The key point to consider is the competitiion between the strong nuclear force and the electrical force. When particles are far apart (separations greater than around a fermi = 10^-15 m!), the electrical force is much larger than the strong force. It is only when the nuclei get less than about a fermi apart that the strong force can overcome the effects of the electrical force and bind the nuclei together.

This is not a trivial exercise. The electrical repulsion for protons one fermi apart is huge -- E ~ 2.5 x 10^-13 Joules. For a gas to have particles with kinetic energies this large, the temperature of the gas must be around T ~ 17 billion K much, much larger than the temperature in the core of the Sun where T = 15 million K.

Having to overcome the humongous repulsive electrical force between the postitively charged nuclei (and its concomittant requirement of high temperature) forms the primary stumbling block for nuclear fusion in stars and on the Earth

So what is going on? The central temperature of the Sun is T ~ 15 million K, well below the limit so indeed,What is the scoop?

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Quantum Mechanical Tunneling

The key to this lies in some odd consequences of Quantum Mechanics. To get a feel for the effects, consider something known as the Heisenberg Uncertainty Principle.

So, what happens is this. Particles cannot overcome the electrical repulsion (classically) because the repulsive electrical force (hill) is too large. However, a small fraction of the time, particles can seemingly tunnel through the barrier and get close enough together to allow the strong force can bind the nuclei together. The shorter the distance the particles have to tunnel (the closer they can approach to each other), the greater the probability that they will be able to tunnel. As a result of tunneling enough nuclear reactions take place in the core of the Sun at the low temperature of 15,000,000 K to allow the shine at its observed rate!

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Evolutionary State of the Sun

Hydrogen is the simplest element (1 proton), it has the weakest electrical charge and so has the weakest electrical barrier to fusion. Because of this, it is the first nuclear fuel to be used. Consequently, we know that the Sun is in the first stage of its nuclear fusion lifetime. After it uses up the hydrogen in its core, the Sun will be forced to burn helium, the next most complex element (2 protons).

An alternate (and, in my mind, a better) definition of Main Sequence stars is that they are stars that are currently using hydrogen to helium fusion in their cores to supply the energy for them to remain in thermal and hydrostatic equilibrium.