Quote from KingOfCharge >>

Quote from 6thLyranGuard >>

The Gravimancer has spells that deal gravity damage, and an actual star has considerably stronger gravity.  And per Spelljammer, stars deal Radiant damage, not Fire. But even if it dealt fire damage, Investiture of Flame requires concentration, so the moment you cast Enlarge/Reduce it's gone.

 But if you cast enlarge/reduce, the problem is gone. Though the rest of your argument is correct

 The problem isn't gone instantly: the spell doesn't cool things down by itself so you've still got to deal with the fact that the corona is still 5000 degrees, a temperature that makes lava look like ice.  And without the sun's gravity to hold it in, it's probably going to come blasting out straight at you at extremely high speed.

Quote from 6thLyranGuard >>

Quote from KingOfCharge >>

Quote from 6thLyranGuard >>

The Gravimancer has spells that deal gravity damage, and an actual star has considerably stronger gravity.  And per Spelljammer, stars deal Radiant damage, not Fire. But even if it dealt fire damage, Investiture of Flame requires concentration, so the moment you cast Enlarge/Reduce it's gone.

 But if you cast enlarge/reduce, the problem is gone. Though the rest of your argument is correct

 The problem isn't gone instantly: the spell doesn't cool things down by itself so you've still got to deal with the fact that the corona is still 5000 degrees, a temperature that makes lava look like ice.  And without the sun's gravity to hold it in, it's probably going to come blasting out straight at you at extremely high speed.

 True. Concentration gone means you die but take the sun out with you. But here’s the best plan. Cast contingency so that if you go next to the star, it gets destroyed by reduce. Get a spelljammer and fly as close to the sun as safely possible. Then true polymorph yourself into a solar dragon, the one creature that can survive being on a star, and fly the remaining distance. Then you can survive the sun’s death and destroy it.

Well, if you permanently True Polymorphed yourself, you wouldn't have to maintain concentration on it.  But then you'd need Subtle Spell to cast Reduce.

Though this still assumes that the sun is a valid target for the spell in the first place.  I don't think an object the size of house is, much less a celestial body.

Well subtle spell is just 1 feat or 3 levels sorcerer so maybe the guy could take it. The sun thing is up to interpretation, but there’s probably a reason nobody’s cast enlarge/reduce on the planet before.

Step 1. Obtain immortality (Lich?)

Step 2. Gather as wizards as possible and indoctrinate them with your plan.

Step 3. Go out there with them (Investiture of flame or something and a spelljammer0

Step 4. Cast ice spells until something interesting happens to the sun

Hopefully the end but if not...

Step 5. Unintentionally but likley be forced to fight the largest coalition of beings in your setting to prevent there from ever being such thing as a sun destroyer

If your setting is religous...

Step 6. Possibly isolate the sun god and annhilate him by focusing the full force of your wizard gang onto him (use their 9th and 8th level spell slots maybe 7th and 6th too)

Step 7. With no god to power or command the sun it either goes out or falls apart.

Step 8. Success

How to destroy a star in Dnd? Well, it depends if it is an actual star. This is a fantasy world. It is based off of tales and legends that we think are cool. There are so many stories from all over the world about what the sun and stars are. Just pick one or a couple that seem cool and then decide how it can be destroyed. Here are some examples:

• In Greek and other mythologies, heroes were often awarded a place in the heavens as constellations. This was a sort of way to grant them immortality, and in Dnd you could translate that more literally. Maybe they have to destroy several of the stars at once, or if they destroy a single one of them, it would be like stabbing a guy's leg. That guy, the celestial hero, will want revenge and might attack. Or, in the attempt to destroy the stars, they stir up the wrath of the gods who created them and aren't going to let that happen. Really there always should be some powerful figure trying to prevent this destruction. 
• Also in Greek mythology, the sun was the chariot of the sun god. To destroy the sun would require going up into the sky, killing the sun god, and tearing apart his chariot. We can see from the myths that this would have terrible effects on the earth though. It would in real life too.
• Maybe the stars are the spirits of the ancestors and killing one will anger the other ancestors. Their immortality is in danger and their friend has been murdered, maybe they will empower someone on earth or come down themselves to seek revenge.
• In Narnia, the stars are an entire people group. Not much is known about them for the ways of the Stars are not for the mortal to know. Going up and destroying one of them would require killing an actual celestial person probably, and this would bring the wrath of the other stars down on you.
• Maybe the stars are lighting bugs stuck to the sky. Imagine having to fight your way up into the heavens, passing all sorts of figures who attempt to stop you from destroying such a sacred thing, and you find a bunch of lightning bugs all squirming on the sticky sky. Destroying a star would simply require you to squish the bugs.
• In one of my worlds, the star is actually a blazing arrow shot by a celestial archer each day. In order to destroy the sun, you would have to break every single arrow in the archer's quiver and smashing one would kill a normal person. 
• In that same world (I have weird worlds), the sun used to be a giant, flaming pterodactyl. Then it was shot down by the sun archer who made arrows out of its bone. 
• In several African and American folk tales, the sun was hoarded by some sort of creature and the hero went to steal it. In those stories the sun could be kept in a basket. Maybe the sun is just a small, tangible object that is quite hot and bright. Destroying it might just require a big hammer, but the challenge lies in getting past the guardians.
• Maybe the sun is the eye of the Sky. Maybe the Sky created the world for his own enjoyment and watches the Earth by staring down at the ground through his massive flaming eye. The moon is his other eye, and the sky is really just his face. Destroying the sun would require poking out his eye, but he would see it coming and is so large and powerful that would be practically impossible. 
• In Terry Pratchett's work, the stars were used by giant flying space turtles to incubate their eggs. Destroying one would mean you would have to get past the protective mother, and once it is destroyed, the eggs would die and that would have bad consequences.
• Maybe the sun is the beating heart of a celestial figure and the other planets are other organs. Destroying the sun would kill the universe.
• It is said that when Fenrir wolf is released from his unbreakable chains, he will swallow the sun. So if you want to destroy the sun, just unleash that world ending puppy.

I recognize this mostly deals with the consequences, challenges, and flavor of destroying the sun, but I hope it was still interesting.