Thermonuclear Reactions in the Sun

Transcription

1 Thermonuclear Reactions in the Sun

2 No Need for Confinement! The enormous self-gravity of the sun holds it together. There is no way the fuel can escape (or the sun can blow itself apart). So the nuclear reactions just keep ticking away in the hot central regions, slowly and steadily releasing the energy that keeps the sun in equilibrium.

3 A Solar Model Astrophysics is Highly Mathematical

4 Let s Turn Hydrogen into Helium 4 H à He + energy (from E = mc 2 ) Protons are consumed, but a big lump of He is the result. (Less than 1% of the mass actually vanishes completely! ) n Atomic masses: Hydrogen 1, Helium 4 n The Hydrogen nucleus is a proton n Helium contains two protons (positively charged) and two neutrons (uncharged)

5 One Obvious Problem How can 4 protons (4 H) merge to produce a Helium with 2 protons and 2 neutrons? Where do the extra positive charges go? (Charge is conserved it cannot simply vanish!)

6 A Second (Less Obvious) Problem What are the chances of four protons running into each other all at the same time to form a Helium nucleus?

7 Exploring The Analogy Two-car accidents are much more common than multiple-car collisions. So too with the atomic nuclei in the stars. A proton has a good chance of running into another single proton, but not three at once!

8 Instead: A Sequence of Events Called the pp (proton-proton) Chain (No need to memorize these steps! But let s count the protons in the figure to make sure that 4H à He)

9 Meet Two New Players In the first step, two protons merge as shown. A positive charge is carried off by a positron (symbolized here as β + ). One proton turns into a neutron! The resulting lump (1 proton + 1 neutron) is called a deuteron. It is an isotope of heavy Hydrogen A particle called a neutrino (ν) is also emitted. We will learn much more about this in a later presentation.

10 After That The new deuteron merges with another proton to form a light isotope of Helium (2 protons, 1 neutron) and releasing a gamma ray (an energetic photon of light) Later still, two isotopes of light Helium merge, yielding regular Helium (and releasing two protons). The net effect, as promised: 4 H à He + energy!!

11 Once Again

12 Another Visualisation

13 and Yet Again:

14 The Critical Points about the p-p Cycle Net effect: 4 H à 1 He + energy (Again, note that only a fraction of the original mass is converted to energy via E = mc 2. Less than 1% of it vanishes, with a big lump of He being left over.) Positrons are created Neutrinos are released (more on this later!)

15 More on Positrons:`Antimatter For every type of particle, there is a corresponding antiparticle. When they meet, they annihilate completely and are converted to energy. They have equal masses. If they are charged, they must have opposite charges (conservation law). The positron (+) is the anti-electron (-). When a positron meets an electron, they annihilate and produce a pair of gamma rays (radiant energy = light of very high energy).

16 Antimatter was Actually Predicted before its Discovery

17 Antimatter in Fiction Remember E = mc 2! Question: could we ever make, save, transport and use antimatter -- say, as an ultimate weapon?

18 The Practical Reality Physicists at CERN managed to capture a small number (some dozens) of antimatter particles for ~1000 seconds (about 16 minutes) in a special magnetic trap

19 So: What Use Is a Positron? It can t be contained: it will annihilate with an electron the instant they meet! We can t build vacuum chambers that are empty enough to prevent this happening quickly! But we use positrons every day in hospitals, in PET scans (Positron Emission Tomography). How??

20 Positrons are Formed Not Only in Thermonuclear Reactions! Positrons are also emitted by some naturally radioactive isotopes. These isotopes can be included in organic molecules (food or drink) and ingested or injected, after which they accumulate in (say) a tumour in the body.

21 PET Scans As they decay, the radioactive isotopes then produce positrons right where we need them! Positrons meet electrons, and the tumour lights up, glowing in gamma rays that we can detect.

22 PET in Diagnostics

23 A Lesson For Us All Positrons ( anti-matter ) were predicted by the pure theoretical mathematics of quantum mechanics, many decades ago There was no obvious marketability Yet they now provide a life-saving medical methodology: a totally unexpected spinoff of pure physics research You just never know