| rdfs:comment
| - Most galaxies fall into three kinds: irregular galaxies are small and formless masses of stars, rich in dust and hydrogen and containing mostly younger stars (see below for star classes); conversely, elliptical galaxies don't have much hydrogen or dust, and their stars tend to be very old; finally, spiral galaxies (such as our Milky Way and our closest neighbour, the Andromeda Galaxy) have a complex structure similar to a pinwheel, with a mixture of both young and old stars. About half of the galaxies (but none of the spirals) are dwarf galaxies.
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| abstract
| - Most galaxies fall into three kinds: irregular galaxies are small and formless masses of stars, rich in dust and hydrogen and containing mostly younger stars (see below for star classes); conversely, elliptical galaxies don't have much hydrogen or dust, and their stars tend to be very old; finally, spiral galaxies (such as our Milky Way and our closest neighbour, the Andromeda Galaxy) have a complex structure similar to a pinwheel, with a mixture of both young and old stars. About half of the galaxies (but none of the spirals) are dwarf galaxies. (Notes: Ms is the mass of the Sun, about 2.0 × 1030 kg; Ls is the luminosity of Sun, about 3.8 × 1026 J) Most likely, life needs heavy elements, a liquid medium and probably a solid ground (also see here and here): only hydrogen and helium, produced by the Big Bang won't be enough. Element heavier than helium, from lithium to iron, are produced by nuclear reaction in the stars, while the rest of the natural elements (until uranium) derive from supernovae. Dwarf galaxies are very poor in heavy elements, and they can be ruled out, and so, partially, irregular galaxies. Most of the stars in elliptical galaxies, being very old, were born right after the Big Bang, and didn't benefit from an interstellar medium already rich in heavy elements as the younger star did; it seems, therefore, that spiral galaxies are the best chance for complex chemistry. Once we have chosen a galaxy, we need to find the "habitable zone" in which to search a life-bearing solar system. A large number of red and yellow stars, mixed with dust and gases, form the Disk, comprehending in turn a number of arms highlighted by hotter blue and white stars. The Halo, a spherical region around the main disk, contains only old, cold red stars very poor in heavy elements useful to build a rocky planet (carbon, silicon, iron). These are much more abundant nearer to the Core, a region extremely rich in young, hot stars. On the other hand, the core is also richer in ionizing radiations, which disrupt atomic bonds and prevents the synthesis of complex molecules; while they'd not be much stronger than the radiation that a planet receives from its own star, the core also presents a higher rate of supernovae within a distance of few light-years. Life could appear in the outer rim of the Core, though not in its centre: planets there would constantly be enlightened, day and night, by the huge number of stars. Still, the most likely location is the Disk, both inside and outside thae arms. In the Milky Way, the best distance from the centre is thought to be 25000-31000 light years (8-10 kiloparsecs), where most stars are between 4 and 8 billion years old. Here there is a high-detail map of the Milky Way, produced by National Geographic.
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![[RDF Data]](/fct/images/sw-rdf-blue.png)