Physics/Astronomy question

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This came up on another web site I read, and it got me to wondering.

The sun is constantly losing mass, as it converts its mass to energy. As it loses mass, its gravitational field weakens, because the gravitational field of a body is directly related to its mass.

The gravitational field is what holds the planets in orbit around the sun. At some point, the mass of the sun decreases to a critical level, at which the sun's gravitational field is too weak to hold the planets in orbit any more, so they would just leave orbit and fly off into space. When does this happen? Has anyone tried to calculate this?

Thanks for any answers or insight.

[SteppingRazor]

1. If I'm not mistaken, I believe the sun finishes converting iron in its core before that happens...

meaning that, long before we go spinning off into the universe, we'll be incinerated as the sun goes red giant.

[Az]

2. It goes red giant and burns the planets to a crisp long before that

The rate of loss is no where near enough to allow the planets out of its grasp in any reasonable time.

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4. Thanks - that brings up another question

I guess I could research this on my own to find out, but since the replies came so quickly... How do we know for sure that the sun will go red giant, and how do we know when that will happen?

[Orrex]

5. Astrology

Duh.

[TreasonousBastard]

7. Because we have a pretty good idea how the different...

types of stars live and die. The Sun is fairly common, smallish, yellow star that has a lifetime of about 10 billion years or so, and this is what happens to smallish yellow stars:

http://en.wikipedia.org/wiki/Sun:

"The Sun does not have enough mass to explode as a supernova. Instead, in about 5 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 million kelvin and will produce carbon, entering the asymptotic giant branch phase.<16>

Earth's fate is unclear. As a red giant, the Sun will have a maximum radius beyond the Earth's current orbit, 1 AU (1.5×1011 m), 250 times the present radius of the Sun.<30> However, by the time it is an asymptotic giant branch star, the Sun will have lost roughly 30% of its present mass due to a stellar wind, so the orbits of the planets will move outward. If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the Sun owing to tidal interactions.<30> Even if Earth escapes incineration in the Sun, its water will be boiled away and most of its atmosphere would escape into space. In fact, even during its life in the main sequence, the Sun is gradually becoming more luminous, and its surface temperature is slowly rising. The increase in solar temperatures is such that in about a billion years, the surface of the Earth will become too hot for liquid water to exist, ending all life.<30><31>"

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8. I guess I better get a few bags of ice.

Thanks for the info!

[phaseolus]

9. a wikipedia link

...because it was easy to find, and wikipedia's pretty reliable when it comes to science:

http://en.wikipedia.org/wiki/Stellar_evolution

[Orrex]

3. It's not losing a lot of mass, relatively (ha ha) speaking

The mass difference between two hydrogens and one helium is pretty darned small.

Granted, we're talking really large-scale here, but there's a whole lot of Sun, and atoms are really small!



Disclaimer: I'm not entirely sure that I know what I'm talking about, but this seemed like a good stab at it at 12:30 in the AM.

[Dead_Parrot]

6. You are sort of right, but there isn't a 'critical level'

As the gravitational field decreases, the Earth (and other planets) move further out because they are traveling a little too fast for the orbit: I think the current rate works out about 1cm per year, but it's too small to measure. The more mass the sun looses, the further out we go, but we never go flying off into space.

[bananas]

10. As the sun loses mass, escape velocity decreases

so at some critical value for the sun's mass, escape velocity will drop below the earth's velocity.

[TheMadMonk]

11. Nope. That's not how orbital mechanics works.

There is no critical threshold. As the Earth moves outwards from the sun it's orbital velocity slows down and it remains in orbit about the sun regardless of how much mass the sun loses.

[bananas]

13. Ok, thanks. nt

[Dead_Parrot]

12. I don't think the sun looses that much mass...

...even as a white dwarf (in however many billion years time) - and we've been eaten by the red giant by then. :scared: It would actually be interesting to see what would happen (from outside, and speeded up) if the planets all survived and the sun did loose that much mass - the increasing eccentricity of the orbits would make for some fascinating interactions. Anyone for planet pool?

[BlooInBloo]

14. F = G M1 M2 / d^2 answers the question.