Hot, Humid July Nights

Globular Cluster Pairs

July in southeastern Virginia brings steamy cloudy nights. There's little opportunity for celestial observing, especially near full Moon. Brief clearing on July 23rd and 25th allowed only a few hours to employ my Seestar50.

Over the past year I've used Seestar to image almost all relatively bright dramatic objects within its range. It's time to try something different. Lately, I've been using Seestar's mosaic mode to capture more than one object in the field of view. One example below is a globular cluster pair in Sagittarius captured on July 23rd with a 40-minute exposure. Enlarge the next image and scroll around to see large globular cluster M22 on the bottom and small, undistinguished globular cluster NGC6642 near the top. An attractive five star group near the right edge contains colorful 5th magnitude red supergiant star, 24 Sagittarii. The crowded Milky Way star field provides a busy background. M22 is separated from NGC6642 by 67 arc minutes. 

What explains the remarkable apparent size difference of these two globular clusters? Answer: M22 is both intrinsically larger and closer than NGC6642. M22 is 97 light years in diameter and 10,400 light years away. NGC6642 is 45 light years in diameter and 26,700 light years away.

Mosaic mode was not required to capture another globular cluster pair in Sagittarius separated by only 16 arc minutes. In the next 10-minute exposure NGC6522 on the right and NGC6528 on the left appear in front of Milky Way stars and some dark nebulosity. The brightest star at the bottom is Alnasl, the 3.6 magnitude star at the tip of Sagittarius' teapot spout.

These disappointing clusters have modest diameters and are relatively far away. NGC6522 is 25,000 light years away and has a diameter of 69 light years. NGC6528 is 26,000 light years away and has a diameter of 37 light years. The sky became increasingly hazy on July 23rd, so I had to stop imaging after obtaining the previous two images. The observing window was open for only about two hours.

At twilight on July 25th the sky seemed nicely clear so I attempted to capture both nebula M17 and open cluster M18 together in mosaic mode. The upper portion of the next 58-minute exposure shows M17, The Omega Nebula, together with open star cluster M18 in the bottom right. M18 is 56 arc minutes from M17. I was disappointed that M18 is only a modest grouping a few stars. I was also disappointed by the sky's increasing milky haze which caused me to limit exposure time and quit early. Once again, the observing window was only about two hours long. 

There are a lot of clouds in July! They obstruct astronomical observing, but often bring late afternoon showers and thunderstorms. Sometimes, conditions are right for rainbows! On July 27th the Sun was still high above the horizon and the rainbow in the next image was correspondingly low. This rainbow was unique because its left end extended down in front of trees about a quarter mile away. I had never seen a rainbow this close to my observing spot before.

Four days later on July 31st a partial rainbow appeared under clouds almost in the same location as the previous rainbow. In the next image you can see its left end behind the trees this time.

I hope for fewer clouds in September and October.