"Carusus" wrote in message
...
Several questions. Hope somebody here can explain.

Each second the sun burns 10 million tonnes of hydrogen. The sun is at
the same time getting gradually denser due to nuclear fusion in its
core. What is the overall effect of these two factors over time on the
sun's gravitational pull: does it increase or decrease? Does it vary
according to the sun's position on the main sequence? Is this different
for non-main sequence stars?

All stars emit vast amounts of energy. From the famous equation e=mc^2 (i.e.
the energy emitted actually has a mass), it follows that all stars are
losing vast amounts of mass. For the Sun, this is 4.25 million tonnes per
second. I'd imagine a pretty vast amount of mass is lost from solar wind as
well. The mass gain from comets/rocks crashing into the Sun is many times
less than this, so overall, the Sun is losing mass. This basically means
that the Sun's gravitational field strength is decreasing.



The earth is at the same time accreting débris from space - from the
solar wind and from solar system residue. What is the effect of this -
and the above changes in the sun's gravitational field - on the earth's
orbit?

The Earth's orbit should increase in radius - mainly due to what I've said
above.


There are also tidal forces on the earth (and to a much less extent on
the sun) which tend to slow its rotation. Does this translate into
angular momentum in the earth's orbit? In how much time will the earth
have one face permanently pointing to the sun

Initially, the Earth will become tidally locked with the Moon, but this will
not happen until the Sun has finished it's red giant phase. The tidal forces
from the white dwarf Sun - perhaps having only around half of its current
mass will mean that the Moon would gradually spiral inwards and get broken
up as it nears the Earth. Not until this happens would there be an opportuni
ty for the Earth to get tidally locked with the Sun. I think we're talking
hundreds of billions of years, if not trillions of years for this to happen.

, with the other looking
away into permanent night? Will this be a problem before the increased
heat of the sun turns the earth into another Venus?

No, it's expected that Earth will begin to have a runaway greenhouse effect
in only a billion years or so,

Ric