OBSERVING DOUBLE STARS -------------------- John Pazmino NYSkies Astronomy Inc firstname.lastname@example.org www.nyskies.org 2009 October 12 initial 2011 March 11 current
Introduction ---------- Double stars, both true binary stars and lined-up unrelated pairs, were a favorite target of stargazing until the late 20th century. One reason was that the usual deepsky objects like nebulae and galaxies were out of reach of the small scopes among home astronomers for most of that century. It was only in the last couple decades that large well-built opticly-good telescopes became affordable to the rank-&- file home astronomer. With the large aperture instrument in hand, home astronomers could realisticly hunt up deepsky objects and skip the easy targets: double stars. Double stars were never fully ignored and they were featured in articles on observing continuously thru today. They just fell out of mainstream favor and were only weakly highlighted at stargazing meets and starparties. There is one niche of stargazer who never left double stars behind in quest of ever fainter and fuzzier targets. The New York astronomer, confined to small scopes by structural and institutional constraints, maintained a lively interest in double stars. Apart from the lesser size of scope, the City astronomer suffers from grayed sky due to luminous graffiti that impedes the view of many nebulae and galaxies. Oh, there are an amazingly large number that can be observed with satisfaction from the City, but orthodox stargazing litterature dissuades against trying for them. For these main reasons double stars remain a significant part of the New York astronomer's target list for his observing efforts.
Mutation ------ Deepsky objects are considered as static fixtures in the sky that can be examined with equal benefit in any year. Changes are due to local observing conditions like haze, air turbulence, bright sky. There are a couple nebulae that have real changes over time, like Hubble Variable Nebula in Monoceros, but such examples are rare for home astronomy A double star is an evolving object. As its component stars orbit each other they change their orientation and separation. This applies to both true binary stars and stars that happen to line up. In the latter case the changes are due to the independent proper motion of the stars. As much as this behavior is now known for quite two hundred years, most observing guides simply fail to appreciate it. They give the orientation and separation as if they were fixed parameters. This is further implied by missing out the year for which these parameters are valid. For the newcomer to observing, such a practice is insidiously disinforming. Unless the specs are really recent and the star evolves really slowly, the aspect of a given double star can be far off that stipulated in the guide. The pair can have widened or narrowed its separation, making the star harder or seem too easy to resolve. The orientation may have rotated to some other angle, confusing your directions in the eyepiece field.
Specification ----------- The most common specification of a double star are its angles of separation and of orientation. The former is cited in arcseconds because the idea of a 'double star' is that under bare eye or low magnification the star looks like a single star. High power breaks it into its two constituents. The orientation is expressed in degrees around a compass rose centered on the brighter of the pair. Celestial north is 0d; east, 90d; south, 180d; west 270d. I never saw an orientation cited as a clock hour, with north being 12 o'clock, as is sometimes done for other celestial alignments. By long convention the brighter star -- regardless of its astrophysical character -- is the primary component and the other, fainter, is the secondary. It is also called the companion or, in Latin, 'comes' (KO-mess). If there be several companions in a multiple star system, they are 'comites' (KO-mih-teez). I rarely see them called by higher ordinal numbers. The specs help you to assess if the star can be split with your telescope. Too close a pair will remain singular. The orientation, or the position angle, helps to look in the proper quadrant of the eyepiece field for the comes. Note carefully the two distinct meaning of angle, which carries over to all other facets of skywatching. One is the 'surface' distance on the celestial sphere between two points. The other is the direction from the one point to an other. Thee angles are similar to the distinction on Earth. The separation of two places can be expressed in degrees along the surface and the direction from the one to the other is in degrees round from north. These are the navigators run and bearing. On Earth you usually cite distance in linear units, kilometers. This is only because the Earth has a fixed known size, so each degree on it correspond to a linear unit. One surface degree is 111 kilometers. One minute of arc is 1,800 meters, the sailor's 'nautical mile'. On the Moon, in comparison, one surface degree is 30 kilometers and one surface minute is 500 meters.
Timescale ------- How rapidly can a double star's aspect change? For some the orbit period is thousands of years or there was no sensible change in the last century or so. There are lots of other stars whose timescale of change is decades. Within a lifetime you can follow the shifting direction and distance. There are clues for the timescale. If the two stars are so far apart that in a small scope you actually see them separately, they are likely to be far apart linearly. This implies a long period of revolution by the same Kepler principle that planets farther from the Sun have longer periods than nearer ones. Hence, an easily split double reasonably has a period so long that you may not sense any change of its parameters over a lifetime. Stars with very close components, discernible only under strong magnification, tend to have shorter periods. There is the chance that you can detect the mutation by keeping careful records of your observations over the decades. Pairs so close they can not be resolved by ordinary telescopes can have periods of months or days. The typical example of this star is an eclipsing binary. The stars are so close that they can mutually hide each other in turn, causing periodic fluctuations of apparent brightness. The absence of a period can not be so causally assigned to an excedingly slow pair of stars. A period more than a thousand years could be missed with the short time of observation in the order of a couple decades or a century. A missing period may also mean a sheer lack of observations. Too few astronomers had the interest or purpose to examine the star and lay out its orbit. A missing period may also mean that the pair is not a true binary. The stars are a perspective pair that happens to sit along our line of sight.
