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Carusus
July 9th 03, 11:41 AM
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?
I seem to remember reading that over time the sun's force of gravity
increases.
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?
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, 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?

That's enough questions!

Richard Bullock
July 9th 03, 12:21 PM
"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

Greg Neill
July 9th 03, 04:01 PM
"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?
> I seem to remember reading that over time the sun's force of gravity
> increases.

The density of the Sun will not affect its gravitational pull
on the planets (provided that its contents remain inside their
orbits). Only a change in mass will do that.

The Solar Constant at the Earth's distance is about 1370W/m^2,
so taking the surface area of a spherical surface at the
same radius, we find that the total energy flux is

1370W/m^2 * 4*pi*(1.496x10^8km)^2
= 3.85x10^26W

or 3.85x10^26 Joules/sec

Using E = m*c^2 and solving for mass, we find that the Sun
loses about 4.3x10^9 kg/sec. Compare this with the total mass
of the sun, some 2*10^30 kg. It's practically negligible. The
time for the Sun to lose 1% of its mass at this rate is about
150 billion years. So there's no reason to worry about it. The
Sun will have been long past its red giant stage by then.


> 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?

As shown above, the Sun's gravitational field will not change
perceptibly, and the Earth, being so much less massive than
the Sun, will not have its orbit affected by any change in
its own mass. Remember, all bodies fall at the same rate in
a gravitational field due to the equivalence principle. The
purtubations caused by the other planets, most notably Jupiter,
have a much greater long term effect than any mass changes of
Sun or Earth.

> 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, 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?

The Earth tides due to the Sun will act to move anglular
momentum from the Earth's rotation to its orbit around the
Sun. But the time scale for any significant change is vast,
certainly longer than the Sun's lifetime.

Robert Ehrlich
July 9th 03, 04:02 PM
What a relief! I thought it was 500 million years or so. Dang! why
did I buy the big air conditioner.

Richard Bullock wrote:

>"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
>
>
>
>

Jonathan Silverlight
July 9th 03, 06:37 PM
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

Odysseus
July 10th 03, 03:11 AM
Carusus wrote:
>
> Many thanks for your prompt answer and clear explanations. However, your
> first answer does not seem to take account of the increasing density of
> the sun, which presumably affects its gravitational pull.
>
As Greg points out (it appears his posting arrived just after you
sent the above) the density or distribution of mass in a body has no
effect on the gravitational force it produces from a distance: only
the total mass counts, and it may be treated as if it were all
located at the body's centre of mass.

So if the sun's density is increasing this implies it will contract
more than one would expect from the loss of mass alone, but the
gravitational force experienced by the earth will continue to be a
simple function of the sun's remaining mass and the distance to its centre.

--Odysseus

Carusus
July 10th 03, 11:31 AM
http://zebu.uoregon.edu/~soper/Sun/mass.html
gives 2x10^30 as the mass of the sun.
Is this correct?
It doesn't change much, other than to reinforce the idea that the
percentage is small.


Jonathan Silverlight wrote:
> 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.

Painius
July 10th 03, 01:59 PM
"Carusus" > wrote...
in message ...
> http://zebu.uoregon.edu/~soper/Sun/mass.html
> gives 2x10^30 as the mass of the sun.
> Is this correct?

Just be careful, Carusus... don't make the mistake *i* did (and i
caught before posting).

Jonathan's units of measurement are "tons," and Greg's units are
in "kilograms." So the mass of the Sun is...

2 x 10^30 kilograms

which is about equal to...

2 x 10^27 tons

> It doesn't change much, other than to reinforce the idea that the
> percentage is small.

If one were using the same units, the difference would be a factor
of a thousand (1,000). That might be significant enough to alter
our views about the Sun's age, term of life, death and other qualities.

And it might give us good questions to ponder... If the Sun's mass
were 1,000 times less (or 1,000 times more), then what would its
life expectancy change to? Would it "die" differently? Would it
now be a different color to our eyes? If 1,000 times more mass,
would it then have enough mass to go nova? supernova?

Studying questions like these might give us more precision as to
the ages of other celestial objects we observe.

happy days and...
starry starry nights!

--
Life without love is
A lamp without oil,
Love without prejudice
A world without soil,
Tool without toil.

Paine Ellsworth

Greg Neill
July 10th 03, 02:39 PM
"Carusus" > wrote in message
...
> http://zebu.uoregon.edu/~soper/Sun/mass.html
> gives 2x10^30 as the mass of the sun.
> Is this correct?

Not without specifying the units. In this case, kg would
be the units, and the value 2x10^30kg would be a reasonable
approximation to the mass of the Sun. I've seen a figure
of 1.989x10^30kg quoted.

Jonathan Silverlight
July 10th 03, 10:06 PM
In message >,
Painius > writes
>
>And it might give us good questions to ponder... If the Sun's mass
>were 1,000 times less (or 1,000 times more), then what would its
>life expectancy change to? Would it "die" differently? Would it
>now be a different color to our eyes? If 1,000 times more mass,
>would it then have enough mass to go nova? supernova?

Well, if current theories are right a star 1000x the mass of our sun is
unstable and explodes, while 1/1000 of the mass of the sun is almost
exactly the mass of Jupiter, so it isn't a star any more.
--
"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

Odysseus
July 11th 03, 01:43 PM
Greg Neill wrote:
>
> "Carusus" > wrote in message
> ...
> > http://zebu.uoregon.edu/~soper/Sun/mass.html
> > gives 2x10^30 as the mass of the sun.
> > Is this correct?
>
> Not without specifying the units. In this case, kg would
> be the units, and the value 2x10^30kg would be a reasonable
> approximation to the mass of the Sun. I've seen a figure
> of 1.989x10^30kg quoted.

NASA's "Planetary Fact Sheet" for the sun agrees; their figure is
1,989,100*10^24 kg. I'm not sure whether this is supposed to have
seven significant figures or five, but I'd guess the latter.

See <http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html>.

--Odysseus