By Bill Pellerin, GuideStar Editor

Houston Astronomical Society

The Astronomical League has a Double Star Club, which I highly recommend (and which I completed some years ago). The program requires you to observe 100 double stars and to record those observations in an observing log with a drawing of the double star (anybody can put a pair, or more, of dots in a circle).

If that is not enough double star observing for you, there are additional resources that you can use to continue your double star observing program. The two books that I will review for this article will keep you in the double star observing business for a long time. Which of these two books is appropriate for you depends on how you like to go about your observing program.

The two books are:

• Double Stars for Small Telescopes – by Sissy Haas
• The Cambridge Double Star Atlas – by James Mullaney and Wil Tirion

The Haas book includes 2100 double stars to see and the Mullaney/Tirion book has 2400 stars. Both books include double stars visible from both the northern and the southern hemisphere, so for most observers not all of the stars in the books will be accessible. Both are large format books useful in the field but only the Mullaney/Tirion book is spiral bound, allowing it to lie flat on the table.

Here are the two books.

The Cambridge Double Star Atlas, by James Mullaney and Wil Tirion (March, 2009)

By Bill Pellerin, GuideStar Editor

Houston Astronomical Society

The Astronomical League has a Double Star Club, which I highly recommend (and which I completed some years ago). The program requires you to observe 100 double stars and to record those observations in an observing log with a drawing of the double star (anybody can put a pair, or more, of dots in a circle).

If that is not enough double star observing for you, there are additional resources that you can use to continue your double star observing program. The two books that I will review for this article will keep you in the double star observing business for a long time. Which of these two books is appropriate for you depends on how you like to go about your observing program.

The two books are:

• Double Stars for Small Telescopes – by Sissy Haas
• The Cambridge Double Star Atlas – by James Mullaney and Wil Tirion

The Haas book includes 2100 double stars to see and the Mullaney/Tirion book has 2400 stars. Both books include double stars visible from both the northern and the southern hemisphere, so for most observers not all of the stars in the books will be accessible. Both are large format books useful in the field but only the Mullaney/Tirion book is spiral bound, allowing it to lie flat on the table.

Here are the two books.

The Cambridge Double Star Atlas, by James Mullaney and Wil Tirion (March, 2009)

The key word in the book title is Atlas and this is what you get. The major part of the book consists of a set of sky atlas pages with double stars identified on those pages. The maps by Wil Tirion are as beautiful as we have come to expect from the premier star map producer of our time. Double stars on the pages are identified in the way that has been standard for a while – there is a black circle representing the location of the double star and a horizontal line through the circle. You can easily look at the map and identify the double stars this way. Like all star atlases that I have ever seen, brighter stars are represented by larger circles on the map. The other identifier of a double star is a catalog identifier, in green. Since there are quite a few double star catalogs there are quite a few forms of the identifier. An identifier of the form Σ (sigma) <number> represents a double from the Struve catalog; an identifier of the form H <number> represents a double from the Herschel catalog, and so on.

This book opens with a short introduction to double star observing which includes a list of 133 ‘showpiece’ double stars. This list is in alphabetical order by constellation (sort of). For some reason, a listing in CVN (Canis Venatici) precedes a listing in CMA (Canis Major) in the list. This list tells you which map the double star is on, the brightness, and separation of the double.

The atlas has other utility as well, many deep sky objects, nebulae, clusters, and galaxies are included in the charts. For example, if you look at chart 2, you will see the double cluster identified, and chart 11 identifies the Ring Nebula and the Dumbell Nebula. Since this is a double star atlas, there is no discussion anywhere in the book of these objects. You will have to look elsewhere for this information.

There are three ways to use this book as a resource for your observing session:

• Find the double star on the chart, move the telescope to the position of the double star, and give it a try. You may not be able to see it because the double star may not be visible in your telescope with your observing conditions. If you’re observing from an urban site and you find out that the double star consists of a 2nd magnitude star with a 10th magnitude companion at 1 arc-second away it is unlikely that you’ll be able to see it. Since you won’t know this until you have interrogated the tables at the back of the book, using this book in this way from an urban location may be challenging.
• Find the double star on the chart, look up the double in the table at the back of the book to get more information (magnitudes and separations), and then decide if you want to pursue the observation of this pair. It can be a bit tedious to be flipping between the map pages and the lists. You have to determine the RA and DEC (or the name) from the map and find the star in the table.
• Start with the tables and pick the star systems that are a good match for your observing conditions and equipment before you find the star on the map. Find the star on the map and move your telescope to that position.