Home observations --------------- More so than ever in the 21st century there are very few campus astronomers taking care of double stars. Their work is increasingly occupied by other fields of astronomy. At the same time, home astronomers are gradually obtaining the means to capture valuable information about double stars with larger aperture of telescope, stable fixed mounts, imaging apparatus, electronic astrometry and photometry, general computation power. There could be a new endeavor for home astronomers to keep tabs on critical stars and compute their orbits. This work has to be coordinated with a facility that has an abiding interest in double stars. In the United States, the US Naval Observatory is one such center for double star work. It could promulgate standards and procedures for observing double stars, collect observations in a competent format, and blend them with other observations. So far it hasn't. In fact, there is a deep absence of campus extension to the home astronomer for double star observations and little effort to render submitted data into orbits or databases. The home astronomer as a result has no easy way to know what a given binary is doing now, as agains reading what is was doing many decades or a century ago. Apart from dedicated recording and documenting double stars, these stars are plain enjoyable as casual targets of admiration. For one thing, they are one of the few celestial objects that show obvious colors, even if two observers do not agree on which colors they are. The planets and bright nebulae have colors but they tend to be subtile and pale. Just There are, to be fair, temporary apparitions with strong color. Their appearance is rare and can not be reliably foretold. These include aurorae, large meteors, bright comets. Just about everything else in the heavens is white or neutral.
Orbits ---- Home astronomers tend to think of 'orbit' as a nearly circular path around a central body. This is based on long experience with the planets around the Sun and the larger moons around planets, Even the orbit of Mars looks pretty circular when drawn to scale on a letter sheet. You have to measure with a ruler to learn that it is really an ellipse. Double stars have orbits that can be extremely excentric, like most comets. Near their closest approach to each other, perisastron, the two stars can swing around rapidly within a decade or so. When far apart, near apoastron, their specs may linger almost fixed for many decades. Unless you know where in its orbit a given binary is now, you can't tell for sure from the period and spec's epoch what the star is doing. You can from a set of observations work out an orbit either by hand or with computer programs. This is usually an exercise, lke in a college astronom class, using canned mesurements culled from observatory records. For well-obseved short-period stars, the data are accessible to try your hand at this work. One ambiguity is that from our remote perspective an orbit by be tilted one way or mirroed to an equibalent tilt in the opposite direction. Supplemental information, like radial velocity data, is needed to obtain the correct tilt.
Gamma Virginis ------------ The wonderful and awesome prototype of rapid change in a binary system is gamma Virginis. This star went thru its periastron in the mid 2-thous. The two components closed to about 1/3 second apart and rotated by quite 120 degrees. Home and campus astronomers watched this star until it collapsed into a single solid point. In 2009 it broke apart again into two stars discernible by small telescopes. For the next, uh, 160[!] years gamma Virginis will be a wide pair ranging up to about 5 seconds separation near apoastron. gamma Virginis is, believe it or not, the ONLY instance where within a couple years you actually witness real gravitational motion of whole stars! For all other binary stars you must wait decades or rely on reports over a century. For a detailed discussion of gamma Virginis, please read my articls about gamma Virginis in the NYSkies website at 'www.nyskies.org/articles/pazmino/gamvir1.htm' and '.../gamvir2.htm'. Barnard's star is an other example but mainly for linear proper motion. It is also not an easy target to find. Its 10 arcsecond per year proper motion results partly from its proximity to us.
Dynamics ------ For many binary stars the orbit is well enough determined to allow elements to be worked up. These are defined similarly to, but a bit differently than, those fo the planets. From them you can plot out the star's aspect for any given date. Dedicated computer program do this, as do some computer planetarium programs. In the early days of home computers, I and others wrote our own orbit programs in BASIC. The main difference between the dynamics for a binary star and a planet is that the stars are of the same order of mass, whole suns apiece. Planets are at the best only one thousandth the mass of the Sun. This means that placing the origin of coordinates in the Sun is a very good approximation to the real situation where you should bank off of the barycenter between the Sun and planet. The barycenter of the solar system is always within the globe of the Sun. When a double star is newly observed, and in the centuries before astrophysics, the masses of the stars are not known. We know now that they are of roughly equal mass so the barycenter is midway between them. Stricta mente, the elements of BOTH stars must be worked up and tied to this barycenter. In practice we hold the brighter, primary, star fixed and put all of the orbital motion into the secondary, dimmer, star. The elements are RELATIVE elements as seen from the primary star. While this sounds sloppy, it does favor the observer on Earth because he instinctively fixates on the primary and looks around it for the secondary. You can see what's going on. It is sort of reversion to the geocentric world where all motion is banked off of a fixed Earth. As a matter of sheer fact, you can do this with all good results. The dynamics, by gravity, are all wrong, but the kinetics, with the planets are massless points, is quite valid. For history sake, I remind that Newton seemed a bit on his high horse to claim his gravity theory was universal. Yet in his day, late 1600s, the only place where gravity was known to exert itself was within the solar system. The stars beyond were truly fixed points. When by the end of the 1700s binary stars were discovered, the Newton theory was applied to them. The first gravity-based orbits were worked up n the early 1800s after enough observations were accumulated for a good arc of motion. Since binary stars are disposed thruout the profundity of space to indefinitely remote distances from Earth, the universality of the Newton theory was first demonstrated.
