Many massive stars end their life cycles as blackholes. Why do astronomers not believe the sun will end its life cycle as a black hole?
Astronomers do not believe that the Sun will end its life cycle as a black hole because its mass is not sufficient to trigger such a collapse. The formation of a black hole requires a star with a mass significantly greater than that of the Sun.
When massive stars exhaust their nuclear fuel, a core collapse occurs, resulting in either a supernova or a hypernova explosion. If the remnant mass left after the explosion is more than about three times the mass of the Sun (known as the Tolman-Oppenheimer-Volkoff limit), it may continue collapsing into a black hole. This collapse is driven by the overwhelming gravitational force that overcomes other forces, such as electron or neutron degeneracy pressure.
The Sun, on the other hand, is a relatively low-mass star, about 1.989 x 10^30 kilograms. While it will go through its own end stages of stellar evolution, it will not have enough mass to undergo core collapse and form a black hole. Instead, it is expected to transform into a white dwarf. A white dwarf is a dense, compact object that gradually cools and fades away over billions of years.
To summarize, the Sun's mass is not sufficient to initiate the core collapse required for the formation of a black hole.