From a dark sky site with adequate equipment to show a wide range of double stars the maps will direct you to double stars in a region of the sky (star-hopping, anyone?) so you can slew around the area to view a variety of double stars.

Recommendation: If you like using a star map and star-hopping, and you are observing from a good site, you will delight in this book and in the maps it includes. If you want to know more about your object before you track it down, you may find the tedium of flipping back and forth a problem. The book is a joy to browse through owing to the quality of the maps by Wil Tirion. As an aside, Wil Tirion was at the 2004 Texas Star Party and was warmly received by everyone there in recognition of his contributions to astronomy.

Double Stars for Small Telescopes: More Than 2,100 Stellar Gems for Backyard Observers (May, 2007), by Sissy Haas, Sky Publishing Corporation

Sissy Haas wrote double star columns for Sky and Telescope for some years, but has not appeared in the magazine recently (in fact the last column that I can find was written in 2002).  She leaves us with this book, a catalog of double stars, so that we can continue to enjoy her efforts for many years to come.

The book is not a collection of her articles. Think of it as a database of double stars from which you can pick and choose the ones you want to try.

The introduction of this book includes information on double stars and an attempt to specify the characteristics of double stars which are visible in different size telescopes. The telescope requirements are based on separation of the stars and magnitude differences in the stars. This is a good try, but there are many more variables that are not included here – telescopic optical quality and seeing, to name two. The ‘showcase’ doubles list in this book only includes 15 objects, many fewer than in the other book described in this article, and includes maps (the only maps in the book) showing the locations of these stars.

The list is organized by constellations, and the names of the constellations are in alphabetical order. It is easy, for example, to look on page 90 in the book to begin the list of double stars in Leo. Here, and throughout the book, you will find the information you need to decide whether this star is worth a try in your telescope under your observing conditions. In addition to name, RA, DEC, magnitudes, and separation there is a description of the double by Sissy Haas or by one of her contributors. About Albireo (beta Cyg), Sissy says, "This wide couple is a stunning pair of deeply colored stars, brilliant citrus orange and vivid royal blue…".

If you want to direct your telescope to any of the stars in this book, you’ll need a map, and there are plenty of good ones on the market. The risk is that some of the objects on the book’s list may not appear on the map and while you may be able to navigate to the coordinates of the double star based on your map you may have to do some looking around to find the star that is being described by the book. According to the introduction, each of the entries in the catalog has a magnitude 8 or brighter primary star, so any atlas that shows magnitude 8 stars will show all the stars in this book. (Obviously, the combined brightness of the primary and its companion stars will exceed magnitude 8.)

Knowing which stars are available to be seen on your observing night is always a problem as well. This is true with both books. Good sky maps, good sky mapping software, or a planisphere can be very helpful here. You may wish to create a database of double stars of interest. This will be a bit tedious to accomplish, but doing so will make your time under the stars more productive. Software exists to help you identify which objects in your observing list are available to be seen; I use SkyTools (www.skyhound.com), but I know that other good software exists.

Recommendation: You will need a good sky map or good sky mapping software to use this book successfully. A go-to telescope will help – you can enter the coordinates of the star system (RA & DEC) and the telescope will point to the location of the object. This book is recommended for the observer who likes to know something about the object he or she is searching for before searching for that object. The only way to use this book is from the list since there are no maps included.

Bottom line: Both books contain what may be a lifetime supply of double stars to view, and there is probably a lot of redundancy between the lists. Either one would be a great addition to your observing library. You can get outside and observe double stars, bright ones at least, under less than ideal conditions (urban skies, bright moon, less than perfect transparency). Which one you choose depends on whether you want to chase down objects to see what they look like (not knowing in advance what to expect) or you want to choose your objects from a catalog and go after them (with information in hand on what to expect). In the first case you will probably prefer the Mullaney/Tirion book, in the second case you will probably prefer the Haas book (knowing that you will need to supplement the Haas book with a good set of maps or a go-to telescope).

The best news of all – either one is a great choice.