# How long before the sun goes red giant?

#### marcus

Gold Member
Dearly Missed
A french poster (ant284) just came to HomeworkHelp with a question about the lifetime of the sun.

Some say the sun will go red giant when it has consumed 12 percent of its hydrogen.

The sun is now 75 percent hydrogen. We know its mass so we can tell how much hydrogen is supposed to be consumed before red giant stage.

I like trying out natural units (c = G = hbar = 1) which seem
surprisingly serviceable in a wide range of contexts, so I calculated the lifetime of the sun in those units as a check for ant284. It turned out we both got the same estimate of the lifetime!

The key thing one must know is the power or "wattage" of the sun. This determines how fast the hydrogen is being consumed,
and allows one to estimate the life.

What do you think the wattage of the sun is? If you like using metric units then of course you will wish to express the sun's power as a certain number of watts. What number?

Ant284 posted a number of watts for the sun, down in Homework section. But why not guess first? It seems that the sun is important to us so shouldn't we have some idea of its power?

Compared to the natural unit of power c5/G, the sun's power is 1.07E-26. That is just what fraction (about E-26) of the natural power unit it happens to be.
And in terms of the natural unit of energy, conversion of that much hydrogen (12 percent of 75 percent of the mass of the sun) will release 6.0E34 units of energy.
So essentially one just divides 6E34 by E-26 to get the life.
Or more precisely 6E34 by 1.07E-26.
But you may prefer to make the corresponding calculation in metric...

Related Astronomy and Astrophysics News on Phys.org

#### marcus

Gold Member
Dearly Missed
A question about the power of the sun

What do you think the power of the sun is?

1----3.9 x 1021 watts

2----3.9 x 1023 watts

3----3.9 x 1026 watts

4----3.9 x 1029 watts

#### Labguy

Re: A question about the power of the sun

Originally posted by marcus
What do you think the power of the sun is?

1----3.9 x 1021 watts

2----3.9 x 1023 watts

3----3.9 x 1026 watts

4----3.9 x 1029 watts

It is 3.86 billion billion megawats, so unless I did the decimal counting wrong, that makes it 3.86 (your 3.9) x 10 26 watts per second.

Also, your original "Red Giant" question is that the Sun will expand to a red giant in less than 4 billion years, after most of the core hydrogen is used (fused?). But, even before that, swelling and energy output will increase to where life on Earth will be wiped out within 1 billion to 2 billion years from now, long before the final expansion to a red giant. I have seen other models showing we have only about 600,000 years to be toasted, but I'll just have to wait and see which one is correct...

Edit: Damn, I hate doing "math stuff". I just think and read, and leave the "mathing" to others.

Last edited:

#### marcus

Gold Member
Dearly Missed
Re: Re: A question about the power of the sun

Originally posted by Labguy

[[What do you think the power of the sun is?

1----3.9 x 1021 watts

2----3.9 x 1023 watts

3----3.9 x 1026 watts

4----3.9 x 1029 watts]]

It is 3.86 billion billion megawats, so unless I did the decimal counting wrong, that makes it 3.86 (your 3.9) x 10 26 watts....
YOU ARE RIGHT! Stop right there, no need to say watts per second

Allen's Astrophysical Quantities, a classic reference, says
3.826 x 1033 ergs per second, which is
3.826 x 1026 watts
so you could have said 3.8
of 3.9 so we went with that, no big difference

Your "billion billion million" might be off but you ended up with

You say: "I have seen other models showing we have only about 600,000 years to be toasted, but I'll just have to wait and see which one is correct... "

It does seem like a good idea to check out some nearby stars for nice-sized watery planets.

"I just think and read, and leave the "mathing" to others"

Appreciate very much your taking the trouble to do a little
"mathing" in this case.

Last edited:

#### heusdens

Re: Re: Re: A question about the power of the sun

Originally posted by marcus
YOU ARE RIGHT! Stop right there, no need to say watts per second

Allen's Astrophysical Quantities, a classic reference, says
3.826 x 1033 ergs per second, which is
3.826 x 1026 watts
so you could have said 3.8
of 3.9 so we went with that, no big difference

Your "billion billion million" might be off but you ended up with

You say: "I have seen other models showing we have only about 600,000 years to be toasted, but I'll just have to wait and see which one is correct... "

It does seem like a good idea to check out some nearby stars for nice-sized watery planets.

