Milky Way steals star cluster from another galaxy

A cluster of stars sparkles like a collection of gems in a stunning image taken by the Hubble Space Telescope. But this celestial jewel box is one that has been plundered from another galaxy.

Terzan 7 star cluster
Hubble’s image of the sparkling stars in the globular cluster Terzan 7. NASA, ESA, and A. Sarajedini (University of Florida)

Studies of the stars in a globular cluster called Terzan 7 show that they used to belong to the Sagittarius Dwarf Galaxy, a smaller neighbour which our own Milky Way is currently busy consuming.

There was much interest shown a couple of years ago when astronomers confirmed that the Milky Way is on course to collide with a larger nearby galaxy called M31. But it was not so well appreciated that we are already engaged in a merger with a very much smaller city of stars.

Famous for being visible to the unaided eye in dark skies, M31, often called the Andromeda Galaxy, currently lies about 2.5 million light-years away from us. However, in around four billion years, M31 and the Milky Way will be involved in a cosmic clash that will rip both galaxies apart.

Tidal forces will pull both galaxies into new shapes. Stars in them will be sent into new paths through space, and it is possible that our own Sun could end up being part of M31 instead of the Milky Way.

That sort of galaxy swap is just what happened to Terzan 7, it appears. And the fact it originated in a different galaxy makes this cluster – discovered by French-Armenian astronomer Agop Terzan – of special interest to professional scientists. The cluster is much younger than similar clusters surrounding the Milky Way, having an age of around eight billion years.

It is also unusual in that all the stars observed seem to have the same age. Generally, globular clusters observed in the Milky Way and other galaxies show two separate generations of stars which started life at different times.

Astronomers want to know whether this is because clusters form differently in dwarf galaxies or whether there is simply insufficient material for two sets of stars to form. Or is the cluster too young for another generation of stars yet to have formed?

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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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