# Thread: The Life of the Sun (and other stars)

1. ## The Life of the Sun (and other stars)

How do we calculate the life of a star like the sun? What model do we use? We obviously cannot tell how much fuel is left to burn in the sun so we are making a guess by observing other Main sequence stars like the Sun (I prefer the term Yellow Dwarf)?? Is that how it works? I know you can generally tell the age of a star by it's size and reading what elements are in it. But how can we be so sure that our Sun wont start the process of turning into a red giant in a 5000 years time?? What makes us so sure?? BTW if you could explain it to me in laymans terms that would help as maths is my weakest point. That and spelling.

2. In simple terms we can use Kepler's third law to deduce the stars mass and we know its size very well and can therefore deduce its density. From that we can deduce the probable chemical ratio of elements. All of this confirms that our "yellow dwarf" is still primarily hydrogen and has a long life ahead of it.

3. So basically it's a educated guess?

4. Originally Posted by dirty_g
So basically it's a educated guess?
Only in the sense that we have not gone there and counted every proton. The science is rock solid.

5. Hooboy! You have opened such a can of worms! Our knowledge of how stars (including the Sun) work is a complex interaction and interlacing of much of physics. As was mentioned, we know the Sun's mass and volume and therefore, its density. We also know how much energy (from nuclear fusion it takes to keep it from collapsing from gravity. Nuclear physics tells us how much energy is released from fusion reactions at different temperatures. These numbers match very well with the calculated temperatures and pressures at the sun's core. We know how old the Earth and Moon are and have a pretty good idea about how long it takes for planetary systems to form after the star ignites. And, that just scratches the surface!

6. I realise that the oldest rock samples are about 4.5 Billion years old from the moon? So we can place the Sun to be at least this old. So then from our current knowlege of main sequence stars we can say we have about another 4.5 Billion years before the Sun expands in size (if it is iteslf around 4.5 billion years old). But have we seen how old rock samples are from Venus or Mars? The same age? How can we calculate the Mass of the Sun? What instrument do we use?

7. Originally Posted by dirty_g
1. have we seen how old rock samples are from Venus or Mars? The same age? 2. How can we calculate the Mass of the Sun? What instrument do we use?
1. We have some pieces of rock that we believe came from Mars (including the famous one that may or may not have bacterial spore fossils). They are much less than 4.6 billion years old. We also have measured the age of meteorites, and only a few of them have grains that are older than 4.6 billion years.

2. We know the mass of the Sun because of knowing the gravitational constant, the sizes of the planetary orbits around it, and the duration of the planetary orbits. This is something we know to a high degree of precision.

8. Originally Posted by dirty_g
But how can we be so sure that our Sun wont start the process of turning into a red giant in a 5000 years time?? What makes us so sure?
The energy producing process of the sun, fusion, is understood. So much hydrogen is currently being fused with itself that it is depleting its resource at a rate that will last, fortunately for us, for billions of years.

(I prefer the term Yellow Dwarf)?
Now that's an educated guess that is probably incorrect.

9. Originally Posted by dirty_g
How do we calculate the life of a star like the sun? What model do we use? We obviously cannot tell how much fuel is left to burn in the sun so we are making a guess by observing other Main sequence stars like the Sun (I prefer the term Yellow Dwarf)?? Is that how it works? I know you can generally tell the age of a star by it's size and reading what elements are in it. But how can we be so sure that our Sun wont start the process of turning into a red giant in a 5000 years time?? What makes us so sure?? BTW if you could explain it to me in laymans terms that would help as maths is my weakest point. That and spelling.
I have some brief tutorials on stellar structure and evolution (written imprecisely at the general education level for non-science majors). Here they are: 1, 2, 3, 4

In particular reference to our Sun-
The late John Bahcall's webpage, looking under the "solar neutrino" links on the left panel, maintains the astrophysical papers and standard solar models. This is one of several other recent technical papers out there that models the Sun from its arrival on the Zero Age Main Sequence to the present time to determine the age.

As noted above, the answer to your question involves a MAJOR field in astrophysical research, which itself is an application of much of physics. Our detailed, if still incomplete, understanding of stellar structure and evolution is far beyond an "educated guess".
Last edited by Spaceman Spiff; 2006-Dec-13 at 12:19 AM. Reason: clarification in the last sentence

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