"I just think and read, and leave the "mathing" to others"

Appreciate very much your taking the trouble to do a little
"mathing" in this case.

We must realy hurry up then. Just 600.000 years. We need to make a space colony, before humanity goes extinct.

Lots of things can change in a half-million years.

But if the speed of spacecraft doesn't, then 'we' need to leave ahead of time.

#### marcus

Gold Member
Dearly Missed
Re: Lots of things can change in a half-million years.

But if the speed of spacecraft doesn't, then 'we' need to leave ahead of time.

If memory serves the increase in temp during next 600,000 would only require going out to Mars orbit. It is not red giant business, just a little heating up.

correct me if I have the numbers wrong.

In 600,000 the moons of jupiter might be a possibility

a way to pump energy into the earth's orbit gradually has been proposed by a couple of qualified (but a bit optimistic) guys.

it uses asteroids.

I dont like any of this. I hope humans get to other stars---much more interesting.

But strictly speaking it is not necessary to leave the solar system for life to continue for the next half a million-----just move out from the fire a little

#### Labguy

Re: Re: Lots of things can change in a half-million years.

Originally posted by marcus
If memory serves the increase in temp during next 600,000 would only require going out to Mars orbit. It is not red giant business, just a little heating up.

correct me if I have the numbers wrong.

In 600,000 the moons of jupiter might be a possibility

a way to pump energy into the earth's orbit gradually has been proposed by a couple of qualified (but a bit optimistic) guys.

it uses asteroids.

I dont like any of this. I hope humans get to other stars---much more interesting.

But strictly speaking it is not necessary to leave the solar system for life to continue for the next half a million-----just move out from the fire a little
Marcus type guy:
If we haven't figured out how to travel elsewhere to live happily in 600,000 years, we wouldn't deserve to survive. It is still hard to imagine, but remember that the Wright brothers flew off less than 100 years ago! Also, the two computers aboard the Apollo 11 lander, 1969, each had a total capacity of a whopping 65K.

marcus,
Thanks for the information, I found it quite fastinating. If they (future generations) have to hop over to other 'dirt-clods' in this solar system it could stall off the inevitable…for a time, but like you I hope humans will be able to get out to the stars…

I hadn’t heard about the possibility of pumping energy into the earth’s orbit until you mentioned it just now. Sounds pretty interesting, how about Rocket Ship Earth? Wouldn’t it be a ‘trip’ if the entire earth was to be relocated into another solar system? Without a sun during the flight is there even the remotest possibility people could survive something like that?

#### ray b

all stars are veryable in their output

ours has been more or less stable for a long time

but latest data points to a slow increase in output, that is behind global warming, in addition to the co2 counts increasing

and history of the global temps points to more iceages then warm spells

but this all will get more interesting as the waters begin to rize

#### marcus

Gold Member
Dearly Missed
Originally posted by ray b
all stars are veryable in their output

ours has been more or less stable for a long time

but latest data points to a slow increase in output, that is behind global warming, in addition to the co2 counts increasing

and history of the global temps points to more iceages then warm spells

but this all will get more interesting as the waters begin to rize
Hello Ray and Majin
Ray did you read about the proposal to steer an asteroid close to the earth every 100 years or so
for the purpose of enlarging the earth's orbit gradually
to compensate for this gradual temperature buildup in the sun?

I saw the article written by the two people who proposed the scheme. It was online. In the past 6 months I believe.

Majin would probably like to read it. But I do not have the URL.

It is very much against my intuition and instincts to consider bringing any asteroid or comet into near-collision with the earth-moon system----I shrink from searching for this article. But it was not a silly article. It was a rigorously calculated and knowledgeable proposal by qualified NASA-type people---wacky though it may sound.

Majin the comets of the Oort cloud contain water and are at a high gravitational potential. One set of thermonuclear explosions could cause a comet to fall 1000 AU to make a tight turn at the sun
and the most efficient use of propulsion would be to use another
thermonuclear thrust at perihelion (closest approach to sun where the propellant payoff is greatest) after which the comet would leave the solar system.
Comets are not as difficult to get to go places. They are not so massive as a planet and they have light elements like hydrogen which can be vaporized or ionized for propulsion.

I do not like your fantasy of moving the earth. think more elegantly (as I have seen you are very capable of doing!)

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving