Final answer:
The star that became supernova SN 1987A originally had a solar mass of about 20 Msun. It lived most of its life on the main sequence, and as a red supergiant, it emitted enormous energy before going supernova.
Step-by-step explanation:
The original solar mass of the star that exploded to form supernova SN 1987A was about 20 times that of the Sun (20 Msun).
This progenitor star of SN 1987A was formed approximately 10 million years ago. Throughout most of its existence, it remained on the main sequence, where it conducted nuclear fusion by converting hydrogen into helium. Its spectral type during this phase was O, with a luminosity 60,000 times that of the Sun. Eventually, as the hydrogen in the star's core was depleted, the core contracted and heated to temperatures sufficient to begin helium fusion. The star became a red supergiant radiating around 100,000 Lsun before it lost some of its mass and went supernova.
Understanding the life cycle of SN 1987A provides invaluable insights into stellar evolution, particularly for massive stars. The stages from the main sequence to supergiant and ultimately supernova are dramatic, with vast changes in size, temperature, and energy output, culminating in one of the most violent events in the universe.