Williams, M.E.W. 1979. "Flamsteed's alleged measurement of annual parallax for the Pole Star." Journal for the History of Astronomy, 10: 102-116.

Young, C.A. 1879. "The distances of the stars." English Mechanic, 29: 130.

SOME WEB PAGES:

(These were checked on August 9, 2001, so undoubtedly several addresses will be outdated by the time you read this.)

Ed. note: Links tested and updated September 21, 2001. All worked on that date except those marked (404).

AMBER Project

Astrometry Department, U.S. Naval Observatory

CHARA (Georgia Stae University's Center for High Angular Resolution Astronomy

COAST (Cambridge Optical Aperture Synthesis Telescope)

DIVA (Deutsches Interferometer fur Vielkanalphotometrie und Astrometrie)

FAME (Full-sky Astrometric Mappping Explorer)

FLUOR--Fiber Linked Unit for Optical Recombination

GAIA (ESA astrometric orbital observatory)

Grand Interferometre a 2 Telescopes (GI2T)

The GSFC Stellar Imager (SI) Homepage

Hipparcos Space Astrometry Mission

The Hyades--So Close and Now, So Familiar

InfraRed Space Interferometry Mission (DARWIN or IRSI)

Interferometric Astrometry with HST Fine Guidance Sensor 3: Parallaxes of the Distance Scale Calibrators\delta Cep and RR Lyr

IONIC (Integrated Optics Near-infrared Interferometric Camera)
and select activities -> High Angular Resolution -> IONIC

IOTA (Infrared Optical Telescope Array)

Keck Interferometer

MIDI (Mid-IR Interferometric instrument for VLTI)

Newcomb Spaceborne Optical Interferometer

NPOI (Navy Prototype Optical Interferometer)

OHANA (Optical Hawaiian Array for Nanoradian Astronomy)

Optical Long Baseline Interferometry News

OSIRIS (Russian Space Optical Interferometer for Astronomy) (404)

Palomar Testbed Interferometer (PTI)

Parallaxes by CCD's (404)

POINTS (Precision Optical INTerferometer in Space; now discontinued)

Scientific American Feature Article: A Sharper View of the Stars: March 2001

SIM (Space Interferometry Mission; prev. OSI)

StarLight (JPL formation flying space interferometer)

The Sydney University Interferometer (SUSI)

3D Universe: Background

TPF (Terrestrial Planet Finder)

Two Astrometric Projects:

MIRA (Mitake optical and Infra-Red interferometer Array) and LIGHT (Light Interferometer satellite for the studies of Galactic Halo Tracers)

The UC Berkeley Infrared Spatial Interferometer

The History of the Discovery of Elementary Particles

Perhaps we can set aside the sad events of September 11 and have an interesting history of science discussion about the fascinating field of elementary particles.

We are all familiar with atoms (which seem so “obvious” to us now) and their constituents: electrons, protons, and neutrons. But how did they figure that out? These infinitesimally small things are not visible to the human eye! I am continually amazed at the cleverness of human beings to be able to experiment and find indirect ways to deduce difficult, non-obvious truths about nature. (Dolphins may be intelligent, but lacking the proper appendages – hands – they can’t make the discoveries that humans can.)

I do not claim to be an expert on this large subject, but perhaps we can begin to explore some of the interesting history and historical figures in this field, and sort out some of the terms used. (We didn’t finish the history of rocketry in 2 sessions, so we probably won’t be able to dissect every tau neutrino and up quark in one.)

As we always do, we start with the obligatory look back to ancient times – to the Greeks and the first idea of atoms – the smallest indivisible pieces of matter. “Atom”, from the Greek “uncuttable”.

Fast-forward to after the birth of modern science (the idea of atoms survived all that time) and John Dalton (1766-1844) who explained the ratios of the weights of the chemical elements and their compounds in terms of the weights of their atoms.

And now we arrive at the late 1800s and early 1900s – the most exciting times in physics – an age of unparalleled discoveries on many fronts – including Einstein’s relativity, radioactivity and quantum mechanics. The story of subatomic particles is intimately tied up with the famous Cavendish Laboratory at Cambridge, England, where all the constituents of the atom were discovered. The first was the electron, discovered by J. J. Thomson (1856-1940) in 1897, using cathode ray tubes. He was experimenting with electric discharges in rarified gases – and his results led him to conclude that there is a particle – the electron – that is both the carrier of electricity (that pesky electricity that is the source of our blackout problems), and a basic constituent of the atom. The terms “negative” and “positive” which we still use were coined by none other than Ben Franklin (1706-1790)! The story of the electron involves simultaneous developments in electricity and magnetism by notables such as Michael Faraday (1791-1867) who studied electric fields, and James Clerk Maxwell (1831-1879) whose equations described electricity and magnetism, which were used to explain the nature of light (electromagnetic waves). Measuring the electric charge was done by Milliken and his oil drop experiment. The mass of the electron turns out to be very small.

