APOD: 2021 July 15 - The Dark Tower in Scorpius

Good morning, Rallypoint. Today's Astronomy Picture of the Day (APOD) is titled "The Dark Tower in Scorpius." The phrase 'Dark Tower' first brings my mind to J.R.R. Tolkien and the Land of Mordor. But now today's APOD has my attention redirected. The term 'OB association' was a new one for me; it refers to two types of stars that are usually found together and have similar spectra when analyzed. Both appear to burn through their hydrogen fuel rapidly, starting off as O-type, becoming B-type, and eventually expiring as supernovae in ~15 million years (compare that to our 5 billion year-old Sun).

NGC 6231 is an OB association of stars clustered near the top of the Dark Tower. The Dark Tower itself is called a 'cometary globule', a dust cloud with a defined head and an extended tail. Here, the tail of the Dark Tower is likely being extended by the radiation coming from NGC 6231. The red glow surrounding the tower is produced by that same radiation as it ionizes hydrogen gas that is present. Light from the stars produces the bluish reflection nebulae seen around some of the members of NGC 6231.

This is the time of year when we find the constellation Scorpius in the southern sky shortly after sunset. NGC 6231 is located at the base of the scorpion's body to the right of where the tail bends upward. The Dark Tower and its star cluster lie about 5,000 light years away.