Final answer:
The star that became supernova SN 1987A was a blue supergiant just before it exploded, providing a unique opportunity for astronomers to study the lifecycle and death of a star with modern instruments.
Step-by-step explanation:
Just before it exploded, the star that became supernova SN 1987A was a blue supergiant. This supernova was significant because it offered astronomers unprecedented insights into the lifecycle and death of a star.
With modern observational techniques, features such as the star's evolution from the main sequence to a red supergiant, and eventually to a blue supergiant, were studied in detail.
Formed around 10 million years ago with an original mass of about 20 times that of our Sun (20 Msun), this star spent most of its life converting hydrogen into helium. After exhausting its hydrogen fuel, the star expanded to become a red supergiant and later lost some of its mass before returning to the blue supergiant stage.
This transition was unusual, as most stars of this type end their lives as red supergiants, but SN 1987A defied this typical pattern which made it a valuable object of study for the scientific community.