If our Milky Way were located in the Virgo cluster instead of the Local Group, chances are we’d already be a “red and dead” galaxy.
What makes a galaxy dead or alive is simple: internal stores of gas.
The low-mass, dusty, irregular galaxy NGC 3077 is actively forming new stars, has a very blue center, and has a hydrogen gas bridge connecting it to the nearby, more massive M81. As one of 34 galaxies in the M81 Group, it’s an example of the most common type of galaxy in the Universe: much smaller and lower in mass, but far more numerous, than galaxies like our Milky Way. The young stars within it have formed from gas reservoirs still present within this galaxy, indicating an “alive” galaxy. (Credit: ESA/Hubble and NASA)
Inside living galaxies, gas is required to enable the formation of new stars.
When massive gas clouds gravitationally collapse, new stars inevitably arise.
As matter fragments, the various clumps grow rapidly, forming new stars and massive star clusters.
Many events trigger galactic star-formation, including: