What makes Sun's light travel as parallel beams towards earth? 
Sun's light appear to travel as parallel beams towards earth $_1$. Sun produces electromagnetic radiations through pp chain and other reactions in Photosphere $_2$.    
I don't see whether these reactions send photons in that neatly arranged parallel rays, or else any other effects make the rays have such parallel beautiful motion.  
So what makes Sun's light travel as parallel beams towards earth?   
Links: A very good news for those who wants to desperately know the answer. I have replaced the word  "ray" with "beam", as both have difference in their meaning. I have extracted a small passage from the webpage Light beam - Wikipedia :  

A light beam or beam of light is a directional projection of light energy radiating from a light source. Sunlight forms a light beam (a sunbeam) when filtered through media such as clouds, foliage, or windows.   

If the passage is saying truth, we are only left with the question, how clouds (the question is concerned with this) form beams?

Reference: $_1$ Crepuscular rays-Wikipedia $_2$ Sun-Wikipedia
 A: The Sun rays are not exactly parallel. They seem parallel as "Phonon" says, see her comments, but at a precise examination they are not so. Are you aware of the Hanbury Brown Twiss experiment? Look in Wikipedia at the site 
http://en.wikipedia.org/wiki/Hanbury_Brown_and_Twiss_effect, 
and see the lower picture in that article. I copied the picture below. The experiment was done by observing the star Sirius but with our Sun it can be done also. In short, rays coming from a points $a$ on the star's disk, reach on the Earth two points, A and B, and so do rays from another point $b$ from the star's disk.

A: Suns rays are isotropic, but if we assume a small area $dA$, then the rays would be perpendicular to that crossection, and parallel with eachother.
A: Reading some of your earlier comments, it seems that there is a slight confusion at play here. From a comment on the original question:

I want to know how these photons are sent in that parallel order. As I understand or imagine, photons should be thrown randomly from Sun's surface.

Technically, you are correct: photons - including streams of photons following the same path, or "back-to-back" as you have said - are emitted in every direction from the surface of the sun. The sun is not a big flashlight, pointing only at the Earth; it's a big flame, shining all around it.
However, just because the photons leave the sun in all directions, does NOT mean that they strike the Earth from all directions. In order for that to happen, the sun would have to be all around us.
If you briefly imagine the Sun and Earth as points, and draw a quick sketch of the sun emitting straight rays of light, you will see that all the rays that actually strike the Earth (rather than missing it) are going the same direction - are parallel, in other words.
Now, the Earth is larger than a point, and the Sun is larger than the Earth, so the average of all the rays that leave the Sun and hit the Earth actually converge, but because the Earth is so small and so far away (i.e. it subtends a very small angle from the Sun) the rays are almost completely parallel, coming uniformly from the direction of the sun.
Concerning the question of how the clouds form sunbeams, the short answer is that they simply cast shadows that provide enough contrast so you can see the sunbeams. The long answer is that under a clear conditions the entire sky looks like sunbeams; sunlight scattering off of dust and gases in the atmosphere: and with the clouds in the way, only certain portions of the sky allow light through to the ground, so you see distinct lit beams along the path the light is taking from the sun to the ground, against a dark background.
A: Close to the surface of the Sun, the light it emits is not in parallel beams at all. It sends photons out in pretty much every direction at random, like this:

But the Sun is far away from Earth. Most of those photons don't reach the Earth because they're heading in the wrong direction. Only those photons that happen to be heading toward the Earth actually get there - and the photons that are heading to the Earth are all heading in more or less the same direction. So the photons that actually reach the Earth are all travelling in more or less parallel directions.
Below I've drawn the photons that reach the Earth in red, and the other photons in grey. You can see that the red ones are more or less parallel. (In reality, the Earth is further away than that, so the photons are more parallel than in the diagram.)

To answer your last question: sunbeams are formed when a gap in the clouds lets some light through, like this:

Photons from the Sun are always travelling in more or less the same direction, because they all came from the Sun, but it's easier to notice this when the light is concentrated in one particular place like this.
Finally, we have to explain why you're able to see the beam at all. This is just because some of the photons in the beam collide with particles in the atmosphere, which scatter them in all directions, and some of these scattered photons happen to hit your eye.
A: Light from the sun actually converges on the earth - the sun is about 220 Earths across, so light from one edge and light from the other edge must converge to reach the same point on Earth. The angle is small enough (about 0.5 degrees) that for most practical purposes we can consider the sun to be either a point source or a uniform flat source as needed.
