The longest-lived stars are expected to undergo fusion reactions for years. a thousand a million a billion a trillion thousands of trillions of

The correct answer for the blank is - A billion years.

The most common star that is considered to be longest lived is Red dwarf (which is primarily formed of hydrogen gas).

During the nuclear fusion of star, tremendous gravity forces the nuclei of atoms to fuse together, thereby liberating large amount of energy, preventing the star from imploding under its own gravity.

Thus, the longest lived stars are expected to undergo fusion reaction for a billion years.

The correct answer would be trillion of years. :

c

explanation: