What is the most efficient way to send an object to the sun?

[tori]

There's an even better way - go to the Scenario Editor and set your debris' velocity components to 0 relative to the Sun. Just make sure you're not in Earth's way when you're doing that, things get nasty real quick when there's a giant orb of rocks and water going at you full speed with just 1 Mm of distance to think in. If you get lucky the universe divides by zero, integrates garbage and your debris item is outta there.

Seriously now, there's no practicality in dumping things into stars. We're people, we like things not being dumped into other things, particularly if they're white hot glowing balls of plasma. If you go through all the trouble of collecting debris then you can just deorbit it back to the Earth and be done with it, it'll get recycled by nature and the propellant company won't force another mortgage on your house. Sun is such a wasteful overkill.
If you feel more like a space pirate, then tow your space booty to your pirate base (and sell it on piratebay).
If you really need to get rid of the object (radioactive goodness inside, etc.), slam it into the Moon. A couple hundred pounds of plutonium dispersed over the area of Mare Vaporum won't even show on our geiger counters, so no biggy there, go ahead.
If it's a classified military spacecraft, then, again - Moon it! It's not like "crashing" into the Earth, there's no atmosphere to tuck you gently into the soil. You go slam, giving off all of your energy in a couple feet of rock solid ground. Good luck trying to find two molecules of the original craft still together.

The only reason that's pro sun-dumping is... drumroll... TV. Stuff heading straight for the Sun makes for good television. A reality show with death row inmates in a derelict skylab, headed for the Sun... imagine the ratings.

 

[Linguofreak]

But what I can't find yet, is the metallicity at which a main sequence star starts to become a red giant.

That depends on mass. From all indications, red dwarves will remain convective from core to surface through their entire lives, and thus will burn themselves entirely to helium and fade out without ever becoming red giants.

 

[RisingFury]

That depends on mass. From all indications, red dwarves will remain convective from core to surface through their entire lives, and thus will burn themselves entirely to helium and fade out without ever becoming red giants.

Ouch! Convection all the way to the surface? I know for Sol, it goes to about a third of the way, from the core out.

 

[Cairan]

Instead of dumping stuff into the Sun, using the moon, Jupiter, Earth-Venus-Venus assists and whatever else is available, I think everyone is missing the obvious point:

If you need to get rid of any "evidence" in the inner Solar system, send it to Jupiter itself. You only need the delta-v to get from Earth to Jupiter, it's freaking hard to miss if you aim for ANY impact with Jupiter and it's much quicker...

Just ask comet Shoemaker-Levy 9... ;P

 

[Urwumpe]

Ouch! Convection all the way to the surface? I know for Sol, it goes to about a third of the way, from the core out.

You mistake radiation zone for convective zone. The sun has the radiation zone one third out of the core, the convection zone is the entire rest until the photosphere.