Double Star Astronomy Bruce MacEvoy May, 2012 http://www.handprint.com/ASTRO/PREZ/DoubleStars.pptx Note This is a stand alone (study) document in PowerPoint format.
Download ReportTranscript Double Star Astronomy Bruce MacEvoy May, 2012 http://www.handprint.com/ASTRO/PREZ/DoubleStars.pptx Note This is a stand alone (study) document in PowerPoint format.
Double Star Astronomy Bruce MacEvoy May, 2012 http://www.handprint.com/ASTRO/PREZ/DoubleStars.pptx Note This is a stand alone (study) document in PowerPoint format. It is intended to provide a brief and general introduction to double stars as a productive focus for amateur astronomy and as essential parts of star formation and modern astrophysics. Slides are generally self contained, and may be deleted as needed to make the deck suitable for presentation. New topic sections are indicated by slides with a green title background. Sources cited only by author last name and date (no title) can be retrieved from the SAO/NASA web site (http://www.adsabs.harvard.edu) using author and date as search terms This document is published without copyright restriction other than author citation to me. Comments, corrections welcomed: please email Bruce MacEvoy at [email protected] version of 07/14/12 Why Double Stars? Majestic displays of universal gravitation ... exotic thermodynamic objects ... a cornerstone of modern astrophysics ... fundamental elements of star formation ... and dynamical “fossils” of dissolved star clusters For the amateur astronomer ... Sheer beauty ... contrast, color, configuration, field, “vastness of space” Huge number and variety of targets, from “easy” to “very difficult” NGC+IC: ~13,200 deep sky objects WDS: ~92,600 double star systems All apertures useful ... 100mm to 250mm are commonly used Unlike planets and moon, bright double stars are robust to mediocre seeing Unlike nebulae & galaxies, most double stars are robust to light pollution Amateurs can contribute to scientific projects Photography/video is a useful measurement tool What’s In a Name? A star system is one, two or more stars bound by gravity as a physical unit; a double star is any combination of two or more bound stars ... a binary is exactly two components Names of nonvisual binaries refer to the basic data used to study them, to types of nonstellar binary components, or to the relationship of binary stars to their Roche lobes DOUBLE STAR optical [illusory] visual BINARY [nonvisual] subtypes BINARY photometric — overcontact MULTIPLE — semidetached — triple, quadruple, etc. — detached (close) — hierarchical — wide — trapezium speckle interferometric basic data visual, IR light curves [Roche lobe subtypes] spectroscopic spectrum Doppler shifts astrometric parallax, proper motion neutron, BH xray, radio emission diffraction images How Many Stars in a Double? Components Count Percent of Systems 2 (binary star) 82760 78.69% 3 (triple star) 12431 11.82% 4 (quadruple star) 4629 4.40% 5 1905 1.81% 6 854 0.81% 7 541 0.51% 8 380 0.36% 9 236 0.22% 1439 1.38% 105175 100.0% 10+ (star cluster) Total Systems Source: Washington Double Star Catalogue (2012) Binary Mass Ratios Rule of thumb: brighter main sequence stars are more massive stars, so stars of the same brightness at the same distance will (usually) have the same mass Measurement bias: close, faint companions are harder to detect when there is a large magnitude difference ... but equal mass binary stars appear to be common Magnitude Difference Nominal Mass Ratio* Count Percent 1.0 > q > 0.90 30470 36.9% 0.45 < Δv.mag. < 1.55 0.89 > q > 0.70 26369 31.9% 1.55 < Δv.mag. < 4.0 0.69 > q > 0.40 20333 24.6% Δv.mag. > 4.0 0.39 > q > 0.01 5485 6.6% 82657 100.0% 0 < Δv.mag. < 0.45 Total *Mass ratio is estimated from binary visual magnitudes as q = 10–(m2-m1/10), where m2 is the magnitude of the fainter star and assuming both stars are on or near the main sequence (luminosity types IV or V). Source: Washington Double Star Catalogue (2012) Scale of Binary Orbits Orbit Radius R☉/AU Distance a = 2” log(P) days Period days/years 0 1.02/0.003 1 8.2/0.027 2 91/0.274 108/0.50 0.25 12.7 circular (Venus R = 0.72 AU) 3 1021/2.74 2.5 1.3 19.7 inner (asteroids R = 2.8 AU) 4 22 10 5 5 250 50 25 6 2800 250 125 0.012 (Heliosphere R = ~120 AU) 7 22,000 1000 500 0.002 stable (widest solved orbits) 8 250,000 5000 2500 . wide (all CPM pairs) 9 2,800,000 25,000 12,500 5.4/0.025 (parsecs) 2500AU 22/0.10 10,000AU Percent of 6th Orbits Category Label 0.006 interacting 0.014 corotating (detached) 43.7 (Saturn R = 9.6 AU) 20.4 median (Kuiper Belt R = 50 AU) . fragile (widest known = ~54,000AU) *Assumes a binary system of two solar masses: M1 + M2 = 2M☉ and a3AU = 2P2yr ; values of period and radius rounded for simplicity. For constant orbital period, orbital distance increases as system total mass increases. Type 0: Interacting (1-10 days, 0.02-0.1 AU) period = 4.78 days orbit radius = 0.07 AU; eccentricity = 0.0016 (est. M = 2.0M☉; est. q = 0.37) semimajor axis = 0.0017” No visual double stars: all are photometric (eclipsing variable), interferometric or spectroscopic systems Tend to have similar mass, smaller total mass; circular orbits, synchronous rotation, tidal star distortion; common plasma envelope, entangled magnetic fields, starspots ... shortest period is ~0.2 days Roche lobe overflow in close pairs leads to asymmetric transfer of mass and momentum, accretion disks, eruptive image from variable stars and Type Ia supernovae 6th Orbital Catalog interferometric images of βLyrae (Sheliak), P =12.9d, MA+B = 12.6M☉ Roche Lobe Dynamics Applied by Kuiper (1941) and Kopal –––– inner critical surface (ICS) (1969) to the analysis of close binary stars An equipotential surface defines the contour of a constant gravitational force, including the centrifugal (orbital) effect as viewed in a corotating frame of reference A surface in hydrostatic equilibrium, such as a star’s photosphere, always conforms to an equipotential surface Binary inner critical surfaces join at the L1 point of gravity/momentum equilibrium If a late (giant) star expands beyond its ICS, equatorial mass flows into L1 occur, due to thermal pressure and Coriolis force Mass loss shrinks the “donor” star ICS, producing more mass transfer at L1 Roche lobe diagram of the semidetached binary TZ Eri: the lower mass star M2 has expanded beyond its ICS and is therefore transferring mass to star M1 If both stars exceed ICS, they form a common photosphere or join physically Interacting/Circularized Binaries W Ursae Majoris (P = 0.33d) RS Canum Venaticorum (P = 4.8d) A fairly common type of contact binary star (~400 known); identified by an “M” shaped light curve with a very short period; dynamical interactions within massive star clusters seem necessary to explain how stars form such close orbits Another common type of variable star that exhibits massive “starspots” in a cycle of ~4 years, likely caused by tangled lines of magnetic flux between the two stars and slow accretion via plasma outflows Interacting/Circularized Binaries βPersei (P = 2.9d) SS Cygni (P = 0.27d) The classic example of a semidetached binary star experiencing mass transfer via Roche lobe flows, visible as erratic variations in the light curve; the receiving star has become a younger “blue” spectral type B through the gain of mass Produced when one star in a binary explodes as a Type II supernova and becomes a massive white dwarf or neutron star; when its companion evolves into a giant star the dwarf draws mass onto an xray bursting accretion disk or flares as a Type Ia supernova Type 2: Short Period (0.27-2.7 years, 0.5-2.5 AU) period = 138.4 days orbit radius = 0.67 AU; eccentricity = 0.11 (est. M = 2.09M☉; est. q = 0.52) semimajor axis = 0.0032” Inner solar system distances; “close” because one or both stars can exchange mass at some point during their evolution Mostly spectroscopic (radial velocity) and astrometric (proper motion) systems Formed together (same rotational direction in orbital plane); orbits nearly circular although there is little tidal dampening; may have interacting magnetic and plasma fields 6th Sampling gap at P = ~10-30 years image from Orbital Catalog; (Type 3) due to limits of spectroscopic 1921 omi Dra spectroscopic detection and optical resolution radial velocity curve Type 4: Median Period (27-270 years, 10-50 AU) period = 186 years orbit radius = 40 AU; eccentricity = 0.53 (est. M = 1.85M☉; est. q = 0.54) semimajor axis = 1.91” Roughly half of all visual doubles, including many “showcase” doubles Outer solar system distances, but within radius of largest protostellar disks (~100 AU) Because P > 10 yrs., solar mass binaries evolve independently (do not exchange mass) At P > 100 years, diverging mass ratios, spectral/luminosity classes and orbital eccentricities (e > 0.5) indicate dynamic interactions with other star systems image from 6th Orbital Catalog Solved orbits include many stars well below naked eye magnitude (v.mag. > 8.0) Type 6: Long Period (2700-27,000 years, 250-1100 AU) period = 3,250 years orbit radius = 277 AU; eccentricity = 0.88 (est. M = 2.01M☉, est. q = 0.86) semimajor axis = 10.2” Outer limit of astrometric/visual double star population; includes many common proper motion/parallax pairs Poor quality orbital solutions due to limited measurement of orbital segment Beyond the probable radius limit of protostellar disks (~100AU): random pairing of rotation, spectral and luminosity classes implies origin in cloud fragmentation or as remnant of multiple system disrupted in a star cluster image from 6th Orbital Catalog One or both stars massive (> 2M☉) Type 8: Fragile (274,000-2.7 million years, 5,000-25,000 AU) Type 8 – Albireo (β Cyg) est. period = 140,000 years est. orbit radius = 5450 AU; eccentricity = ? (M = 8.2M☉; est. q = 0.64) semimajor axis = 34.6” Orbital scale about 1/100th to 1/10th of average interstellar distance (~1 parsec) No orbital solutions: identified by similar astrometric or photometric parallax, or common proper motion (CPM) Wide range of mass ratios; one or both stars are often massive and/or binary stars About 15% of v.mag. 9-12 field stars around the NGP are fragile doubles at separations up to ~0.15 parsec Most likely formed when released in parallel from dissolving parent star cluster or by tidal forces from a third component Bahcall & Soneira (1981) sample area at the NGP Multiple Stars Are Hierarchical Most multiple stars have “multi- level” rather than “solar system” orbits Ratio of periods between levels is very large, roughly 1 : 20,000 to 30,000 Corresponding ratio of semimajor axes between levels is 1 : 10 to 2000 Multiple systems consist of subunits: two or more “hard” binaries, or a close binary and a distant single star Observed “trapezium” systems are young and do not endure Close binaries (spectroscopic or interferometric) are the base elements of the multiple star orbit hierarchy Nomenclature: the largest visual (primary) star is labeled A; companions B,C, and so on; nonvisual pairs are indicated by lower case letters (Aa,Ab) or numbers (Ba1,Ba2) hierarchy diagrams from Raghavan, McAlister et al. (2010) Common Visual Configurations Double stars appear in a wide variety of combinations, but some of these are especially striking or memorable due to a unique arrangement, close separation, vivid brightness and/or color contrasts, or many surrounding field stars Law of Universal Attraction “I offer this work as the mathematical principles of philosophy, for the whole burden of philosophy seem to consist in this — from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena.” (Preface, 1687) Kepler’s Three “Laws” Derived from measurements of Mars made c.1590 by Tycho Brahe; the “laws” culminate in Kepler’s Epitome of Copernican Astronomy, Books IV-V (1621) 1. The orbit of every planet is an ellipse with the Sun at one of the two foci (ƒ1, ƒ2) 2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time (area a1 = area a2) 3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit (p2 ∝ r3) Not really scientific physical laws, but empirical generalizations: force and mass are missing Newton’s Mechanics The Principia laid down “axioms or laws of motion” derived from Galileo: If no external force is applied, a body moves at a constant speed in a straight line (Law I) — this is its momentum Law I reveals the accelerating force required to deflect a moving body into a curved path (moving from P to Q instead of P to R) Newton proved that a circular or elliptical orbit requires “a centripetal force that varies inversely as the square of the distances” ... ... and that bodies with net positive kinetic energy must follow a parabolic or hyperbolic “escape orbit” around the center of mass Newton equated the amount of attractive force (G) to the sum of the two masses (m1+m2), and developed methods to calculate an orbit from changes in celestial position Herschel’s Search for Parallax Newton’s laws of motion described the Earth’s moon, the planets, comets, and the moons of Jupiter and Saturn — but double stars were considered to be optical binaries The earliest (17th & 18th century) double star discoveries were mere oddities, found during transit measurements or observations of comets and nebulae Christian Mayer (1779) catalogued about 70 “sidereal comites” [stellar companions] discovered with his 2.5 inch ƒ/39 quadrant telescope, but Bode published them as a basis to measure stellar proper motions (discovered by Halley c.1717) William Herschel originally wanted to calculate the distance to the stars (On the Parallax of Fixed Stars, 1781), using a double star parallax method proposed by Galileo His systematic double star survey (1779 to 1784 — the first ever) was made with a 7 inch ƒ/14 Newtonian reflector and catalogued over 700 double star systems John Michell (1783): “If we apply the doctrine of chances” then “by far the greatest part” of Herschel’s discovered stars must be gravitationally bound At first skeptical of this conjecture, Herschel affirmed it when he described “binary sidereal systems” in the “Remarks on the Construction of the Heavens” appended to his Catalogue of 500 new Nebulae ... (1802) Herschel’s Binaries “I shall therefore now proceed to give an account of a series of observations on double stars, comprehending a period of about 25 years, which, if I am not mistaken, will go to prove, that many of them are not merely double in appearance, but must be allowed to be real binary combinations of two stars, intimately held together by the bond of mutual attraction.” – William Herschel, “Account of Changes that have Happened...” (1803) Herschel’s Stars Observed Change Herschel’s Estimate of Period (years) Current Estimate of Period (years) α Gem (Castor) PA 342 445 γ Leo (Algieba) PA, Sep 1200 510 ε Boo (Izar) PA, Sep 1681 [? several thousand] ζ Her Sep . 