Seven objects of the Messier's list are generally classed as gaseous nebulae, although Diffuse Nebulae would perhaps be a better description. They are: M8 (Lagoon Nebula), M16 (Eagle Nebula: but Messier saw the ssociated cluster, not the nebulosity), M17 (Omega Nebula), M20 (Trifid Nebula), M42 and M43 (Great Orion Nebula) and M78 (reflection nebula in Orion).
The diffuse nebulae are clouds of interstellar matter, rarefied but very extensive agglomeration of gas and dust. They are often, if sufficiently large and massive, regions of star formation: so, they are frequently associated with star clusters. Some of the young stars are sometimes so massives and hots that their high radiative energy excite the gas of the nebula, to the point that it emit light: these are the emission nebulae. M8, M16, M17, M20 and M42 are all of this type.
If the stars are not sufficiently hots, their light is simply reflected or diffused by the dust, and it can be seen bluish as reflection nebula. It is the case of M78, and the nebulosity surrounding M45. M43 emits both emission and reflection spectra.
The dark nebulae, at last, are clouds of interstellar matter that shade the light of the stars on the other side of them.
Emission, reflection and dark nebulae are often associated (as in the Orion' region).
Globular clusters are heaps of stars, more or less spherical in shape. As much satellites, they populate the halo of several galaxies, both spirals, as our Milky way, and ellipticals.
About a hundred or so globular clusters are known, enveloping our Galaxy everywhere. They form a more or less spherical system, whose centre coincides with the centre of the Galaxy. Harlow Shapley (first authority on these objects) discovered it. The system of M31 counts some 140 globulars (the biggest, named G1, finely resolved into stars by Hubble Space Telescope), and the one of M87 even over 1,000.
Since the Solar System is some 30,000 l. y. away from the centre of the Galaxy, the globular clusters appear to be arranged asymmetrically in the sky: they crowd round the regions toward the centre of the Milky Way, in the constellation of Sagittarius, Scorpius, Ophiuchus and Ara, while in the opposite direction the sky seems lacking in such objects.
The nearest globular is M4 in Scorpius. The gratest is w Centauri. In the northern sky, the most conspicuous globular is M13 in Hercules, followed from M3 (Canes Venatici) and M5 (Serpens).
Although many of the globular clusters appear irregular as seen through the telescope, photographs showing all the fainter stars demonstrate that the total outline is rather circular. An average globular contains some 100,000 stars and has an absolute visual magnitude of about -7 or -8. The true diameters are more or less 150 l. y. and the average brightness is 100,000 times the Sun's one.
The degree of concentration of the stars is the most prominent feature
(work in progress...)
