# Life in universe

1. Mar 14, 2015

### Guarana

1. The problem statement, all variables and given/known data
For the development of life time is needed. The period during which the star remains at the main sequence depends primarily upon its mass. Type stars, our sun "yellow dwarf" remain on the main sequence about 10 billion years. Weight is often measured in multiples of the mass of the Sun called. M-Sun.

Life on earth needed about 3.5 billion years to the local civilization could explore the universe, so if life anywhere else go to the same state, there must:

a) Weight of stars to be less than about 1.3 m-Sun.

b) Weight of stars to be less than about 3 m-Sun.

c) Weight stars to be less than about 13 m-Sun.

2. Relevant equations

3. The attempt at a solution
I don't understand why some of that has to be true...

Last edited by a moderator: Mar 15, 2015
2. Mar 14, 2015

### DaveC426913

Because the lifespan of a star is closely (and inversely) related to its mass.

That should be a good push in the right direction to help you with the answer.

3. Mar 14, 2015

### Guarana

Less m-Sun means longer lifespan, right?

4. Mar 14, 2015

### DaveC426913

Are you guessing or are you researching?

5. Mar 14, 2015

### Guarana

Researching! I think it's a, because if you have star with 0,8 m-Sun it stays on main sequence for 25000 milion years and that's close to 35000 - like us.

6. Mar 14, 2015

### DaveC426913

Check your figures. 25000 million years is longer than the universe has existed.

7. Mar 14, 2015

### Guarana

Isn't Universe 13,8 bilion years old?

8. Mar 14, 2015

### DaveC426913

Yes. And the number you wrote is 25 billion years.

9. Mar 14, 2015

### Guarana

My apologies. I meant 2,5 bilions.

10. Mar 14, 2015

### Staff: Mentor

But 2.5 billion years is not the lifetime of a star with 0.8 solar masses...

Stars with a smaller mass live longer, but I think the other direction is more relevant here.

11. Mar 15, 2015

### vela

Staff Emeritus
That's not really a problem, though, is it? It would just mean no low-mass stars have burned out yet in our universe. 2.5 billion years, on the other hand, is a problem because it would be inconsistent with the lifetime of our Sun.

12. Mar 15, 2015

### DaveC426913

Agreed. It's not a problem, it just doesn't help answer the question. The question is about the required minimum lifespan of a star to support life.

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