Author Topic: Dumb question but what is a Double Star and how do you find them?  (Read 358 times)

Stanley Elliott

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Are they Star's that look together visually although they may actually not be shut or are they actually close to one another?  Is there a list of those?



Chris Smale

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #1 on: December 29, 2017, 03:38:23 AM »
True double stars are gravitationally linked and relatively close to each other. "Visual doubles" are stars that happen to be in nearly the same direction so appear to be close together but are actually far apart. A list of double stars will include only gravitationally linked stars.

kerolero

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #2 on: December 29, 2017, 03:39:15 PM »
A double star is two stars that are orbiting each other otherwise known as a binary star system. Sometimes people call stars that just happen to be near each other from our perspective double stars when they really aren't true double stars. There are plenty of lists of them I would advise you to look at Albireo or Mizar/Alcor. Those are some nice bright double stars that are easy to tell apart. Albireo is my favorite because the stars are nearly opposite colors (blue and yellow).

Tim Jauregui

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #3 on: December 30, 2017, 06:01:52 AM »
It's not a stupid question. And they can actually be both. Two stars that merely appear close to one another, but are actually far apart in space, is called an optical double, while two stars that are in actual orbit around each other is called a binary star. There can be two or several members, in which case it's called a multiple star. There are many catalogues, most from the 19th and early 20th century, when studying double stars was a big thing in astronomy. One of the most famous catalogs is the Struve catalogue, which consists of ~2500 pairs, most of them discovered with a 9" refractor, which makes it ideal for amateurs. There are many other catalogues. Just Googling for double star catalogues should get you started. There's also many observing lists, assembled from various catalogues by observers.Clear skies!
Thomas, Denmark

Colin Ramadan

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #4 on: January 09, 2018, 01:44:24 PM »
This catalog costs ten bucks *shipped* and has a great list of double stars that are do-able for most amateur instruments. It also has a generous serving of galaxies, nebulae, planetary nebulae, globular clusters, and what not.

Greg N

acbacema

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #5 on: January 10, 2018, 09:35:22 AM »
The Astronomical League has a list of doubles that should keep you entertained. See their Doubles program for details.\

Alex

James Przystup

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #6 on: January 11, 2018, 12:50:18 AM »
The AL's list of double stars includes many of the best binary and multiple stars.

https://www.astrolea...ar/dblstar2.pdf

Some other noteworthy binaries that are currently visible include Almach (Gamma Andromedae), Cor Caroli, Eta Cassiopeiae, Gamma Delphini, Alpha Herculis, Epsilon Lyrae (the Double Double), and Struve 2470/2474.

Dave Mitsky

corloconre

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #7 on: January 12, 2018, 05:58:49 AM »
Starting out as a nebula hunter, double stars came as a bit of a surprise for me. My experience is simple aesthetics. Many are just beautiful, often colorful and curiously engaging. Most star atlases use the same convention for marking double stars, the usual star dot with a horizontal line through it. This is how I started locating them. I soon sought out some deeper references like the Cambridge Double Star Atlas and Sissy Haas' Double Stars for Small Telescopes. The latter has no charts but contains a great deal of data and some interesting observing notes.

charnhoupito

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #8 on: January 12, 2018, 10:30:52 AM »
Thank you! I'm going to learn more about these Double Stars and start looking for some! I'm only setup for visual observation and with the Nexus DCS on my Double Star mount and these resources I should be able to find some!

middbankrecra

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #9 on: January 12, 2018, 06:59:27 PM »
One of the nice things about doubles is that you can generally study them from the city.

Alex

Chris Castillo

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #10 on: January 13, 2018, 03:49:07 PM »
Quote
One of the nice things about doubles is that you can generally study them from the city.

Alex

+1. As a novice, I've found that from my suburban Chicago location, they are great targets. The first one I found, not intending to but kind of stumbling onto it, was the Double Double, and that whetted my appetite. I plan a good portion of my sessions around them at this point.

vicareeti

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #11 on: January 13, 2018, 10:25:25 PM »
See this Cloudy Nights post:

http://www.cloudynig...-4#entry7235365

Thomas Homer

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #12 on: January 14, 2018, 04:30:11 AM »
Quote
A list of double stars will include only gravitationally linked stars.
Not necessarily. Many double-star lists include some visual pairs; there happen to be a number of very attractive examples of these. For instance, the striking (and challenging) naked-eye pair of Alpha1 and Alpha2 Cap.There are also many cases where it's impossible to determine whether any given pair is actually bound gravitationally or not. Two cases that people have been arguing about for more than a century are Mizar/Alcor in the Big Dipper and Albireo in Cygnus.It's probably true that most of the entries in the enyclopedic Washington Double Star catalog are visual doubles rather than true binaries.

birchzufhyro

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #13 on: January 18, 2018, 04:40:53 AM »
A few thoughts, comments, experiences:

Some apps and programs have extensive double star data bases. The two I use are Skytools 3 and Sky Safari 4 Pro. Both these programs use the orbital data for short period binaries to compute the actual separations and position angles. Binaries like Porrima, Sirius and Castor have short enough orbital periods they slowly change. For example, the current separation of Porrima (Gamma Virginis) is about 2.5 arc-seconds, 5 years ago it was 1.7 arc-seconds, 10 years ago it was about 0.5 arc-seconds.

Porrima is a relatively close double. The resolving power is related to the aperture of the telescope and the stability of the atmosphere are both critical for close doubles. Today, at 2.5 arc-seconds, it's a relatively easy double and splittable is a 60mm telescope. 5 years ago it required reasonable good seeing and a telescope around 80mm. 10 years ago it required excellent seeing, 0.5 arc-second seeing is rare in most parts of the country and a telescope at least 10 inches.

I use these programs because they provide current information rather than information for a fixed date like a list or some other soffware does.. They are not always precise but they are close. Both SkyTools 3 and SkySafari have extensive search functions with the ability to turn the searches into observing lists. Both are capable of search by magnitude and separation as well as by constellations... I often create double star observing lists with Sky Safari when I am observing. I might pick a constellation or two based on their location in the sky, a range of separations and magnitudes based on the seeing and the telescope I am using. The actual search takes maybe 15 seconds. I then choose for each object on the list to be marked with a circle in the chart view and away I go, moving from one to the next.

As others have said, double stars, triple stars, these are great fun, most are visible from a light polluted back yards, there is a great variety of doubles, some are just stunningly beautiful, some have wide separations, some are very close and represent a challenge, some are equal magnitude pairs, some differ in brightness by several magnitudes or more. There are double stars for every aperture and they are a great many of them..

If one is splitting close double stars, the resolving power of the telescope is important. There a couple of numbers that are commonly used to determine just how close a double star a telescope of a given aperture can resolve. The Dawes limit is probably the most common but a Dawes limit Double is actually very challenging. In a Dawes limit double, the two Airy disks are overlapping, there is no clean separation, there is only a minima of about 5% drop between the two centers. The Rayleigh Criterion is more reasonable as a limit, the Airy disks are overlapping but the center of one disk and the first minima coincide.

The equations for these limits:

Dawes (arc-seconds) = 4.56 inches/D  The Dawes limit for a 4 inch scope is about 1.14 arc-seconds, for a 10 inch about 0.46 arc-seconds.

Rayleigh Criterion (arc-seconds) = 5.45 inches/D, the Rayleigh criterion for a 4 inch scope is 1.36 arc-seconds, for a 10 inch, about 0.55 arc-seconds.

These numbers apply to equal magnitude doubles that are about 5th magnitude. Unequal doubles are generally more challenging.

If one is hoping to split close double stars, the seeing is an important consideration as is the thermal state of the telescope. Because one is pushing the telescope and it's optics very close to the theoretical limit, everything has to be just right.. For very close double stars, very high magnifications are often necessary, beyond the normal 50X/inch limit. This is because the Airy disks are overlapping and so the observer is actually inspecting the two disks as if they were extended objects. I often use magnifications of about 80X/inch, sometimes even more if the seeing is excellent. Last night I was viewing Zeta Herculis which is a 1.3 arc-second magnitude 2.8-5.7 double, not an easy one in a 120 mm refractor. I got the best view at 360x which is about 76X/inch though I did try 514x..

There is a double star forum on Cloudy Nights... Doubles are the great equalizer, No matter what scope one uses, no matter what the conditions are, there a doubles to be seen and enjoyed.

Jon Isaacs

Dan Perez

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Re: Dumb question but what is a Double Star and how do you find them?
« Reply #14 on: January 21, 2018, 02:30:08 AM »
Quote
It's probably true that most of the entries in the enyclopedic Washington Double Star catalog are visual doubles rather than true binaries.
From what I know, it's actually the other way around. I think it was proven statistically, that the number of faint stars close to one another is much greater than what should have been the case, had they been distributed at random.Clear skies!
Thomas, Denmark