The Delta Twins of Lyra – Delta-1 (δ-1) and Delta-2 (δ-2)

Lyra, the Lyre, in all it’s double star glory! (Stellarium screen image with labels added; click on the chart for a larger view)

When you look up high into the summer sky at Lyra, if you let your eyes linger in the northeast corner of the parallelogram that marks the constellation’s outline, you’ll find your eyes are attracted to the delightful Delta (δ) Lyrae .  (If you find stellar directions — which are not the same as land-based directions — confusing, take a look at this post by Greg).

And, as with all of the other stars that form that outline (with the exception of Gamma (γ) Lyrae at the southeast corner), it’s a multiple star — although in this case the two primary stars are visual doubles, as opposed to being gravitationally linked.  (Actually, they’re moving in opposite directions from each other!)  But as a bonus, each of the two main components of Delta (δ) Lyrae are also doubles — which means we have here yet another “double-double” within the boundaries of Lyra.  In this case, however, the four main components don’t have quite the visual impact of Epsilon Lyrae and Struve 2470-2474.  What they do display, though, is a unique beauty which is all their own.

Delta-1 ( H VI 3)        HIP: 92728   SAO: 67537     
RA: 18h 53.7m   Dec: +36° 58′
Magnitudes:  5.6, 9.9
Separation:   175.2″
Position Angle:  20°   (WDS 2009)
Distance: 1080 Light Years
Spectral Classification:  B2.5, A

Delta-2  (ES 2028)    HIP: 92791   SAO: 67559
RA: 18h 54.5m   Dec: +36° 54′
Magnitudes   A: 4.3      B: 11.2      C: 11.6
Separation    AB: 87.9″       BC: 2.1″
Position Angle   AB: 349°   (WDS 2010)    BC: 135°   (WDS 2008)
Distance:  899 Light Years
Spectral Classification:   M4, G, G

Delta-1 and Delta-2 both stand out well against a rich background of stars because of their brightness.  In particular, the deep red color of Delta-2 will  quickly attract your attention.  The two of them are surrounded by a collection of stars which form an open cluster known as Stephen 1, usually abbreviated on the charts to Steph-1.  There is some question as to how many of the stars in the immediate vicinity belong to the cluster, but in general, they all are about 1100 light years distant, which puts them at the same distance as Delta-1.

Image as seen in a refractor, north at top, west at left. Click on the image for a larger view. (STScI photo with labels added)

Of the two pairs, Delta-1 is by far the easiest to split.  In the case of a 60mm scope, you’ll find that you have to look closely to see the 9.2 magnitude companion, but it isn’t all that difficult.  On this particular night, I was using two refractors, a 60mm and a 105mm.  In both of them, Delta-1 appeared white with a trace of blue, while the secondary was too faint to be able to really detect any color.  Even though it has a radius 5.6 times greater than the sun, Delta-1 is classified as a white dwarf, as is the secondary.  There is a hint of a spectroscopic companion linked to this system, but not enough data so far to confirm it’s existence.

Delta-2, a giant with a diameter equal to that of earth’s orbit, is a star of a different color — and I’m not referring only to its beautiful deep red.  The first time I viewed this system  several years ago in a 102mm refractor, I looked and looked for Delta-2’s “B” component and never found it.  I came to the conclusion that either it didn’t exist, or it was fainter than the listed magnitude.  However — there’s always a however — it’s there, and the published magnitude is about right.  The “however” trick is to use averted vision, and like magic, the elusive little devil will pop right into view.

In my 105mm refractor at 83x, averted vision was a necessity for extracting the 11.2 secondary from the glare of it’s much brighter companion.  Moving up to 94x, I could begin to see it when looking directly at it, and had no problem at all in seeing it at 107x and 150x.  In my f16.7 60mm scope, I had to look very hard to see it at 50x, even with averted vision, and even then, the focus had to be perfect to detect it.  Increasing the magnification to 71x made it a bit less of a battle, and at 100x it began to stand out.  I stopped at 111x, where it was reasonably easy to pick out of the glare with direct vision.

The 11.2 magnitude secondary of Delta-2 also has a companion just two arc seconds from it, and a bit fainter at 11.6.  It would be interesting to hear of anyone who has been able to overcome the red glare of the primary and the challenge of the two seconds of distance in order to see it.  I know of a devoted double star fanatic in Massachusetts with a fifteen inch Obsession who just might be able to get a glimpse of it!

Over the past few years, I’ve developed a fondness for Steph-1 which results from using it frequently to see how faint I can go with various combinations of scopes and eyepieces.  There is a good range of tenth through fifteenth magnitude stars for this kind of testing.  Of course you can go fainter than that, but I stopped at fifteen because it’s about the limit of my equipment.

When you look at a star field frequently, you begin to pick up patterns in the stars that stay with you.  For example, there is a line of four north-south stars to the southwest of Delta-2, and just east of those is another pair — together they make an attractive and easily recognized asterism.  Within that very small grouping are stars ranging in brightness from eighth magnitude to as faint as fourteenth.  It’s always interesting to aim whatever scope I’m using on a given night at this tight little group and see what the faintest star I can detect is.  The best part, though, is simply being able to recognize the star field after having seen it so many times that I no longer even need a chart to determine magnitudes.

Achieving that kind of familiarity with a very small part of the night sky is something every serious observer treasures — and it’s one of the things that keeps me coming back for more.