The progression of twilight

 There are three defined stages of twilight:  Civil Twilight ends when the sun is 6° below the horizon, Nautical Twilight ends when the sun is 12° below the horizon, and Astronomical Twilight ends when the sun sinks to 18° below the horizon.  The significance of Astronomical Twilight is that the sky is finally dark enough to begin observing faint objects (supposedly).

The following four images were taken with a Sony A7iii camera , Sony 20mm f/1.8 lens,  and a Sparkle-6 filter .  The first two were obtained about 20 min before the end of Astronomical Twilight, and the second set very close to twilight's end. 

Leo, Mars, and Gemini setting over Bristol Head (SW view).

Gemini (Castor and Pollux) and Auriga (Capella) setting over Bristol Head (NW view).

Twenty minutes later, at twilight's end:

 

 One feature of interest in these images are the bands of green and purple airglow.  This airglow is never easily visible to the eye but is always picked up by the camera sensor.  Until I started photographing the night sky I had no idea how much color was present in the seemingly dark sky, just below the level of perception.

The sky brightness measured over an hour after the end of Astronomical Twilight was sqml=21.45.  This is about 0.2 mpsas brighter than earlier in the week.  The likely explanation is the airglow illustrated above, but there was also some haze in the air this morning, so distant wildfires might be contributing to the decrease in darkness. 

Looking toward the southern horizon, the giant globular cluster Omega Centauri was just emerging from behind the flank of Snowshoe Mountain:

Olympus E-M1iii + Sigma 56mm f/1.4 lens. ISO 1600, 30 s, softon filter.

 Some obtrusive cabin lights have been blacked out in the lower left corner.  These lights are unfortunately just below the line-of-sight to the cluster.

Looking almost straight up, the constellation Coma Berenices is dominated by the star cluster cataloged as Melotte 111.  This cluster is one of the nearest open star clusters, about 300 light years distant.

The Coma Star Cluster, Mel 111.  Sigma 56mm + softon filter.