We have used a combination of ACS HST high-resolution and wide-field Subaru data in order to study the blue straggler star ( BSS) population over the entire extension of the remote Galactic globular cluster NGC 2419. The BSS population presented here is among the largest ever observed in any stellar system, with more than 230 BSSs in the brightest portion of the sequence. The radial distribution of the selected BSSs is essentially the same as that of the other cluster stars. In this sense the BSS radial distribution is similar to that of ! Centauri and unlike that of all Galactic globular clusters studied to date, which have highly centrally segregated distributions and, in most cases, a pronounced upturn in the external regions. As in the case of ! Centauri, this evidence indicates that NGC 2419 is not yet relaxed even in the central regions. This observational fact is in agreement with estimated half-mass relaxation time, which is of the order of the cluster age.

The Lense-Thirring effect is a very small shift of the orbit of a test particle due to the spin of a body. It may be described as an orbital drag due to the gravitomagnetic field generated by the spin of the central body. Gravitomagnetism, a fundamental weak-field prediction of Einstein's theory of general relativity, is generated by currents of mass and owes its name to its formal analogies with magnetism, generated by currents of electric charge. Then, according to general relativity, the Earth's spin should influence the motion of its orbiting satellites. Indeed, we have analysed the laser-ranging observations of the orbits of the satellites LAGEOS and LAGEOS II and have obtained the first direct measurement of the Lense-Thirring effect, or 'frame-dragging', due to the Earth's spin. We measured µ Lense−Thirring ∼ = 1.1 and estimated the total error δµ Lense−Thirring ∼ = ±0.3, whereas the general relativistic value is µ Lense−Thirring ≡ 1.

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Gravitomagnetism is a fundamental, weak-field, prediction of Einstein's theory of general relativity, is generated by currents of mass and owes its name to its formal analogies with magnetism, generated by currents of electric charge. Then, according to general relativity, Earth's spin should influence the motion of its orbiting satellites. Indeed, we analysed the laser ranging observations of the orbits of the satellites LAGEOS and LAGEOS II and obtained the first direct measurement of the gravitomagnetic orbital perturbation due to the Earth's spin, known as the Lense-Thirring effect.