The Naked Scientists

The Naked Scientists Forum

Author Topic: How big does a ball of ice need to be in order to start atomic fusion?  (Read 5053 times)

rhlopez

  • Guest
Ricardo Hernandez Lopez asked the Naked Scientists:

How big a ball of ice needs to be in order to start atomic fusion?

What do you think?


 

Offline Supercryptid

  • Hero Member
  • *****
  • Posts: 606
    • View Profile
    • http://www.angelfire.com/sc2/Trunko
The part of the ice that is going to be fused first is the hydrogen. So let's look at something that fuses hydrogen. It is estimated that stars need to have a minimum mass of 75 to 87 times the mass of Jupiter in order to fuse hydrogen (this may be lower if you consider brown dwarf stars, which can fuse less common isotopes of hydrogen like deuterium).

Here are my thoughts: An ice ball around that mass range would have an enormous gravitational field that would try to crush it. Whenever something is compressed, it gets hotter. If gravity can compress it enough, then the heat and pressure will rise enough to turn the center into a super-heated gas of plasma that may undergo fusion.

So my guess is 75-87 times the mass of Jupiter is how large you need to make your ice ball. Someone correct me if I'm wrong, though.
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
My feeling is that you would need more ice mass for hydrogen fusion to take place.

I'm not sure how, or at what pressure/temperature it may happen, but wouldn't the oxygen and hydrogen separate? If so, then as oxygen has more mass than hydrogen, wouldn't that make it sink to the centre of the mass? The hydrogen would, therefore, be further from the centre of the mass, not under as much pressure and, consequently, not at such a high temperature. To get the hydrogen up to fusionable temperature you would need to increase the pressure/temperature which means more initial mass.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
a quick guess would be about the size of the sun or a bit smaller (down to about 10% of the size of the sun.  It is interesting to note that the mean density of the sun is about equal to water.  The effect of the presence of a large quantity of Oxygen is possibly a problem stars need a bit in amongst some carbon and nitrogen to get the normal fusion cycle going.  it is possible that large quantities could make the star unstable.
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
The effect of the presence of a large quantity of Oxygen is possibly a problem stars need a bit in amongst some carbon and nitrogen to get the normal fusion cycle going. 

How can that be? When the first stars formed there was only hydrogen, helium and a teensy-weensy bit of lithium. Oxygen, carbon & nitrogen and products of the nuclear fusion in stars, not a pre-requisite for it.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
The very first generation of stars were significantly different from current stars and they needed to have significantly larger masses to get direct helium fusion reactions started and fusion up to the higher atomic weights without the presence of carbon nitrogen and oxygen as a catalyst.

see   http://www.astro.yale.edu/larson/papers/SciAm04.pdf
« Last Edit: 28/12/2008 23:01:48 by Soul Surfer »
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
I know that early stars were metal-poor compared to later stars; and the very first stars had no metal at all apart from, maybe, a tiny amount of lithium. Would it be the relative abundance of metals in the later stars that makes oxygen, carbon & nitrogen necessary? I must say, I have never come across that requirement in any literature I've read on the subject.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Look up the carbon - nitrogen cycle for helium fusion in stars.  It was first proposed by Fred Hoyle and is generally accepted as the most likely process in most main sequence stars beyond the very first generation. 

You have to remember that in nuclear reactions in general only one thing can happen at a time ie one particle can join a nucleus.  To convert hydrogen into helium requires four particles to be linked together ie two protons and two neutrons.  remember also neutrons are not totally stable in isolation and decay back to protons and electrons.  Also to convert a proton into a neutron is a weak interaction process that is much slower than the strong interaction processes.

In nuclear synthesis of higher elements getting beyond the beryllium gap is quite difficult because two helium atoms cannot join together to form beryllium  because that needs an extra neutron to be stable
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
I know of the beryllium gap and that it was Hoyle who proposed a solution; but I wasn't aware what the solution was. Thank you.
 

Offline Mr. Scientist

  • Neilep Level Member
  • ******
  • Posts: 1451
  • Thanked: 2 times
  • http://www.facebook.com/#/profile.php?ref=profile&
    • View Profile
    • Time Theory
Hoyle was amazing. He found how carbon was produced inside of stars involving a special type of nuclear reaction with three alpha-particles, which themselves are made up of two protons and two neutrons.

Fred Hoyle was one of the min scientists who grabbed my imagination in my youth.

 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums