the death of a low-mass star the gravity of a large nebula the collapse of a high-mass star the combining of material from planets
Astronomers have found evidence for the existence of a super-massive black hole in the center of the giant elliptical galaxy M87. What is thought to cause black holes to form?

Options:
- the death of a low-mass star
- the gravity of a large nebula
- the collapse of a high-mass star
- the combining of material from planets

the collapse of a high-mass star