Source for specs -------------- When an orbit of record exists, you can compute the specs of the star for a given date and use it to evaluate the aspect of the star. This is good if the orbit is well established, as by having a short period and strong observation record. This is hardly the normal situation. Because double stars can have long periods, their history spans several lifetimes. I mean not only human lifes but the lifes of an observatory. Its mission and purpose may shift, leaving the star un observed for many decades. The orbit of record could be based on too short an observed arc. Specs generated from it for a current date can be well off of the mark. Believe it or not, this was the case with gamma Virginis! As late as 2005, when the stars were obviously closing together month by month, there were still in circulation old orbits that put off the periastron for a few more years! This is for a very important star, the very first one to witnessed thru a periastron back in 1838. The location of the secondary could also be nn observed spec, measured by photographs or micrometer. This is both a good practice and not. It's good in that it adds to the history of the star. When combined with other measures, the orbit is ever better defined over the years. Its not so good if the observation is in isolation and then quoted as a precise parameter in observing guides. Some authors try to update their books by hunting for recent observations of the star. Doing so for the sake of capturing the latest, rather than the best fit trend, leads to erratic specs for the star. You may look in double star records and see how the observations whack around so badly. Errors up to 1/3 of the instant radius of the orbit are annoyingly common. If you look up in an observatory journal for the one newest report you don't know where it sits on the fitted orbit and you may then cite a reckless specification. The miserable harm is that almost never is the source of the spec given in observing guides. Is it computed form a such-&-such orbit? Is it merely an entry in some laboratory's observing logbook?
Selecting double stars -------------------- Because the prime factor in seeing a star as double is the separation, many guides select stars wider apart than a certain number of arcseconds. The author explains that he wants to offer stars within range of certain instruments assumed for the reader. Such reasoning is consistent with that for picking out only the brighter star clusters or deeper range of variable star. With the mutation of a double star's specs, the roster may be inaccurate because the author used, often by ignorance, ediurnate specs. He can include stars that were once wide enough to qualify for his list but are now too close together. Conversely, stars falling short of the author's qualification may actually meet them because they are now wider apart. This is why you can not beat up an author with 'the such-&-such book has a star that should be in your list'. While there can be honest omission by mistake of a good specimen, you better make the inquiry about your suggested entry. You could be reading ediurnate parameters. Note well that you can not be clued by the issue date of the book! The author may copy data from older material. A new work can still have out of date information about a double star. There aere many attemtps to screen pairs for the prospect of being real binaries. None work well. The problem is that a candidate star will call for years and decades of further examination, so a simple quick screen would be deeply welcome. One common method is a formula with star magnitudes and separation as input. If the formula value excedes, or decedes, a decalred limit, the star is a a possible binary. There are as many such formulae as astronomers who propose them, with the outcome for a given star being at the mercy of each. Other methods include orbital motion, proper motion, and radial velocity. If the star has a short period, comparing measurements of over years could reveal one component to orbit the other. This is in fact how binary stars were discovered in the late 1700s. Before than it was assumed the pairs were all chance alignments. If the star has a long period, orbital motion may not yet be evidenced. The two components will travel thru space together as a unit, sharing the same proper motion. Comparing proper motion measures can uncover such pairs. In some double star catalogs these stars are tagged 'cpm' for 'common proper motion'. The radial velocity, or Doppler, method is not as secure a test but a clue. If the suspected pair has the same radial velocity, within some dispersion limits, it could be a binary. The dispersion comes from the one star running in its orbit to or from us relative to the other. Taking Doppler meassurements over many years or decades will receal, like orbital motion, the binary. The real difficulty comes in selecting perspective, line-of-sight, doubles. With no physical attachment between the components there is coordianted study of them. There is no positive way to hunt for them or cull them from catalogs. You make do with anecdote: ' There's a star just north of the nebula with a timy adjacent companion in a pretty field'. It could be a binary or a perspective double. There could be other cnadidates in the vicinity that chnaced to be overlooked. All in all, there simply is no uniform consistent catalog of double -- binaries or line-of-sight pairs -- for the home astronomer to enjoy. There are hundreds of well known stars that turn up again and again in observing guides. Among these the selection is erratic, some stars in one are missed form an other bu some, often unknown, author's judgement.
