THE MILKY WAY, OUR GALAXY

Image of our galaxy; you can see the plane of the disc

The structure and the dynamics of the Milky Way

The Galaxy is composed of a central bulge, a disc, the spiral arms and the halo. The bulge is 16,000 light years thick, it is situated in the centre of the disc which is 2,000 light years thick, and has a diameter 100,000 light years long. The disc contains the spiral arms, like "stripes" of gas and stars that originate in the nucleus and wind all around it. The disc is surrounded by a halo, almost spherical, made of globular clusters, that spreads up to a diameter of 150,000 light years.
Three arms were discovered in our galaxy: the Orion one (where our Sun is), Perseus and Sagittarius. The absorption of light by the gas and dust prevents us from observing the arms on the opposite side of the galaxy.
Half the stars of our galaxy are isolated, and half gathered in open and globular clusters. The former can be found mainly in the arms of the disc, the latter in the halo.

The M3 globular cluster in the constellation of the Canis Venatici. (SEDS)

Besides, there are clouds of gas and dust, that form various kinds of nebulae . The most famous are those of the Vela, the Crab, (remains of the explosion of supernovae), but also the dark "Horse Head" nebula in the Orion constellation (made of dust that absorbs the stellar light). The interstellar matter is mostly concentrated in the spiral arms: it is the raw material for the formation of new stars.
The stars can be divided in two populations: the population II, that is of first generation, that date back to the early phases of the life of our galaxy, when the gas consisted of Hydrogen almost completely; the population I, the second generation, made of gas already enriched by the first generation stars, that exploded as supernovae or released the gas in form of stellar wind. The latter have a composition richer in metals. The old stars are situated mainly in the bulge or in the halo, the young ones mainly in the spiral arms.

As in the other spiral galaxies, all the stars in our galaxy rotate around the centre along elliptic or almost circular orbits; the speed increases with the distance from the centre, reaches a maximum and then decreases gradually. The halo does not take part in the rotation motion of the Galaxy.
Our Solar system is situated at the periphery of the galaxy, at approximately 28,000 light years from its centre, around which it rotates with a speed of approximately 250 Km/s, and it is 30 light years high on the galactic plane.
The analysis of the rotation curve of the Galaxy (that is the trend of the rotation speed of the stars, as a function of the distance from the galactic centre) demonstrated that, at large distances from its centre, such speed remains quite high, instead of decreasing as expected. This means that the gravitational field exerted on these stars is not that expected according to the measurement of the galactic mass. There must be some other kind of matter, not visible, able to produce a strong gravitational field: it is supposed that the galaxies are surrounded by massive halos of dark matter, that often connects the galaxies, like an actual "bridge".

The centre of the M4 globular cluster, the nearest to us, contains over 100 thousand stars. This cluster has many evolved stars, red giants and white dwarfs, and therefore it represents a sort of "cemetery" of stars. (HST)

 

In mosaic of pictures of the M15 globular cluster you can count over 30 thousand stars. M15 is the most compact and dense globular cluster of our galaxy. Due to its compactness, a black hole might have originated inside it, due to a gravitational collapse. (HST)

The centre of the Galaxy

The centre of our galaxy is in the Sagittarius constellation; in this direction you can see many stars, stellar clusters and nebulae. The nucleus is very hard to observe, because its light is absorbed by the gas and dust clouds present in the galactic plane; the only possible observations are those carried out in the infrared and radio wavelengths, to which such material is a lot more transparent.
In the centre of the Galaxy, the stars are very close: if we lived in that region, the brightness of these stars would be so intense that there would be no night. Besides, the speed of their rotation around the galactic centre is so high that we would notice a change in the shape of the constellation in a few decades.

The centre of our galaxy emits very intensely in the radio band; its emission has a non thermal, that is non stellar, origin. Apparently, there is a gas mass with a diameter of a few light years, in rapid rotation around a small and massive object: the astronomers suppose that there could be a black hole in the heart of the Galaxy. This situation seems very likely, and recently received some confirmations.
The black hole "swallows" the surrounding gas, emitting a radiation in the radio and X bands; the same thing might happen, on a larger scale, according to the modern theories, in the active galaxies and in the quasars.

The stars of the Milky Way
in the background of the NGC6603 open cluster,
in the Sagittarius constellation.
In that direction you can see a large number of stars,
because you are looking towards the nucleus of the Galaxy. (SEDS)
 

The M55 globular cluster in the Sagittarius constellation. (SEDS)