# Basic question about sun's mass & orbit of planets

## Main Question or Discussion Point

This came up on another web site I read, and it got me to wondering, so I am certain someone here could answer this.

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? Or does some other event intervene before this happens?

Thanks for any answers or insight.

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malawi_glenn
Homework Helper
Sun will not loose that much mass, calm down, don't panic ;-)

Sun will blow up to a red giant and die before it have lost so much mass due to fusion and solar winds that planet's can't be bound anymore.

An interesting thing is that due to the sun's mass loss, planets will get a larger radii in their orbits, thus maybe the most inner planets will survive to be swallowed by the volume increasing sun.

One of my teachers in astrophysis told me that such calculations have been performed.

Thank you for your reply. This brings up another basic question. (I know I could get the answers just by studying on my own and I will study it more.) How do we know for certain that the sun will go red giant? How do we know when that will happen? Just based on evidence of other stars? I need to buy a couple bags of ice the night before.

Astronuc
Staff Emeritus
This came up on another web site I read, and it got me to wondering, so I am certain someone here could answer this.

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? Or does some other event intervene before this happens?
Actually such calculations are around here at PF somewhere.

It's a common question, especially in some introductory courses in astrophysics or stellar astrophysics.

The Sun also produces a 'solar wind' of particles at a rate of about 10^-14 solar masses per year. In 4 billion years this amounts to about 0.001 percent of the Sun's mass . . .

Our understanding of stellar (and solar) evolution come from a combination of observation and theory, of which part is modelling (simulation) of the physics of stars. Is one familiar with the term 'main sequence' and Hertzsprung-Russell diagram?

I am familiar with the terms supernova, red giant, white dwarf, neutron star and black hole, but not the terms 'main sequence' or 'Hertzsprung-Russell diagram.'

Astronuc
Staff Emeritus