Massive stars are born, just like average stars, out of clouds of dust called nebulae. When a nebula collects enough mass, it begins to collapse under its own gravity. ... If the cloud of dust is large, it will create a massive star. A star is considered massive if it is at least eight times more massive than our Sun!

What makes a star more massive?

In the process of dying, most stars eject some matter, enriched in heavy elements, into interstellar space where it can be used to form new stars. Each succeeding generation of stars therefore contains a larger proportion of elements heavier than hydrogen and helium.

Why does a star become so massive and dense?

Its inner layers start to collapse, which squishes the core, increasing the pressure and temperature in the core of the star. While the core collapses, the outer layers of material in the star to expand outward. The star expands to larger than it has ever been – a few hundred times bigger!