34 δ Ser PA 375 1038 γ Vir (Porrima) PA, Sep 708 169 Early Measurement Catalogs Herschel’s announcement that stars behaved according to the laws of attraction made cataloguing the locations and measuring the motions of double stars a new focus of astronomical research. Observer Active Systems in WDS WDS Catalog Code [Old Catalog Symbol] Willam Herschel c.1790-1815 139 [805]* H + class number John Herschel James South c.1820-1840 c.1820 4720 168 HJ [h] S, SHJ [Sh] Friedrich Wilhelm Struve c.1830-1850 2824 STF, STFA, STFB [Σ] Otto Wilhelm Struve c.1840-1860 609 STT, STTA [ΟΣ] Sherburne Burnham c.1870-1900 1445 BU, BUP [β] Rev. T.E. Espin c.1900-1920 2545 ES Robert Jonckheere c.1910-1915 2834 J Robert Grant Aitken W.J. Hussey c.1900-1930 3019 1570 A [ADS] *Actual number of discoveries. See “Herschel Double Star Catalogs Restored” Measuring Binary Orbits Position angle (PA, θ) is measured in counterclockwise degrees from the line to celestial north Separation (Sep., ρ) is angular width in arcseconds (= 1/3600º ... the visual width of a golf ball at 5½ miles) Visual magnitudes (mA, mB and Δm) have been estimated by eye, measured photometrically, or both Several measures are taken across different nights, then averaged “Aitken’s Rule” is sometimes misapplied to identify optical doubles: optical ... if log(ρ”) > 2.8–0.2mA (e.g., if >40” when mA = v.mag. 6.0) Every catalog has magnitude and separation limits on included stars Orbital Elements There are currently about 2100 orbital solutions in the 6th Catalogue of Orbits The apparent orbit is the image on the sky of the relative orbit (relative to the more massive star), which is usually tilted (foreshortened) to our direction of view. orbits of iota Leonis (Type 4) Both orbits can be reconstructed by means of seven orbital elements: Dynamical elements (of relative orbit): Semimajor axis (a, in arcseconds) Eccentricity (e) Period (P, in days or years) Time of Periastron Passage (T) Campbell’s elements (of apparent orbit): Position Angle of Node (Ωº) Argument of Periastron (ωº) Inclination (iº) “Solar Type” Orbital Elements Period (P) median = 250 years range = 10–3.2 to ~106 yrs Orbital Radius (a) median = 40 AU range = 0.05 to >14,000 AU ... average solar type binary mass is 1.5M☉ Eccentricity (e) median e = ~0.0 if P <~10d median e = ~0.5 if P > 100d “forbidden” – short P, high e Mass Ratio (q = M2/M1), M2 < M1 flat distribution to q = 0.2, but maxima at q = 1.0 and q = ~0.4 ... q distributions vary, depending on type of study, simulation or spectral type of primary Campbell elements, orientation of rotational or orbital axes: random All data from Raghavan et al. (2010) Center of Mass Newton showed that an absolute orbit is the motion of both stars around a mutual center of mass (barycenter) as observed from an external frame of reference The center of mass occupies one focus within each orbit, and is the “pivot point” for the line of mutual gravitational attraction Orbital radii and velocities are proportional to the system mass ratio; orbital eccentricities are equal and foci lie on a common semimajor axis 50 yr. proper motion cycle of Sirius discovered by Friedrich Bessel, 1844 From Earth, orbital velocities can be measured as the Doppler shift of strong absorption lines in a star’s spectrum The center of mass follows a straight line trajectory through space, creating an astrometric “wobble” in the proper motion of the brighter star (e.g., Sirius) The Mass Luminosity Relation The Sun is the only star actually to have been weighed. But eclipsing binary light curves and spectroscopic Doppler shifts have been used to estimate stellar mass and luminosity at great distances among hundreds of binary systems, unlocking the relationships that underlie astrophysics and explain stellar evolution Summary of the Key Equations Quantity Formula/Value Gravitational constant (G) 6.678 x 10–11 kg–1 / m3 / sec–2 Period (P) sqrt[4π2a3/G(M1+M2)] in Solar standard units Pyears = sqrt[a 3AU / M☉] System Mass (M = M1+M2) Source Data (Cavendish, 1798) solved orbits 4π2a3/GP2 in Solar standard units M☉ = a 3AU /P 2years (Kepler’s 3rd Law) Separation (from orbital velocity v) aAU = v∙P/2π Doppler shifted spectra Mass Ratio (q) M2/M1 = v1/v2 = aCM1/aCM2† mass/velocity ratio Stefan-Boltzmann constant (σ) 5.67 x 10–8 W m–2 K–4 Star Radius (R) sqrt[L/4πσ∙1/T2] in Solar standard units L = L☉(R/R☉)2 ∙ (T/T☉)4 Luminosity (L) (R ∝ M)* 4πR2σT4 in Solar standard units L = L☉(M/M☉)3.5 Surface Temperature (T) (Kelvins) photometric curves (2.9nm∙106 /λmax)K (Wien’s Law) (L ∝ M4)* (T ∝ M½)* *Approximation valid for FGK main sequence stars. †Ratio of mean orbital radii from common center of mass Blackbody Temperature Is Color A blackbody curve (Planck, 1900)describes the electromagnetic flux from a “black” object at a specific temperature (energy); Wien’s Law yields the peak energy wavelength Luminosity increases as the 4th power of temperature and the 2nd power of radius (surface area) The correlated color temperature (CCT or rK) is the temperature (K) of a blackbody profile that best matches an actual flux profile A color index is the difference in a star’s apparent magnitude as measured with 2 different filters Johnson System: U (UV, 365nm), B (blue, 440nm) or V (green, 550nm) — e.g., B-V color index The Hertzsprung Russell Diagram A graphical display of temperature vs. luminosity proposed c.1910; can also be plotted as a color vs. magnitude diagram: →Temperature is color Surface temperature T → B-V color index →Luminosity is magnitude Luminosity → absolute magnitude Mass/radius was identified in 1923 and proved in 1926 as the origin of the temperature/luminosity relationship Luminosity Classes I-III are giant (late evolved) stars; Class V is the dwarf main sequence (most stars, including the Sun) Summarizes the evolutionary distribution and aggregate frequencies of spectral and luminosity types produced by the galactic initial mass function (IMF) Source: Hipparcos Yale Gliese catalogs Stellar Fundamentals “Mass is destiny” Mass (M☉) Type B-V Index Temp. (K) Radius (R☉) Luminosity (L☉) Lifetime (years) ~150 to 18 O –0.45 >30,000 >9.3 ~106 to 53,000 O5 = 3.6x105 18 to 2.9 B –0.17 30,000 to 9500 9.3 to 2.5 52,500 to 54 B5 = 7.2x107 2.9 to 1.6 A 0.16 9500 to 7200 2.5 to 1.4 54 to 6.5 A5 = 1.1x109 1.6 to 1.05 F 0.45 7200 to 6030 1.4 to 1.05 6.5 to 1.5 F5 = 3.5x109 1.05 to 0.8 G 0.70 6030 to 5250 1.05 to 0.85 1.5 to 0.4 G5 = 1.5x1010 0.8 to 0.5 K 1.11 5250 to 3850 0.85 to 0.63 0.4 to 0.08 K5 = 5.3x1010 0.5 to 0.06 M,S,C 1.61 3850 to 2640 0.63 to 0.13 0.08 to 0.001 M5 = 1.9x1011 ~1.0 to 0.01 w . 