Resolution -------- There arises from time to time a fetish for the resolving power of a telescope. This was a major source of good arguments when double stars were a larger part of home astronomy, It persists today for discerning details ob planets and even on the moons of other planets. The common sense of resolution is that you must positively tell if two point sources are really two or are blended into one. The most conservative criterion is that in the instrument the two stars must be clearly tangent to each other. A nudge farther apart will collapse the notch between them into a clean break. That its, they must be separated by the sum of their radii. Points do got radii. A 'point' of a star's image is really a tiny dot of finite diameter. This is the Airy disc and is a result of the wave nature of light. Without going into the theory, this disc is only a very few seconds across so under low power it does look pretty like a pinpoint. The formation of optical images from a point source into a finite dot is one example of how Nature avoids the singularity problem. If the image was in fact a true point, the energy density there would be infinite: (energy flux)/(area of image) --> (infinity). Since this constitutes a singularity, Nature forces the image to occupy a finite area so the ratio is a finite value, not infinity. If the stars are about of equal brightness their Airy discs are of equal diameter, so it is often noted that resolution is achieved if the stars are apart by one diameter (two equal radii) of their Airy discs. Opticly the Airy discs are of equal size, but blooming in the eye's perception cna make the brighter star appear to have a bigger disc. Some light from its disc splashes over to adjacent cells on the retina to excite them into vision. This is how a double star of very mismatched brightness can be a lot harder to resolve than one of more equal brightness. A less stringent rule is that the stars must stand only one radius apart. In this case the discs overlap with a notch along the sides halfway between the ends of the elongated image. You can demonstrate this with two coins or other discs. When overlapping, the common figure is a sort of dimpled ellipse. It is by interpretation, knowing that there should be two round dots, that allows you to claim there are two stars. Without such prior expectation you could be looking at a funny-shaped sharp-edged nebula. If by some chance, the object really is a single odd-shaped thing, you would be fooled. Staying with stars in proximity, the optical theory allows that resolution is achieved if the separation of the stars is greater than
(resolution in arcsec) = (120) / (aperture in mm)
This is rounded from the theoretical formula and is a lot easier to mentally do the math for the common apertures among telescopes. A 60mm aperture of scope should by this formula resolve double stars as close as 2 seconds apart. A 250mm scope should split stars of only 1/2 sec separation. And so on. From the ground, the atmosphere is hardly so stable and still to allow perfect optical images. The ideal Airy disc is more likely to be a wretched spot of a couple seconds diameter. Thus it is common to experience when among many different telescopes that none penetrate deeper in resolution than, say, 3 seconds on a given night. Such is the case at starparties where there are impromptu competitions among telescopes. For telescopes outside of the atmosphere, things are vastly better. The ideal limit for resolution based on aperture is achieved. That's why as example Hubble can see such amazing detail that a similar size of scope on the ground can not see. Hubble can resolve to about 5/100 arcsecond, allowing for huge enlargement of its images while keeping its star images as neat round dots. The equal and greater size of ground scope can hardly achieve 3/10 -- one sixth of Hubble -- under the best of atmosphere above it. This is based on the skills and arts of telescope building when Hubble was under design in the 1970s. Today with adaptive optics, ground scopes equal and excede Hubble. For photography when the camera shutter was open to incoming light for many minutes or hours, the movement of the air made the star image /wander over the film. The result was a blotch a coup;e arcsecond across. This so severely limited the detail in pictures that the traditional rubric was that the eye can always capture, in the instant of still air, much finer texture than a photograph can ever get. For home astronomers, the practical limit from most observing sites is one arcsecond. This is based on long experience with many scopes and sites and is rounded for simplicity sake. If a double star is closer than that, even for larger scopes, it congeals into a single image with no definite evidence of duplicity. I know there can be chance moments when the air calms dowm to let a better image be formed, For this instant the combined blurry dot condenses into two points, but that demands lots of patience and a quick eye. Unless you are favored by a site with exceptionally stable air, you may pass up double stars of separation less than one second.
Optics ---- Achieving the ideal resolution, as limited by your air, calls for optics that in fact render a point source, the star, as an ideal Airy disc. This is a function of both the quality of the optics and your maintenance of them. The former used to be an issue among commercial telescopes. thru the mid 20th century, the lack of affordbale good quality optics was one motivation for building your own scope. Today, however, commercial scopes typcily have theoreticly perfect optics for affordable prices. Where many scope fail down is the observer's maintenance. The instrument must be clean, collimated, free of damage. Altho it is easy to avoid damage and to keep the optics clean, it is a real chore to keep collimation within specs. Most home astronomers do not have the tools, test rigs, skill, patience to look after colimmation. Yet loss of collimation is among the most common defects in home telescopes. The usual situation is that the optics creep slowly over years without the observer noticing a deterioration of the images they produce. Only when he views thru a scope known to be well aligned does he realize his own instrument is out of spec. A scope out of collimation will not send all the incoming rays from a point into an ideal Airy disc. Some will spray outside of it. Those within it may form a oval or irregular disc. Such behavior degrades the image and resolving power. Double stars can not be split so closely because, perhaps unknown to the observer, the image is swollen beyond the diameter of the ideal Airy disc. In spite of the necessity to keep collimation, there are so few really simple and reliable ways to do so. In some telescopes, there is no way to adjust the optics, like screws, spring bolts, shims. In many cases the instrument is sealed against tampering. If you eventually discover loss of collimation, chances are the scope must be sent to the manufacturer for adjustment and, by the way, its warranty expired. Serious and detailed inspection of double stars requires really good optics, both by design and construction, maintenace under your care. A shortfall in optics will ruin double star observing.
