Our experts answer your questions

Galactic mergers can give rise to new galaxies

© NASA

GALAXIES

Can galaxies die?

It very much depends on how you define a galaxy being alive! A common definition is that a galaxy is alive if it’s forming new stars, and by that definition, galaxies can and often do – die. Any galaxy we see that is very red in colour has mostly stopped forming stars – the blue stars that formed with the red ones die off the quickest, leaving the red ones behind.

There are many reasons a galaxy might stop forming stars – maybe it’s out of gas, needed to form new stars. Maybe the gas is there, but simply too hot to be able to cool and collapse down into a star. The end result is the same in both cases: no new stars. On the other hand, you could choose a more lenient definition: if it exists, it’s alive. To die would thus be to have the galaxy disappear entirely.

The closest we get is if the galaxy were to merge with another galaxy. It would then build a new galaxy with its companion, marking the end of its previous existence – though it hasn’t disappeared; it’s had all of its pieces folded, like clay, into a new form.

Dr Jillian M. Scudder, department of physics and astronomy, Oberlin College and Conservatory

© Jennifer Manna

STARS

What’s the difference between a globular cluster and open cluster?

Globular clusters are much more massive, and they are usually much older. Open clusters have comparatively fewer stars and are comparatively young. Well, young in astronomical terms that is… There is pretty much nothing in between these two extremes, and the reason for this is as interesting as the question itself. Globular clusters are relics of an ancient time when the universe had more gas to fuel intense star formation. These conditions no longer exist (or are very rare) in the present universe. Forming a new globular cluster would be a feat, and thus new star clusters are comparatively smaller. There are no old open clusters simply because they have not survived billions of years of interactions with their host galaxies and environment; instead they have been split apart by gravitational interactions, and their stars now populate their host galaxies. Globular clusters survived their cosmic journey because they were massive enough to hold onto their stars for billions of years.