A star that exploded in a galaxy 21 million light-years from Earth is a red supergiant, a British astrophysicist said.

The explosion was discovered about 10 days ago and has excited astronomers because the supernova appears once every decade.

Red supergiants are very large stars approaching the late stages of their lives.

The supernova, called SN 2023ixf, appeared in the Rollercoaster galaxy (M101) and was first seen by an amateur Japanese astronomer on May 19.

Joan Pager, an astrophysicist at the University of Central Lancashire, identified the star as a red supergiant after anglizing data collected over a decade for the Carousel galaxy.

"When we identify a red supergiant, we know the star is in the later stages of its existence, but of course we don't know when it will become a supernova it could be next year or it could be in a million years."

Pleger said it was the closest supernova mankind had seen in more than a decade, making the event "very exciting."

The original star was about eight to 12 times the mass of the Sun and between 30 to 50 million years old.

The explosion was a Type II supernova that occurred when a massive star ran out of fuel and swelled into a red supergiant.

Red supergiants persist for several million years by burning various elements such as hydrogen or helium.

Data from the Hubble Space Telescope show that much of the area around the star's core is rich in hydrogen, which Pleger says is what was seen in the explosion. She added that the star is still visible in the night sky and can be observed with a relatively small telescope. The light will fade in about two years. After that, there will remain a small but very dense object, such as a neutron star, in which material around a teaspoon weighs 10 million tons.