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The incredible fate of a star ejected from our galaxy

The incredible fate of a star ejected from our galaxy
Space telescope Hubble has studied a sun which is speeding away from our Milky Way at 2.5 million km/h and has suffered a most unusual fate...

HE 0437-5439: a blue star breaking away from our galaxy at an extraordinary speed (illustration). This star should theoretically have burnt out well before it reached its current distance from us. Astronomers have come to an astonishing conclusion which initially involved 3 suns!
Credit: NASA, ESA, G. Bacon (STScI)

Our Sun is an ordinary star sheltered in a galaxy that we call the Milky Way. On looking up at the night sky, where the stars appear to be stationary in relation to one another, it always comes as a surprise to know that the galaxy turns on its own axis and that the relative position of all these suns changes over very long periods of time. And that some quite violent events can occur!

Three stars in one
Studies have, for instance, already shown that our galaxy is able to “eject” stars, about one every 100,000 years. And what is responsible? The gigantic black hole that is located in the centre of our galaxy which, due to its considerable gravitational attractive force, is able to draw certain stars towards it and put them on a trajectory such that they are forced to leave the Milky Way via a “gravitational slingshot” effect. Astronomers have, moreover, identified 16 suns said to be hyperfast as they move away at high speeds.

The star HE 0437-539 as observed by Hubble. A star with a perfectly ordinary appearance which is the result of a rather eventful past.
Credit: NASA, ESA, Z. Levay (STScI)

But the case of the star HE 0437-5439 examined with the space telescope Hubble has proved rather strange. It is drawing away from our galaxy at a speed of 2.5 million km/h and is already 200,000 light years away (our galaxy spans 100,000 light years in diameter). The problem is that such a distance means that the star left the Milky Way 100 million years ago. And yet its characteristics (9 times the mass of our Sun and blue in colour) relate to a star that should have totally burnt itself out within 20 million years…

The incredible fate of HE 0437-539 summarised in 5 steps. 1: a triple-star system was drawn to the black hole in the centre of our galaxy. 2: one of the three stars was grabbed by the black hole and the two others ejected. 3: the duo broke away from our galaxy. 4: on ageing, the two stars merged. 5: the merger gave rise to a blue straggler which continues to move away from our galaxy.
Credit: NASA, ESA, E. Feild (STScI)

Using the observations from Hubble, and notably accurate measurements of the position of HE 0437-5439 in 2006 and 2009, astronomers have come to an astonishing conclusion. They believe that it started out as a triple-star system with 3 stars. The trio was drawn to the black hole in our galaxy. Said hole grabbed one of the 3 stars and sent the other 2 packing out of our Milky Way. The two breakaways continued to follow the usual stellar evolution with one of them becoming a red giant. Finally, the latter enveloped its companion and the two of them transformed into what is known as a “blue straggler”, a type of sun that has been rejuvenated through the merger of two ageing stars. Clearly, the fate of HE 0437-5439 is far from being commonplace! The Universe is obviously full of surprises…

Publié le 23 juillet 2010