Like most Lagoons, the Lagoon Nebula is definitely deepest in the middle!
The superb picture at right really hints at what I think is the true structure of the Lagoon. Note at lower left a billowing red ridge (with a dark dust "man" standing near the lower left corner, staring to the upper left with his one misplaced eye). Above the red ridge is cluster NGC 6530, and the hot stars of NGC 6530 are undoubtedly responsible for carving out this red ridge and clearing a cavity behind it. A long dark "waterfall" (or "dust flow"?) separates most of the members NGC 6530 from bright blue star 9 Sagittarius and slightly less bright blue star HD 164816.
Note the blue light surrounding 9 Sagittarius. I interpret this as a faint reflection nebula, which in turn means (or so I think) that 9 Sagittarius is so young that it hasn't had time to blow away all the dust in its vicinity, and the ambient dust reflects blue light from this blisteringly hot O4V star.
By contrast, the stars on the left side of the long dark dust flow don't show obvious signs of being surrounded by reflection nebulas, even thought the lit-up dust wall behind it displays a purplish hue. These stars have had time to (mostly) clear away the gas and dust that they were born out of from their immediate vicinity, but the red ridge in front of them and the purplish wall behind them shows where gas and dust has piled up.
According to this paper
, NGC 6530 is only about 2.3 million years old. The paper also made the assumption that 9 Sgr is part of NGC 6530. My amateur opinon is that 9 Sgr appears younger than the stars on the other side of the dark dust flow.
Can you see the bottom of it?
Now take a look at the bright center of the Lagoon Nebula
, where the hot massive star Herschel 36 has just been born, and its tattered birth nebula, known as the Hourglass Nebula, is brilliantly lit up. There can be no doubt that Herschel 36 and the Hourglass Nebula are situated in the (currently) deepest past of the Lagoon Nebula. In the heart of Sagittarius, star formation progresses farther and farther away from us, deeper and deeper into the hidden depths of this deep cosmic Lagoon.
Star formation in Corona Australis. Photo: Marco Lorenzi.
Star formation typically takes place at one end of elongated dust structures, like the one you can see in the picture at right of a region in Corona Australis. Stars are being born at one end of a long "tube" of gas and dust, and the dusty filament of star forming material goes on for light years behind the actual birth site of the stars. In some cases, star formation "eats" its way deeper and deeper into the cosmic river of yummy molecules, and the "river" gets "pregnant" again and again with more and more stars. The dust lanes, however, wither away as more and more of their atoms and molecules get locked up inside stars.