**Ask a monkey a physics question thread**

[MadMojoMonkey]

Evaporate? You mean cease to exist? That's a mere thought experiment that cannot happen in the real world.

No. The sun is "evaporating" in multiple ways.
Yes. None of these ways will cause the sun to cease to exist - as such.

As a back of the envelope calculation, it gives off over 4 million tons per second in light energy by nuclear fusion converting mass into heat and light.
As a back of the envelope calculation, it throws off over 1.6 million tons per second in protons, neutrons and electrons.

As a simple observation, these numbers are so many orders of magnitude smaller than the sun's mass that the sun is within 0.05% the mass it had 4.5 billion years ago, counting only these losses.

Matter doesn't just disappear, its gravitational influence will always exist.

Definitely no. Mass-energy changes forms from one expression of mass-energy to another and back again all the time.

All
the
time.

The sun will "evaporate" over a period of hundreds of billions of years.

Well, given stellar evolution, this is mostly true. The sun will eventually start fusing helium and become a red giant, and ultimately will experience significant mass loss during the later stages of this process, leaving behind a white dwarf.

As the sun expands, its density will reduce, and as such it will warp spacetime to less of an extent... it will decrease in gravity.

This is a very confusing topic to find good information on as a layman.

I suspect there are many planets worth of heavy metals in the core of any non first generation star like our sun. When people talk about the core of a star, they're not talking about this. They're talking about first generation stars which started as mostly pure Hydrogen with a smattering of pre-fused Helium in the core. Meaning that the only components of the star's core are it's own fusion products.

Regardless:
The core is increasing in density over time.
The fusion processes increase in activity over time.
The outer layers are pushed harder by the increased activity, so the size of the star increases.
More material is ejected as more energetic fusion processes occur in the core.

The inner part of the star is getting more dense, but the overall size of the star is increasing. The rate at which it loses mass increases, too. The total density goes down and the total mass goes down. Then at the end of the star's evolution, it sheds its entire "atmosphere" causing its total size to drop dramatically, leaving only the core. Its total density goes up dramatically.

Density is only related to gravity when you're up close.

Far away:
If the sun were replace with a hypothetical equivalent mass of different density, the Earth would orbit that object just the same. Replace the sun with a black hole. Earth don't care that it's much more dense than a star, just that the total mass is the same and in the same place.

Up close
To a very good approximation, stars are made of shells which have nearly constant density throughout that shell. The symmetry of that situation means we can treat the mass like a single particle. That is, the density is a function of the radius from the center and how much mass is in the imaginary sphere represented by that shell, M. All the mass outside that shell cancels out, due to [mathematical awesomeness]. The gravity felt at any position on the shell is that of a single particle with mass M located at the center of the shell. So the density of the shells inside effect the gravity, but not the density of the shells outside.

But this will be the least of our problems on Earth, we'll be long gone before the decreased gravity results in our planet drifting away from the sun.

The Earth is already drifting away from the sun.

I mean, real slow like.

It's gonna end up inside the red giant version of the sun, even though it will be receding more quickly then. Once it gets close enough, friction will drop it to the core to hang out with Mercury and Venus.

Trying to imagine the gravitational consequences of the sun suddenly disappearing is a waste of time. You'd be better off trying to figure out how to rig the lottery... at least that might be possible.

Don't be jaded, man.

There is so much to gain by asking these kind of hypothetical questions. Sometimes you don't even know where to start, so you start somewhere you know is silly, but you then take it seriously and see what you learn.