Hubble watches galactic dance

Seyfert's Sextet is really a foursome

By Dr David Whitehouse BBC News Online science editor

The Hubble Space Telescope is witnessing a cluster of galaxies perform a slow dance of destruction that will last billions of years.

The galaxies' gravity is beginning to rip stars from them and distort their shapes. Eventually, they will merge to form one large galaxy.

The grouping, called Seyfert's Sextet, implies that six galaxies are coming together but in reality only four are.

The small face-on spiral with the prominent arms of gas and stars is a background galaxy almost five times farther away than the other four.

The sixth member of the sextet is not a galaxy at all but a river of stars torn from one of the galaxies.

River of stars

The galaxy grouping spans just 100,000 light-years, occupying less volume than our Milky Way galaxy.

Each galaxy is about 35,000 light-years wide. Three of them show evidence of close interactions with each other, or perhaps with an interloper galaxy that is unseen.

There is much evidence of stars being torn away from their home galaxies.

Although part of the group, the nearly edge-on spiral galaxy at top-centre remains relatively undisturbed, except for the slight warp in its disc. Most of its stars have remained within its galactic boundaries.

Unlike most other galaxy interactions observed with the Hubble telescope, this group shows no evidence of the characteristic blue regions of young star clusters, which generally arise during galaxy interactions.

The explanation may be that astronomers are seeing the cluster at the beginning of its interaction, before much has happened.

Billions of years from now, all four may merge and form a single galaxy. Astronomers believe that most elliptical galaxies are the result of galactic mergers.