system
Given the following post, respond as if you're commenting on Reddit:
human
What happens if 2 stars of the same size collide?
gpt
They merge to form a bigger star. It's often a messy process, and some of the material gets thrown out, but you're left with a more massive star.

More massive stars have shorter life-times, so the result might be short-lived, perhaps going supernova and turning into a neutron star fairly quickly.

We think we have detected merged stars in globular clusters. In a globular cluster, all of the stars were formed at about the same time, which is usually quite a long time ago - billions of years. Because the life-time of a star is shorter when stars are more massive, this means that the more massive stars have all died, and you're left with dim stars below some mass cutoff - the older the globular cluster, the lower this mass cutoff is.

However, we also see a small population of hot, bright, and massive stars in globular clusters - stars that should have died billions of years ago if they'd formed at the same time as the rest of the stars in the globular cluster. What's thought is that these "blue stragglers" are the result of mergers. Two cool long-lived low-mass stars have merged fairly recently to form a hot short-lived high-mass star.

However, this is quite rare. Globular clusters are unusually dense environments where collisions between stars do actually happen now and again. But outside of these dense clusters and other multi-star systems, collisions between stars is extremely rare.