Thursday, August 20, 2009

Stars 101: The Fundamentals

Stars are perhaps the most fundamental building blocks of the universe. Without stars our universe would be a dark, cold (a few degrees above absolute zero) mixture of 72% hydrogen, 28% helium, and less than a tenth of a percent lithium. Period.
Stars are also where the four fundamental forces in the universe come together to fight for celestial supremacy. In one corner, coming in as the weakest force in nature - but the only one that can work on astronomical scales - is gravity which tries to crush the star down into a black hole. Gravity is responsible for bringing together clouds of gas and dust that collapse down to what are known as proto-stellar nebulae. Eventually as gravity compresses the cloud it will become hot and dense enough in the middle to bring the other major contender into the ring and a star will be born.

In the other corner, coming in as the unification of all of the other fundamental forces in nature, is the triple-threat of electromagnetism, the weak nuclear force, and the strong nuclear force. Despite being vastly more powerful than gravity, these three forces can only wield their full power at very small scales. They need environments where pressure and temperature become high enough to bring atoms close enough for these forces to work. The “Big Three” lead to nuclear fusion, which tries to blow the star apart and will eventually be the death of all stars. Let the battle begin.

So if stars are simply the result of the battle between gravity and the Big Three, what makes one star different from another? Every known celestial object is electrically neutral, so electromagnetism can’t produce differences between stars. The nuclear forces only care about the types of elements present (via the protons and neutrons in the nuclei) and since all stars are born form the Big Bang’s leftovers, they are all made of essentially the same two elements - hydrogen and helium - in the roughly the same proportions. That leaves gravity as the one thing that can be changed. The more massive a star is the stronger gravity pulls, resulting in higher pressures and temperatures in the core of the star, which increases the rate at which fusion occurs, bringing the forces back into balance and making the star burn hotter and brighter.

So in a rough sense, stars are who they are because of their mass and a balance between the most basic forces in nature.


  1. only gravity and mass tell you what type of star you will have? That is awsome. Is that why the earth is not a star?

    didn't someone around here say they read a book on how the sun will kill us? How does that happen?

  2. Wow Nick, this is really well written. I too am amazed how much physics can result from a star's mass alone.

  3. What is the estimated ratio of hydrogen and helium in the Universe now?

  4. Stan, good question. See here:

    H ~ 70.57%
    He ~ 27.52%
    O ~ 0.5920%
    C ~ 0.3032%

    So the universe has not strayed far from the exact numbers predicted by the Big Bang, and all "straying" is do to stars. So we have to thank stars for the carbon, oxygen, and so forth that gives us life. :)

  5. Sorry, the numbers I quoted were for the solar system. See the link I mentioned to find better numbers.

    For the galaxy we know it is:
    H ~ 73.9%
    He ~ 24.0%
    O ~ 1.04%
    C ~ 0.46%

    Most galaxies have similar percentages to ours so it is safe to assume the universe as a whole is approximately this. Nick can correct me if I am wrong.

  6. To a very good approximation, the only thing that differentiates stars is their mass. I'll devote a future post to how mass changes stars and another on how stars die.

    And yes, our planet didn't become a star largely because it didn't have enough mass to hang onto its hydrogen and helium. After our sun turned on, the Earth was too light and too close to the sun, which blew away almost all of our hydrogen and helium.

    And Joe is right - the chemical composition hasn't changed much since the Big Bang, but most of what has changed is due to, you guessed it, stars.


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