This huge stellar census is providing the data needed to tackle an enormous range of important open questions relating to the origin, structure and evolutionary history of our Galaxy.įor example, Gaia is identifying which stars are relics from smaller galaxies long ago ‘swallowed’ by the Milky Way. Gaia is creating an extraordinarily precise three-dimensional map of nearly two billion objects throughout our Galaxy and beyond, mapping their motions, luminosity, temperature and composition. 2016).What's special? The density of stars from Gaia’s Early Data Release 3 The least-squares estimates of □ and □ will be small If the quasars are reasonably distributed over the celestial sphere, the correlation between Version of Equation 3.96 with six unknowns, Known instead the components of □ in the ICRS should be introducedĪs additional unknowns when solving for □. It is not wise to assume that this effect is precisely Where □ = □ / c and c is the speed of light. The effect on the quasars isĪn apparent streaming motion towards the galactic centre, described by Pointing more or less towards the galactic centre. □ should have a magnitude of g ≃ 2 × 10 - 10 m s - 2, Of stellar aberration due to the galactocentric acceleration of the solar-systemīarycentre (Kopeikin and Makarov 2006). Proper motion pattern for quasars is expected to be produced by the secular variation Including the peculiar motions of quasars and optical variability causing centroidĭisplacements (Bachchan et al. Latter are expected to be very small, but not zero. The determination of □ can use a much larger set of quasars, becauseĮquation 3.96 does not require their precise positions in ICRS toīe known, only their proper motions in ICRS ( μ α *, μ δ). The same equations can thenīe used to correct the source parameters so that they refer to □ instead of In □ ~ and □, these equations can be used to obtain a least-squaresĮstimate of □ 0 and □. Given the position and proper motion differences for the same sources expressed These relations can be written in matrix form as Where □, □, □ are the unit vectors introduced in Obtained in the astrometric solution, denoted ( α ~, δ ~ )Īnd ( μ ~ α *, μ ~ δ ), are similarly defined by the Where the components of the vectors □ ¯ B ( t ep ) andĭ □ ¯ B / d t | t = t ep are actually the Of a source in the ICRS (relative to □) are defined by means of The celestial position ( α, δ ) and proper motion ( μ α *, μ δ ) Note that □ is time-dependent,ĭue to the rotation of □ ~, and that it is defined in the sense of a Μas precision is aimed at, that | □ | must consequently be Meaning that terms of the order of | □ | 2 can be neglected. The last term indicates that we are in the regime of the small-angle approximation, At any particular time t the difference between the two systems canīe represented by a rotation vector □ such that Moreover, □ ~ may rotate slowly with respect Resulting from the astrometric solution can similarly be represented by a vector The reference system of the source and attitude parameters Since ICRS is non-rotating relative to distant quasars, the directions of these Where □, □, and □ are unit vectors pointing towards The ICRS may be represented by the vector triad □ = ,
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