Sagittarius Dwarf galaxy is colliding with Milky Way


Don't look now but our galaxy is being invaded by the Sagittarius Dwarf. Even worse, our Milky Way is on a collision course with the Andromeda galaxy, and there is nothing we can do about it.

There isn't much reason to panic - our crash with Andromeda isn't due for another four or five billion years. And while the Sagittarius Dwarf is already here, it seems to be having surprisingly little impact.

Delegates were told yesterday about colliding galaxies. These may sound like cataclysmic events and big things can and do smash into one another, but most galaxies have a lot of open space and individual stars tend to glide past one another.

"You could sleep through the whole thing," suggested Dr Mark Dickinson of Johns Hopkins University and Space Telescope Science Institute. "The evidence seems to be the merger rate was higher early in the history of the universe than now."

In fact we are already in collision with a small galaxy, the Sagittarius Dwarf. With its tens of millions of stars it is 10,000 times smaller than our Milky Way. Dr Rosemary Wyse, also of Johns Hopkins, explained that the Dwarf orbits through our galaxy once every billion years or so and is currently embedded near the "bulge" at the centre of the Milky Way.

"The Sagittarius Dwarf is already interacting strongly with the Milky Way," she stated. Its presence was only discovered by accident in 1994 so it can't be that big a deal. Colliding galaxies clearly have astronomers stirred up, however, and they are using new earth telescopes and the Hubble space telescope to scan the skies.

"You have to understand what happens when galaxies collide if you want to understand how galaxies function," said Dr Bradley Whitmore of the Space Telescope Institute. He searches for the "fossil record" of earlier smashes to assess what is going to happen when the Milky Way does its head-to-head with Andromeda.

"We don't have to worry about stars hitting each other," Dr Chris Mihos suggested helpfully, but adding all the same, "We might be expelled from the galaxy in a tidal tail." These are the telltale jets of material spewed out that show when two galaxies have toughed it out.

Being ejected out into the cosmos mightn't be such a bad thing, he believes. We live on the outskirts of the Milky Way, well away from its urban centre. Things tend to get hot - and dangerous - in the middle during collisions, with new star formation and a remaking of the old. Thank God for the suburbs.