Located in the constellation of Sagittarius, in the middle of the Milky Way and in the direction of the galactic center, the nebulae M8 (“the Lagoon”) and M20 (“the Clover”) constitute 2 “star” targets of the summer sky. Two very bright nebulae and easy to observe with a simple pair of binoculars, and clearly visible already to the naked eye for the Lagoon under a dark sky.
If each of these two nebulae constitutes in itself one of the most beautiful objects in the sky that it is possible to observe, their association sublimates them even more. The astrophotographer will therefore often be tempted to associate them in the same shot.
The M8 nebula, in the Lagoon, is located just over 4,000 light-years away and extends over 110 light-years, for 50 light-years wide. It is therefore a nebula of considerable size, whose apparent size in the sky is greater than 3 times that of the Full Moon.
The M20 nebula, the “Clover” or “Trifid”, is slightly further away, at about 5000 light-years. Of more modest dimensions, it largely compensates for its reduced size by its fascinating beauty, which it reveals to both observers and photographers. It combines emission (red), reflection (blue) and absorption (dark bands) zones.
Essentially, these two nebulae are in “emission”: they are large clouds composed of 90% hydrogen, whose atoms are ionized by the radiation of the stars newly created within them. True stellar nurseries, the immense gas clouds condense in multiple places to give birth to as many new stars, whose radiation then illuminates the nebula. The HII regions (ionized hydrogen) have an emission line at 656 nm (Ha line), which explains their characteristic red color.
For the Lagoon, the newly created cluster (NGC 6530), about 2 million years old, consists of several hundred young and hot stars. The nebula is also strongly ionized by the radiation of the blue supergiant 9 Sag . For the Trefoil, the emission nebula is mainly caused by the radiation of a triple star system at its core, the main component of which is a giant star of more than 20 solar masses. But an open cluster of 3000 stars is located around the Trefoil; and a few hundred stars are still hidden by the dark nebulae…
The Clover Nebula also has a magnificent area of reflection nebula, of intense blue color. The mechanism at work here is different from the emission areas: the radiation is not energetic enough to ionize the atoms of the cloud, and the light is simply diffused by reflection. The angle of reflection undergone by the light is different depending on the size of the molecules of the cloud and the wavelength of the incident light: the shorter wavelengths are diffused more, which explains the blue color of these nebulae (it is also the same mechanism that explains why the sky is blue on Earth).
In the Lagoon, as in the Trefoil, many dark (or absorption) nebulae are also present; mainly visible by contrast in front of the emission nebulae. It is these bands of compact dust and gas that draw, for example, the characteristic “trefoil” shape in the heart of the nebula M20 (Barnard 85). These areas are so dense and compact that visible light cannot pass through them… on the other hand, thanks to infrared radiation, it is possible to reveal the presence within them of many newly created stars as well as active star-forming areas.
The nebulae themselves obscure a large part of the stars, as demonstrated in this comparison between visible and infrared light carried out by the ESO. These infrared images have notably made it possible to discover 2 Cepheids hidden by the nebulae, which allow us to better determine the precise distance of the latter.