Free Astronomy Magazine March-April 2015

SOLAR SYSTEM comparison Proxima Centauri is more than twice fainter), hence about 50 times fainter than the faintest stars visible by the naked eye. However, since the star at issue is a red dwarf of spectral type M (namely M9.5, and T5 its companion), among its peculiarities it has that of being magnetically very ac- tive and of producing intense flares capable of increasing by 9-12 magnitudes its total brightness (i.e. from a few thousands to tens of thousands of times), over time scales rang- ing from several minutes to a few hours. This implies that the star could have also been visible to the eyes of our distant ances- tors, and this for a sufficiently long period of time during which it was moving with re- spect to other stars at the astounding speed of 70 arcseconds per year, which is equiva- lent to traversing the full Moon’s disk in about 26 years. Obviously, neither Homo sa- piens nor Homo neanderthalensis will have appreciated that kind of star movement, which is for us instead macroscopic, consid- ering that the “extremely fast” Barnard’s Star reveals a proper motion among stars of just over 10 arcseconds per year. But to what extent is Sholz’s Star unique in its kind? How many other similar stars are still hiding from the astronomers’ view? The answers to these questions will come from the Gaia mission of the European Space Agency, which is already operational and whose main task is to measure distances and velocities of 1 billion stars to reveal which of them passed or will pass near the Sun. So far the most precise astrometric catalogue from which astronomers have drawn information was that of the Hippar- cos satellite, which has not, however, paid due attention to red dwarfs. Gaia will be able to fill that gap and possibly provide new, interesting candidates. T his image shows the motion in the sky of Scholz's Star (centre) with re- spect to the back- ground stars. To obtain it were superimposed three different, previously filed images taken in three different colours and shot at intervals of several years: the red dot 60 years ago; the blue dot the current posi- tion. The white arrow shows the star’s trajectory in the next 200 years. [V. D. Iva- nov, DSS, and 2MASS] Below, Scholz’s Star view- ed in a much nar- rower field. have transited close to the Sun in the last 2 million years, but almost certainly they are not that large, which is a comforting thing to know if we consider that big surges in cometary fluxes (over 10 times the normal flux for long-period comets) are solely attri- butable to the transit through the Oort Cloud of stars much more massive than red dwarfs and within 10,000 AU from the Sun; events that are considered extremely rare (1 every billion years). It may be interesting to note that when Scholz’s Star passed close to the Sun it had an apparent magnitude of about 10.3 (as a n