Un miliardo di stelle (di nuovo) …A billion stars: what are they good for?

L’articolo è in inglese, ed è la traduzione di un articolo apparso tempo fa. E’ una prima prova per una sezione anglofona 😉

First I would like to reassure my faithful readers: the question is much less vague than you think! The catalog we are talking about should be created by ESA’s mission GAIA that we have already discussed a few times.

GAIA spacecraft will be launched in May of 2013, and under the five years of activity planned, it will record the positions and velocities of a large amount of astral objects; mostly stars in our Milky Way (but not only). Thanks to the particular system of scanning the sky (derived from the same principles as Hipparcos probe) the eye of GAIA will scan a significant portion of the entire sky. Moreover, a large number of astral objects will be observed not once, but dozens of times. All to the advantage of the accuracy of data collected and the possibility of conducting studies about objects of variable brightness.

At the end of the mission it is expected a catalog of objects of about a billion “rows”; that’s not bad at all if we consider that the census of Hipparcos – to which we still owe much of our current understanding of the Galaxy – numbered “only” 120,000 objects!

Reserving the treatment of extragalactic objects for a next post, let’s see now what Gaia will be able to do for our knowledge of the Milky Way. The points which scientists expect to clarify are the following:

• distance and velocity distribution of all stellar populations of the Galaxy. We know that the Milky Way is a very wide and complex, where there are clusters of stars of different type and origin (even some with external shunt, engulfed by the gravitational pull of our galaxy). A detailed investigation will surely bring many fruits.

• spatial and cinematic structure of the disk and the halo, very important to understand the exact dynamics of formation of the Milky Way and so of galaxies similar to it (large spiral galaxies). a detailed map of the distribution of dark matter derived from analysis of the velocities and

•positions of the stars. Streams of stars in certain directions can say much about the distribution of gravity and consequently of matter, whether it be light or “dark”.

• a rigorous framework for studying stellar evolution and structure formation: studies of stellar evolution are becoming more and more detailed, and they require observational data even for samples of stars that are very “rare”, to verify the predictions concerning, e.g., fast or peculiar evolutionary stages. a large-scale survey of extra-solar planets and small bodies in the Solar System (even one quarter of a million): we will be able to determine the orbits, the rotation period, the main parameters, obtaining a range of information that are essential for understanding the formation of our Solar System and planets in general.

For each object in the catalog we will have, as mentioned, not only the location but also the color and the radial velocities. In case of partly superimposed objects along the line of sight, algorithms are being studied to be able to “separate” the luminous fluxes of the various sources. Will we get “basic” information also about the chemical composition of the stars observed, due to a wideband spectrometry.

Here then is the meaning of the question: what can we do with a billion stars? It’s more than likely, however, that the time taken to the astrophysics community to “digest” the data will be much longer than the time necessary for GAIA to acquire them.

Our own way of thinking about the Milky Way will be soon destined to change, under the pressure of all these new data.

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