Fast stars may be join­ing us from other gal­ax­ies

All About Space - - Launch Pad -

Gaia has helped astronomers spot hy­per­ve­loc­ity stars that ap­pear to come from out­side the Milky Way

Sci­en­tists study­ing stars trav­el­ling at high speeds within the Milky Way be­lieve a good num­ber of these ce­les­tial ob­jects could ac­tu­ally be on a jour­ney from an­other gal­axy. Re­search us­ing data from the Euro­pean Space Agency’s Gaia mis­sion shows that most ob­served hy­per­ve­loc­ity stars are rapidly mov­ing to­wards the galac­tic cen­tre. It had pre­vi­ously been thought such stars moved out­wards.

The fresh find­ings have been re­ported by sci­en­tists at Lei­den Univer­sity in the Nether­lands fol­low­ing ob­ser­va­tions made by the Gaia space­craft be­tween 25 July 2014 and 23 May 2016. By look­ing at the po­si­tions, dis­tance in­di­ca­tors and mo­tions of some seven mil­lion stars with full 3D ve­loc­ity mea­sure­ments, the re­searchers found 20 were fast enough to even­tu­ally es­cape the Milky Way.

In the past such be­hav­iour has been pinned on the black hole at the cen­tre of our gal­axy pro­pel­ling these stars to­wards the Milky Way’s edge. Here, how­ever, some­thing un­usual was hap­pen­ing. “Rather than fly­ing away from the galac­tic cen­tre, most of the high-ve­loc­ity stars we spot­ted seem to be rac­ing to­wards it,” ex­plains study co-au­thor Tom­maso Marchetti. “These could be stars from an­other gal­axy, zoom­ing right through the Milky Way.”

One pos­si­bil­ity is that the hy­per­ve­loc­ity stars have come from a rel­a­tively small gal­axy or­bit­ing the Milky Way called the Large Mag­el­lanic Cloud. “Stars can be ac­cel­er­ated to high ve­loc­i­ties when they in­ter­act with a su­per­mas­sive black hole,” says co-au­thor Elena Maria Rossi. “So the pres­ence of these stars might be a sign of such black holes in nearby gal­ax­ies.”

Al­ter­na­tively, the stars may have been part of a bi­nary sys­tem, flung to­wards the Milky Way when their com­pan­ion star ex­ploded as a su­per­nova. “Study­ing them could tell us more about these kinds of pro­cesses in nearby gal­ax­ies,” Rossi con­cludes.

An artist’s im­pres­sion of the po­si­tions and or­bits of 20 high-ve­loc­ity stars inthe Milky Way

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