Double Star Question

I have a question regarding the magnitude differences between stars in
a double and how it affects the ability to split them.
One of my main reference books is an older one, "How to use an
Astronomical Telescope," by James Muirden. In the chapter on
double/multiple stars, he gives the basic information: magnitudes,
seperations, etc., and a brief discussion about each. I understand
that the seperations are mainly for 1985, but that's not important for
my question.
Under 38 Lyncis (AB 3.9, 6.6; 2".8), he writes, "This close pair can
just be divided with an aperture of 100mm, but it is more distinct
with larger instruments."
Under v Ursae Majoris (AB 3.7, 10.1; 7".2), "The primary is a lovely
full yellow, but the companion may be found difficult to see with an
aperture of less than 150mm or so."
The second pair is more difficult to split, even though it's
relatively much wider than the first, because of the difference in
magnitudes between the doubles in each pair.
I understand the basics of the resolving power of differing apertures,
that's not the question. What I'd like to know is, is there any "rule
of thumb" that one can use to determine the difficulty or even the
possibility of splitting two stars that are "x" magnitude at "y"
distance apart in any given aperture telescope (in my case, a 114mm
refractor)?
BTW, does anyone know of a reference source (book, website) that can
tell me the current (or close enough) distances between double stars?
Thanks!

"Lisa" wrote in message
om...
I have a question regarding the magnitude differences between stars in
a double and how it affects the ability to split them.

No..theoretical angular resolution has nothing to do with magnitude. Only
aperture.

The minimum resolvable angular separation= 206265 (wavelength of light/D)

On 22 Sep 2003 15:42:04 -0700, (Lisa) wrote:

I understand the basics of the resolving power of differing apertures,
that's not the question. What I'd like to know is, is there any "rule
of thumb" that one can use to determine the difficulty or even the
possibility of splitting two stars that are "x" magnitude at "y"
distance apart in any given aperture telescope (in my case, a 114mm
refractor)?

Sky & Telescope had some articles on this subject a few years ago.
You could search their web site. Dawes limit is for approximately
equal 6th magnifude stars. Faint companions and so on need a
larger telescope.

BTW, does anyone know of a reference source (book, website) that can
tell me the current (or close enough) distances between double stars?

Look for the Washington Double Star Catalog. Several computer
programs include WDS data. Cartes du Ciel (free download!) is one.
The WDS itself is on line.

"Mick" wrote in message
...

"Lisa" wrote in message
om...
I have a question regarding the magnitude differences between stars in
a double and how it affects the ability to split them.

No..theoretical angular resolution has nothing to do with magnitude. Only
aperture.

No.. she asked about real stars. Unequal magnitudes can make a big
difference. See Sirius for example.

The minimum resolvable angular separation= 206265 (wavelength of light/D)

Formulas make little sense without specifying the units. Generally, few
people use wavelength and would be more familiar with mm or inches. That is
(without getting too technical about airy disks etc), 4.5 divided by the
aperture (in inches) gives resolvable separation in arc seconds. Of course,
this is assuming decent optics and fair observing conditions. Top optics and
very stable air will do a little better. Of course, once you move beyond
4-6" seeing will usually limit you before aperture does.

You might also try the doubles group at:
http://groups.yahoo.com/group/binary-stars-uncensored

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

Lisa,

magnitude differences will have a significant influence on the
difficulty of splitting doubles. A few general rules:

The closer they are to each other in magnitude the easier they will
be.

Even if they are similar in magnitude, the closer they are to the
limiting magnitude of your scope the more difficult they will be.

One post above refers to the wavelength of light formula. That
formula boils down to the Rayleigh criterion. 5.45/D inches (138/Dmm).
Without the technical gargon, It is a measure that gives a good
indication of what you could see "split". it measures the distance
between the centers of two stars Airy disks separated by the radius of
the Airy disk. Since the central bright spot is slightly smaller than
the Airy disk, this usually shows a fine split.

The other numerical reference made above, actually 4.56/D (116/Dmm) is
Dawes Limit. It gives a measure of the limit you would be able to see
two stars as double if you separate the centers of two stars by the
less than the radius of the Airy disk, greater than 50% overlap. This
may of may not show a fine split.

Neither Rayleigh or Dawes provides a true measurement of a split
between two stars. Only if the light in the central bright spot of
the Airy disk is small enough will you be able to SEE a split. The
answers these two formula give as results is in arcseconds of angular
measure.

A great reference book for double stars and other deep sky objects is

Sky Catalogue 2000.0 Volume 2, Double Stars, Variable Stars and Non
Stellar Objects.

edz

edz wrote in message . ..

Lisa asked in message . ..
BTW, does anyone know of a reference source (book, website) that can
tell me the current (or close enough) distances between double stars?
edz replied -
Sky Catalogue 2000.0 Volume 2, Double Stars, Variable Stars and Non
Stellar Objects.
W Hamblin replied -
Look for the Washington Double Star Catalog. Several computer
programs include WDS data. Cartes du Ciel (free download!) is one.
The WDS itself is on line.

Lisa, in addition to these sources, I found the following useful:

Mullaney, Jack. 1998-2002. Celestial Harvest. Dover Books.

Mullaney culled 300 celestial objects from a number of catalogues, but he has a "thing" for doubles and multiple systems. Half his
catalogue are binaries and multiples. His binaries were chosen for asthetic value - e.g. color contrasting pairs like eta Cas and
itoa Cas or the classic beta Cygnus (Albireo). Visually striking binaries are typically those with wide spectral classes like O
(white) and B (blue white) contrasted with G (yellow) or M (red).

You may have to order this book from a used book dealer.

Hello there Lisa,

In Jan 2002 issue of S&T, there are two articles about double
star by two experts, Luis Arguelles and Sissy Haas. I think
they'll answer your question very nicely. Unequal doubles can
be quite challenging: Antares is one that comes to mind.

Ron B[ee]
--------------

"Lisa" wrote in message
om...
I have a question regarding the magnitude differences between stars in
a double and how it affects the ability to split them.

Thank you to everyone for your input! I will look for these reference sources.


(Lisa) wrote in message . com...
I have a question regarding the magnitude differences between stars in
a double and how it affects the ability to split them.
One of my main reference books is an older one, "How to use an
Astronomical Telescope," by James Muirden. In the chapter on
double/multiple stars, he gives the basic information: magnitudes,
seperations, etc., and a brief discussion about each. I understand
that the seperations are mainly for 1985, but that's not important for
my question.
Under 38 Lyncis (AB 3.9, 6.6; 2".8), he writes, "This close pair can
just be divided with an aperture of 100mm, but it is more distinct
with larger instruments."
Under v Ursae Majoris (AB 3.7, 10.1; 7".2), "The primary is a lovely
full yellow, but the companion may be found difficult to see with an
aperture of less than 150mm or so."
The second pair is more difficult to split, even though it's
relatively much wider than the first, because of the difference in
magnitudes between the doubles in each pair.
I understand the basics of the resolving power of differing apertures,
that's not the question. What I'd like to know is, is there any "rule
of thumb" that one can use to determine the difficulty or even the
possibility of splitting two stars that are "x" magnitude at "y"
distance apart in any given aperture telescope (in my case, a 114mm
refractor)?
BTW, does anyone know of a reference source (book, website) that can
tell me the current (or close enough) distances between double stars?
Thanks!