OBSERVING DOUBLE STARS
--------------------
John Pazmino
NYSkies Astronomy Inc
nyskies@nyskies.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.