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Red Giant Sun

SalixIncendium

अग्निविलोवनन्दः
Staff member
Premium Member
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?
 

Revoltingest

Pragmatic Libertarian
Premium Member
The Sun's mass would remain constant (approximately)
during expansion. I'd expect the gravitational field to
remain the same (approximately).
 

Regiomontanus

Ματαιοδοξία ματαιοδοξιών! Όλα είναι ματαιοδοξία.
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?


Gravity Force of a Spherical Shell

"This proves that the force from any spherically symmetric mass distribution on a mass outside its radius is the same as if the total mass were a point mass concentrated at the center of the sphere."
 

Regiomontanus

Ματαιοδοξία ματαιοδοξιών! Όλα είναι ματαιοδοξία.
The Sun's mass would remain constant (approximately)
during expansion. I'd expect the gravitational field to
remain the same (approximately).


upload_2021-3-5_8-39-58.jpeg



Though, it seems as if singularities may not actually exist (which would make sense to me; infinities for physical quantities should raise red flags).

https://phys.org/news/2014-09-black-holes.html
 

Altfish

Veteran Member
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?
Will I have time to have my hair done?
 

Revoltingest

Pragmatic Libertarian
Premium Member
View attachment 48244


Though, it seems as if singularities may not actually exist (which would make sense to me; infinities for physical quantities should raise red flags).

https://phys.org/news/2014-09-black-holes.html
I've no math background in this stuff. But I read about time
slowing down as matter enters the black hole (due to gravity
& speed) & wondered about the problem of angular velocity
& slowed time preventing matter from ever forming a singularity.

I only know enuf to ask ignorant questions. We're lucky
that the universe is so interesting.
 

WalterTrull

Godfella
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?
Well... as the sun expands it will tend to get silly. Some planets just won't appreciate the loss of gravity and flee to other systems. Some will simply join the silliness and be consumed.
 

Heyo

Veteran Member
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?
As others already pointed out, gravitation would largely stay the same. There are two minor factors that could change the planets orbits.
1. For planets that are engulfed with the expanding sun, the attraction shrinks as a (small) part of the mass of the sun is then outside the orbit. That would widen the orbit.
2. The coronal mass causes (a tiny bit of) friction, slowing the planets down, contracting the orbits.

In short, not much will happen. When the sun has gone through the red giant phase and shrunk down to a White Dwarf, the planets will be where they were before, slightly singed. At least if no other event kicks them out of orbit.
 

Regiomontanus

Ματαιοδοξία ματαιοδοξιών! Όλα είναι ματαιοδοξία.
I've no math background in this stuff. But I read about time
slowing down as matter enters the black hole (due to gravity
& speed) & wondered about the problem of angular velocity
& slowed time preventing matter from ever forming a singularity.

I only know enuf to ask ignorant questions. We're lucky
that the universe is so interesting.

It would only slow down from the point of view of an observer. If you and Mrs. Revoltingest, holding hands as I know you always do in public, crossed the event horizon things would seem normal to you (until you were turned into spaghetti of a diameter of the order of magnitude of molecules, because of the steep gradient).
 

Revoltingest

Pragmatic Libertarian
Premium Member
It would only slow down from the point of view of an observer. If you and Mrs. Revoltingest, holding hands as I know you always do in public, crossed the event horizon things would seem normal to you (until you were turned into spaghetti of a diameter of the order of magnitude of molecules, because of the steep gradient).
I share that understanding.
So from our perspective, matter falling in at the event
horizon could still possibly remain there for what
seems an eternity
 

Regiomontanus

Ματαιοδοξία ματαιοδοξιών! Όλα είναι ματαιοδοξία.
No use, The hair will get burnt, possibly evaporate.

Yes and no. The accretion disk of gas and dust around an active black hole is hot. But the BH event horizon itself is very cold (and BHs do not always have accretion disks anyway).


"The most massive black holes in the Universe, the supermassive black holes with millions of times the mass of the Sun will have a temperature of 1.4 x 10^-14 Kelvin. That's low. Almost absolute zero, but not quite.

A solar mass black hole might have a temperature of only .0.00000006 Kelvin. We're getting warmer.

Since these temperatures are much lower than the background temperature of the Universe – about 2.7 Kelvin, all the existing black holes will have an overall gain of mass. They're absorbing energy from the Cosmic Background Radiation faster than they're evaporating [Hawking radiation] and will for an incomprehensible amount of time into the future.

