Correction: An earlier version of this article incorrectly referred to a 1574 supernova as being seen by Johannes Kepler. The supernova visible in that year, commonly known as the supernova of 1572, was associated with Danish astronomer Tycho Brahe, not Kepler. Also, in listing recently seen supernovae, the article omitted supernova 1987a. That stellar explosion was a “core collapse,” or Type II, supernova, and thus of less interest to cosmologists than the currently visible supernova, which is a Type Ia. This version has been corrected.

Twenty-one million years ago in the Pinwheel Galaxy — an elegant, spiral-armed neighbor of our Milky Way — an old, dim star had a very bad day. It exploded and began to blaze like a billion suns.

On Aug. 24, Peter Nugent had a very good day.

The Lawrence Berkeley National Laboratory astrophysicist was about to grab lunch when he checked to see if a robotic telescope had spied anything of interest the night before. Bingo. Boom. Supernova.

And not just any supernova. Every night, astronomers spot several exploding stars across the universe. But this one was so close — in cosmic terms — and seen so soon after its light reached Earth that astronomers are calling it the supernova of a generation.

An automated sky survey that searches for new nighttime objects with a telescope at California’s storied Palomar Observatory first flagged the supernova.

Soon, telescopes around the world — and beyond, including the Hubble Space Telescope — swiveled to take a peek.

“This is a special event,” said Ken Sembach of the Space Telescope Science Institute in Baltimore. “Everyone wants a piece of it.”

Nugent said the blaze will continue to brighten until Wednesday or Thursday, visible just above the handle of the Big Dipper to backyard astronomers wielding binoculars.

The last supernova to generate such buzz flared into view in 1972. Before that, a 1937 explosion caught everyone’s attention. In 1987, supernova 1987a was a “core collapse,” or Type II, supernova, and thus of less interest to cosmologists than the currently visible supernova, which is a Type Ia.

“This is not an every-year event,” said Robert Kirshner, a veteran Harvard University super­nova hunter. “This is an every-decade-or-four event.”

Three hours after Nugent’s discovery, the Grand Canary Telescope off the coast of Africa, where the sun had just set, took a look. The jackpot grew bigger when that scope’s data poured in. The Pinwheel supernova, known as SN2011FE, was a Type 1a, of special interest to astronomers.

Type 1a’s all blaze with nearly the same brightness. That makes them the perfect yardsticks for measuring cosmic distances. Their apparent brightness tells us how far away they are.

In the 1990s, Kirshner led a team that leveraged this property to make one of the biggest discoveries of the past century: The universe is flying apart, rapidly accelerating.

To explain this, cosmologists were forced into an uncomfortable conclusion.

Either gravity does not work the way it is supposed to, or a mysterious force is pushing galaxies apart at a quickening pace. They called this unknown force “dark energy” and have little idea what it is, even though they are able to calculate that it constitutes an astounding 73 percent of all mass and energy in the universe.

Cosmologists hope the new supernova will help refine estimates of the universe’s acceleration. The data might even hint at the nature of dark energy.

Professional stargazers also point to the Pinwheel supernova as the best candidate to fill another gap in their knowledge: the precise sequence of events that cause such explosions. They are fairly certain Type 1a’s start with old, dim stars called white dwarves. Such stellar husks have blown away their outer layers after running out of fuel. This fate awaits our own sun in about 5 billion years.

For our sun, because it is isolated from other stars, that will be the end. But some white dwarves reincarnate as interstellar pilferers. Their high gravity sucks material from any nearby companion stars.

Laden with cosmic booty, the thieving dwarf reaches a critical mass, about 1.4 times that of our sun. Atoms fuse. Hell is set loose. A runaway explosion ensues — a hydrogen bomb wrought massive.

That’s one possibility. In another, two white dwarves might circle each other in a deadly dance until they collide and explode.

“This is where the mystery lies,” said Roseanne Di Stefano, a Harvard University astrophysicist. “There’s still a lot of uncertainty in the field as to whether one or both happen.”

Peering at the Pinwheel supernova should provide clues as to which scene unfolded, Di Stefano said.

Already, astronomers have pored over Hubble images of the Pinwheel Galaxy, also known as M101, taken years ago.

They have seen no stars where the super­nova appeared. This absence is itself a clue as to what happened; it eliminates the possibility that a large, bright star “donated” its mass to the presumed white dwarf that went boom.

Hence, the double-dwarf scenario — in which both stars were too dim for Hubble to see — has taken the early lead.

In any case, astronomers will stare for years, maybe decades, at the cloud of gas the Pinwheel supernova will soon leave behind.

Like Galileo, who spotted a supernova in 1604, and Danish astronomer Tycho Brahe, who saw the so-called supernova of 1572 in 1574, and the Chinese, who recorded such a sight in 1054, sky watchers remain in thrall of the biggest bangs in the universe.

“You’d be hard pressed to find a telescope that hasn’t turned to it,” Nugent said.