100,000 to . ~0.01 ~0.001 n.a. Source: Hester, Smith, Blumenthal et al., 21st Century Astronomy (2010) How Many Stars Are Double? Kuiper (1942) Heintz (1969) Abt & Levy (1976)* 274 n.a. 123 Stars as Singles 70% 30% 45% Binary 25% 47% 3 4% 4+ Systems (N) All Double Star Systems Nordström et al. (2004) Raghavan et al. (2010) 164 16682 454 57% 66% 56% 46% 38% 34% 33% 16% 8% 4% . 8% 1% 7% 1% 1% . 3% 30% 70% 55% 43% 34% 44% 50 Median R Stars in Doubles Duquen -noy & Mayor (1991) 35 AU AU 52% *As revised by Abt (1978, 1983) 85% 40 73% 62% AU 51% 65% Roughly 60% of all local, “solar type” (~F5 to ~K5), main sequence stars are in double star systems ... The Heintz (1969) 85% estimate is a clear outlier ... but roughly 60% of all star systems are single stars Recent surveys of the solar vicinity (< 25 pc) have exhaustively looked for close, faint companions, brown dwarfs and CPM binaries Solar vicinity is unusual: Sun is between Galaxy spiral arms and ~120 pc from the nearest star forming region Are All Spectral Types Double? O B A F G K M C Total 0.23 8.9 16.0 22.0 19.6 27.6 5.0 0.14 Multiple 0.55 10.3 21.6 26.8 21.1 16.2 3.9 0.05 Ratio M/T 2.45 1.15 1.34 1.22 1.08 0.59 0.59 0.37 Sources: (Total) Hipparcos/Yale/Gliese Catalog; (Multiple) Washington Double Star Catalog As the mass of a primary star increases, there is a higher binary frequency and mass biasing (trend to more massive components): 75% of OB stars in or near galactic clusters and 60% of OB field stars are O+OB binary or multiple stars; only 40% of “runaway” OB field stars have companions Only about 20% to 30% of K and M stars have companions ... the 50% inflection is around spectral type G0 — stars like the Sun Accretion limit at ~10M☉ and high binary fraction imply unique OB evolution by merger (collision) events or competitive accretion near center of dense star clusters Brown dwarf desert — Less than 1% of solar type binary stars include brown dwarf companions, indicating exclusion during the star formation process or frequent ejection by dynamic interactions in natal star clusters How Do Double Stars Form? The overall binary frequency, mass biasing and the higher frequency of double star systems in protoclusters, indicate that double stars form when the individual stars form both gravitational capture of one single star by another and rotational fission of a rapidly spinning single protostar are inefficient or implausible paths to binary formation The separation, period, eccentricity, mass ratio, rotational speeds and orientation of rotational axes are “fossils” of both star formation processes and dynamical encounters Star formation theory describes a complex process in which critical transitions are controversial, implausible or unknown, because of: observational data that can only yield “snapshot” rather than dynamic information or confirmation; thick gas/dust clouds that hide critical star formation processes from view difficulty of writing numerical simulations (software) that can model the full interaction of gravitational, turbulent, thermal, magnetic, angular momentum, opacity and fusion effects, and the chaotic, random and self similar nature of the processes involved insufficient computer resources (computation time, resolution) needed to run software that can model physical scales of 106 years, 1014 km, 107 K and 1020 cm–3 number densities The stellar initial mass function (IMF), binary frequencies by spectral type, and binary orbital characteristics are three critical constraints on star formation theory Three Stages of Star Formation In outline, star formation occurs across three distinct stages: 1. Spiral arm shock waves and supersonic ——— H II region turbulence fragment and compress cold, giant molecular clouds (GMCs) into filaments, which contract gravitationally into dense cloud cores 2. At critical density, cloud cores collapse into binary protostars; these burn deuterium, accrete mass and dissipate angular momentum inside an accretion disk and dust cloud “cocoon”, forming hot, luminous protostellar objects (PSOs) 3. PSOs contract to fusion temperatures (107 K), end accretion, slowly disperse their dust clouds and accretion disks and enter the main sequence The entire process takes <107 years GMC ——— These three stages are specific problem domains currently studied with numerical simulations and infrared/radio telescopes spiral arms of M51 (UMa) Life Cycle of a GMC Source: E.E. Barnard (1913) The Galaxy consists of ~4% (~109 M☉) interstellar medium (ISM) made of atomic hydrogen, molecular hydrogen, helium and “dust” (grains of ice and graphite/silicates) ISM revolves at different speeds than galaxy spiral density waves; as a result, the spiral waves sweep up the ISM into cold (10K) giant molecular clouds (GMCs, ~104-106 M☉, radius 50-150 pc) Turbulently compressed and gravitationally collapsing cloud cores (102-103 M☉, radius 0.5-1 pc) transform a small fraction (<5%) of the total GMC mass into stars, almost all within star clusters Heated to 104 K and ionized by the UV radiation from young OB stars, GMCs light up as hydrogen emission nebulae or H II regions Remnant gas & H II regions are dispersed by photoevaporation and supernova shock waves within ~3x107 years, leaving a naked star cluster Numerical Simulation of Spiral Shock Waves Source: Kim & Ostriker (2006); Shetty & Ostriker (2006) ––—– Orion, Taurus & Perseus Star Forming Regions Orion Complex (450 pc) ––—–––– –––– Perseus Complex (350 pc) Taurus Complex (140 pc) This microwave (CO emission at 115 GHz) panorama stretches from Cassiopeia to Canis Major, and shows the nearby (visually larger) Taurus complex and the farther, denser Perseus and Orion complexes, three active (and intensively studied) areas of star formation. Source: Dame, Hartmann & Thaddeus (2000) Turbulence, Supernovae and HII Regions 15º 12º 0º M17 nebula (Sagittarius) Source: Spitzer Space Telescope “Prompt” Fragmentation During Cloud Collapse Simulations indicate that star formation in a collapsing dense cloud is particularly sensitive to the cloud gas density distribution (α) and angular momentum (β). Cold filamentary clouds — decoupled from magnetic support, lacking very dense central cores but having some differential rotation — readily produce binary stars Scale: Image width = 80,000 AU (0.4 pc). Source: Bate, Bonnell & Bromm (2002) Cloud Collapse Forms Protostars Above the Bonnor-Ebert density, gravity overwhelms thermal/magnetic support: a fragmentation and collapse of core occurs density/radius[← G] vs. [K →]← cloud pressure Isothermal free fall collapse creates a disklike, rotating protostellar object (PSO), with a radius of ~1-5 AU and mass ~10–3M☉ “Late” infalling gas forms a rotating accretion disk up to R =~100 AU around PSO Accretion disks near ~1M☉ form spiral arm shock