# Thread: What limits the size of stars

1. I've watched a show on discovery and seen how smaller stars can burn for longer than larger stars, but then fade out slowly, rather than the larger ones ending with a massive force.

However how small can a star be, and how large can one be, how do stars become the size they are?

2.

3. it really depends which part of the star lifecycle you mean.

however from birth to main sequence
as far as i know a star must accumulate enough mass so that the internal energy of the star increases until the temperature is large enough for the fusion of hydrogen.

I'm not sure about the maximum size of the star. I think it would be an interesting function though.
The mass it attracts would be due to the gravitational attraction of the star. and therefore you could argue that the a star would accumulate mass exponentially...however like you said. the larger stars burn out quicker therefore reducing the time period for accumulation...interesting mathematical question.

4. The stellar quantities are well described by the Hertzsprung-Russell-Diagram. Here is a nice example (Sonne = Sun, heißer = hotter, kälter = colder, heller = brighter, dunkler = darker in German; horizontal axis: surface temperature in Kelvin, vertical axis: luminosity in solar units):

The stars start as fairly large objects before they begin the fusion process. They collapse until their core temperature is high enough to support the fusion. As soon as this happens, the star produces a balance between the radiation pressure pushing from the inside and the gravitational pressure pushing from above determining the final size of the star. In the HRD, it is described as the "main sequence", where they have the smallest size during their lifetime. It is still not clearly known, what starting conditions are needed to form either a low-mass or a high-mass star.

At the end of their lifetime, the stars bloat and develop into the "giant" and eventually "super giant" range. The path in the HRD, however, strongly depends on their mass. Low mass stars (< 8 solar masses) expell their outer shells, while the core slowly cools down as a "white dwarf". High-mass stars evolve into a supernova leaving only a neutron star or a black hole.

5. Originally Posted by rjc79
I've watched a show on discovery and seen how smaller stars can burn for longer than larger stars, but then fade out slowly, rather than the larger ones ending with a massive force.

However how small can a star be, and how large can one be, how do stars become the size they are?
The smallest stars have a mass of about 0.04 that of our Sun. Any smaller than that and not enough pressure exists at the center to maintain fusion. These are called Brown Dwarfs

The largest stars, the Hyper Giants, can get up to about 150-200 times the mass of the Sun. The limiting factor comes in during formation. Once that cloud of gas forming the star colapses enough for fusion to start at the center, the release of energy tends to push the outer layers of the cloud away. Since larger stars burn faster and hotter, there is a limit to how much of the cloud can collapse to form a star before its energy output blows away the rest of the cloud.

6. An interesting site:

http://www.rense.com/general72/size.htm

The Size Of Our World

7. Originally Posted by Dishmaster
.... ..... radiation pressure pushing from the inside and the gravitational pressure pushing from above ... ... ...
Can this explain the cause of temperature inside Earth?

8. Originally Posted by sak
Originally Posted by Dishmaster
.... ..... radiation pressure pushing from the inside and the gravitational pressure pushing from above ... ... ...
Can this explain the cause of temperature inside Earth?
No, because the earth is composed of an incompressible fluid, quite contrary to the gaseous plasma of a star. The internal temperature of the earth is caused by natural radioactive decay, predominantly happening in the core.

9. Originally Posted by Dishmaster
... ....No, because the earth is composed of an incompressible fluid, quite contrary to the gaseous plasma of a star. The internal temperature of the earth is caused by natural radioactive decay, predominantly happening in the core.
Can you explain more, give reference of some web? I wonder how radioactive decay is the reason for such a huge temp.?

10. Originally Posted by sak
Originally Posted by Dishmaster
... ....No, because the earth is composed of an incompressible fluid, quite contrary to the gaseous plasma of a star. The internal temperature of the earth is caused by natural radioactive decay, predominantly happening in the core.
Can you explain more, give reference of some web? I wonder how radioactive decay is the reason for such a huge temp.?
The core of the earth contains some amount of natural radio isotopes like Uranium and Thorium that decay slowly under heat dissipation, just like in a commercial nuclear power plant. However, it seems that this is not enough to account for the total heat flow observed in the earth. There are models that attribute quite some amount to the dissipation by the interaction of the terrestrial magnetic field with the earth's inner layers. An interesting publication can be found here:
http://www.pnas.org/content/98/20/11085.full

A general start into the topic can be:
http://en.wikipedia.org/wiki/Inner_core

There is also a number of references to found there. I hope that helps for now.

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