Highest power ----------- Can't you separate a pair of stars, how ever closely spaced, just by applying a high enough magnification on the telescope? No. As noted above the image of points in space are finite dots. It is only in geometric, ray tracing, Gauss optics that points in space map into points in the image. When you magnify this image, you enlarge the dots. If the dots overlap, you see only bigger overlapping dots. Think of a regular photograph. It contains a certain depth of texture. If you do a photographic enlargement of this picture you do not acquire more texture. The existing texture is made bigger with no new information revealed. In fact, you can apply too much magnification. The texture in the image can be so enlarged that its smallest feature occupies a sizable angular extent before your eye. It occupies several retina cells. At this point the image is blurred, grainy, diffuse, and unattractive and unappealing. You have to step down to a lower power to obtain a more pleasing, smaller, view. The highest magnification is usually capped by the state of the air, which governs the size of the star images. Turbulent air yields larger dot images, forcing you to be content with lower power. Good and stable air approaches the ideal image formation, permitting higher powers before the view turns lousy. There is no determinist figure for the highest feasible power of a telescope because the evaluation of the image is subjective by each observer. The general rule is that highest magnification you can expect to apply is
(highestt power) = (2) * (aperture in mm)
A 200mm scope has a highest power of 400. This rule assumes good optics, a reasonable assumption with modern telescope manufacture, and good collimation, not nearly so good an assumption for many scopes. This does not mean you must not excede this limit. There will be occasions when a higher power can be usefully employed. Please have in your observing kit one or two high power eyepieces. The essential point to appreciate is that the property of a telescope often hawked is its magnification, as if this was the prime purpose of the instrument. That's why low-end scopes banner their highest power. You will soon find that it about never can you usefully enjoy that high power on this scope. Because double stars can have close separations, high power may be needed to resolve them. Resolution comes only when the optical image of that star in fact consists tf two distinct dots tangent or with empty space between them. Else you will obtain only a larger, more blurred and diffuse, single elongated spot.
Lowest power ---------- There is a LOWEST magnification limit for double stars. Recall that by eye a certain star looks single and it becomes double under telesocpic resolution and magnification. The eye alone has too little power to resolve the star. The situation here is in the retina, composed of cells of light reception. These cells have a finite size, about 10 micron diameter in normal eyes. If the image on the retina spans several cells, you see texture in the image, a bit from each cell. If the image, however finely textured, sits entirely on one cell, you get only a blended dot for its visual impression. In the normal eye, the daytime resolution is about one arcminute. Targets in space angularly smaller than that register in the brain as a dot with no internal detail. Since we observe double stars as night, we are viewing with the night vision cells of the eye, the rods, which are larger and less numerous than the day cells, the cones. For the normal eye the cell size of rod cells is about 30 microns, resulting in a night resolution of about 3 minutes. If even in the telescope the images of two stars fall within 3 minutes of each other before your eye, you see only one star. The two images are exciting only one rod and send a single combined signal to the brain. By applying some magnification, the two images are made to sit on their own cells and produce two impressions in the brain. Thus, there actually is a lowest power below which the double star remains unresolved. There are several ways to determine this minimum power but the one I use is useful and simple. It asks:'What power is needed to put a double star's image on the retina at least 3 minutes apart'? The resolution of a telescope, to create two distinct Airy dots in the image is (120)/(aperture in mm) in arcseconds. 3 arcminutes is 180 asrcseconds, so we have (180)/((120)/(aperture)) = (180)*(aperture)/(120) = (1.5)*(aperture). Thus
(lowest power) = (1.5) * (aperture in mm)
This resolution-based minimum power is a different parameter fro an other lowest power based on aperture. This works on capturing the outcoming beam of light from the eyepiece. In astronomy every photon counts. You want to get as many of the ones entering the telescope to then enter your eye. The eye's own aperture of about 5mm at night for a normal adult. The diameter of the exit beam of light from the eyepiece is (aperture ub nn)/(magnification). To make sure the exit beam is no larger than 5mm, else you spill photons over the edge of your pupil, you must apply a minimum power of (aperturein mm)/(5mm). A 150mm scope has a lowest power of 30. Less than that and you spill some of the photons collected by the instrument. They miss your eye by falling betond the edge of the eye pupil. Note well that this aperture-based power will not exploit the full resolution of the scope! The 150mm scope needs 225 power to split the closest pairs. For wider pairs, lower power may be used to improve the pleasure of the view. This is particularly the case if the star is in a pretty field of other stars. For some targets there is an abundance of light like the Moon. Some targets need a wide field of view like an open cluster, large comet, or the spread of Jupiter's moons. Powers less than the minimum limit are handy to have in your kit for these targets.
Visual magnitude -------------- Human vision, appearing so real and physical, is really a response to the stimulus of radiation impinging on the eye. Without eyes in the form we do have there is no vision, even tho the incoming radiation still exists. This fact causes severe angst among those studying life on other worlds. It is also a primne factor in anthropic cosmology. The failure to realize that light is not a physical construct built up an edifice of misleading, erroneous, and at times ridiculous lore about human vision. Here I discuss only a couple aspects relating to double stars. The 'magnitude' system by which we rank the apparent brightness of stars, is speficily defined only within the wavelength range of human eye sensitivity. Because that sensitivity varies widely among observers, a standard curve of response to radiation was adopted for photometric and illumination work. Your individual eye may diverge severely from this standard model. You likely know a person claiming a lamp is too dim while you assert it's a too bright for comfort. Valiant attempts are made to build devices that emulate the eye and record star magnitudes automaticly. With lots of sweat and tears these do work but there is yet to arrive a unit that you point at a star and get a reading 'magnitude '+7.54' directly. You get an electric current, electron count, sensor value that you must interpret as a magnitude for the star that excited the device. That's why photometry in astronomy is so prevalently differential, not absolute. You box in the target star between other stars of known or declared magnitude. You then trend the device readings on a graph and pick off the target's magnitude. Altho this chore can be performed automaticly, there must still be the standards to calibrate the plot. Observing guides are inconsistent with magnitudes. Some give the primary's brightness as the combined brightness of the two stars while others give the individual magnitude. The former is likely for stars intended for naked eye view where the double remains single. You do get the combined illumination from the two stars. The individual magnitudes are common for stars both listed in a photometric catalog. Be careful when a guide gives a magnitude for the primary and a DIFFERENCE of magnitude for the secondary. The observer took the primary's magnitude from some source and estimated how much fainter the comes is; 'Hmmm, it looks 2.5 magnitude dimmer.' A gurther variation is to offer the RATIO (illumination, not magnitude) between the two stars. The secondatu is a certain frction, decimal, of the primary. As an example suppose a double star consists of two stars of 4.0 an 4.7 magnitude. We can read in the various guides: --------------------- 1st 2nd meaning -- --- --- ---------------------------- #1 4.0 4.7 each star with own magnitude #2 4.0 0.7 1st with own magnitude, 2nd is offset from it #3 3.3 4.7 1st is combined magnitude, 2nd is own magnitude #4 3.3 4.0 1st is combined, 2nd is magnitude of primary #5 3.3 0.5 ist is combined magnitude, 2nd is ratio (2nd/1st) --------------------------------------------------------------- The correct meaning may be buried in the letterpress or left for the the reader to guess. The magnitude can be deliberately approximate. It usually isn't all that important what the separate brightnesses are. What counts is the dispersion of the two. It is a lot easier to see the pair if the stars are about equally bright than if they are deeply different. The brighter star may overshine the fainter, even tho they may be separated enough to resolve in your scope.
Sensitivity --------- The human eye needs a certain minimum influx of radiant energy to trigger perception. In astronomy this is expressed by saying the threshold brightness of star for the normal eye is 6th or perhaps 6- 1/2 magnitude. A star chart that reaches this faintness is a 'naked- eye' chart and you're supposed to perceive all stars on it, within constraints of altitude, latitude, and sky darkness. Falling short of doing so marks you as somehow imperfect. Human sensitivity varies all over the map. Some people genuinely can see 8th magnitude stars in the zenith from a darksky site while others barely make out 5th magnitude from the same place. The lower, deeper, limit is rare, but among many hundreds of observers at a large starparty there can be a couple who achieve it. For them the Sky Atlas 2000 is their naked-eye chart! As an observer ages his eye sensitivity declines. The falloff is extremely gentle. The astronomer assigns it to deteriorating sky conditions over his site. There is increasing suspicion that reports of worsening sky transparency, judged by the raising of the threshold limit of star magnitude by long time residents, may be the result of the observer's own aging. Adding to the perceptin of worsening skies is the lack, since the mid 20th century, of new, keen-eyed, astronomers into the progfssion. On the whole, the profession is aging and the threshold magnitude becomes brighter. This phaenomenon can seriously compromise the tracking of luminous graffiti, routinely monitored by assessing the threshold star magnitude. The situation feeds on itself. With brighter limiting magnitude, observing become more infrequent as many nights are passed up. When the night is taken, fewer challenge targets are tried. The astronomer sticks to the easier ones under the 'brighter' skies. There sets in a discouragement from attracting new astronomers to the 'vanishing' opportunities fo stargazing. Clubs shrink, astronomy readership dwindles. public awareness and support for the profession evaporates. Some pessimists fear that home astronomy by the mid 21st century may resemble the ghost seas of Mars. Altho double stars are quite resistant against bright skies, the constricted selection of targets infects them as well. Fainter double stars, still quite visible with satisfaction, are skipped.
