The Milky Way is 'still hungry' - how our galaxy keeps devouring its smaller neighbours

By Rob Waugh

Last updated at 2:56 PM on 2nd December 2011

The Milky Way bar was always advertised as the chocolate bar that you could eat without filling you up.

But the galaxy after which it's named - our own - has an insatiable appetite.

It's still devouring neighbouring dwarf galaxies at a frightening rate - and astronomers recently captured two 'streams' of stars 'torn off' a nearby dwarf galaxy into our own.

An artist's impression of the four tails of the Sagittarius Dwarf Galaxy (the orange clump on the left of the image) orbiting the Milky Way. The bright yellow circle to the right of the galaxy's center is our Sun (not to scale).

A team of astronomers led by Sergey Koposov and Vasily Belokurov of the University of Cambridge recently discovered two streams of stars in the Southern Galactic hemisphere that were torn off the Sagittarius dwarf galaxy.

'We have long known that when small dwarf galaxies fall into bigger galaxies, elongated streams, or tails, of stars are pulled out of the dwarf by the enormous tidal field,' said Sergey Koposov.

The Sagittarius dwarf galaxy used to be one of the brightest of the Milky Way satellites. Its disrupted remnant now lies on the other side of the Galaxy, breaking up as it is crushed and stretched by huge tidal forces.

It is so small that it has lost half of its stars and all its gas over the last billion years.

A map showing streams of stars being pulled from the Sagittarius dwarf galaxy - which has lost half its stars as huge forces from our own galaxy tug at it

'Perhaps the Sagittarius dwarf galaxy has suffered an encounter with an object in the game of galactic billiards'

Sergey Koposov and colleagues analyzed density maps of over 13 million stars in the latest release of Sloan Digital Sky Survey data, including the crucial coverage of the Southern Galactic sky.

The new data show that the Sagittarius stream in the South is also split into two, a fatter and brighter stream alongside a thinner and fainter stream.

'Sagittarius is like a beast with four tails,' observed Wyn Evans, from the Institute of Astronomy, University of Cambridge.

Co-author Geraint Lewis of Sydney University says, 'Perhaps the Sagittarius dwarf galaxy has suffered an encounter with an object in the game of Galactic billiards. Maybe a collision with a massive clump of dark matter, or even another satellite galaxy, has split each of the streams into two

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