Until the background temperature of the Universe goes below the temperature of these black holes, they won't even start evaporating.

A black hole with the mass of the Earth is still too cold.

Only a black hole with about the mass of the moon is warm enough to be evaporating faster than it's absorbing energy from the universe.

As they get less massive, they get even hotter. A black hole with the mass of the asteroid Ceres would be 122 Kelvin. Still freezing, but getting warmer.

A black hole with half the mass of [asteroid] Vesta would blaze at more than 1,200 Kelvin. Now we're cooking!

Less massive, higher temperatures."


https://phys.org/news/2016-09-cold-black-holes.html
 

Revoltingest

Pragmatic Libertarian
Premium Member
Yes and no. The accretion disk of gas and dust around an active black hole is hot. But the BH event horizon itself is very cold.


"The most massive black holes in the Universe, the supermassive black holes with millions of times the mass of the Sun will have a temperature of 1.4 x 10^-14 Kelvin. That's low. Almost absolute zero, but not quite.

A solar mass black hole might have a temperature of only .0.00000006 Kelvin. We're getting warmer.

Since these temperatures are much lower than the background temperature of the Universe – about 2.7 Kelvin, all the existing black holes will have an overall gain of mass. They're absorbing energy from the Cosmic Background Radiation faster than they're evaporating [Hawking radiation] and will for an incomprehensible amount of time into the future.

Until the background temperature of the Universe goes below the temperature of these black holes, they won't even start evaporating.

A black hole with the mass of the Earth is still too cold.

Only a black hole with about the mass of the moon is warm enough to be evaporating faster than it's absorbing energy from the universe.

As they get less massive, they get even hotter. A black hole with the mass of the asteroid Ceres would be 122 Kelvin. Still freezing, but getting warmer.

A black hole with half the mass of Vesta would blaze at more than 1,200 Kelvin. Now we're cooking!

Less massive, higher temperatures."


https://phys.org/news/2016-09-cold-black-holes.html
Could it be that this low temperature is only from the
perspective of one outside the event horizon?
Rapidly rotating matter just within the event horizon
could be extremely hot...particularly from the local
perspective.

Just ignorant speculation on my part.
 

Regiomontanus

Ματαιοδοξία ματαιοδοξιών! Όλα είναι ματαιοδοξία.
Could it be that this low temperature is only from the
perspective of one outside the event horizon?
Rapidly rotating matter just within the event horizon
could be extremely hot...particularly from the local
perspective.

Just ignorant speculation on my part.

Right, those low temperatures would be measured just outside the event horizon.
 

Twilight Hue

Twilight, not bright nor dark, good nor bad.
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?


https://phys.org/news/2016-05-earth-survive-sun-red-giant.html

I like this summary of what's going to happen.
 

Polymath257

Think & Care
Staff member
Premium Member
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?

The gravity from a spherical object acts as if the whole mass is concentrated at the center.

So, while an expanded sun would be less *dense*, it would still have the same *mass* (excluding loss from the higher solar winds at that point). And so the gravitational effect of anything *outside* of the sun would remain the same.

Now, the surface of the sun would be at a larger distance from the center, and so the 'surface gravity' would be lower, but that isn't what is relevant for the motion of the planets.
 

Brickjectivity

Turned to Stone. Now I stretch daily.
Staff member
Premium Member
It would appear that science generally accepts that the sun will eventually become a red giant, consuming the planets closest to it.

Red Giant Stars: Facts, Definition & the Future of the Sun | Space

Which leaves me with the following question:

The planets of the solar system stay in orbit because of the sun's gravity. Why would it be concluded that as the sun expands that the gravitational pull would remain constant and the planets would remain in their orbit. Wouldn't it stand to reason that as the sun expands, the sun's gravity would weaken resulting in orbit of the planets expanding outward?

Why or why not?
The Sun puts out a solar wind of particles, and you can come up with an average rate per thousand years. That rate might relate to its surface temperature, surface activity, surface area. Over time this will slightly decrease the mass of the Sun and therefore its gravitational force, but by how much?

Is the Sun Losing Mass? | by Brian Koberlein

The above article says its losing 174 trillion tons per year. You just multiply that by 5 billion years (or however many years) and then recalculate the strength of the Sun's gravitational pull. Then you can recalculate the shape and size of the planetary circuits.
 
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