In message , Richard
Bullock writes

"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.

Over 4.5 billion years you're talking about 4500 x 4.25 x 31 = 600
thousand million million million tons lost by fusion, roughly. Call it
10^24 tons.
But the Sun's mass is 2 x 10^27 tons, so you're losing less than one
part in 2000. The rate goes up drastically at the end of the Sun's life,
and it's been argued the Earth may survive the red giant phase.
And going back to "Carusus" post, non-main-sequence stars - including
the Sun at the end of its life, as well as a lot of giant stars - lose a
lot of mass, Several percent, at least.
--
"Roads in space for rockets to travel....four-dimensional roads, curving with
relativity"
Mail to jsilverlight AT merseia.fsnet.co.uk is welcome.
Or visit Jonathan's Space Site http://www.merseia.fsnet.co.uk