Galaxies that speed through the intergalactic medium will have their gas and material stripped away, which will lead to a trail of stars formed in the wake of the expelled material, but will prevent new stars from forming within the galaxy itself. This galaxy, above, is in the process of being stripped away of its gas entirely. The stripping is much more pronounced in the environments of rich galaxy clusters, as illustrated here. (NASA, ESA ACKNOWLEDGEMENTS: MING SUN (UAH), AND SERGE MEUNIER)

Galaxy Clusters Are Where Galaxies Like The Milky Way Go To Die

If we were in a place like the Coma Cluster instead of the Local Group, we’d already be dead.

In a living spiral galaxy, like the Milky Way, the rich gas inside enables the ongoing formation of new stars.

The brightest, closest galaxy confirmed to be beyond the local group is NGC 300, at just 6 million light years distant. The pink regions found along the spiral arms are evidence of new star formation, triggered by the interaction of internal gas and the density waves of the internal structure. (ESO / WIDE FIELD IMAGER (WFI))

When enough gas gets concentrated in a single location, it collapses under its own gravity.

Star-forming regions, like this one in the Carina Nebula, can form a huge variety of stellar masses if they can collapse quickly enough. Inside the ‘caterpillar’ is a proto-star, but it is in the final stages of formation, as external radiation evaporates the gas away more quickly than the newly-forming star can accrue it. (NASA, ESA, N. SMITH, UNIVERSITY OF CALIFORNIA, BERKELEY, AND THE HUBBLE HERITAGE TEAM. STSCI/AURA)

Various matter clumps will grow, faster and faster, leading to new stars and star clusters.

Hubble space telescope image of the merging star clusters at the heart of the Tarantula Nebula, the largest star-forming region known in the local group. The hottest, bluest stars are over 200 times the mass of our Sun. The new star cluster, shown here, is less than 2 million years old. (NASA, ESA, AND E. SABBI (ESA/STSCI); ACKNOWLEDGMENT: R. O’CONNELL (UNIVERSITY OF VIRGINIA) AND THE WIDE FIELD CAMERA 3 SCIENCE OVERSIGHT COMMITTEE)

This can be triggered by internal dynamics, an external gravitational influence, or a merger with another galaxy.