Next, the very existence of atoms was proven by the renowned Sir Ernest Rutherford (1871-1937), a New Zealander, who succeeded Thomson as Cavendish Professor of Experimental Physics. He began his work on radioactivity; his discovery of the atomic nucleus, with the famous metal foil experiments (1906-09), and the conclusion, through large deflections, that the core of the atom is concentrated in a small dense mass, and that most of the atom is empty space. So one can think of the nuclei of atoms as consisting of heavy, positively-charged particles that Rutherford in 1920 named “protons”.

But there was a problem – the helium atom has a mass 4 times that of hydrogen, but only twice the electric charge. The correct answer was not found until 1932, with the discovery of the neutron, by James Chadwick (1891-1974), the last great discovery out of the Cavendish. It solved the problem of atomic weights – that they weren’t all protons in the nucleus, but another heavy, neutral particle. His method (a neutron chamber) was the beginning of the idea of the accelerator – to find out what’s in the nucleus, it was necessary to break it up and see what comes out. There was the first cloud chamber (C. Wilson’s) which makes the trails of ionizing particles visible.

More conclusions about the nucleus and particles were derived from the study of the new phenomena of radioactivity, and its release of alpha and beta particles, which turned out to be helium nuclei and electrons, respectively. An enormous amount of energy is tied up in the nucleus (remember E = mc2)!

Now the plethora of other particles begins. A little extra energy comes off: A particle besides the electron is emitted in beta decay; W. Pauli hypothesized a particle, the neutrino, “little neutral one” (nearly massless, chargeless particles that are produced when particles collide, and account for the extra mass, and are found to be very common in the universe).

Before the development of accelerators, the only way to study other new exotic, short-lived elementary particles (such as pions, muons, mesons) was through high energy cosmic rays, hitting atoms and molecules in Earth’s upper atmosphere. Huge circular accelerators (such as the Bevatron and the Cyclotron at Berkeley) and linear accelerators (SLAC at Stanford) which slam highly accelerated particles (either electrons or protons) into other particles, are used to study the tracks of the products of these collisions. Other facilities are Fermilab in Illinois (a 4-mile-diameter ring), Brookhaven in New York, and CERN in France and Switzerland. Particles too numerous to mention have been discovered in the past 20 years.

Anti-particles (“antimatter”) for each particle, such as the positron for the electron (first recorded track: 1933) and the antiproton (by Owen Chamberlain in 1955) have also been discovered.

Among the more interesting groups are the quarks (named by Nobel Prize winner Murray Gell-Mann, and his 8-fold way), which were proposed in 1960 by Gell-Mann and Ne’eman: that hadrons (particles such as protons and neutrons which are involved in the strong nuclear interaction) are all composites of “really” elementary particles called quarks. 3 quarks lie inside each proton and neutron.

And on and on we go – upon each elephant another elephant stands.

We will also try to sort out terms such as: hadrons, leptons, fermions, baryons, bosons, spin, parity, charge, cross section of the atom, the standard model, and particles such as mesons, pions, muons, neutrinos, tau quarks and gluons.

They have even been able to start to unglue the gluons from the quarks, and try to re-create the conditions at the Big Bang, before particles coalesced.! (Too bad about the demise of the Super-Collider!)

Bibliography There are many, many books on this subject. Here are just a few:

The Discovery of Subatomic Particles, by Steven Weinberg, W. H. Freeman, NY, 1990. By the Nobel Prize winner and author of The First Three Minutes. Gives a very good history of the discoveries at the Cavendish Lab, and the methods used to deduce them, and formulas, and the personalities involved. Many historical photographs.

The Particle Hunters, by Yuval Ne’eman and Yoram Kirsh, Cambridge University Press, 1996. A more recent book, goes into detail about the plethora of particles now known, and the use of accelerators. Ne’eman is a noted particle researcher himself.

From Quarks to the Cosmos: Tools of Discovery, by Leon Lederman and David Schramm, W. H. Freeman, NY (Scientific American Library), 1995. More for the general reader, but gives good introduction to key concepts. Profusely illustrated.

The Penguin Dictionary of Physics, 2nd edition, Valerie Illingworth, ed., 1991. Or any physics dictionary that will help you sort out the terms.

Other titles: The Particle Explosion, and, less authoritatively, The Dancing Wu Li Masters and The Tao of Physics.

The World Wide Web has lots of references, including an excellent poster of the Standard Model (and how to obtain it). Lawrence Berkeley National Lab above UC Berkeley publishes the Handbook of Particles.

Norm asks: Could someone bring nesting “matryoshka” dolls?