What were the "shooting stars" that Muggles saw on the day Voldemort died?

Question

Vernon sees on the news that people have reported a lot of shooting stars all over England.

Here's the relevant paragraph:

"Well, Ted," said the weatherman, "I don't know about that, but it's not only the owls that have been acting oddly today. Viewers as far apart as Kent, Yorkshire, and Dundee have been phoning in to tell me that instead of the rain I promised yesterday, they've had a downpour of shooting stars!"

-Harry Potter and the Philosopher's Stone, Chapter 1

In the same chapter, McGonagall also says to Dumbledore:

"... Shooting stars down in Kent — I’ll bet that was Dedalus Diggle. He never had much sense.”

So what exactly were these shooting stars? Are witches and wizards able to summon shooting stars at will, or were they some celebratory spells that the Muggles thought were shooting stars?

Answer 1

They were probably magical fireworks.

The Muggle weatherman certainly seemed to believe they might be fireworks.

"Well, Ted," said the weatherman, "I don't know about that, but it's not only the owls that have been acting oddly today. Viewers as far apart as Kent, Yorkshire and Dundee have been phoning in to tell me that instead of the rain I promised yesterday, they've had a downpour of shooting stars! Perhaps people have been celebrating Bonfire Night early - it's not until next week, folks!"
_{(Philosopher's Stone, Chapter 1, The Boy Who Lived).}

The stars in fact have a magical source, and are listed by McGonagall as taking place amongst wizarding exuberance.

"Oh yes, everyone's celebrating, all right," she said impatiently. "You'd think they'd be a bit more careful, but no - even the Muggles have noticed something's going on. It was on their news." She jerked her head back at the Dursleys' dark living-room window. "I heard it. Flocks of owls...shooting stars...Well, they're not completely stupid. They were bound to notice something. Shooting stars down in Kent - I'll bet that was Dedalus Diggle. He never had much sense."
_{(Philosopher's Stone, Chapter 1, The Boy Who Lived).}

So we know that the stars are magical in origin and were set off in celebration.

The stars are consistent with Dr. Filibuster’s Fabulous Wet-Start, No-Heat Fireworks. These are known to produce stars when set off.

Fred and George rounded off the evening with a display of Filibuster fireworks; they filled the kitchen with red and blue stars that bounced from ceiling to wall for at least half an hour.
_{(Chamber of Secrets, Chapter 5, The Whomping Willow).}

...in the case of Fred and George Weasley, trying to find out what would happen if you fed a Filibuster Firework to a Salamander...The sight of Percy bellowing himself hoarse at Fred and George, the spectacular display of tangerine stars showering from the Salamander's mouth, and its escape into the fire, with accompanying explosions, drove Filch and his Kwikspell envelope from Harry's mind.
_{(Chamber of Secrets, Chapter 8, The Deathday Party).}

The wizarding community setting off Filibuster fireworks en masse may have produced the effect of shooting stars in the various places around the country where wizards and witches were hiding, or something similar to it.

Answer 2

They could be from spells that shoot off sparks.

There are some spells whose function is to shoot out sparks. Some of these spark-creating spells seem to have defensive applications. Harry and Ron use them to keep a Blast-Ended Skrewt back.

“Don’ frighten him, now!’ Hagrid shouted, as Ron and Harry used their wands to shoot jets of fiery sparks at the Skrewt, which was advancing menacingly on them, its sting arched, quivering, over its back. ‘Jus’ try an’ slip the rope round his sting, so he won’ hurt any o’ the others!”
_{^{- Harry Potter and the Goblet of Fire, Chapter 21 (The House-Elf Liberation Front)}}

There are also spells that shoot up red and green sparks, which seem to be used mainly as signals. Hagrid tells the students in the Forbidden Forest to use green sparks if they find the injured unicorn they’re looking for and red sparks if they’re in danger and need help.

“All right, but I warn yeh, he’s a coward,’ said Hagrid. ‘So me, Harry an’ Hermione’ll go one way an’ Draco, Neville an’ Fang’ll go the other. Now, if any of us finds the unicorn, we’ll send up green sparks, right? Get yer wands out an’ practise now – that’s it – an’ if anyone gets in trouble, send up red sparks, an’ we’ll all come an’ find yeh – so, be careful – let’s go.” _{^{- Harry Potter and the Philosopher's Stone, Chapter 15 (The Forbidden Forest)}}

Triwizard competitors are told to send red sparks up if they’re in danger.

“We are going to be patrolling the outside of the maze,’ said Professor McGonagall to the champions. ‘If you get into difficulty, and wish to be rescued, send red sparks into the air, and one of us will come and get you, do you understand?”
_{^{- Harry Potter and the Goblet of Fire, Chapter 31 (The Third Task)}}

When Harry uses the spell, sparks hover above Krum.

“He’d deserve it,’ Cedric muttered, but all the same, he raised his wand and shot a shower of red sparks into the air, which hovered high above Krum, marking the spot where he lay.”

These spells are also used as signals by the Order when transporting Harry, so it seems like they’re used mainly as signals, but it’s possible that some wizards used it in celebration as well. It’s also likely something just about any wizard can do, since first-years are expected to be able to do it.

There are also spell effects that look like stars.

At Bill and Fleur’s wedding, the wizard marrying them showered them in silver stars that floated around them. It’s possible that the spell could also be used to shoot them upwards, or that a different spell could.

“The tufty-haired wizard raised his wand high over the heads of Bill and Fleur and a shower of silver stars fell upon them, spiralling around their now entwined figures.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 8 (The Wedding)}}

In addition, the Dark Mark when conjured in the sky looks like it’s made out of stars.

“Then he realised that it was a colossal skull, composed of what looked like emerald stars, with a serpent protruding from its mouth like a tongue. As they watched, it rose higher and higher, blazing in a haze of greenish smoke, etched against the black sky like a new constellation.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 9 (The Dark Mark)}}

While this is undoubtedly not the spell used, it does show that spells can create things that look like stars.

They could also be fireworks, but spells only need a wand.

The Dark Lord’s answer says the “shooting stars” were probably fireworks. That seems both possible and logical (matching the description well), but that would require the wizard to already have the fireworks. The events at the Potters’ were entirely unexpected, so no one would have really been planning the celebrations in advance. Right up until then, the wizarding world was very tense and fearful.

“Dark days, Harry. Didn’t know who ter trust, didn’t dare get friendly with strange wizards or witches … Terrible things happened. He was takin’ over. ’Course, some stood up to him – an’ he killed ’em.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 4 (The Keeper of the Keys)}}

It seems a bit odd for adult wizards in that situation to be buying fireworks. It’s certainly possible someone did have them, though. Casting a spell, however, requires only a wand, which wizards would certainly keep on them. It seems like it would be more likely for wizards who suddenly get this news to start casting excited spells than to find fireworks somehow.