Stardate:07:15:99

Clusters, Clusters Everywhere

With it's strange sounding Arabic name, Sadr is one of the brightest, easiest stars to find at this time of year. Shining at magnitude 2.3, it is about as bright as a typical approach of the planet Saturn. At about 10pm this week, look for this star just 20 degrees north of due east, nearly 48 degrees above the horizon. That's over one half of the distance to straight above our heads! This will be our starting point for this week's dance across the skies.

There are two open clusters right near this area. The first is just 1-3/4 degrees away from Sadr, almost in a perfect diagonal to the east, and upwards above the horizon. This is an open cluster of stars known as M29, or NGC 6913. This object is nearly bright enough to been seen with the naked eye from a dark sky location. However, at only 7 arcminutes across, it fills only about 1/9 of a degree side to side as seen from Earth.

Now, head on back to Sadr for the journey to our next object of the week, NGC 6910. This is another open cluster which can be found just 1/2 degree away from Sadr, in the opposite direction as M29. NGC 6910 is not quite as bright, or even as large as M29, but it is comparable.

One question you may ask yourself is how are these objects are alike, and how they are different? Are the stars which comprise them similar colors, or different? The answer to this question is not as trivial as it may seem. There is a lot one can tell just from the colors of the stars about the cluster or galaxy which you are viewing. That, however, is the subject of another week's article.

Our next leap of the evening requires a jump of 5 degrees away from NGC 6910 to NGC 6866. It can be found 4-1/2 degrees above our last object, and three degrees to the north. Number three in our succession of open clusters, this one is about as bright as NGC 6910, and appears as large across as M29.

One method of easily keeping track of how far you travel across the sky (and the easiest method, in my opinion) is to measure how much area of the sky you can see through the instrument you are using. For instance, the Moon stretches about 1/2 degree from horizon to horizon. Some time when the Moon is full, go out and measure how many full Moons stretch from one side of your field of view to the other. A typical pair of binoculars may have a field of view perhaps five degrees across. A small backyard telescope will have a field of view of perhaps one degree at medium power.

Once you know how much of the sky can be seen at one time, when you are observing, you can simply count how many "fields of view" you will need to travel. If you have to move two degrees, for instance, and the telescope/eyepiece combination you are using has a field of view of one degree, it is a simple matter of traveling two fields of view from one object to another.

Traveling one quarter of a degree north of NGC 6866, and 3 1/2 degrees further above the horizon brings us to the bright (magnitude 3) star Delta Cygnus. The name is derived from the fact that Delta is the fourth letter of the Greek alphabet, and this star is the fourth brightest star in the constellation Cygnus. It has the even more unromantic name SAO 48796.

This star is a good launching point to find our next object, NGC 6811. This object is another in our treasure chest of open clusters, and brighter than any other open cluster we have seen this week, except for M29. It is also the largest by far, appearing nearly twice as wide across as M29. This is NGC 6811, and it is found close enough to the pole star to be classified as a circumpolar object, meaning it can be always be found somewhere in the northern sky, as seen from anywhere in the world at our latitude. Look for this cluster 2-1/4 degrees further north from Delta Cygnus, and an additional degree and a half above the horizon.

Those people who may enjoy a final challenge of the week can turn their attention 7-1/2 degrees further north (you will now be exactly 30 degrees north of east). Lower your sights just 1/2 degree toward the horizon, and see if you can spot a small, dim planetary nebula. This is called the "Blinking Planetary", or NGC 6826. This is another circumpolar object, meaning it can be seen year round, and it never sets.

Clear skies, and good viewing.

"Understanding is joyous" - Carl Sagan