waves; these create binary/multiple protostellar objects by disk fragmentation At T > 2000 K, H2 is ionized and gas becomes opaque, creating a “photosphere”: a second core collapse to ~0.3 AU occurs Deuterium fusion raises core to 106 K and photosphere luminosity from 5 L☉ to ~1000 L☉ as accretion from disk continues Protostars Form Accretion Disks Protostellar accretion disks are the second focus of numerical simulations; here turbulence, grativation, mass, density, angular momentum, temperature and magnetic flux combine to influence star formation. Disks near solar mass fragment into binary star systems, which evolve through accretion episodes, energetic cold mass outflows, gravitiational and magnetic torques, and dynamic encounters with other protostars and their disks. (Left) Proplyds in Orion Nebula; (above) artist’s concept and simulated disks No Fission Occurs In a Contracting PSO Computer simulations indicate that the increasing rotational speeds produced by accretion and contraction elongate a “liquid” protostellar object (PSO) into a oblong “bar”; however the ends of the bar develop spiral arms that release large amounts of angular momentum by shedding a relatively small amount of mass, braking the rotation and allowing further accretion and contraction to occur (Source: Bonnell, 1994) Accretion Disk Fragmentation Is Common ... Fragmentation of the protostar accretion disk is believed to be a frequent if not the most common path to binary formation at distances of around 40 AU (Type 4) ... a massive spiral arm forces the protostar off the center of mass to produce a binary structure; the spiral arms draw more mass into the accretion disk while reducing the binary orbital momentum via gravitational (and possibly magnetic) torque (Source: Bonnell & Bate, 1994 [a]) ... And Disks Can Form Multiple Systems Greater cloud core turbulence, density, kinetic energy and metallicity content can increase the fragmentation of massive and/or rapidly accreting protostar disks, which can produce a variety of multiple star systems (Source: Bonnell & Bate, 1994 [b]) Accretion Increases the Binary Mass Ratio Numerical simulations suggest that protobinaries which form at relatively close separations (a < 5 AU, P < 3000 days) or near solar masses are more likely to form circumbinary accretion disks. Stars with small companions (q < 0.3) evolve by accretion toward more equal masses (larger mass ratio) and closer separations, and brown dwarf companions of ~1M☉ stars are rare (the “brown dwarf desert”). In this simulation the collapsing cloud cores have a uniform rotation (longer period at larger radius) and a density that is either uniform or decreases with radius (but more gradually than ρ = 1/r). Comparable rotations and densities are commonly observed in dense cloud cores. (Source: Bate, 2000) Binary Formation Likely a Hierarchical Process Simulations that rely on single process models (random draws from an IMF, or biasing by cloud fragments) do not well describe binary frequencies, velocity distributions and mass ratios across all spectral types. A better fit appears in models that assume a “two-step” role for (1) the mass distribution of fragmenting “clumps” and (2) the mass distribution of single stars produced by a single clump. In addition, dynamical interactions between protostars may both create brown dwarfs and eject them from evolving multiple systems. (Sources: Sterzik & Durisen, 2001 [brackets summarize observed binary frequencies]; and Reipurth & Clarke, 2001.) Simulations Reveal Chaotic Formation Pathways Protostar formation is turbulent and chaotic, therefore difficult to model or predict. (Note the resemblances to merging spiral galaxies.) Source: Matsumoto & Hanawa (2003) PSOs Contract Into Stars (right) infrared image of triple YSO T Tauri; (below) infrared images of energetic mass outflows from XZ Tauri, a binary HerbigHaro object Protostar sheds ~99% of GMC angular momentum via disk gravitational/magnetic torque, binary orbital energy, and dynamic interactions with other protostars Energetic mass outflows at ~100 km/s (Herbig Haro objects) relieve excess accretion and inject turbulence into the contracting cloud, stimulating bursts of star formation Surveys show >80% of mass outflows and polar jets occur in binary protostellar systems Protostar gains >90% of its mass through irregular accretion events, and contracts into a young stellar object (YSO) or T Tauri star After ~106 years of accretion, YSO begins hydrogen fusion, may form planets from remnant disk, evaporates gas/dust envelope and moves onto the main sequence Roughly one in four protostars never reach the hydrogen burning limit (HBL, ~107 K) and instead become brown dwarfs or “failed stars” Turbulent Spectra of Young Stellar Objects ~3 x105 years ~5 x105 years ~106 years (left) As a protostar accretes mass and depletes its accretion disk, infrared radiation gradually declines onto the blackbody curve of a hydrogen burning star. (top) Spectra of T Tauri stars show strong emission lines from accretion and significant lithium absorption lines that indicate very young age (lithum burns above ~2.5 x 106 K). (bottom) The VY Tauri visual light curve shows enormous and erratic fluctuations, likely caused by accretion events comparable to merger with Jupiter sized masses. (Sources: Lada 2004; Basari, 2007; Herbig 1977) Protobinaries Accrete From Asymmetric Clouds Spitzer Space Telescope (SST) imaging at 8000 nm demonstrates that protobinaries evolve in accretion clouds that are typically asymmetrical and rarely spherical. These clouds are often connected to larger filamentary or cloud core structures that are more likely to produce rapid and episodic rather than slow and continuous accretion. (Source: Tobin, Hartmann & alia, 2010 and 2011) Visual vs. Infrared Imaging of M42 Protocluster Advances in infrared and radio astronomy permit an exhaustive survey of the Trapezium protocluster and study of early star formation hidden behind dense gas clouds. (Source: Lada & Lada 2003) NASA’s Hubble Space Telescope in visual (RGB = 672 [SII], 656 [Hα] & 508 [OIII] nm) ESO’s Very Large Telescope in JHK infrared (RGB = 2200 [K], 1650 [H] & 1250 [J] nm) HR Diagrams Reveal Bursts of Star Formation line of zero age main sequence (ZAMS) — ————— (right) In the 10 million year old Scorpius association, star formation peaked 2 m.y.a. and has ended (Sco OB is free of molecular hydrogen); massive stars have already reached the line of zero age main sequence (ZAMS) and the latest group of protostars is within a narrow isochrone band (equal age boundary, dashed line) indicating a recent burst of star formation. (left) The Perseus complex (IC 348) has been forming stars for ~10 m.y. and is still mixed with natal gas; it has also produced a dense group of YSOs within the past 1 million years, including very small objects (at far right). (Source: Palla & Stahler, 2000) Protocluster Initial Mass Function The initial mass function (IMF) is the distribution of stars and brown dwarfs being created in the Galaxy at any time IMF of embedded Trapezium Cluster Distribution first defined by Salpeter (1955) using bright, mixed age field stars (excluding brown dwarfs) Protoclusters (embedded clusters) are believed to best reveal the primordial IMF All stars are the same young age and brown dwarf luminance is at its peak — as bright as a Type B star, but in infrared Most stars form at masses of spectral types late G, K or M — smaller than the solar mass (1M☉) but above the hydrogen burning limit (HBL = ~0.07M☉, 107 K) The secondary peak at ~0.016M☉ is just above the deuterium burning limit (DBL =~13MJ = 0.013M☉; 5 x 105 K) Source: Lada & Lada (2003) Five Paths to Binary Formation Most if not all stars form as double systems, and most if not all double systems form as members of a star cluster Cloud fragmentation (a > 100 AU): Binaries form “in place” during the turbulent collapse of a massive, dense cloud core Disk fragmentation (a < 100 AU): Binaries form within protostar disks where they modulate angular momentum, accretion rates and mass ratio Competitve Accretion & Mass Segregation: Forced accretion of gas and dust in the gravitational well of a dense cluster core are a plausible origin for the largest (OB) stars and their high binary frequency Dynamical interaction: Multiple systems in a star cluster eject brown dwarf components, capture higher mass stars and “harden” binary orbits Parallel Dispersion (a > 1000 AU): “Soft” or “fragile” binaries bind when released in parallel trajectories from dissolving natal star cluster Star Clusters Dissolve Quickly As new stars form, cold mass outflows and OB radiation disperse ~95% of the GMC mass; loss of mass unbinds the protocluster into field stars and expanding OB stellar associations Supernova explosions in the most massive O stars also disperse mass and create “runaway” (high proper motion) single stars The process repeats as radiation and turbulence from a new cluster collapses nearby cloud fragments within an extended GMC About 90% of star clusters disperse within a few 107 years — very few clusters last longer than 108 years Numerical simulations suggest that galactic binary frequencies, dynamical elements, mass ratios and the stellar mass distribution (IMF) are equivalent to stars formed in star clusters of ~200 binaries in a 0.8 pc half mass radius Binaries Remain in Galactic Clusters Many galactic clusters display a variety of binary systems, especially near the cluster center. Look for mass segregation (bright and/or red giant stars and binary stars near cluster center, e.g. M47); also look for close, “matched” binary systems (highlighted in purple below) Observing Double Stars We never see the stars themselves, only a diffraction artifact created by the wave nature of light The diffraction artifact is formed of two components — the Airy disk and the diffraction rings. The rings are more easily deformed or erased by atmospheric turbulence A large magnitude difference between two stars makes identification of the faint companion more difficult Star color is more elusive than it seems — but it can be used to calculate binary distance and separation (“orbital type”) Three eyepiece focal lengths — equivalent to 0.33, 1.0 and 2.0+ times the relative aperture (ƒ ratio) — are needed for different tasks Telescopes of modest aperture (150mm to 250mm) are entirely suitable and even optimal; larger apertures suffer more from atmospheric turbulence and glare Inexpensive measurement tools are available Look twice! — there is often more to discover than you expect Star Diffraction Artifact Down to a limiting magnitude, under good seeing, a bright star appears artifactually as a tiny disk and rings of light. The diameter of the disk and the spacing of the rings are determined by the aperture (D). Observing the Airy Disk At focus, Airy disk angular diameter (in radians) depends on reciprocal aperture (1/D); Airy disk image diameter (in millimeters) depends on relative aperture (N = ƒ/D): A” = ~2.44λmm/Dmm∙ 206265 vs. Amm = ~2.44λmm∙ N To observe the Airy disk clearly, use an aperture mask to make aperture smaller and relative aperture larger; 19th century preference was for a stop ~1/5 of full aperture Notice that seeing (air turbulence) has a noticeably subdued effect on the Airy disk Visually, the diameter of the Airy disk depends on the star magnitude: it is smaller in fainter stars, and disappears entirely in stars too faint to be seen with the fovea of the eye Aperture mask for a Schmidt Cassegrain, with cutout for secondary support Observing the Resolution Limit To visualize the resolution limit, use an aperture mask with opposing circular cutouts A bright star image will be smeared into a series of light and dark bands, susceptible to atmospheric turbulence (seeing), and separated by the Rayleigh Diffraction Limit: R” = 1.22λmm/Dmm∙206265 (more simply, R” = 140/Dmm at λ= 555nm) Visual resolution is 70”<V”<140”, so minimum magnification is found as the ratio 70”/R” to 140”/R” Source: Bob Argyle (ed.), “Observing and Measuring Double Stars” (2004) Three Powers of Magnification Three levels of magnification are routinely useful in visual astronomy: Wide Field: Magnification <0.5Dmm (ƒe = >2N; 70º or more AFOV) Standard: Magnification 1.0Dmm (ƒe = N; 70º to 60º AFOV) Nutcracker: Magnification >2Dmm (ƒe = <0.5N; 60º or less AFOV) Swap eyepieces often to examine a star’s environs and to look for faint companions Limiting magnitude increases with magnification: faint stars are easier to see Unlike lunar/planetary astronomy, higher magnification can often improve detection and resolution in poor seeing The target may not be what you’re expecting! ... take time to look over and into the star field ... and indulge in some leisurely wide field wandering STF 541 (Taurus) ... Perhaps the only “double double” with one binary pair inside the other! Resolution Beyond the Limit Astronomers use standard visual criteria and descriptive labels to report the appearance of a close, equal magnitude binary star: Separate – a dark gap is clearly visible between the two stars (the stars are “resolved”) Contact – the star is clearly two disks, but a gap between them is not visible (Rayleigh Criterion) Notched – the star appears as an elongated bar with distinct notch (Dawes Criterion, 116/Dmm) Elongated – the star appears prolate or “rodlike” (below Sparrow Criterion, 109/Dmm) Most apertures can identify doubles well below the nominal resolution (e.g., ~60% of the Rayleigh limit) with very high magnification Most visual astronomers report that a double star is recognizable on first inspection; in fact, the diffraction gap is often just detectable at magnifications near the lower resolution limit of the eye (M = ~0.5Dmm). Simple test: visually estimate the star ρ and θ (near meridian helps), then check these in WDS; a match within ±15º of PA is confirmation that you have correctly identified the pair Detection & Magnitude Contrast A magnitude difference of Δv.mag. > 2.0 between two closely spaced stars makes a faint companion difficult to detect ... the flux ratio also limits speckle interferometry A larger central obstruction ratio (η = d/D) pushes obscuring light into the diffraction rings; too much magnification or poor seeing will smear away faint stars Bruce’s Rule of Thumb: theoretical resolution is possible when (1) Δv.mag. < ~1.0; beyond that glare can wash out faint stars, so limit is roughly Δv.mag. < rho/R” Glare has a greater obscuring effect as aperture or star brightness increase Averted looking, tapping the scope, star drift or masking the brighter star with the eyepiece field stop can make a faint companion easier to see “Calculators” or complex formulas are poor predictors of visual limits or optical capabilities — target difficulty, optical equipment, observing conditions and observer skill are important! Source: P.J. Treanor, “On the Telescopic Resolution of Unequal Binaries” (1946) Evaluate Atmospheric Turbulence A Simple Seeing Scale Observe a single “white” mag. 3-5 star at least 45º above the horizon with at least 2.0Dmm magnification F – Star image is enlarged by “boiling” speckles and no Airy disk is visible ... go read a book D – Airy disk is recognizable but sometimes obscured, surrounded by flashing speckles C – Airy disk and first gap are round, crisp and continuously visible; diffraction ring(s) are continuously broken into short or long arcs B – First diffraction ring is continuous but in constant motion and occasionally broken A – Airy disk and diffraction rings are distinct, unbroken and occasionally motionless Seeing is the distortion of an optical image by atmospheric thermal turbulence Warm air refracts light less than cool air: boundaries between layers of warm and cool air bend light like a lens Turbulent layers originate in heat from the telescope mirror or nearby pavement and buildings; it is created by passing weather fronts, wind and the high altitude jet stream Cool down a telescope for at least one hour before use, longer if daytime storage temperature is > 80º; set up to observe far from pavement and buildings Seeing is usually best about 1-2 hours after sunset and again before sunrise Fried’s r0 is (informally) the aperture at which a telescope is optically limited by the seeing— a typical value is 10 cm! What is the Best DS Telescope? Larger aperture (D) increases both resolution (as 1/D) and light grasp (as D2) Larger D increases susceptibility to thermal turbulence (in atmosphere and in telescope), increases dimensions, weight (reducing portability) and cool down time Longer focal length (ƒ) increases image magnification Smaller relative aperture (N = ƒ/D) provides a brighter image, wider field of view and a shorter OTA for a given D; but it increases collimation issues, optical aberrations, field curvature, and reduces magnification In refractors, objective lens cools down well and is free of central obstruction, but can show chromatic aberration at high magnification and is costly per inch of aperture In reflectors, mirror cools down slowly; a large secondary obstruction ratio (η = d/D) lowers Strehl ratio, but this can improve (!) resolution of close, matched binaries Both refractors and “unfolded” reflectors are unwieldy at high N/long ƒ In pursuit of reliable portable aperture, commercial telescopes push the low N/short ƒ limits Best compromise ... 8” to 10” aperture, long ƒ, high N, smallη Cassegrain format A mount with accurate GOTO computer and celestial coordinate input is extremely helpful! Resolution & Aperture Rayleigh Resolution Limit (arcseconds) by Aperture (cm) Aperture & Light Grasp at limit magnitude of 6.5 at limit magnitude of 4.5 Source: Bradley Schaefer, “Telescopic Limiting Magnitudes” ASoP (1990). Note that increased magnification increases light grasp, up to about M = 1.0Dmm ... the main effects are related to objective condensation (“dewing up”) and naked eye limiting magnitude — light pollution, diffusion, moonlight Aperture Reach in WDS In terms of resolution, light grasp, performance under poor seeing and number of resolvable systems in WDS, modest aperture (6” to 10”) instruments are entirely suitable and in some respects (for example, minimizing glare and rendering star color) even optimal for double star astronomy Measurement Methods video capture transit timing brass filar micrometer c.1925 Lyot Carmichael wedge micrometer Micrometer Eyepiece circular position angle scale linear separation scale Relatively inexpensive — purchase new ~$100 Minimal magnitude limitation on observations Simple to use, but requires skill and patience Can be highly accurate and reliable Basic Micrometer Method The scale unit value (arcsecond width of one separation) is determined by repeated star drift timings across the entire scale The scale is centered on the primary star, then rotated to measure the double star separation; observed units are multiplied by the scale unit value The mounting drive is turned off and the primary star is allowed to drift to the perimeter PA scale PA scale must be reversed if mirror diagonal is not used Three or four measures of PA and separation should be taken on two or three different nights, then averaged to obtain the final values Refined Micrometer Method An alternative, more accurate method uses the PA scale to “foreshorten” the separation scale after the stars are exactly bisected by whole scale unit markings (z” = arcsecond value of a separation scale unit, n = number of separation scale units used) θ= (θ1+θ2)/2 ρ= (n ∙ z”)/cos(θ) ......n=3 first orientation measures θ1 second orientation measures θ2 Source: Tom Teague, “Measuring Double Stars,” Sky & Telescope (July, 2000) Neglected Doubles The US Naval Observatory publishes lists of “neglected doubles” that have been observed only once or twice since their discovery ... 23% of pairs in WDS have been measured only once, some not since the 19th century ... even secondary school students can contribute! Star Color ... Again Visual appearance depends on several factors, including: Blackbody temperature (radiance profile, luminosity) Visual brightness (aperture + magnitude) Separation, magnitude difference & color contrast of components William Herschel: Here I must remark, that different eyes may perhaps differ a little in their estimations [of star colors]. I have, for instance, found, that the little star which is near α Herculis, by some to whom I have shewn it has been called green, and by others blue. Nor will this appear extraordinary when we recollect that there are blues and greens which are very often, particularly by candle-light, mistaken for each other. (Preface to Catalogue of Double Stars, 1782) Yes, but ... “I can tell the spectral type of a star by the star color” How Well Do Observers Agree? The observational record proves that star color reports are subjective, idiosyncratic and unreliable. System Component Colors (Observer 1) Component Colors (Observer 2) 26 And bright blue, faint blue white, red 41 Aqr yellowish peach, pale violet reddish, blue 15 Aql amber yellow, bluish turquoise white or yellow white, red lilac εAri