How big would a star have to be to instantly explode?

[36grit]

I've heard that the bigger the star the shorter it's life span. so, how big would a star have to be live a single instant in time?

[Schrödinger's hat]

I've heard that the bigger the star the shorter it's life span. so, how big would a star have to be live a single instant in time?

 

Well, there's speed of light to consider. The star has to actually have time to form. This takes a fairly long while by our standards.

 

On top of that, there's a factor called metallicity to consider.

The large percentage of the heavier elements there are in a star, the less fusion occurs at a given temperature.

The less fusion that occurs, the less radiation pressure there is holding the star up.

The less pressure there is holding it up, the denser it gets.

And once it gets to a certain level of density it'll overcome degeneracy pressure and form a neutron star or black hole, exploding in a supernova as it does so.

 

The modern universe has a higher proportion of heavier elements, so smaller stars form now than they did in the earlier universe.

I don't remember exact figures, but low-metallicity stars in the early universe were up to 100s of times bigger than our sun. I wouldn't think that anything 1000s of times as large would last very long (ie. after the fusion started, it would slow the collapse of the gas cloud, but never stop it, and the cloud would never form a stable star before exploding).

[baric]

I've heard that the bigger the star the shorter it's life span. so, how big would a star have to be live a single instant in time?

 

Well, stars cannot get so big that they fuse and go supernova instantly!

 

There is something called the "Eddington Limit". That is a level of mass where the rate of hydrogen fusion in the core is so great that the outward force it generates overcomes the gravitational pull on the outer layers.

 

This is because, as mass increases, the rate of fusion scales up faster than the rate of gravitational pull.

 

Essentially, the young, massive star blows away its outer layers until its overall mass drops below the Eddington limit (slowing the rate of fusion). At that point, it simply becomes a huge star with a very short life (but not instant!)

 

The actual limit is not precisely known and it varies based upon the chemical composition of the molecular cloud that the star forms from. Heavier elements in the cloud lower the limit because they provide more powerful sources of energy than basic hydrogen fusion.

 

Hope that helps. Here's a wiki link for the Eddington Limit: http://en.wikipedia....Eddington_limit

Edited October 20, 2011 by baric