Eye care ----- The size and density of cells on the retina ranges widely thru the human population. You surely know of heard of a person having extra keen sight to read newsprint at 30 meters (in daylight, please) while others must bring the print to within one meter to begin reading it. At night some observers can see double stars, opticly resolved ones, with lower power than other observers. Their smaller denser rods allow the image to span two or more cells and impress in their brain as distinct points. The astronomer with less dense rods must use higher power to push the two images onto separate cells. If the loss of acuity is disease-induced degeneration of the retina, relief could come from medical treatment or delicate surgery. These methods so far can only slow or stop the progress, not reverse it back to good vision. Such a situation requires entering into continuous ophthalmic care. A corollary condition is clarity, the ability to see stars close together without hazing, misting, glaring from each other. It depends on the eye being truly transparent, with no nebulous obstruction. Young eyes tend to be really clear, to the point of revealing the Moons of Jupiter when near elongation from the planet. Older eyes tend to be clouded, tho not so much as to impede ordinary life. While a person can still function by day with a moderate loss of clarity, it takes but the merest defect of clarity to punch holes in the view of the night sky! Even for nonastronomy functions, it is common to know someone who has only a 'daytime' automobile licence or complains about uncertain goings about at night. The archetypical cause of clouded vision is cataracts. The lens inside the eye gets milky. This causes a mist around bright lights (stars and street lights), veiling of darker objects near bright ones, whiting out the scene in the face of sunlight or headlights, obscuring signs and print, diffusing colors, impeding navigation in the dark. The effect is like looking thru a dusty window. The remedy is surgery. The defective lens is bodily removed and replaced by an artificial one that is mechanicly attached inside the eye. If you wear eyeglasses or contacts, the correction can be molded into the new lens. The prescription is now INSIDE your eyeball! You discard the exterior glasses after the surgery. In double star work, a faint comes may be veiled, perhaps at first unnoticeably so, by a mist around the primary. The sky isn't really black, but gray with the comes buried behind it. You lose the pleasure offered by the star's colors, seeing see only a drab pair of dots.. As you examine double stars, you'll be astounded, maybe scared, to learn that your eye has a patch work of sensitivity, clairty, acuity over the retina! In day vision you don't notice this but at night with only points of light hitting the eye in a dark background, vision defects show up harshly. In fact, one instrument used in ophthamic tests looks like a planetarium! You sit in front of a dark titled dome. Over its surface are lamps that blink on and off in a random pattern. You click a button when you see a blink, keeping your head still on a chin rest. The record of clicks and blinks maps out your visual field. There are other causes for loss of clarity that only a proper ophthalmic examination can determine. Good astronomers do an eye check once or twice each year. Many health insurances cover these exams as part of prevemtive care. Please inquire after this feature.
Night vision ---------- There is a yin-yang about vision at night. For ordinary tasks, the eye shifts from day vision to night vision during nautical twilight, roughly 1/2 to 1 hour after sunset. The shift is reersed an equal time before sunrise. This is away from artificial nighttime illumination like within a town. The smaller, color-sensing, close-packed cone cells shut off and the larger, looser-packed color-dull rod cells take over. The rods are far more sensitive to dim light at the cost of losing resolution, and color perception. The scene is grayscale at night and texture issigns harder to interpret. In addition, the rods at the edges of the retina are even more sensitive to low lighting, making the center seem 'blind'. The trick of averted vision to see extra faint objects in the telescope exploits this feature of night vision. For earthly tasks, some persons have a gtadation of night vision from center to edge that can be positively dangerous. The dullness of sight in the center impedes confident view of the scene. Such perople may miss, or hit!, curbs, steps, poles, walls, furntiure, trees, fences, cliff edges, cuts and pits, standing water, pipes and chains, and more. For astronomy thry may have trouble reading maps and markings, working dials and knobs, finding dropped items, checking around for cleanup, aiming a telescope, looking thru a narrow field eyepiece. For double stars, the situation can be seriously obstructive. It is real hard to concentrate on a target set off to the edge of the visual field. But when centered it is too dim for comfort or is vanished completely. Using a high power eyepiece, that typicly also has a narrow field of view, the double star may be missed, making the astronomer think he isn't aiming at it. There follows a spell of frustration as he tries again and again to acquire the star.
Night color --------- If night vision is grayscale vision, how can you see distinguish colors in stars? Allowing for the dispersion of the claimed color seen by different observers, why is any color noticed at all? Most observing litterature is silent on this fact. They have a chapter on night vision, like the section above, and then procede without a blink to assert the colors present in double stars. Stars have colors but only weak tints like pastel. Even the brighter stars do not have vivid hues, altho you read that Arcturus is an 'orange' star. To me it does look sort of orange but hardly so deep a hue as a traffic cone, the fruit itself, or anything else that's orange in sunlight. Before astrophysics there was no known cause for the colors of stars. One early thought was that the color came from the Doppler shift of their rest color due to recession or accession. Such an idea sounds idiotic today but long before the nature of light was understood, the Crazy-Eddie speeds worked out for such immense Doppler shifts seemed plausible. In the mid to late 19th century there was a flap to carefully assess the colors of stars and keep records of them. One thing found soon was that red stars are prevalently variable stars. We know now that this is a result of the location of red stars in the HR diagram. Since we can only see the more luminous red stars by eye or small scope, the red stars are among the giants and supergiants on the chart. Such stars are indeed prevalently variable by the energy processes associated with this part of the HR diagram. It is true that there is little color in night vision because the light level is below the trigger threshold for the color-sensitive cone cells. This applies to the general field of view where overall the retina suffers too low an influx of radiation. Cone vision switches to rod vision. If a PARTICULAR spot on the retina has enough illumination for day vision, THAT spot switches to cone vision and you discern color in the illumination! This happens when in an otherwise dark field of view there are singular points of light hitting the eye. Each point strikes the retina under its image and could, if bright enough, cause it to hand the vision function to the cones. A sky at night, by direct view or thru a telescope, is exacta mente such a field of view. The spot on the retina lighted by a star could be illuminated enough to excite the cones into action and pick up a color in the star. The retina nearby, still in darkness, remains under rod vision. Under a general night vision you see colors in stars! The effect is momentary. Soonest you move the star away the rods come back into play and cones under the new location of the star's image act up. There still is the absolute trigger influx that restricts color perception to the brighter images. Faint stars in the field remain grayscale.