pearly white, vaguely blue pale yellow, whitish 26 Aur straw yellow, atlantic blue pale white, violet δ Boo citrus orange, silvery green bright yellow, fine blue γDel yellow, light emerald reddish yellow, greyish lilac 32 Eri grapefruit orange, silvery blue topaz yellow, sea green 95 Her both pure gold apple green, cherry red N Hya both grapefruit orange lucid white, violet tint γLeo bright orange, greenish yellow gold, greenish red Source: Sissy Haas, Double Stars for Small Telescopes (2006) Perceptual Color Effects Two opponent dimensions create hue: yellow/violet (b+/b–) red/green (a+/a–) Complementary hue contrast: adjacent hues shift toward opposite positions (opponent values) on hue circle All star colors are whitish and easily altered by shifts in the white point Magnitude differences are typically overestimated as contrast increases Reduced brightness shifts hue: orange becomes red, yellow becomes green, cyan becomes sea green, white becomes faint blue, blue becomes violet, etc. Blackbody temperatures (as their correlated color hues) on the a+/a–, b+/b– opponent dimension hue circle Use minimal, simple color descriptions: bright/dim, intense/pale, and hue or hue+hue Color Can Reveal Distance Although observers cannot reliably perceive the visual colors of double stars, the catalog listed star spectral/luminosity class is useful to estimate the absolute magnitude of a primary star, which can be used to estimate the system distance and separation A schematic color/magnitude lookup diagram Photometric Parallax With current technology, parallax distance estimates are accurate out to only about 50 parsecs (~160 light years); beyond that, distance can be calculated from estimates of a star’s absolute magnitude as determined from its spectral type and luminosity class Distance modulus Spectral/luminosity class → Absolute magnitude Absolute magnitude (M) / actual magnitude (m) → radial distance from Earth: Estimated distance (parsecs) δ = 10[((m - M)/5)+1] Distance x angular extent → minimum component separation Estimated orbit radius (AUs) a = δ∙R”, where log(R”) = log(ρ”)+0.13 ... Interstellar extinction, errors in measurement of angular separation and magnitude, inaccurate star spectra, projection of actual orbit and magnitude variations within spectral/luminosity classes make photometric parallax estimates only approximate Minimize error: interpret the estimated AU radius as the binary type! (slide 7) Type based on distance estimate usually errs on the low side (gives minimum orbital radius) ... If parallax distance (δ) is available from planetarium software or star catalog, use it! Illustrative Distance/Orbit Estimates System mv* Spectral Class* Sep.* ρ Est. Ma Est. Dist. δ Est. Orbit Radius Actual Dist.† Error ratio Type Est. (Act.) γ Leo 2.4 K0 III 4.7” 3.1 7.2 46 38.5 0.19 4(6) α Gem 1.9 A1 V 4.8” 0.7 17.4 113 15.8 1.10 5(5) ε Boo‡ 2.6 K0 III (A0?) 2.8” 3.1 (0.3?) 7.9 (28.8?) 30 (109?) 64.3 0.12 (0.45) 4(6) 5(5) ζ Her 3.0 G1 IV 1.3” 3.1 9.5 17 10.8 0.88 4(4) δ Ser 4.2 F0 IV 3.9” 1.8 30.2 159 64.4 0.47 5(6) γ Vir 3.5 F0 V 1.6” 3.0 12.5 27 11.8 1.06 4(4) ι Leo 4.1 F4 IV 1.8” 2.3 22.9 56 24.2 0.95 5(5) γ Del 4.4 K1 IV 9.0” 2.8 20.9 254 31.3 0.67 6(6) β Cyg 3.2 K3 III 34.8” 0.5 38 1784 118 0.32 7(8) β Ori 0.3 B8 I 9.3” –6.9 275 3450 237 1.16 7(7) 5.2, A4V,A6IV 210.0” 1.8 47.9 13,570 49.4 0.96 5.3 *From WDS. †From Hipparcos/Yale/Gliese. ‡Conflicting spectral classes given in WDS and HYG. ε1,2 Lyr 8(8) Basic Double Star References Brian Mason & Bill Hartkopf, Washington Double Star Catalog (WDS, ~107,000 entries, ~97,100 systems, updated frequently; all data and dataset notes are available online at http://ad.usno.navy.mil/wds/wdstext.html) WDS ID, historical IDs, epoch, position angle (θ), separation (ρ), magnitudes, etc. My edited spreadsheet version in “night vision” red on black type and distance calculator is available at http://www.handprint.com/ASTRO/XLSX/WDS.xlsx Sissy Haas, Double Stars for Small Telescopes (2008, 2100 systems) Informative, reliable and even inspirational; excellent observing list Ian Cooper & George Kepple, The Night Sky Observer’s Guide (2008, 2100 systems) Compiled by skilled amateurs, with selected double stars by constellation (in 3 volumes) James Mullaney & Wil Tirion, Cambridge Double Star Atlas (2010, 2300 systems) Star charts and preface are invaluable, but observing list is full of ID and measurement errors Robert Burnham Jr., Burnham’s Celestial Handbook (c. 1966) Badly out of date in many respects, but a great source of astronomical romance (3 volumes) Alan Hirschfield, Roger Sinnott, Sky Catalogue 2000, Vol. 2 (1985, 8,100 systems) Out of print, out of date (c.1976), limited selection (to mag. 8) and expensive Additional References Bob Argyle (ed.), Observing and Measuring Visual Double Stars (2008) An indispensable reference for serious observing Eric Chaisson & Steve McMillan, Astronomy Today, 7th Edition (2011) One of many introductory textbooks on astronomy and cosmology Paul Couteau, Observing Visual Double Stars (1978) Informative and reader friendly; geared to refractor observations Wulff Heintz, Double Stars (1978) Comprehensive, detailed and concise; academic and somewhat dated Webb Deep Sky Society Double Star Section ... http://www.webbdeepsky.com/ SAO/NASA Astrophysics Data System ... http://www.adsabs.harvard.edu Wil Tirion, Sky Atlas 2000 (1981) 26 medium scale star charts ... large format and attractive, with durable binding Ian Ridpath, Norton’s Star Atlas (2010) A trustworthy, compact and up to date reference ... 8 small scale (double page) star charts Roger Sinnott, Millennium Star Atlas (1997, in 3 Volumes) Splendid atlas, 1100 large scale star charts, now out of print and regrettably expensive Spreadsheets Help WDS Night Vision Version StarPlotter Look Twice! Habits and outdated catalogs can lead an observer to ignore the obvious ... and succumb to the dreaded BINARY BIAS! One astronomer’s observing notes: 23.35 I began with Cancer, revisiting Praesepe (M44), but giving M67 a miss this time, due to the Moon. Despite its faintness, Cancer was surprisingly full of fine doubles. Iota was a splendid yellow and blue pair at low power, doing a very passable impersonation of Albireo. Less striking, but similar in colour, was 57 Cancri, whilst STF 1245 was yellowish and white. I then used high power to split the white pairs of STF 1177 and the well-matched Phi-2, and revisited Zeta ... ... In fact, the STF 1245 system comprises seven stars — two visual binaries and three single companions — “fossil” indications of its star cluster origin STF 1245 (Cancer) Clear Skies! “Binary formation is the primary branch of the star-formation process.” —Mathieu (1994) “Binaries are the basic building blocks of the Milky Way as galaxies are the building blocks of the universe. In the absence of binaries many astrophysical phenomena would not exist and the Galaxy would look completely different over the entire spectral range.” —Portegies Zwart, Yungelson & Nelemans (2000) drawing of S 404 AB (gamma Andromedae)