 Deep inside a star's core, protons are colliding at a tremendous rate, but few of these collisions result in a fusion of the two protons. That's because when protons collide, they have to overcome a repulsive electric force due to the fact that they are both carrying a positive charge. This is called the Coulomb barrier. In order to understand why star luminosity is so sensitive to small increases in temperature, we need to see how this barrier is breached. We'll start with a look inside the Sun's core. Using the known relationship between temperature and kinetic energy, we can calculate the average thermal energy and velocity of the protons in the Sun's core. It depends entirely on the temperature, 15 million degrees Kelvin. We find that each proton has on average 2 kilo-electron volts of kinetic energy and travel at just over 600,000 meters per second. That's well over a million miles per hour. With this we can calculate the number of times a proton will collide with another proton. The number depends on the proton density, cross-section, and thermal velocity. To cover cross-section in the How Small Is It? video book, it represents the target area for determining a collision versus a miss. We calculate that each proton experiences over 1 trillion collisions per second. We can also calculate the Sun's fusion rate per proton. Dividing the mass of the Sun's core by the mass of a proton gives us the number of protons in the core. Dividing the fusion rate calculated earlier by the number of protons in the core gives us the fusion rate per proton. And if we divided this by the trillion collisions per second, we get the number of fusions per collision. We see that the probability that any particular collision will result in a fusion is extremely small. That's why a proton's trillion collisions per second can go on for billions of years before one of them results in a fusion event.