Saturday, July 29, 2017

Catching speeding stars - UNIVERSE

This animation reveals the evolution of stars in our Galaxy over the past million of years.

It starts from the positions of stars in the sky 1 035 000 years ago, which were calculated using data from the Tycho-Gaia Astrometric Solution, or TGAS, one of the products of the first Gaia data release. The video follows the evolution of stellar positions until the present day, ending with a view of the sky as measured by Gaia between 2014 and 2015.

Highlighted in yellow are the trajectories of six special stars: these are hypervelocity stars, moving through the Galaxy at several hundred of km/s. While it might not be apparent from the video, which shows the motions of stars as projected on the sky, they are moving through space much faster than the galactic average.

Scientists spotted these speeding stars from the TGAS data set of two million stars with the help of an artificial neural network – software that mimics a human brain – and they are looking forward to finding many more in future Gaia data releases.

These stars owe their high speeds to past interactions with the supermassive black hole that sits at the centre of the Milky Way and, with a mass of four million Suns, governs the orbits of stars in its vicinity. Having travelled great distances through the Galaxy, they provide crucial information about the gravitational field of the Milky Way from the centre to its outskirts.

One of the six stars (labelled 1 at the end of the video) seems to be speeding so fast, at over 500 km/s, that it is no longer bound by the gravity of the Galaxy and will eventually leave. The other five stars are somewhat slower (over 400 km/s for the stars labelled 2, 3, 4 and 6, and 360 km/s for the star labelled 5) and are still bound to the Galaxy.

These slightly slower stars are perhaps even more fascinating, as scientists are eager to learn what slowed them down – the invisible dark matter that is thought to pervade the Milky Way might also have played a role.

The stars are plotted in Galactic coordinates and using a rectangular projection: in this, the plane of the Milky Way stands out as the horizontal band with greater density of stars. The stripes visible in the final frames reflect the way Gaia scans the sky and the preliminary nature of the first data release; these artefacts are gradually washed out in the video as stars move across the sky.

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