Color clash --------- Altho the physiology of color perception under night vision is still loosely known, one fascinating effect is well documented. When two points of color are close together, on adjacent or almost adjacent retina cells, their colors amplify. A resolved double star, with sky between the stars, sits on at least three cells (star A, empty sky, star B). By some physiological mechanism the color of each intensifies into vivid hues. Because most celestial targets are grayscale or have only weak tints, a double star attracts attention by what sure as hell looks like fiery hues. In some cases the effect is so lovely that vocabulary to describe it can be awfully, erm, colorful. With no generally credible way to quantify color for home astronomy, there arose a broad diversity in double star description. Authors tried to find regular words to define precisely what color a double star presented to his sight. Or he copied the description from some authority or more experience observer. It is impossible, unless there comes a way to link up brains between two persons, to know what an other person sees compared to your impression. The other astronomer can not 'show' you his color rendition to compare with yours. The honest truth is that each and every person has his unique color perception. This is part of the natural diversity of spectral sensitivity and rendition from person to person. You can not be taught to see a certain color. No amount of training or practice will let you confidently see a prescribed hue. Even if you somehow manage to call certain samples of color correctly, this technique can not be applied to new, not previously exercised, examples. You will fail to call them properly. The color you see IS the true color for you and that's that. The lurid terms used to specify double star colors are actually useless for a given observer. What can happen is that when a new astronomer fails to see what is supposed to be seen, like the color stated by an experienced observer, he feels failed as an observer. The bottom line is that the crazy words to portray double star colors are really silly and may be discarded from your expectations.
Schematic color ------------- My own practice is to cite the schematic colors according to the spectral class of the two stars. This is artificial in that these colors are themselfs not what you 'should' or 'must' see in a star of a given spectral type. When I say Arcturus is an orange star I mean that is has a K type spectrum, whose schematic color is orange. Or, to invert the sense, an 'orange' star is a k type of star. The colors scheme is listed below -------------------------- spec | color | temp -----+--------------+----- O | blue | hotest B | blue-white | | A | white | | F | yellow-white | \|/ G | yellow | solar-type K | orange | \|/ M | red | coldest ---------------------------- These colors are commonlyy used to tint stars in a chart or lamps in a projection planetarium. The color gradation is conitnuous thru the spectral classes, but binned. Any finer distinction would be beyond normal people to pick out the colors. This would be a very fair and consistent manner of specifying double star colors but for one major hitch. The spectra of double stars are not well cataloged. Usually only that for the primary or a blend of the two is stated. Individual spectra are on the books for both components of only the brighter double stars, Unless there was a pointed interest in the individual spectra for dimmer pairs, chances are they were never captured and cataloged. Stars in a binary system have different spectral types because the two stars are of different mass. While formed together, so they are of the same age, they are generally of different mass. Heavier stars sit on higher points of the main sequence in the HR diagram and go thru their life cycle faster. The most massive stars evolved into the red giant phase of life, leaving their less massive companion still on the main sequence. For this reason, among the more important uses of double stars is astrophysics. Since the two stars were formed together, the age factor is eliminated from the cause of divergent spectral types. Also eliminated is chemistry, being that the two stars came from the same nebula of a single composition. Eventually it was learned that mass was the dominant factor in dispersing spectral types within a binary star.
Concentration ----------- Because it can take some concentration to inspect a double star, it helps immensely if your telescope tracks the star. Traditionally this required a conventional equatorial mount, but modern scopes can follow the star with a computer controlled altazimuth mount. Having the star sit still in the field, or at worse drift only slowly, is a huge facilitation for inspecting the double star. Any distraction, like to twiddle axis knobs or press motor buttons, upsets your concentration. This is specially true if your hoping for a brief instant of still air on a turbulent night. Tracking also helps in perceiving color. By holding the star still in the eyepiece field you can hunt around your eye's field of vision, placing the image here and there on the retina, to find the overall best view. Other distractions must be banushed. Things like street noise, wind and rafts, ground vibration, other people walking around, headache, muscle stiffness, thirst and hunger will chip away at your observing comfort and concentration. In deed, the best of serious observing is done alone or with highly cooperative partners. At a starparty or public viewing it is just about impossible.
Conclusion -------- Double stars were and can still be a substantial part of the home astronomer's stargazing. There are so many in all sectors of the sky that there is always an ample selection when and where ever you observe. There are specimina for all size of telescope, even for binoculars, allowing enjoyment under just about all circumstances of being outdoors at night. The main obstacles for confidently selecting targets is the ediurnation of published parameters. These, due to orbital motion of the stars, can be so baldy wrong after a few decades that many stars now are out of range of the instrument to hand. Other stars omitted from old lists could be widened into good targets. Double stars present intriguing insight to human vision. They may help you understand your own vision regime and seek appropriate eye care. They also point out some of the misapprehensions of vision that still circulates thru astronomy. In spite of it all, double stars are a most rewarding and enjoyable target, offering beauty and color wanting from most other celestial objects.